11

The word corrosion is as old as the earth, but it has been known by different

names. Corrosion is known commonly as rust, an undesirable phenomena which destroys

the luster and beauty of objects and shortens their life. A Roman philosopher, Pliny

(AD 23–79) wrote about the destruction of iron in his essay ‘Ferrum Corrumpitar’.

Corrosion since ancient times has affected not only the quality of daily lives of people, but

also their technical progress.

The term “Atmosphere Corrosion” comprises the attack on metal exposed to the air

as opposed to metal immersed in a liquid. Atmospheric corrosion is the most prevalent

type of corrosion for common metal [1].

Corrosion is a chemical phenomenon which is related to metals. Corrosion is one

of the most interesting fields of electrochemistry. The word Corrosion comes from the

Latin word “Corroder”, which means “gnaw away”. Corrosion is defined in many ways.

For simplicity, it can be defined as “the spontaneous process of degradation and

deterioration or destruction of metallic construction in the course of their chemical,

biochemical or electrochemical interactions with the surroundings.” In terms of

thermodynamics, “Corrosion is the reversion or partial reversion from the meta stable

condition of the metal to stable condition of its compound accompanied by the reduction

in the free energy of the system”. Common examples of corrosion are rusting of iron and

steel, tarnishing of silver, dulling of brass, fogging of Nickel, etc. In some cases, chemical

and electrochemical attack may be accompanied by physical deterioration and is described

by the terms such as corrosion-erosion, corrosive-wear or fretting-corrosion [2-3].

According to these general definitions materials, other than metals such as

ceramics, plastics and concrete may also corrode. Mild steel finds its application in many

industries due to its easy availability, ease of fabrication, low cost and good tensile

strength, besides various other desirable properties. It suffers from severe corrosion when

it comes in contact with acidic solutions during acid cleaning, transportation of acid,

descaling, storage of acids and other industrial processes. The heavy loss of metal as a

result of its contact with acids can be minimized to a great extent by the use of corrosion

inhibitors. Inorganic compounds like chromates, phosphates, molybdates etc., and a

variety of organic compounds containing heteroatom like nitrogen, sulphur and oxygen are

being used as corrosion inhibitors for mild steel [4-10].

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If the metal (or) alloy structure is not properly maintained, they deteriorate slowly

by the action of atmospheric gases, moisture and other chemicals. This is the main reason

for metallic corrosion [11]. The secret of effective engineering lies in controlling rather

than preventing corrosion, because it is impracticable to eliminate corrosion [12].

Atmospheric corrosion is a subject of global concern because of its importance to

the service life of equipment and durability of the structural materials. While there is a

general agreement on the possible types of parameters that may lead to corrosion, these

studies suffer severely from the lack of generality in the sense that their predictive

capability is extremely poor.

Atmospheric corrosion is a very important practical process that causes

deterioration of structures, machines and materials placed at external environments

[13-16]. It constitutes a relatively complicated electrochemical process that consists of a

metal and its corrosion products, an electrolyte (a thin wet film on surface) and the

atmosphere (more or less polluted). Electrolyte’s composition depends on the air

pollutants deposition rate, and changes with the humidity conditions of the atmosphere.

In order to study the atmospheric corrosion effects several outdoors studies are

frequently developed [17-19]. These studies normally involve metals and alloys exposition

to the action of several atmospheres in different geographical regions for certain period of

time. Recompiled data could be used to obtain corrosion maps that may allow to evaluate

the atmospheric corrosion effect as a function of exposition time and several climate

factors (i.e. relative humidity, temperature, pollutants content) [20].

Mathematical models developed to explain atmospheric corrosion at a particular

region cannot be extrapolated to other places. Possibly, because the number of variables

considered by these models is very low in comparison with the great number of variables

that really influence atmospheric corrosion processes. Therefore, it is necessary to develop

corrosion studies for each particular zone or region.

‘Rust never sleeps!’ So says a popular song. As the metals contain more free

energy than the corresponding metallic oxides, hydroxide, carbonates, etc. from which

they originates, there is an intrinsic tendency for metals to revert to such compounds and

give off energy in the process. Corrosion or the conversion of a metal back into its oxide,

etc. is a surface chemical reaction only, and there are, therefore, a number of ways of

slowing down or even stopping this reaction.

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Corrosion can be fast or slow. Corrosion rate vary from place to place, from hour

to hour and from season to season.

[1.1] DEFINITION OF CORROSION:

The term “corrosion” has its origin in Latin. The Latin word rodere means“gnawing,”

and corroder means “gnawing to pieces.”

Corrosion is a complex form of materials deterioration. It has been defined many

ways, some of which are:

(1) “Corrosion is the destructive attack of metal by chemical or electrochemical reaction

with its environment” [21].

(2) Corrosion may be defined as unintentional attack on a material through reaction with

a surrounding medium [22].

(3) “Eating away” of metals.

(4) “Corrosion is the deterioration of a substance or its properties because of a reaction

with its environment”[23]. There have been recent tendencies to include the

destruction of materials other than metals such as glass, plastics, ceramics, leather,

and cloths etc., which find usage in modern society.

[1.2] DEVELOPMENTS IN CORROSION SCIENCE:

During the Gupta Dynasty (320–480 CE), the production of iron in India achieved

a high degree of sophistication, as attested by the Dhar Pillar, a 7-tonne (7000 kg),

one-piece iron column made in the fourth century CE. The existence of this pillar implies

that the production of iron from oxide ore was a well-established process, and the

personnel involved in the production of the iron pillar were aware of the reverse reaction

involving the oxidation of iron to produce iron oxide (the familiar rusting of iron) and of

the need to minimize the extent of this reverse reaction. Copper nails coated with lead

were used by the Greeks in the construction of lead covered decks for ships. The Greeks

probably realized that metallic couples of common metals are undesirable in seawater.

Protection of iron by bitumen and tar was known and practiced by the Romans.

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TABLE – 1. Timeline of Developments in Corrosion Science

No. Name of scientist Year Development

1. L.J. Thenard 1819 Enunciated electrochemical nature of corrosion

2. Sir H. Davy 1829 Principle of cathodic protection

3. A. de la Rive 1830 Established best quality of zinc for galvanic batteries

4. M. Faraday 1834-1840 Established relations between chemical action and generation of electric currents based on what were later called “Faraday’s laws”

5. S. Arrhenius 1901 Postulated the formation of microcells

6. W.R. Whitney 1903 Confirmed the theory of microcells

7. A.S. Cushman 1907 Confirmed the theory of microcells

8. W. Walker 1907 Established the role of oxygen in corrosion as a cathodic simulation

9. A. Thiel Luckmann 1928 Investigated the attack of iron by dilute alkali with liberation of hydrogen

10. U.Evans 1928 Observed increased corrosion rate when a small anode is connected to a large cathode

11. T. Finnegan 1939 Investigated attack of iron

[1.3] COST OF CORROSION:

A well-known study performed by the National Bureau of Standards and Battle

Memorial Institute at the request of US Congress found that the cost of metallic corrosion

alone approaches 4.2% of gross national product, or roughly $ 180 billion in 1985 [24].

The most basic community to the most sophisticated, atmospheric corrosion has the ability

to influence a nation’s economic health.

Corrosion represents a tremendous economic loss and every effort is now being

taken to reduce it. Corrosion is probably the greatest consumer of the metal known to the

man. The tonnage of metals like steel, copper, aluminum, lead, zinc and tin lost through

corrosion is extremely high. The cost of corrosion is enormous considering the many

costly processes involved in the manufacture of metals. The estimated annual cost of

corrosion in the United States varies between $ 8 billion and $ 126 billion. Corrosion of

bridges is a major problem of the age and requires replacement which cost billions of

dollars [25]. Corrosion is a cancer of metal and alloys and causes direct and indirect

15

losses. The cost of direct loss due to corrosion has been estimated to be about 3 to 4% of

GNP (Gross National Product) in developed countries. In India, the amount is calculated

to be roughly 2-3% of GNP. The estimated cost of losses in India is about more than

hundred crore rupees every year [26]. Indirect damages of corrosion are loss of good will,

personal injuries, and health hazards etc. Economic aspects of corrosion are Capital

investment, Operating cost, Maintains and Overhead cost.

Daeves and Trapp, who are quoted by Barton [27] have calculated that 2% of steel

production in Germany in 1937 was being converted back to rust by atmospheric corrosion

each year. It is estimated that the financial loss in the U.K. alone caused by corrosion is

approximately £ 600 million each year. Often it is the failure of quite small components

which make a large capital installation useless.

Corrosion in automobile fuel system alone costs 100 million dollars per year.

Auto-radiators account for about 52 million dollars. The estimated cost of corrosion

automobile exhaust system is 500 million dollars. Approximately 3 million home water

heaters must be replaced each year [28].

According to Rajgopalan, direct cost of corrosion in various sectors of Indian

economy in 1984-1985 can be summarized as under (Table-1.1).

Table-1.1 No. Sector Direct cost of corrosion

(Rs. in crores) 1. Agriculture, live stock forestry and fishery 1800 2. Extraction of mineral resources 84 3. Manufacture of fold leather and textile products 60 4. Wood and paper products 42 5. Petroleum refining and chemical products 125 6. Glass refractory and chemical products 8 7. Manufacture of ferrous and non-ferrous metals 140 8. Fabricated metal products 50 9. General machinery 55 10. Special machinery 40 11. Transport equipment 94 12. General and special chemical:

- Appratus - Ordinance - Passenger traffic - Public utilities - Construction including highways

- Trade and business services

96 40 250 240 300 600

16

In India, due to its tropical climate, the corrosion problem is more serious than in

the colder countries. Losses are divided into (1) direct losses, and (2) indirect losses. By

direct losses are meant the costs of replacing corroded structures and machinery or their

components. Direct losses include the extra cost of using corrosion-resistant metals and

alloys. The indirect losses may include contamination of the product, shut-down of the

unit or plant while repairs or replacement of some part of the plant, loss of efficiency

occur due to the diminished heat transfer through accumulated corrosion products, loss of

production and safely. Corrosion in automobile fuel systems, auto-radiators, automobile

exhaust systems etc. costs million dollars per year. The costs to other industrialized

nations are similar.

[1.4] HISTORICAL REVIEW:

The history of atmospheric corrosion research may be divided into the following

stages:

1. Studies by Graedel and Leygraf (90th) [29] :

Most recently, studies by Graedel and Leygraf have again concentrated on the

adsorption of gaseous species into the corroding surface and subsequent chemical

reactions occurring in the thin electrolyte layer. These studies mainly aim to an

understanding of the atmospheric corrosion of electronic materials.

2. Studies by Stratmann (80th – 90th) [30] :

Stratmann concentrated also first on studies referring to rust reduction and

reoxidation using rust covered Au electrodes and found very specific reactions taking

place in the potential scale of interest.

3. Studies by Misawa (70th) [31] : He complemented his investigations by IR-spectroscopy and showed, that a high

number of thermodynamically metastable phases exist in particular so called green rust

phases which are quite important in an understanding of the formation of stable oxides.

17

4. Studies by Vernon and Buckowiecki (30th to 50th) [32-33] :

During this period of time was realized, that thin water layers being present on the

steel surface are responsible for the atmospheric corrosion. It was proven, that the critical

humidity necessary to trigger the corrosion reaction it linked to the presence of

hygroscopic salt particles. This was the first evidence that the atmospheric corrosion

cannot be described as a simple oxidation reaction but has to be discussed in the

framework of electrochemical reaction kinetics.

5. Studies by Schikorr (60th) [34] :

Schikorr was among the first to discuss a unique electrochemical mechanism of the

atmospheric corrosion.

6. Studies by Evans (70th) [35] :

U. R. Evans was among the first to develop an electrochemical reaction model,

which took explicitly into account the continuous wetting and drying of the surface during

atmospheric corrosion. He considered for the first time besides oxygen reduction the

reduction of iron (lll) oxides as a potential second cathodic reaction and realized that this

reaction could only be of importance for in stationary corrosion conditions like wetting of

a dry surface.

7. Studies by Pourbaix (70th) [36]: Pourbaix was also interested in the cyclic corrosion behaviour of steel surfaces

during atmospheric corrosion and in addition tried to explain the specific corrosion

mechanism of weathering steel.

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[1.5] BASIC PRINCIPLES OF ELECTROCHEMISTRY AND CORR OSION:

Stable Meta Stable Stable

Metals occur in nature most commonly as oxide or sulphide ores in which they are

in a higher oxidation state than that of the free metal. Extraction of metal from the ore

involves reduction of the oxidized form to free metal, resulting in an increase in internal

free energy. Consequently, the metal will try to lose its excess energy by becoming

oxidized again, through loss of electrons. This oxidizing tendency of a metal is the driving

force for the corrosion and it is found in virtually all metals except very noble metals such

as Gold and Platinum.

Whether metals will corrode in certain environments or not depends upon the

thermodynamics concept. According to thermodynamic theory to complete any reaction of

a system, the value of free energy changes (∆G°) should be negative.

∆G° = -nFE°

Where, ∆G° = free energy change,

n = number of electrons involved in the reaction.

F = Faraday constant,

E = Cell potential.

If the value of ∆G is negative then corrosion will take place. Thus, for a reaction to

be feasible, the cell potential E must be positive.

Different types of electrochemical reaction, depending upon the chemical nature of

the environment are as follows [28].

METAL CORROSION PRODUCT ORE Extraction

Reduction

Corrosion

Oxidation

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NEUTRAL MEDIA

ANODE : M → Mn(+) + ne- Mn(+) + n(OH)- → M(OH) n

Corrosion product (Rust) CATHODE : O2 + 2H2O + 4e- → 4(OH)-

ACID MEDIA

ANODE : M → Mn(+) + ne-

CATHODE : 2H+ + 2e- → H2↑ Definition of anode and cathode:

The electrode at which chemical oxidation occurs (or positive current leaves the

electrode and enters the electrolyte) is called the anode. Examples of anodic reactions are :

Zn → Zn2+ + 2e-

Al → Al3+ + 3e-

Fe2+ → Fe3+ + e-

Cu → Cu2+ + 2e-

[Corrosion of metals usually occurs at the anode]

The electrode at which chemical reduction occurs (or positive current enters the

electrode from the electrolyte) is called the cathode. Examples of cathodic reactions are :

H+ + e- → ½ H2

Fe3+ + e- → Fe2+

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Electrochemical series:

To determine which of the two metals in a pair is likely to become the anode and

cathode, and which is likely to remain the cathode, reference is made to the standard

electrochemical series [37]. This gives the e.m.f. (electro motive force) in volts at 25°C in

relation to NHE (Normal Hydrogen Electrode).

Standard Reduction Potentials:

Electrode Reaction E° V Au+3/Au + 1.498 Pt+2/pt + 1.200 Ag+/Ag + 0.799 Cu+2/Cu + 0.337 2H+ + 2e- → H2 ± 0.000 Pb+2/Pb - 0.126 Sn+2/Sn - 0.136 Ni+2/Ni - 0.250 Cd+2/Cd - 0.403 Fe+2/Fe - 0.440 Cr+3/Cr - 0.744 Zn+2/Zn - 0.763 Al+3/Al - 1.662

It is also possible for metals or alloys that have intrinsically higher E° values than

iron to be cathodic when in contact with iron. Fresh aluminium, which is fairly active, has

a distinctly anodic position to iron and steel, the same is not true for aluminium where a

good passive film has formed.

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[1.6] TYPES OF ATMOSPHERES:

Metals are commonly exposed in various atmospheres which may be divided into

the certain groups like:

(1) Industrial:

These atmospheres are associated with heavy industrial manufacturing facilities

and can contain concentrations of sulfur dioxide, chlorides, phosphates and nitrates. An

industrial atmosphere is characterized by pollution composed mainly of sculpture

compounds and nitrogen oxides. Sulphur dioxides from burning coal or other fossil fuel is

picked up by moisture on dust particles as sulfurous acid. This is oxidized by some

catalytic process on the dust particles to sulfuric acid which settle in microscopic droplets

on exposed surfaces. The result is that contaminants in an industrial atmosphere, plus dew

or fog, produce a highly corrosive wet acid film on exposed surfaces.

(2) Urban:

Similar to the rural type in that there is little industrial activity. Additional

Contaminants are of the SOx and NOx variety, from motor vehicle and domestic fuel

emissions. A typical urban atmosphere has much heavier pollution from the products of

domestic combustion and transport, resulting mainly in an increase in sulphur dioxide,

sulphuric acid and dirt with a slight increase in CO2 and chloride. The direction of

prevailing wind and shielding from wind can have a very marked effect on the distribution

of corrosion on structures even in urban areas.

(3) Marine:

Marine atmospheres are usually highly corrosive and the corrosivity tends to be

significantly dependent on wind direction, wind speed, and distance from the coastal area.

Fine windswept chloride particles, deposited on surfaces, characterize this type of

atmosphere.

Marine atmospheres may be expected at sea or in a rural area in proximity to the

sea. A marine atmosphere is laden with fine particles of sea salt carries out by the wind to

settle on exposed surfaces. The quantity of salt contamination decrease rapidly with

distance from the sea, and is greatly affected by wind currents. The marine atmosphere is

also including the space above the sea surfaces where splashing and heavy sea spray is

encountered. Structures and even relatively specimens may be much severally corroded on

the seaward than on the landward side.

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(4) Rural:

This category is generally the least or less corrosive and normally does not contain

chemical pollutants, but does contain organic and inorganic particulates. The principal

corrodents are moisture, oxygen and to a lesser extent carbon dioxide. Arid or tropical

types represent special extreme cases in the rural category.

A truly rural atmosphere is free from pollution, but very slight pollution by

domestic products is not ruled out in the general use of the term. A rural atmosphere does

not contain any strong contaminants, but does contain organic and inorganic dusts.

(5) Tropical:

This type of atmosphere is found in hot countries. In the tropics, in addition to the

high average temperature, the daily cycle includes a high relative humidity, intense

sunlight and long periods of condensation during the night.

(6) Industrial-marine:

Quite moderate industrial or urban pollution appears to be sufficient to mask the

marine effect of some coastal atmospheres. For example, the work of Vernon and Whitby

[38], show that in a purely marine atmosphere the patina on copper consisted essentially of

basic copper chloride.

(7) Urban-marine:

This type of atmosphere is found in city area situated near sea. (8) Tropical-marine:

Atmospheres differ from rural and marine atmospheres respectively in higher

temperatures and humidity which prevail.

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[1.7] BASICS FORMS OF ATMOSPHERIC CORROSION:

There are many forms of corrosion, but it is rare that a corroding structure or

component will suffer from only one.

(1) General or uniform corrosion:

General corrosion can be even or uneven and is the most common form of

corrosion. It is characterized by a chemical or electrochemical reaction in which corrosion

takes place uniformly over the entire exposed surface area of the material without

appreciable localization. In general corrosion surface layers of metal is converted to

corrosion products in such a way that the thickness of the section is uniformly decreased.

(2) Crevice corrosion:

Crevice corrosion is an intensive localized corrosion occurs within crevices and

other shielded area on metal surfaces exposed to a corrosive environment.

Crevice corrosion is more likely to occur in holes, gasket, surfaces, lap joints,

surface deposits and crevices under bolt and rivet heads thjat retain solutions and take

longer time to dry out. This form of corrosion is sometimes called as deposit or gasket

corrosion.

(3) Pitting corrosion:

Pitting corrosion is a form of localized attack that results in localized penetration of

the metal. This is one of the most destructive and insidious forms of corrosion.

Pitting is a deep, narrow attack that can cause rapid penetration of the substrate

(metal) wall thickness. The corrosion is caused by the potential difference between the

anodic area inside the pit and the surrounding cathodic area.

(4) Corrosion fatigue:

Corrosion fatigue [39] may be defined as the reduction of fatigue strength by

corrosion environment. In this, repeated cyclic stress such as shaking, vibration, tapping,

shuttering and flexing, in the presence of corrosive environment although the stress is well

below the normal fatigue limit cause failures. Coating such as electro-deposited zinc,

chromium nickel, copper and nitride coating can improve corrosion fatigue resistance.

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(5) Fretting corrosion:

Fretting corrosion is defined as metal deterioration caused by repetitive slip at the

interface between two surfaces in contact that were not intended to move in that fashion.

A related type of corrosion occurs in the atmosphere is known as fretting. If two

faying surfaces exposed to the atmosphere under load are subjected to vibration incipient

with respect to each other, which produces slight or incipient slip. Fretting corrosion will

develop at the areas of contact. It is presumed that protective corrosion films are destroyed

by the vibration, thus respectively exposing fresh metals to the continuing corrosion

process.

(6) Intergranular Corrosion:

This form of corrosion consists of localized attack at and adjacent to grain

boundaries, causing relatively little corrosion of grains, but resulting in disintegration of

the alloy and loss of strength.

(7) Galvanic corrosion:

Galvanic corrosion occurs because of the Potential differences usually exist

between two dissimilar metals when they are immersed in a corrosive or conductive

medium, if these metals are placed in contact (or otherwise electrically connected).

Several investigations [40] have shown that galvanic corrosion is directly proportional to

the area ratio of the cathodic metal to the anodic metal. Corrosion of the less corrosion

resistant metal is usually increased and attack on more resistant material is decreased.

(8) Erosion Corrosion:

Erosion corrosion is the acceleration of corrosion owing to relative movement of

the corrosive fluid and the metal surface. It is characterized by grooves, waves and valleys

in the metal surface, and short time periods to unexpected failures. Erosion corrosion is

promoted by high fluid velocity, turbulent tow and the impingement of those high-velocity

fluids on metal surfaces, for example, at elbows in pipelines. Erosion corrosion is

obviously minimized by reducing fluid velocities, promoting less turbulent flow and

by the avoidance of sharp changes in flow direction.

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(9) Microbiologically Induced Corrosion (MIC):

MIC is corrosion involving the action of bacteria on metal surfaces, most

commonly in stagnant water. Slime-forming bacteria are aerobic and thrive in most

cooling-water systems. As they metabolize dissolved oxygen from the water, they create

an anaerobic environment at the metal surface. Anaerobic bacteria can then attack the

metal surface. Some bacteria can oxidize or reduce metal species directly, for example

Fe(II) to Fe(III). The ferric compounds precipitate in pipes. Concentration gradients form

under these deposits, resulting in corrosion. Other bacteria can reduce ferric iron to the

more soluble ferrous form. This strips off the ferric compounds which normally stabilize

the surface of mild steel, leaving it reactive. Corrosion is thus accelerated. Other bacteria

can metabolize chromium, there by corroding stainless steels. A major factor in

minimizing MIC is the elimination of stagnant water. A clean metal surface with

sufficiently high fluid velocities will also prevent bacteria from establishing a

foothold.

(10) Stress Corrosion Cracking (CSS):

Stress corrosion cracking is the formation of cracks where localized corrosion has

combined with steady tensile stresses in the metal to cause the damage. This effect has

been seen in low pressure turbine disks and blade roots and also in boiler tubes. The

hostile electrolytic environment can attack particular metals or alloys, for example,

chloride and stainless steels. Excessive SCC can cause failure, typically sudden and

without warning.

Two general theories are used to explain the SCC mechanism. The electrochemical

theory centers on galvanic cell action in the grains and between grain boundaries. The

stress sorption theory suggests that sec proceeds by weakening the cohesive bonds

between surface metal atoms. The source of tensile stresses may originate during

manufacture or from in-service conditions. Lowering tensile stress by decreasing applied

load, stress relieving or introducing residual compressive stress through procedures such

as shot peening will minimize SCC.

SCC is also minimized through chemical control of the water in the system, or

applying coatings to reduce or eliminate contact between the metal and the hostile

ion.

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[1.8] CLASSIFICATION OF ATMOSPHERIC CORROSION:

Atmospheric corrosion can be conveniently classified as follows:

(a) Dry atmospheric corrosion

(b) Damp corrosion

(c) Wet corrosion

(a) Dry atmospheric corrosion (Dry oxidation):

This takes places in the atmosphere with all metals that have a negative free energy

of oxide formation. Here atmospheric water vapour is either virtually absent or if present,

seems to play no essential part in the reaction. In the absence of atmospheric pollution, all

the common metals develop invisible films of oxide rapidly which reach a limiting

thickness since ion diffusion through the oxide lattice is extremely slow at ambient

temperatures. For example, those on iron are typically 30 A° thick. For certain metals and

alloys these films are so fault-free or rapidly self-healing that they confer remarkable

protection on the substrate, e.g., stainless steel, titanium, chromium. In the presence of

traces of gaseous pollution (e.g. H2S) copper, silver and certain other non-ferrous metals

undergo a visible film formation, even at ordinary temperatures; generally known as

tarnishing. If H2S gas is in trace, it may retard the process and may increase when H2S

gas is in excess [41].

(b) Damp atmospheric corrosion:

It is characterized by the presence of a thin, invisible film of electrolyte solution on

the metal surface. It requires moisture in the atmosphere, and suddenly becomes more

serious when the humidity exceeds a critical value. It is favored by the presence of volatile

acidic substances, certain dispersed solid particles in air and also presence of hygroscopic

substance on the surface of the metal, this is sometimes a corrosion product e.g. fogging of

nickel in air containing moisture and sulphur dioxide. The bronze diseases which on

corrosion cause damage in museums also receive consideration.

(c) Wet atmospheric corrosion:

It is characterized by the presence of visible deposits of dew, rain and sea spray

etc., e.g., white rusting of zinc. When metal is exposed to rain, the mechanism of the

attack becomes close to that developed under immersed conditions, although when a thin

stream of rain-water flows continuously over a metal surface, the replenishment of oxygen

27

will be better than when a metallic specimen is immersed in water or when water flows

through a pipe. A drop of rain water held by capillarity, at a place where two plates come

close together, provides a situation analogous to that of a drop placed on a horizontal

metal place and surrounded with moist air so as to prevent evaporation.

[1.9] FACTORS AFFECTING ATMOSPHERIC CORROSION: The rate of corrosion of metal depends on interaction of several environmental

parameters, such as, 1) Atmospheric contaminants (a) Dust particle, (b) Gaseous pollutant.

2) Time of wetness include (a) Critical relative humidity (b) Dew and (c) Rain.

3) Temperature

4) Wind direction and velocity;

5) Effect of corrosion products on corrosion rates.

(1) Atmospheric contaminants: (a) Dust particles:

Vernon [38] classified suspended solids as:

(i) Harmless (Producing no rust) or inert non-absorbent particles e.g., Silica,

which can only affect corrosion by facilitating differential aeration processes

at points of contact.

(ii) Intrinsically corrosive (producing rust where they settle) e.g. (1) Ammonium

sulphate particles which is formed in heavily industrialized areas where

appreciable concentrations of ammonia and SO3 or of H2SO4 aerosol co-exist.

It is a strong stimulator of the initiation of corrosion, being hygroscopic and

acidic, (2) marine salt (saline particles) such as NaCl. Chlorides are also

hygroscopic and Cl- is highly aggressive to some metals, e.g. stainless steel.

(iii) Indirectly corrosive; e.g. charcoal, carbon particles and soot which, by

adsorbing acidic sulphur gases (SO2), can profoundly stimulate corrosion in an

unsaturated atmosphere by catalyze the formation of a corrosive acid

electrolyte solution. Dirt with soot assists the formation of patina on copper

and its alloys by retaining soluble corrosion products long enough for them to

be converted to protective, insoluble basic salts. Studies with carbon particles

have shown the sorptive properties for water and SO2, the catalysis of the

cathodic reduction of O2 and the promotion of SO2 corrosivity [42-43].

28

On a weight basis, dust is the primary contaminant of many atmospheres. The

averages city air contains nearly 2 mg/m3, with higher values for an industrial atmosphere

reaching 1000 mg/m3 or more [44] In contact with metallic surfaces, this dust influences

the corrosion rate in an important way as follows:

(1) They can form galvanic cells with the sheet steel.

(2) They can obscure part of the surface of the steel and thus induce differential

aeration corrosion.

(3) They absorb moisture from the unsaturated air because of that hygroscopic nature

and therefore form an electrolyte on the metal surface.

(4) They can absorb acid materials such as H2SO4 on their surfaces.

(5) On the stainless steel, dust particles may produce local screening from oxygen so

that the protective film fails to kept repair.

(b) Gaseous pollutants: (i) Sulphur dioxide (SO2):

The most important corrosive constitute of industrial atmosphere is SO2, which

originates predominantly from the burning of coal, oil, gasoline and from the fossil-fuel

power plants. Emissions of SO2 in Europe [45] sharply increased after 1950 due to the rise

of oil consumption and amounted about 25 million tons of sulphur by 1970. In 1968, an

estimated 33×106 tons of sulphur oxides were emitted in the U.S.A.

The solubility of SO2 in water is very high (about 40 volumes of SO2 in 1 volume

of water in ordinary condition). SO2 is non-flammable colorless gas having pungent

irritating odour. The concentration required for taste detection ranges from 0.3 ppm to 1

ppm in air and the odour threshold is about 0.5 ppm. Both the chemical composition and

the physical state of the pollutants change during their transport in the atmosphere. The

life time of SO2 in the atmosphere is usually 1.5 to 2 days, which corresponds to a mean

transport distance of a few hundred kilometers.

The term SOx is used to denote the mixture of sulfur oxides emitted into the

atmosphere. SO2 reacts with water to form H2SO3 (sulfurous acid) as,

29

SO2 + H2O → H2SO3 ↔ H+ + HSO3-

SO3 reacts with water vapour to form sulphuric acid as, SO3 + H2O → H2SO4

It is known that the oxidation of SO2 is affected by the presence of metallic oxides

and metallic ions.

SO2 is oxidized on moist particles or in droplets of water to sulfuric acid, SO2 + H2O + ½ O2 → H2SO4

The sulfuric acid can be partly neutralized, particularly with ammonia derived

from the biological decomposition of organic matter. This results in the formation of

particles containing (NH4)3H(SO4)2[46].

The main processes of deposition of sulfur compounds are [47] (i) adsorption of

gas (SO2) on material surfaces (dry deposition), and (ii) removal of gas and aerosols by

precipitation (wet deposition) sulphate is deposited primarily by wet deposition and has a

lifetime of 3-5 days.

So, dry deposition of sulphate is of minor importance compared to SO2 in areas

with large emissions [48,32] Most of the sulphur derived from burning of fossil fuel is

emitted in gaseous form as SO2. Rural atmosphere present SO2 deposition rates lower than

10 mg m-2d-1. In Urban atmospheres however these values range between 10 and 100 mg

m-2d-1, while industrial zones show values higher than 100 mg m-2d-1[49-50].

It is well known that SO2 pollutant substantially enhance the corrosion rates of

metals exposed in the atmosphere. Rozenfeld [51] has suggested that because of its greater

solubility (SO2 is about 2600 times more soluble than oxygen), it might be reduced at

cathodic sites more rapidly than oxygen, consequently increasing anodic dissolution rates.

(ii) Oxides of nitrogen (NOx) :

Nitrogen oxide emissions originate from combustion processes other than those

emitting SOx. Road traffic and energy production are the primary sources. Most of the

nitrogen oxides are emitted as NO in combustion processes. In the atmosphere oxidation

to NO2 takes places successfully according to reaction,

30

2NO + O2 → 2NO2

As the pollutant moves further from the source it is further oxidized by the

influence of ozone:

NO + O3 → NO2 + O2

Near the emission source nitrogen dioxide is considered to be the primary

pollutant. The NO2/ NO ratio in the atmosphere varies with time and distance from the

source. Allowed enough time the NOx may be further oxidized according to the reaction.

2NO + H2O + 3/2O2 → 2HNO3

Since this reaction occurs at a very slow rate [52] NO and NO2 have a low

solubility in water[53] The amounts of deposited NOx and NO3- were found to be 10 to

100 times lower than the amount of SO2[54] Investigations in climatic chambers have not

revealed that 0.05 or 0.5 ppm of NO2 has any significant effect on the corrosion of

weathering steels or zinc[55] Field tests of carbon steel in Japan failed to show an

appreciable effect of NOx on the corrosion rate[54]. This study also showed that

deposition rate of NOx in comparison with SO2 is very much smaller, of the order of one

hundredth. This may be main reason why NOx seems to have a very limited influence on

corrosion of steel under outdoor conditions.

(iii) Carbon dioxide (CO2):

The concentration of carbon dioxide in the atmosphere is about 350 ppm[56]. The

effect of CO2 on the atmospheric corrosion of zinc was investigated by Falk, et al [57] and

Lindstrom, et al [58]. They reported that ambient concentrations of CO2 inhibit the NaCl

induced corrosion of zinc.

Carbon dioxide has an effect on the corrosion metal, since carbonic acid will form

when it dissolves in thin films of moisture, such as dew, or in rain. There is some evidence

to suggest that CO2 might reduce the effect of SO2 on the corrosion of steel and copper

presumably of the nature of the corrosion product formed [59].

The small amount of CO2 normally present in air, contrary to the impression of

early investigation, neither initiates nor accelerates corrosion. Experiments by Vernon [32]

on iron showed that the normal CO2 content of air actually decreases corrosion rate.

However, basic zinc carbonate is frequently found in the corrosion products of zinc and

small amounts of silerite (FeCO3) are found in ferrous rusts [60].

31

A laboratory study of the effect of CO2 on the atmospheric corrosion of aluminium

is reported. The samples were exposed to pure air with 95% relative humidity and the

concentration of CO2 was <1 and 350 ppm, respectively. Atmospheric corrosion of

aluminium is about 10-20 times faster in CO2 –free humid air compared to air containing

ambient levels of CO2. In the absence of CO2, bayerite, Al(OH)3, forms. Only minute

amounts of carbonate were found on the surface after exposure to CO2 containing air [61].

(iv) Chlorides:

Chlorides are deposited mainly in the marine atmosphere as droplets or as crystals

formed by evaporation of spray carried by the wind from the sea. Other sources of

chloride emissions are coal burning and municipal incinerators. Most coals have a chloride

content of 0.09 to 0.15%. In high chlorine coals, values of 0.7% are found. In the burning

of coal, most of the chlorine is emitted as gaseous HCl.

Atmospheric salinity distinctly increases atmospheric corrosion rates. Apart from

the enhanced surface electrolyte formation by hygroscopic salts such as NaCl and MgCl2,

direct precipitation of chloride ions in the electrochemical corrosion reactions is also

likely.

In marine environments chloride deposition usually decreases strongly with

increasing distance from the shore, as the droplets and crystals settle by gravitation or may

be filtered off when the wind passes through vegetation. Chloride deposition rates in

marine areas are reported to be in the range between 15 to 1500 mg Cl-m-2d-1[62], using

the wet candle method [63-64]. Chlorides also play an important role in accelerating the

corrosion rate of carbon steel [64,66]. Those atmospheres in which the chloride

concentration is less than 15 mg Cl-m-2d-1 belong to rural atmospheres, however, in the

different test sites situated in the province of Santa Cruz de Tenerife [67].

(v) Hydrogen sulphide (H2S):

Trace amount of hydrogen sulphide is present in some contaminated atmospheres.

Hydrogen sulphide is known to be extremely corrosive to most metals and alloys.

A trace amount of H2S in contaminated atmospheres causes tarnish of silver

composed of Ag2S films and may also cause tarnish of copper composed of a film of a

mixture of Cu2S + CuS +Cu2O. Iron became covered with a very thin coating of rather

loose dark-brown rust within two weeks. Brass showed iridescence locally and soon

became dull, but tin and aluminium remained unchanged.

32

(vi) Ammonia (NH3):

Impurities in liquid ammonia such as air or carbon dioxide can cause stress

corrosion cracking of mild steel. Ammonia is highly corrosive towards copper and zinc.

Ammonia is formed in the atmosphere during electrical storms, but increase in the

ammonium ion concentration in rainfall all over Europe in recent years is attributed to

increased use of artificial fertilizers. Ammonium compounds in solution may increase the

wettability of metal [44] and the action of ammonia and its compounds in causing season

cracking of cold worked brass.

Iron suffered no change whitest copper rapidly darkened, soon shedding a violet-

blue liquid which showed the reactions of nitrite and probably contained the amine

[Cu (NH3)4](NO2)2.

(vii) Hydrochloric acid (HCl) vapour:

The first major study of atmospheric degradation of metals by HCl was carried out

by Feitknecht’s [68] exposed iron and nickel plate to a vapour of HCl in cylinders

indicates the break-down of an oxide film at isolated points, whereas on zinc and

cadmium, the whole film undergoes change, producing a smooth layer of basic salt.

Feiktnecht regards the mechanism as electrochemical, with the oxide-film as cathodes and

small areas of metal exposed at breaks as anodes; the interaction between the OH- ions,

formed by the cathodic reduction of oxygen, and the metal ions, formed by the anodic

reaction, leads to hydroxide or basic chloride. Barton and Barton ova [69] carried out a

much more extensive investigation of the corrosive effect of HCl gas at concentrations

between 7 and 10 ppm on zinc, mild steel at temperatures between 20°C and 50°C.

(viii) Sulphuric acid (H2SO4) vapour:

Trace amount of H2SO4 vapour of industrial area should seriously shorten the life

of metal structures. The effect is most pronounced for zinc, cadmium and nickel. It is less

pronounced for metals that are more resistant to dilute sulphuric acid, such as lead,

aluminum and stainless steel.

33

(2) Time of wetness:

Time of wetness is a key parameter, directly determining the duration of the

electrochemical corrosion process. This variable is a complex one, since all the means of

formation and evaporation of an electrolyte solution on a metal surface must be

considered.

The time of wetness is strongly dependent on the critical relative humidity. The

relative humidity of the air varies in large limits, in function of geographic zone, of season

and daily time. Apart from the primary humidity, associated with clean surfaces,

secondary and tertiary critical humidity levels may be created by hygroscopic corrosion

products and capillary condensation of moisture in corrosion products, respectively.

A capillary condensation mechanism may also account for electrolyte formation in

microscopic surface cracks and the metal surface-dust particle interface. Other sources of

surface electrolyte include chemical condensation (by chloride, sulphates and carbonates),

Adsorbed molecular water layers, and direct moisture precipitation (ocean spray, dew,

rain).

Atmospheric corrosion takes place in corrosion cells which can only act when an

electrolyte film exists on the metallic surface. The sum of all the times in which the

metallic corrosion is possible, is called time of wetness (TOW). For practical reasons,

TOW was determined using the temperature and relative humidity (R.H.) measured. TOW

was considered to occur when the relative humidity (R.H.) is more than or equal to 80%

and the temperature is above 0°C [67].

The length of time during which the metal surface is covered by a film of water

that renders significant atmospheric corrosion possible. It determines the duration of the

electrochemical corrosion process. The time of wetness varies with the climate conditions

at the site. It depends on the relative humidity (R.H.) of the atmosphere, the duration and

frequency of rain, fog and dew, the temperature of the air and the metal surface, as well as

the wind speed and hours of sunshine.

(a) Critical relative humidity:

The primary value of the critical relative humidity denotes that humidity below

which no corrosion of the metal in question takes place [70]. It is known that the corrosion

can occur at relative humidity as low as 35% [71]. However, it is important to know

whether this refers to a clean metal surface or one covered with corrosion products. In the

34

latter case a secondary critical humidity is usually found at which the rate of corrosion

increases markedly. This is attributed to the hygroscopic nature of the corrosion product.

In the case of iron and steel, it appears that there may be a tertiary critical humidity [72].

Thus, at about 60% R.H. rusting commences at a very slow rate (primary value)

[73], at 75%-80% R.H. there is a sharp increase in corrosion rate probably attributable to

capillary condensation of moisture within the rust [32,74]. At 90 % R.H., there is a further

increase in rusting rate [72] corresponding to the vapour pressure of saturated FeSO4

solution[75], FeSO4 being identifiable in rust as crystalline agglomerates[76]. The primary

critical R.H. for uncorroded metal surfaces seems to be virtually the same for all metals,

but the secondary values vary quite widely. The critical humidity varies with the metal,

e.g., 70% for copper[77]. 70% for nickel[78], and 80% for aluminum[79]. In a complex or

severely polluted atmosphere a critical R.H. may no longer exist [80]. The pollution often

lowers the critical humidity level.

(i) Adsorbed electrolyte layers:

In this case, the water molecules are bound to the metal surface by van der waal’s

forces. The amount of water adsorbed on metal surface depends on the R.H. of the

atmosphere and on the chemical and physical properties of the corrosion products. It is

estimated that by increasing the R.H. from 55% to just below 100%, the film on polished

iron is increased in thickness from 15 molecular layers to 90 molecular layers. Such films

are capable of supporting electrochemical corrosion processes [81].

[1] Capillary condensation:

Capillary condensation may also contribute to the formation of adsorption layers of

electrolyte on the surface [81]. Although, its importance in corrosion process has not yet

been established.

The vapour pressure above a concave meniscus of water is less than that in

equilibrium with a plane water surface. It is, therefore, possible for moisture to condense

in narrow capillaries from an atmosphere of less than 100% R.H. Ferric oxide gel is

known to exhibit capillary condensation characteristics[82] and pore size deduced from

measurements of its adsorptive capacity are of the right order of magnitude to explain a

secondary critical R.H. ≈ 70% for rusted steel[73].

35

[2] Chemical condensation:

This occurs when soluble corrosion product (obviously sulphate, chloride and

carbonates) or atmospheric contaminants are present on the metal surface. When the

humidity exceeds that in equilibrium with a saturated solution of the soluble species, a

solution, initially saturated is formed until equilibrium is established with the ambient

humidity.

(b) Dew:

Laboratory experiments performed by Knotkova et al. [83], which sprayed

specimens periodically with distilled water, showed that wetting increased the corrosion

rates very substantially. One factor contributing to the high corrosivity of dew may be the

large content of atmospheric contaminants in dew [84].

Dew formation occurs when the temperature of the metal surface is below the dew

point (which can be calculated from the known ambient humidity) of the atmosphere. This

may occur outdoors during the night, when the surface temperature may decrease by

radiant heat transfer between the metal structure and the sky. Another reason for dew

formation may be the conditions in the early morning, when the temperature of the air

increases faster than the temperature of the metal, if the heat capacity of the metal is high.

(c) Rain:

Rain creates even thicker layers of electrolyte on the surface than dew. Rain may

reduce corrosion by washing from the metal a foreign matter and corrosion products which

are directly corrosive (such as ferric sulphate) [72,76] or which are hygroscopic, thus

aiding the early formation of a continuous protective film[85] and reducing the danger of

local breakdown of that film. On the other hand, rain may facilitate corrosion by helping to

maintaining the metal in a wet condition, or by washing off corrosion products which

would otherwise offer some protection [86]. On balance, the effect of rain most often

appears to reduce corrosion. These effects may vary not only with the weather, but also

with the situation of the particular metal surface. For example, the lower sides of

specimens show more corrosion than the upper surface.

36

(3) Temperature:

The effect of temperature on atmospheric corrosion rates is also complex [87-88].

It seems, however, that the influence of temperature on atmospheric corrosion is greater on

carbon steel than on zinc and copper [89]. On the one hand, an increase in temperature

will tend to stimulate corrosive attack by increasing the rate of electrochemical and

chemical reactions as well as diffusion processes; while on the other hand, an increase in

temperature leads to more rapid evaporation of surface moisture films created by dew or

rain. So, when the time of wetness is reduced in this manner, the overall corrosion rate

tends to diminish [90-91].

For a constant humidity level, an increase in temperature would lead to a higher

corrosion rate. Raising the temperature will, however, generally lead to a decrease in

relative humidity and more rapid evaporation of surface electrolyte.

For closed air spaces, such as indoor atmospheres, it has been pointed out that the

increase in relative humidity associated with a drop in temperature has an overriding effect

on corrosion rate [92]. This implies that simple air conditioning that decreases the

temperature without additional dehumidification will accelerate atmospheric corrosion

damage.

The rate of drying of electrolyte solution from the metal surface directly into the

atmospheric or through layers of corrosion product is strongly temperature dependent. In

these regards the metal surface temperature is probably more important than ambient

temperature although the latter obviously strongly influence the former. However, many

other factors will affect the metal temperature, including the thermal capacity of the metal

structure, its orientation with respect to the sun, the intensity of sunlight, the reflectivity of

the metal surface or its corrosion products, wind velocity and direction.

The solubility of oxygen and corrosive gases in the electrolyte layer also decreases

with increasing temperature. At temperature below 0°C the electrolyte films may freeze.

This leads to a very pronounced decrease in the corrosion rate which may be illustrated by

the low corrosion in sub-arctic and arctic regions [93].

(4) Effect of corrosion products on corrosion rates:

Rust films formed in the atmosphere tend to be protective, that is, the corrosion

rate decreases with time. The corrosion rate eventually reaches steady state and usually

changes little on further exposure [94].

37

The corrosion products may be soluble or insoluble. If insoluble, they usually

reduce the rate of corrosion by isolating the substrate from the corrosive environment.

Less commonly, they may stimulate corrosion by offering little physical protection which

retaining moisture in contact with the metal surface for longer periods. Soluble corrosion

products may increase corrosion rates in two ways. Firstly, they may increase the

conductivity of the electrolyte solution and thereby decrease “Internal resistance” of the

corrosion cells. Secondly, they may act hygroscopically to form solutions at humidities at

and above that in equilibrium with the saturated solution. The fogging of nickel occurs in

SO2 containing atmospheres, due to the formation of hygroscopic nickel sulphate.

However, where the corrosion products are soluble or insoluble, protective or non-

protective, the corrosive atmosphere experienced by the substrate (i.e micro-environment).

For this reason, corrosion rates are rarely constant for extended periods of atmospheric

exposure.

The change in corrosion rate with time varies markedly for different metals due to

the differing degrees of protection conferred by the corrosion products. Aluminium and

copper corrode initially but eventually form completely protective films [81]. The

corrosion rate of zinc appears to become linear after an initial period of decreasing

corrosion rate [81]. The behaviour of steel depends very much on the alloying elements

present for any given environment. Thus, the decrease in corrosion rate with time for

mild-steel is very much slower than for low-alloy steel.

Zincite, ZnO, is the first product formed when the naked metal is exposed to the

air, creating a protecting film that inhibits corrosion process. Under humidity conditions

higher than 80%, zinc is oxidized forming zinc hydroxide. If the pH on the surface is high

enough this hydroxide can react with atmospheric components such as CO2, SOx, Cl-, etc.

forming, in the hydroxide/air interface. The corresponding zinc basic salts [95]. Some of

these products form a compact film that protects the metal against later corrosive

attacks[96]. An important intermediate in the subsequent formation of other corrosion

products, is hydrozincite Zn4CO3(OH)6.H2O[97-98]. This hydrocarbonate that strongly

sticks to the metal and behaves like a semi-permeable barrier that protects zinc against

corrosion, is formed a very fast process (i.e. minutes or hours) [99]. If the pH of the humid

surface is low, neither hydroxide nor basic salts are formed [95].

Weather conditions at the time of initial exposure of zinc and steel have a large

influence on the protective nature of the initial corrosion products [72]. This can still be

detected some months after initial exposure. Finally, rust on steel contains a proportion of

38

FeSO4 which increase with increase in SO2 pollution of the atmosphere. The effect of this

on corrosion rate is so strong that mild-steel transferred from an industrial atmosphere to a

rural one corrodes for some months even though it was exposed to the industrial

environment [34].

(5) Wind direction and velocity:

The prevailing wind direction is also an important factor in relation to increase in

corrosion rates to be expected from the proximity of large industrial plants producing

appreciable concentrations of potentially corrosive pollutants. Wind also carry the

corrosive constituents of urban and marine atmospheres and may cause seasonal and

annual variations.

[1.10] EFFECT OF SO2 ON VARIOUS METALS: (a) Iron:

SO2 is highly soluble in water [27] and is adsorbed on both metals [100,102].

SO2 molecules adsorbing on the iron surfaces are rapidly oxidized to SO4-2, due to

catalytic action of iron oxide or hydroxide [103-107]. In turn, SO4-2 ions promote the

anodic electrochemical reaction [27, 35, 108].

FeS is the stable corrosion product in the pH range from 3 to 6 [109-110]. When

SO2 concentration is in the range of 0.01 to 0.2 ppm reduction of SO2 does not take place.

This applies especially to rusty surfaces, which strongly catalyze the oxidation of SO2.

In presence of SO2, iron and steel became dark brown within a few hours, and

nearly black after a day. After two weeks there was a brown or black deposit about 0.1

mm thick, with local excrescence of yellow-brown ferric hydroxide, which was definitely

wet, despite the fact that the specimens were not immersed in the liquid; the dark deposits

showed the reaction of a sulphate (indicating oxidation of the sulphur from the 4-valent to

the 6-valent state), it contained Fe+3 as well as Fe+2 ions, although the latter preponderated.

Johanson and Vineberg [109-110] made an extensive analysis of the

thermodynamic conditions for reaction of SO2 in moisture layers on metal surface. At low

and medium SO2 concentrations (1-10 ppm) following reaction occurs as all sulphur

(iv) species in aqueous solutions are thermodynamically unstable.

SO2 (g) + H2O + ½ O2 → H2SO4

The rate of oxidation of SO2 by O2 in water is rather low. The reaction is, however,

catalyzed by Fe+2(aq) and Mn+2

(aq) as well as by oxides and hydroxides of iron which are

39

present at the steel surface. The reaction is strongly pH dependent, the reaction rate

decreasing with pH [103,110]. At low SO2 concentrations, the pH value will be higher

than at high SO2 levels, thus promoting the oxidation reaction.

Rozenfeld [111] found that SO2 in high concentration has a marked depolarizing

effect on cathodic reaction at polished metal surfaces. SO2 contents > 10 ppm, reduction of

SO2 to dithionite may take place according to the following reaction:

2SO2(aq) + 2e- → S2O4-2

Where the protective rust layer may be partly destroyed, leading to a drop in the

corrosion potential.

The reactive dithionite is then either oxidized or further reduced by the metal to

sulfide.

S2O4-2 + 8H+ + 10e- → 2S-2 + 4H2O

(b) Zinc:

Upon exposure, the primary corrosion products [ZnO and Zn(OH)2] form and

rapidly cover the zinc surface but they are not protective in the presence of SO2 and

secondary reactions occur almost immediately. SO2 promoting the formation of a more

conductive electrolyte than with CO2 producing basic sulphates and soluble ZnSO4; thus

Zn(OH)2 + SO4-2 → ZnSO4 + 2OH-

Under extreme conditions, it is known that if the sulphur species concentration

accumulates to about 0.5 vol. %, ZnSO4.7H2O may form as the predominant corrosion

product [112].

Several days of exposure to SO2, all zinc surfaces showed corrosion products

indistinguishable either visually or microscopically. However, for upto 24 hours exposure,

zinc surfaces were entirely covered by various shades and shapes of grayish-white

corrosion products. The corrosion mechanisms and phenomena for zinc in SO2 containing

atmospheres appear quite different from the mechanisms suggested for iron, where pitting

is more prevalent [113].

For zinc, the sulphur acids probably act by dissolution of the protective basic

carbonate film. This reform, consuming metal in the process, redissolves, and so on. Zinc

sulphates are formed in polluted winter conditions whereas in the purer atmospheres of the

summer the corrosion products include considerable amounts of oxide and basic

carbonate. Thus, for non-ferrous metals, SO2 is consumed in the corrosion reactions [114].

40

Due to heavy air pollution with SOx, ZnSO4 is formed as: Zn + SO2 + O2 → ZnSO4 Zn(OH)2 + SO2 + ½ O2 → ZnSO4 + H2O ZnOH(CO3)0.5 + SO2 + O2 + 2H+ → ZnSO4 + 1.5H2O + 0.5CO2

This ZnSO4 is soluble in water. It may be washed by rain and will then, of course,

give no protection. In consequence, the corrosion rate will then be high [115]. Zinc slowly

developed a most white paste about 0.2 mm thick after 2 weeks which contained much

sulphate and sulphite. The identification of zinc atmospheric corrosion products is of

special importance to explain the corrosion mechanism that takes place in the metal.

(c) Aluminium:

SO2 is less adsorbed on aluminium than other metals [114]. Anions such as SO4-2

deposited on the oxide surface of aluminium may react with the oxide with the formation

of water soluble salts, e.g., Al2(SO4)3 and may also be incorporated on the lattice to form a

variety of basic salts and complexes, about which little is known so far. High SO2 level,

causing a very low pH value in the moisture film, will lead to dissolution of the protective

coating [116].

(d) Copper:

The Gaseous pollutants like Sox, NOx, CO and H2S generated by geothermal fields

electricity Industry and motor Vehicles burning fossil fuels leads to appearance of

corrosion on metals surfaces. Copper suffers in particular due to attack by sulphur

containing pollutants H2S and NOx forming copper sulphide and oxide [117-118].

Oxidation:

2Cu →2Cu2+ +4e-

O2 + 2H2O +4e- → 4OH-

2Cu + O2 + 2H2O → 2Cu(OH)2

2 Cu (OH)2 → Cu2O +2H2O (cuprite)

Sulphidation:

Cu → Cu2+ +2e-

H2S → H+ +SH-

2SH-→ 2S- +H2

Cu + H2S → CuS +H2

41

[1.11] EFFECT OF CHLORIDE ON VARIOUS METALS:

Compton [119] reported on increased corrosion due to air-borne chlorides in

tropical and desert areas.

(a) Iron:

In marine environments heavily polluted with chlorides, a protective patina does

not form and the corrosion rate may be high, especially close to the shore. Practical

experience from Sweden shows, however, that the distance about 1 km from the shore the

chloride deposition does not negatively affect patina formation [120].

(b) Zinc:

In moist outdoor conditions the zinc is oxidized with the formation of zinc

hydroxide:

2Zn + H2O + ½O2 → Zn (OH)2

Zn(OH)2 react with Cl- form basic zinc salts (patina) at the hydroxide/air boundary,

provided the pH value of the surface moisture is sufficiently high. This patina protects the

surface from further attack.

2Zn (OH)2 + 0.6Cl- + 0.6H+ → Zn(OH)1.4Cl0.6 + 0.6H2O

(c) Aluminium:

In the presence of chloride, the oxide coating is more permeable to ions. The Cl-

ions are believed to migrate into the oxide layer and lower its resistance to outward

migration of Al+3, and pitting may also initiate [114]. In the propagation stage Al is

dissolved anodically to Al+3 ions with in the pit. The cathodic reaction takes place either

outside the pit close to its mouth or inside the pit and consists of the reduction of oxygen

or H+ ions, respectively. The passivity oxide layer has low electronic conductivity, but the

cathode reaction may locally destroy the protective oxide layer due to alkalization, which

lowers the electrode potential and may even make hydrogen liberation possible [111]. By

hydrolysis of Al+3 ions-acid conditions are created within the pit and a cap of Al(OH)3

and/or Al2O3 is formed over its mouth: the corrosion products finally block the operation

of the pit. So during long-term exposure the pit depth approaches a nearly constant value.

Thus, aluminium also has very good corrosion resistant in polluted atmospheres.

42

[1.12] ATMOSPHERIC CORROSION MECHANISM OF METALS: (a) Carbon steel:

Investigations have shown that the corrosion process of carbon steel is an

electrochemical phenomenon in nature and takes place in cells of macroscopic dimensions

with very distinct anodic and cathodic areas.

Initiation: (1) In a dry, clean atmosphere the steel surface becomes covered by 20-25 A° (2-5 nm)

thick oxide film which appeared to limit further oxidation. This oxide film consists

of an inner layer of epitaxically grown Fe3O4 and layer of polycrystalline γ-Fe2O3

or FeOOH [121]. If water is present, then γFeO(OH) may also be produced [113].

(2) In non-contaminated atmosphere initiation may occur at specific sites where lattice

mismatch and at surface inclusions such as MnS, which dissolve when the surface

becomes wet in presence of water vapour.

(3) Settled air borne dust promotes corrosion by adsorbing SO2 and water vapour from

the atmosphere. Hygroscopic salts, such as, chlorides or sulphates which form a

corrosive electrolyte on the surface and carbonaceous particles can also start the

corrosion process as they may form cathodes in micro cells with the steel

surface[81].

Propagation:

During the initiation period anodic spots surrounded by cathodic areas are created.

This happens even if the surface is covered with oxide containing crystalline magnetite

(Fe3O4), because magnetite is a good electrical conductor. The corrosion reaction may be

described in terms of an electrochemical cell of the type.

Fe / Fe(aq) // OH- / O2(aq) “Fe3O4”

Besides magnetite, hydroxides containing both divalent and trivalent ions, i.e.

green rust, may serve as cathodes as they possess appreciable electrical conductivity.

The following equations may in principle describe the reactions taking place in the

corrosion cells.

43

At the cathode: (1) The main cathode reaction is considered to be reduction of oxygen dissolved in the

electrolytic film [122-123].

O2 + 2H2O + 4e- → 4OH-

This processes causes a local increase in pH at the cathodes and promotes

precipitation of corrosion products at some distance from the anodes, allowing the

corrosion processes to continue, and causing the formation of a semi-permeable

membrane of FeOOH over the corrosion sites [35] with expansion leading to

corrosion protuberances.

4Fe2+ + 4OH- + 2O2 → 4FeOOH

(hydrated ferric oxide, i.e. rust)

OR

4Fe+2 + 6H2O + O2 → 4FeOOH + 8H+

This so-called oxidative hydrolysis plays an important role in most of the proposed

mechanisms of atmospheric corrosion.

(2) As soon as ferric corrosion products have been formed another cathodic process may

take place.

Fe+3 + e- → Fe+2 (3) The cathodic step of reduction of ferric rust to magnetite (Fe3O4).

8FeOOH + Fe+2(aq) + 2e- → 3Fe3O4 + 4H2O

OR

4Fe2O3 + Fe+2(aq) + 2e- → 3Fe3O4

This process takes place during wet periods and has been verified through

cathodic polarization measurement on rusty steel specimens in sulphate solution

[124].

(4) According to Evans and Taylor [35], the magnetite produced by cathodic reduction

is reoxidized by O2 in the presence of water.

4Fe3O4 + O2 + 6H2O → 12FeOOH

OR

3Fe3O4 + 0.75O2 → 4.5Fe2O3

44

At the anode:

Evans [125] considers that the basic anodic reaction,

Fe ↔ Fe+2 + 2e-

is balanced by the reduction of ferric rust to magnetite (Fe3O4) under wet condition when

access of O2 is limited.

Presence of SO2 stimulates the propagation of the corrosion process. SO2 is

adsorbed and oxidized to SO4-2 in the rust layer. In the corrosion cells SO4

-2 accumulates

at the anodes and thus creates so-called sulphate nests in the rust, which were first

described by Schwarz [76]. In the initial stage, the surface is covered by a great number of

small sulphate nests. With increasing exposure period the nests grow larger and their

number per unit area decreases [76,124,126]. After four months of outdoor exposure the

average diameter of the nests was about 0.5 mm; on the further prolonged exposure it

increased to about 1 mm [76].

The sulphate nest becomes enclosed within a semi permeable membrane of

hydroxide formed through oxidative hydrolysis of the iron ions. The electric current in the

corrosion cell causes migration of SO4-2 ions in to the nest. This will stabilized existence

of the nest.

Schikorr [87] proposed a theory of atmospheric corrosion of steel based on the

“acid regeneration cycle”. Sulfuric acid is formed by oxidation of SO2.

2 SO2 (aq) + 2H2O + O2 → 2H2SO4

This H2SO4 absorbed in the rust layer attacks the steel according to the overall

reaction.

4H2SO4 + 4Fe + 2O2 → 4FeSO4 + 4H2O

Sulfuric acid is then reformed by oxidative hydrolysis.

4FeSO4 + O2 + 6H2O → 4FeOOH + 4H2SO4

Even if Schikorr’s theory does not explain the detailed mechanism of the corrosion

process, oxidative hydrolysis seems to very important in the process of atmospheric

corrosion of steel [127-128].

Rozenfeld [111] presented a theory stating the acceleration of atmospheric

corrosion of steel by SO2 is due to its cathodic reduction to dithionite or sulfide.

45

Formation of corrosion products are as follows:

Hydrated ferric oxide (FeOOH) is orange brown. Magnetite (Fe3O4) is black. Iron

hydroxyl complex cation species [Fe(H2O)5(OH)]+2 is yellow[129].

(b) Zinc:

In normal indoor atmospheres, zinc corrodes very little. The high corrosion

resistance of zinc in the atmosphere is largely due to the formation of a surface barrier film

of corrosion product such as zinc carbonate [130].

Generally, a visible tarnish film forms slowly, starting at spots where dust particles

have fallen on the surface. Once such a film completely covers the zinc surface the

corrosion continues at a reduced rate, often but not always, the rate is linear, that is the

total corrosion is directly proportional to the duration of exposure. Under other

circumstances, for example, in highly polluted industrial atmospheres, the corrosion rate

may increase somewhat on continued exposure. Corrosion rate of zinc in many

atmospheric exposure test have been found to correlate with two major factors, time of

wetness and concentration of air pollutants [131-132]. The yearly averaged corrosion rate

of zinc does not vary much for a given atmosphere [132-134]. Many other factors, such as

wind direction and velocity [135], have also been found to affect the corrosion rate of zinc.

There is no single set of rules for a reliable estimation of the corrosion rate of zinc for

atmospheres at every geographic location. Field exposure data still provide the best

information for reliable prediction of real corrosion rates of zinc and its alloys in the

atmospheric environments.

The effect of rainfall alone is less pronounced, since such water is not usually acid.

Rainfall may at times be somewhat beneficial, because it washes away corrosion

accelerants, such as chlorides as sea-coast locations and dust particles at industrial

locations.

Zinc corrosion products formed as zincite or zinc oxide [ZnO], Hydrozincite or

zinchydrocarbonate [Zn4CO3(OH)6.H2O], zinc hydroxychloride [NaZn4Cl(OH)6SO4.H2O],

zinc oxysulphate [Zn3O(SO4)2] and zinc hydroxosulphate [Zn4SO4(OH)6] [67].

46

(c) Aluminium:

In clean outdoor atmosphere, aluminium will retain its shiny appearance for years,

even under tropical conditions. When a fresh aluminium surface is exposed to dry air it is

immediately covered with a dense, thin amorphous oxide coating (100-200 A° thick)

which is protective. In moist atmospheres, the oxide coating grows thicker. It consists of

one dense, protective barrier layer next to the metal and one more permeable bulk

layer [136].

In polluted outdoor atmospheres, small pits develop which are hardly visible to the

naked eye. The pits become covered with crusts of aluminium oxide and hydroxide. The

growth rate of the maximum pit depth is relatively high during the first few years of

exposure, but decreases gradually so that the pit depth approaches a nearly constant value.

The pitting is more significant, but in moderately polluted atmospheres the maximum pit

depth rarely exceeds 200 µm even after one or two decades of exposure [137].

Rozenfeld [111] showed that the atmosphere corrosion at a relative humidity of

98% is little affected by moderate SO2 contents probably due to the low adsorption

tendency for SO2 on aluminium surfaces [100]. Only at excessive SO2 concentration

(above 0.01% by volume) severe corrosion effects occur [81].

Dust may accelerate corrosion by absorbing moisture and SOx from the

atmosphere, thus for long periods producing an acid medium on the surface; under such

condition the protective alumina coating is not stable. Further, carbonaceous dust may

initiate pitting by galvanic action [111].

Anions, such as SO4-2 or Cl- deposited on the oxide surface may react with the

oxide with the formation of water soluble salts e.g. Al2(SO4)3, and may also be

incorporated in the lattice to form a variety of basic salts and complexes, about which little

is known so far. The oxide coating is protective in urban atmospheres with SO2 pollution,

producing a relatively low pH value in the moisture film.

The following species have been identified in corrosion products formed on

atmospheric corrosion of aluminium under outdoor conditions; amorphous Al(OH)3,

α- Al(OH)3 (bayerite), and γ-Al 2O3, the latter with varying amounts in the lattice[84].

47

[1.13] CONTROL OF ATMOSPHERIC CORROSION:

The most effective way to minimize atmospheric corrosion is to remove the

atmosphere under vacuum and seal the components in impervious envelopes. The

protection is only effective while the integrity of the covering is maintained.

The most common methods of corrosion control are as follows [28].

[A] Material selection:

(1) Metals and alloys:

The most common method of preventing corrosion is the selection of the proper

metal or alloy for a particular corrosive service, e.g. selection of stainless steels for

construction on the basis that they are the “best”. Stainless steels represent a class of

highly corrosion-resistant materials of relatively low cost which should be carefully used.

(2) Metal purification:

The corrosion resistance of a pure metal is usually better than that of one

containing impurities or small amounts of other elements. However, pure metals are

usually expensive and are relatively soft and week. In general, this category is used in

relatively few cases which are more or less special.

(3) Non-metallic’s:

This category involves integral or solid non-metallic construction and also sheet

linings or coverings of substantial thickness. The five general classes of non-metallic are –

(a) rubbers, natural and synthetic, (b) plastics, (c) ceramics, (d) carbon and graphite, and

(e) wood.

[B] Vapour phase inhibitors:

These materials can be used to inhibit atmospheric corrosion of metals without

being placed in direct contact with the metal surface. The vapour-phase inhibitors are

usually only effective if used in closed spaces such as inside packages or on the interior of

machinery during shipment.

48

[C] Use of coatings:

Corrosion prevention can be attained by use of protective coating which is as

under:

(1) Metallic and other inorganic coatings:

Relatively thin coatings of metallic and inorganic materials can provide a

satisfactory barrier between metal and its environment. The chief function of such coatings

is to provide an effective barrier. Metal coatings are applied by electrodeposition, flame

spraying, cladding, hot dipping and vapour deposition, e.g., automobile bumpers and trim,

household appliances and fixtures, and galvanized steel. Inorganic are applied by spraying,

diffusion or chemical conversion.

(2) Organic coatings:

These involve a relatively thin barrier between substrate material and the

environment. Paints, lacquers and similar coatings doubtless protect more metal on a

tonnage basis than any other method for combating corrosion. Exterior surface are most

familiar, but inner coatings or linings are also widely utilized.

[1.14] PHYSICAL PROPERTIES AND USES OF METALS:

Iron is the basis of structural steels and zinc is the basis of the galvanic coating on

galvanized steel. Hence, study of both iron and zinc is required. Aluminium has good

electrical and thermal conductivity. It is very active in the e.m.f. series but becomes

passive on exposure to water. Although, oxygen dissolved in water improves the corrosion

resistance of aluminium. It has good corrosion resistance even in polluted atmospheres.

The physical properties of Fe, Zn and Al and Cu are as under in table-1.2.

49

TABLE – 1.2

The physical properties of metals [138,139]

Physical properties Metals

Fe Zn Al Cu Atomic number 26 30 13 29

Atomic weight 55.847 65.37 26.982 63.546

Melting point (°C) 1535 419.60 660 1084.62

Boiling point (°C) 2750 907 2467 2562

Density (solid at 20°C) (g/cm3)

7.87 7.14 2.70 8.94

Electronegativity 1.6 1.7 1.6 1.90

Valencies 2,3,6 2 3 1,2

Valance electrons [Ar] 3d6, 4s2 [Ar] 3d10, 4s2 [Ne] 3s2, 3p1 [Ar] 3d10, 4s1

Crystal structure (at 20°C)

2 3 1 1

Standard reduction potential (at 20°C)

Fe→Fe+2+2e-

E°= -0.44 V Zn→Zn+2+2e-

E°= -0.76 V Al→Al+3 + 3e-

E°= -1.66 V Cu→Cu+2+2e-

E°= -0.337 V (a) USES OF MILD-STEEL:

Mild-steel is one major material of construction extensively used in chemical and

allied industries [140]. Nelson [141] and King [142] suggested mild-steel as an important

material of construction and is used for storage tanks and pipe fittings.

The common system of classification of steel on the basis carbon contents is as

follows:

1) Very mild and mild-steels (upto 0.25 % carbon):

Such steels are used for making sheet-steel, boiler plates, various engineering

structures, rivets, bolts, soft wires, nails etc.

2) Low and medium carbon steels (0.25 % to 0.6 % carbon):

These are used for making machine parts, railway axles, rails, springs, siftings etc.

50

3) High-carbon steels (0.6 to 2.0 % carbon):

These are used for making tools, files, knives, saws, wires and cables, ball bearings

etc. Mild-steel and alloy-steel are of paramount interest for India where these materials are

extensively used in industries as well as various sectors like agriculture, manufacture of

fold leather and textile products, transport equipment, wood and paper products,

manufacture of ferrous and non-ferrous metals etc.

(b) USES OF ZINC: Corrosion resistance property of zinc is so important that nearly half the world’s

annual consumption of the metal about four million metric tons in all is used to protect

steel from rusting.

Zinc is widely used as a coating for carbon steel because of its good corrosion

resistance and relatively low price [143]. Due to its practical use, zinc atmospheric

corrosion has been studied in field exposures as well as in laboratory with controlled

environments [144-145].

Zinc coated steel is commonly used as a construction of material, for example, for

highway guard rails, various kinds of towers and structural steelwork in bridges, roofs and

sidings for farm and industrial buildings, pipes, fencing and tubing. Zinc-coated steel is

also used in such fabricated parts as automobile bodies, pole line and marine hardware,

pails, cans, nails, hooks, bolts and nuts and other small parts.

Zinc is used in contact with many organic chemical and chemical specialities such

as detergents, insecticides, agricultural chemical and similar materials. In most cases, zinc

comes in contact with such chemicals during the handling, packaging and storage of the

commercial products.

(c) USES OF ALUMINIUM:

Aluminium is a light weight silvery white metal. It is used in the manufacture of

chemical apparatus, electrical conductors, capacitors etc. Aluminium alloys are widely

used in building of vehicles, ship and river-vessels, air-craft, etc. Aluminium has been

used as a protective coating for steel many aqueous media.

51

(d) USES OF COPPER:

The major applications of copper are in electrical wires (60%), roofing and

plumbing (20%) and industrial machinery (15%). Copper is mostly used as a pure metal,

but when a higher hardness is required it is combined with other elements to make an

alloy(5% of total use) such as brass and bronze[146]. A small part of copper supply is

used in production of compounds for nutritional supplements and fungicides in

agriculture. Machining of copper is possible, although it is usually necessary to use an

alloy for intricate parts to get good machinability characteristics.

[1.15] CORROSION-RATE EXPRESSIONS:

Metals and non-metals are compared on the basis of their corrosion resistance. To

make such comparisons meaningful, the rate of attack for each material must be expressed

quantitatively. Corrosion rates have been expressed in a variety of ways in the literature

and relationship between some usual units for corrosion rate shown in table-1.3.

TABLE – 1.3

Relationship between some usual units for corrosion-rate

Given unit Factors for recalculation to mdd g/m2.day mm/year mpy Ipy

mg/dm2.day

(mdd) 1 0.1 0.0365/p 1.44/p 0.00144/p

g/m2.day 10 1 0.0365/p 1.44/p 0.00144/p mm/year 27.4.p 2.74.p 1 39.4 .0394 mil/year (mpy)

0.696.p 0.0696.p 0.0254 1 0.001

inch /year (ipy)

696.p 69.6.p 25.4 1000 1

1 mil = 10-3 inch = 25.4 µm 1 µm = 10-6 m = 10-3 mm p = density of the metal, g/cm3

52

[1.16] REPORTED WORK ON ATMOSPHERIC CORROSION:

P. C. Okafor and U. J. Ekpe (Calabar, Nigeria) et al.[147] measured a corrosion

rate of Mild Steel in cross river state environment are 35 µmpy for exposure period of one

year.

H. K. Kadiya and R. T. Vashi[148] measured the corrosion rate of mild steel to the

monthly exposure in Vapi is 650 to 2014 mg/sq.dm/month and yearly exposure in vapi are

9109 to 23495 mg/sq.dm/yr.

The corrosion rate of steel corresponding to the first year exposure in cuba are

2019.1 to 7000.0 g/m2 and campeche CRIP station are 2257.3 g/m2[149].

Corrosion rates of steel, zinc and aluminium (g/m2) for different exposure

conditions and periods of exposure at Viriato coastal (cuba)[149] found that:

Exposure period in

month Steel Zinc Aluminium

6 1154.65 61.65 2.15 12 3566.2 79.1 3.2 18 - 100.5 3.3

D. D. N. Singh et al. reported[150] corrosion rate of steel exposed for two years at

different locations of India found that, Digha (Saline) 24.2 µm/y, Chennai

(industrial,urban) 19.0 µm/y, Jamshedpur (industrial, hot dry) 12.9 µm/y and Delhi (dry,

urban polluted) 13.9 µm/y.

Juan A. Jaen, Josefina I. & Cecillio Hernandez measured corrosion rate of mild

steel exposed for one year is 37.2 µm/yr at Panama [151].

Raman et al.[152] and Misawa[153] suggest that protective layer of steel is a

mixture of γ-FeOOH and FeOX(OH)3-X which is attacked by chloride anions to form first,

α-FeOOH and then Fe3O4 (but also possible β-FeOOH) [88].

Indira, et al. [154] studied the atmospheric corrosion behaviour of steel and

aluminium in marine environment at Kochi (India) for a period of one year and found the

corrosion rate of steel and aluminium are in the range of 0.0025 to 0.0314 mpy and 0.0 to

0.0014 mpy respectively.

53

Very high corrosion rate is reported for coastal atmospheres all over the world, but

particularly in tropical and subtropical conditions [67,155-157].

Mohan et al. [158] reported that the rate of corrosion of copper is affected by

relative humidity, temperature and pollutants like sulphur dioxide and chloride etc.

Wanida et al. [159] studied the atmospheric corrosion behaviour of steel in marine

environment at Thailand for a period of one year and found the corrosion rate of steel is

111.0 g/m2/yr.

In foreign countries, the corrosivity of the various sites has been systematically

studied. In an industrial atmosphere of Altoons, Pennsylvania, galvanized steel sheets

(0.381 Kg zinc/m2, 0.028 mm thick) began to rust after 2.4 years, whereas in the rural

atmosphere of State College, Pennsylvania, rust appeared only after 14.6 year [160].

In the marine atmosphere of Nigeria, Ambler [161] recorded the corrosion rate of

carbon steel was 5.6 g/dm2/month at 50 yards from the open sea, but only 0.24 at a spot 2

miles inland.

Upham [162] exposed 100 gm mild-steel test panels in Chicago at seven different

places and noted 4 to 6, 7 to 11 and 11 to 17 gm weight loss corresponding to 3, 6, and 12

months exposure respectively.

Larrabee and Ellis[163] reported the yearly corrosion rate of steel plate (4×6 inch)

exposed in various atmospheres at different places of North America as follows: 1.1 mil/yr

at State College (rural atmosphere), 6.52 mil/yr at New York (industrial atmosphere), 2.15

mil/yr at Kure Beach (marine atmosphere), 3.30 mil/yr at Kearny (industrial atmosphere),

1.36 mil/yr at middle town (semi-industrial atmosphere), 7.85 mil/yr at Daytone Beach

(marine atmosphere), 2.46 mil/yr at Sandy Hook (industrial atmosphere), 1.13 mil/yr at

Ottawa (rural atmosphere), 1.62 mil/yr at Montreal (industrial atmosphere), 1.67 mil/yr at

Halifax (marine atmosphere) and 1.59 mil/yr at Trail (industrial atmosphere).

Larrabee and Ellis[163] exposed zinc plate (4×6 inch) in industrial atmosphere of

various places in North America and found corrosion rate as follows: 0.199, 0.093, 0.144,

0.086, 0.123 and 0.086 mil/yr corresponding to New York city, Kure Beach, Kearny,

Sandy Hook, Montreal and Trail, Canada respectively.

54

According to the literature, zinc corrosion rate values in different types of

atmospheres, expressed as penetration rate, range between 0.2 and 2 µm/year for rural

atmospheres, between 0.5 and 8 µm/year for marine atmospheres and between 2 and 16

µm/year for urban and industrial ones[164-165]. In rural and urban atmospheres (like the

ones of the islands of the province of Santa Cruz de Tenerife) zinc penetration thickness is

approximately, a linear function of the exposure time [166].

Ferro and L. Velva [167] was found yearly corrosion rate of Copper at Peruvia port

are 2.88 gm/dm2 (0.032 mm/yr). At Kure Beach, North Carolina, specimens of steel

located 24 m from the ocean where salt water spray is frequent, corroded about 12 times

more rapidly than similar specimens located 240 m (800ft) from the ocean [168].

Schikorr [169-170] confirmed that zinc exposed to Berlin or Stuttgart atmospheres

corrodes more rapidly in winter than in summer. For iron, he found that corrosion rate was

also higher in winter, but unlike zinc a decrease in rate occurred during cold spells,

presumably either because FeSO4 solution on the surface freezes or because oxidation of

FeSO4 is much related at low temperatures.

Hudson [171] exposed zinc plate (98.7 % purity) for one year at different places of

England and measured a corrosion rate as follow: 0.39, 0.26, 0.19 and 0.11 mil/yr

corresponding to Birmingham (urban atmosphere), Wakefield (industrial atmosphere),

South Port (marine atmosphere), Bourneville (sub-urban atmosphere) and Cardington

(rural atmosphere) respectively. He also noted the corrosion rate of rolled high grade zinc

specimens exposed for 10 years at different places of America as follows: 0.27, 1.4, 0.9

and 0.6 mdd corresponding to Palmerton, New York city, Pittsburg and Montauk Point

respectively.

Hudson and Stunners[172] exposed zinc (rolled) sheet for 5 years at different

locations and measured a corrosion rate as follows: 0.09 mil/yr at Llanwrtyd wells (rural

atmosphere) of Wales; 0.13, 0.18, 0.16 and 0.20 mil/yr corresponding to Calshot (marine

atmosphere), Motherwell (industrial atmosphere) respectively of England; 0.02 and 0.03

mil/yr corresponding to Aro (tropical atmosphere) and Congella (marine-industrial

atmosphere), Motherwell (industrial atmosphere) respectively of England; 0.02 and 0.03

mil/yr Corresponding to Aro (tropical atmosphere) and Congella (marine-industrial

atmosphere) of Nigeria, 0.01 mil/yr at Khartoum (tropical atmosphere) of Sudan; 0.01

mil/yr at Abisco (subpolar atmosphere) of Sweden; 0.01 mil/yr at Basrah (dry-subtropical

atmosphere) of Iraq; 0.03 mil/yr at Singapore (marine-tropical atmosphere) and 0.006

mil/yr at Delhi (dry-urban atmosphere) of India.

55

Schikorr[173] exposed pure zinc plate for one year in various atmospheres and

measured a corrosion rate as follows: 0.21 to 0.27 mil/yr at Berlin (urban atmosphere),

0.14 mil/yr at Stuttgart (urban atmosphere); 0.73 mil/yr at Bitterfeld (industrial

atmosphere), 0.19 to 0.24 mil/yr at Hamburg (industrial atmosphere) and 0.04 mil/yr at

Westphalia (rural atmosphere).

Anderson[174] exposed a high grade zinc in different atmosphere for 10 years and

reported a corrosion rate as follow: 0.21 mil/yr at Altoona, Pennsylvania (heavy industrial

atmosphere), 0.22 mil/yr at New York (heavy industrial atmosphere), 0.04 mil/yr at State

College, Pennsylvania (rural atmosphere), 0.06 mil/yr at Sandy Hook, New Jersey (marine

atmosphere), 0.02 mil/yr at Key West, Florida (marine atmosphere), 0.07 mil/yr at Lajolla,

Callifonia (marine atmosphere) and 0.008 mil/yr at Phoenix, Arizona (semi-arid

atmosphere).

A report of ASTM (American Society for Testing and Materials) sponsored study

is shown in table-1.5[175]. Several facts regarding the listing are worth nothing: the steel

weight losses vary between 0.73 and 336.0 g/panel; the zinc losses vary between 0.07 to

6.8 g/panel; the ranking with respect to zinc losses is quite different from that with respect

to steel losses; the steel: zinc loss ratio is not constant and varies from low of 9.8 to high

of 364.0. These corrosion loss data (table-1.5) represent average losses for three exposure

periods at each site.

TABLE – 1.5

Ranking Location 2-years exposure (g lost)

Steel-zinc loss ratio

Steel Zinc Steel Zinc (1) (2) (3) (4) (5) (6) 1 1 Norman Wells, N.W.T., Canada 0.73 0.07 10.3 2 2 Phoenix, AZ 2.23 0.13 17.0 3 3 Saskatoon, Sask, Canada 2.77 0.13 21.0 4 4 Esquimalt, Bc, Canada 6.50 0.21 31.0 5 15 Detroit, Ml 7.03 0.58 12.2 6 5 Fort Amidor Pier, Panama, C.Z. 7.10 0.28 25.2

(1) (2) (3) (4) (5) (6) 7 11 Morenci, Ml 9.53 0.53 18.0 8 7 Ottawa Ont., Canada 9.60 0.49 19.5 9 13 Potter Country, PA 10.0 0.55 18.3

56

10 31 Waterbury, CN 11.0 1.12 9.8 11 10 State College, PA 11.17 0.51 22.2 12 28 Montreal, PQ, Canada 11.44 1.05 10.9 13 6 Melbourne, Australia 12.7 0.34 37.4 14 20 Halifax (York Redoubt), NS Canada 12.97 0.70 18.5 15 19 Durham, NH 13.3 0.70 19.0 16 12 Middletown, OH 14 0.54 26.0 17 30 Pittsburgh,PA 14.9 1.14 13.1 18 27 Columbus, OH 16.00 0.95 16.8 19 21 South Bend, PA 16.20 0.78 20.8 20 18 Trial, BC, Canada 16.9 0.70 24.2 21 14 Bethlehem, PA 18.3 0.57 32.4 22 33 Cleveland, OH 19.0 1.21 15.7 23 8 Miraflores, Panama, C.Z 20.9 0.50 41.8 24 29 London (Battersea), England 23.0 1.07 21.6 25 24 Monroeville, PA 23.8 0.84 28.4

26 35 Newark, NJ 24.7 1.63 15.1 27 16 Manila, Philippine Islands 26.2 0.66 39.8 28 32 Limon Bay, Panama, C.Z. 30.3 1.17 25.9 29 39 Bayonne, NJ 37.7 2.11 17.9 30 22 East Chicago, IN 41.1 0.79 52.1 31 9 Cape Kennedy, ½ mi from ocean 42.0 0.50 84.0 32 23 Brazos River, TX 45.4 0.81 56.0 33 40 Pilsey Island, England 50.0 2.50 20.0 34 42 London (Startford), England 54.3 3.06 17.8 35 43 Halifax (Federal Building), NS,

Canada 55.3 3.27 17.0

36 38 Cape Kennedy, 60 yd from ocean, 60-ft elevation

64.0 1.94 33.0

37 26 Kure Beach, NC 800-ft lot 71.0 0.89 80.0 38 36 Cape Kennedy, 60 yd from ocean, 30-

ft elevation 80.2 1.77 45.5

39 25 Daytona Beach, FL 144.0 0.88 164.0 40 44 Widnes, England 174.0 4.48 39.0 41 37 Cape Kennedy, 60 yd from ocean,

ground level 215.0 1.83 117.0

42 34 Dungeness, England 238.0 1.60 148.0 43 17 Point Rayes, CA 244.0 0.67 364.0 44 41 Kure Beach, NC, 80-ft lot 260.0 2.80 93.0 45 45 Galeta point Beach, Panama, CZ 336.0 6.80 49.4

57

These data shows that the corrosion of zinc and steel varies considerably from one

atmosphere to another. The corrosion rate of zinc is lowest in dry, clean atmosphere and

highest in wet, industrial atmosphere. Sea coast atmospheres, not in direct contact with salt

spray are mildly corrosive to zinc [163,176]. The corrosion rate of zinc measured at 26

sites in one rural area in Spain has been found to vary from 0.7 to 2.4 µm/yr [177].

Santa Cruz de Tenerife (Canary Islands, Spain) and measured a corrosion J.

Morales et al. [178] studied the corrosion of zinc in subtropical areas like rate ranging

from 0.77 to1.68 µm/yr for exposure period of four years. In CEGB survey [179]

corrosion rates of zinc in the U.K. showed no correlation with atmospheric averaged SO2

concentration which strongly suggests that, although unrecorded, more important factors

were varying from site to site. Mattson[180] reported the corrosion rate of aluminium in

the range of 1 to 3 g.m-2y-1 in industrial atmosphere and 1 to 2 g.m-2y-1 in marine

atmosphere.

American Society for Testing and Materials [181] exposed aluminium plates and

measured a corrosion rate as follow: 0.032, 0.028 and 0.001 mil/yr corresponding to New

York, Lajolla and State College, Pa, respectively. Carter and Campbell[182] found that

aluminium (99.9 purity) react differently to the initial environmental condition. He noted

82 mg/sq.dm (summer start) and 90 mg/sq.dm (winter start) corrosion loss in industrial

atmosphere where 10 mg/sq.dm (summer start) and 6 mg/sq,dm (winter start) in marine

atmosphere.Joanna Kobus[183]\ measured the corrosion rates for one year exposition of

metals on three sites in Poland. The corrosion rates of carbon steel 64.5, 37.5 and 27.5

µm/yr, zinc 1.70, 0.93 and 1.12 µm/yr and aluminium was 0.43, 0.18 and 0.19 µm/yr

corresponding to Kotowice (urban-industrial), Warasw (urban) and Borecka Forest (rural),

respectively.In India, data regarding the relative corrosivity of atmospheres at

Ahmedabad[184] (urban), Bombay[185] (industrial cum marine), Balasore[186] (marine),

Baroda[187] (industrial), Calcutta[188] (industrial),Cochin[189] (marine), Jodhpur[190],

Kanpur[191] (semi-industrial), Mandapam Camp[192] (tropical marine), Patan[193]

(tropical rural), and Tezpur[194] (rural) Surat[195] (industrial), Ankleshwar[196]

(industrial), Valsad[197] (Urban), Vapi[148] (Industrial), Tithal[198] (Marine) and Mota-

Vaghchhipa[199] (Rural) are available. It is almost 32 years since the first corrosion map

of India was brought out. Over these years, Lot of environmental changes has occurred

due to industrialization, population growth and enormous vehicle population. Places of

Indian cities where atmospheric corrosion study of different metals were done as (fig.-1.1).

58

Fig.-1.1 : Places of Indian cities where atmospheric corrosion study of different metals where done.

59

Corrosion rate of mild-steel, zinc and aluminium varied from city to city and from

month to month. Monthly and yearly corrosion rate of mild-steel, zinc and aluminium was

observed at different Indian cities as shown in Table-1.6, Table-1.7 and Table-1.8.

TABLE – 1.6

Corrosion rate of mild-steel

places Monthly (mg/sq.dm)

Yearly (mg/sq.dm)

Ahmedabad[184] 681 to 2003 -

Bombay[185] 169 to 1338 2108 to 4385

Balasore[186] 61 to 930 -

Baroda [187] 145.6 to 1022.7 7438.5 to 9413.0

Calcutta[188] 397 to 1385 1399 to 3397

Cochin[189] 303 to 1278 3446 to 5883

Jodhpur[190] 20 to 286 312 to 529

Kanpur[191] 46 to 324 1073 to 2090

Mandapam Camp[192] 1950 250 to 5300

Patan[193] 7 to 572 802 to 974

Tezpur[194] 83 to 635 1473 to 3409

Surat[195] 335 to 2200 1150 to 20076

Ankleshwar[196] 356 to 1625 13109 to 14925

Valsad[197] 77 to 824 2575 to 3667

Vapi[148] 591 to 2014 9109 to23415

Tithal[198] 119 to 1132 2743 to 4286

Mota-Vaghchhipa[199] 24 to 504 1795 to 2549

60

TABLE – 1.7

Corrosion rate of Zinc

places Monthly (mg/sq.dm)

Yearly

(mg/sq.dm) Ahmedabad[184] 109 to 408 -

Bombay[185] 12 to 40 111 to 545

Balasore[186] 23 to 345 1977

Baroda [187] 10.7 to 42.5 97.5 to 185.0

Calcutta[188] 2.1 to 40 -

Cochin[189] 9.5 to 65 129

Jodhpur[190] - 3 to 54

Kanpur[191] 3.2 to 10.8 27

Mandapam Camp[192] 120 to 390 440

Patan[193] 2.2 to 66.4 -

Tezpur[194] 14 to 101 -

Surat[195] 20 to 119 173 to 268

Ankleshwar[196] 67 to 167 285 to 415

Valsad[197] 19 to 109 181 to 459

Vapi[148] 54 to 179 211 to 890

Tithal[198] 24 to 134 244 to 541

Mota-Vaghchhipa[199] 11 to 79 164 to 403

61

TABLE – 1.8

Corrosion rate of aluminium

Ahmedabad[184] 1.7 to 2.8 -

Balasore[185] - 47.0

Baroda [187] 0.6 4.9

Cochin[189] - 9.0

Jodhpur[190] - 1.4

Patan[193] 0.4 to 1.6 1.4 to 3.1

Surat[195] 3.0 to 15.0 55 to 131

Ankleshwar[196] 4.0 to 30.0 65 to 126

Valsad[197] 1.5 to 17.8 8.3 to 37.2

Vapi[148] 1.9 to 27.5 18.9 to 58.0

Tithal[198] 2.0 to 21.8 15.9 to 47.8

Mota-Vaghchhipa[199] 1.1 to 11.8 6.9 to 32.1

62

[1.17] THE OBJECTIVE OF THE PRESENT STUDY ARE AS FOLLOWS: 1) To determine the corrosion rate of mild-steel, zinc and aluminium (monthly,

yearly, progressively and seasonal basis etc.) in industrial, urban, marine and rural

environments.

2) To determine the corrosion rate of Copper (yearly) in industrial, urban, marine and

rural environments.

3) To study the positional effect, fully outdoor and partly sheltered condition of mild-

steel.

4) To study the current atmospheric salinity rate of industrial and marine

environments.

5) To study the current atmospheric sulphation rate of industrial, urban, marine and

rural environments.

6) To determine the effect of temperature, rainfall and number of rainy days, relative

humidity, corrosion rate, sulphation rate and salinity rate on mild-steel, zinc,

aluminum and Copper.

7) To study the type of atmospheric corrosion, X-ray diffraction study, SEM-EDX

Analysis of mild steel scraped materials.

8) To evaluate the corrosion rate ratio of MS:Zn, MS:Al and Zn:Al for different

Environments.

63

REFERENCES

1. X. Naixin, L. Zhao, C. Ding, C. Zhang, R. Li and Q. Zhong; Corros. Sci. (2002)

44-163.

2. H. A. E1-Dahan, T. Y. Soror, R. M. El-Sherif., Mater. Chem.Phys. 89 (2005) 260.

3. S. A. Ali, M. T. Saeed,S. V. Rahman, Corros. Sci., 45 (2003) 253.

4. S. A. M. Refaey, Appl Surf Sci., 240 (2005) 396-404.

5. G. Gunasekaran, L. R. Chauhan, Electrochim. Acta., 49 (2004) 4393.

6. A. A. Sorkhabi, B. Shaabani and D. Seifzadeh, Appl Surf Sci., 239(2) (2005)154-164.

7. M. Bouklah and A. Ouassini, et al., Appl Surf Sci.,252(6) (2006) 2178-2185.

8. Y. Abboud, and A. Abourriche et. al., Desalination, 237 (2009) 175-189.

9. E. E. Oguzie and B. N. Okolue et. al., Mater Chem Phy., 87(2-3) (2004) 394-401.

10. R. Ramanauskas, R. Juškénas, A. Kalinièenko, L. F. Garfias-Mesias,

Jou. Solid State Electrochem., 8 (2004) 416.

11. P. Bothi Raja and M. Gopalakrishnan Sethuraman., Mater Lett., 62(1)(2008)113-116.

12. R. A. Prabhu, A. V. Shanbhag, T. V. Venkatesha, J. App. Electrochem, 37 (2007) 491.

13. J. R. Viche, F. E. Varela, G. Acuna, E. N. Condaro, B. M. Rosales, G. Moriena,

A. Fernandez, Corros. Sci., 37 (1995) 941-961.

14. J. R. Viche, F. E. Varela, E. N. Condaro, B. M. Rosales, G. Moriena, A. Fernandez,

Corros. Sci., 39 (1997) 655-679.

15. F. Corvo, C. Haces, N. Betancourt, L. Maldonado, L. Veleva, M. Echevarria, O. T. de

Rincon, A. Rincon, Corros. Sci. 39 (1997) 823-833.

16. M. Morcillo, B. Chico, L. Mariaca, E. Otero, Mat. Perform, 38 (1999) 72-77.

17. A. R. Mendoza and F. Corvo, Corros. Sci., 42 (2000) 1123-1147.

18. J. J. Santana Rodriguez, F. J. Santana Hernandez, J. E. Gonzalez, Corros. Sci.,

45 (2003) 799-815.

19. I. T. E. Fonseca, R. Picciochi, M. H. Mendonca, A. C. Ramos, Corros. Sci.,

46 (2004) 547-561.

20. F. Corvo, N. Betancourt and A. Mendoza, Corros. Sci., 37 (1995) 1889-1995.

21. H. H. Uhlig, “Corrosion and Corrosion Control”, Ed. II, John Wiley & Sons, Inc.,

USA (1971) 1.

22. Gosta Warnglen; “An introduction to corrosion and protection of metals”, Butler and

Tanner Ltd., Frome and London, 19 (1972) 13.

64

23. Anton de S. Brasunas; “Symposium on Corros. Fundamentals”,The University of

Tennessee Press, 13 (1950).

24. N. B. S. Publication; ‘Economic Effect of Metallic Corrosion in the United States’,

Publication No. 511, National Bureau of Standards, Washington, DC (1987).

25. B. Anand, V. Balasubramanian, Int. Jou. of Advanced Mat. Sci., 1 (2011), 1–7.

26. M. Lagrene´e, B. Mernari, N. Chaibi, M. Traisnel, H. Vezin, F. Bentiss,

Corros. Sci., 43 (2001) 951.

27. K. Barton: Protection against atmospheric corrosion, Theories and Methods, John

Wiley, Chichester (1976).

28. M. G. Fontana and N. D. Greene: “Corrosion Engineering”, Ed. II, Mcgraw-Hill

International Book Company, NewYork, 1 (1967) 196.

29. C. Leygraf, T. Graedel; Atmospheric Corrosion ISBN 0-471-37219-6, (2000).

30. M. Stratmann; Ber. Bunsenges. Physik. Chem. (1975).

31. T. Misawa, K. Hashimoto, S. Shimodaira; Corros. Sci., 14 (1974) 131.

32. W. H. J. Vernon; Trans. Faraday Soc., 31 (1935) 1668-1678.

33. A. Buckowiecki, Schweiz. Archiv Angew. Wiss. Techn.; 23 (1957) 97.

34. G. Schikorr; Werkst. Korros, 14 (1963) 69.

35. U. R. Evans and C. A. J. Taylor; Corros. Sci., 12 (1972) 227.

36. M. Pourbaix; Corros. Sci., 14 (1974) 25.

37. A. J. De Bathane and N. A. S. Loud; a standard aqueous electrode potentials and

temperature coefficient at 25°C, Clifford A. Humpel Skokie 1-11 (1964).

38. W. H. J. Vernon and L. Whitby; J. Inst. Metals, 44 (1930) 389.

39. J. R. Mc.Dowell, ASTM., Special Technical publication., 144, (2003) 104.

40. L. J. Berchmans, V. Sivan, S. V. K. Iyer, Mater. Chem. Phys., 98 (2006) 395.

41. W. H. J. Vernon; Trans. Faraday Soc., 1923-24, 19 (1927) 839.

42. G. Agabio and A. Tamba; Br. Corros. J., 5 (1970) 112.

43. R. Ericsson, B. Heimler and N. G. Vannerberg; Werkstoffe Korros., 24 (1973) 207.

44. A. Kutzelnigg; Werkst. Korros., 8 (1957) 492.

45. Acidification Today and Tomorrow, Swedish Ministry of Agriculture, Stockholm

(1982).

46. C. Brosset; Acid Particulate Air Pollutants in Sweden, Swedish Water and Air

Pollution Research Laboratory, Gothenburg (1975).

47. H. Dovland, E. Jorangar and A. Semb; Wet and Dry Deposition of Pollutants in

Norway, Norweigian Institute for Air Research, Kjeller (1976).

65

48. R. Preston and B. Sanyal; J. Appl. Chem., 6 (1956) 26.

49. H. Guttman, P. J. Sereda, in : Metal corrosion in the atmosphere, ASTM STP, vol.435,

ASTM, Philadelphia, (1968).

50. L. Rozenfeld, Atmospheric corrosion of Metals, NACE, (1972).

51. I. L. Rozenfeld, Proc. 1st Int. Cong. Metallic Corros., Butter worth, London, (1961)

243.

52. P. Grennfelt; Nitrogen Oxides in Swedish and Norwegian Cities, Swedish Water and

Air Pollution Research Laboratory, Report for project KHM EM 705, Gothenburg

(1982).

53. I. Smith: Nitrogen Oxides from Coal Combustion Environmental Effect, Ref.

ICTIS/TR, IEA Coal Research, London (1980).

54. K. Katoh, et al.; Boshoku Gijutsu., 30 (1981) 337.

55. F. H. Haynie, et al.; Mater. Perform, 15 (1976) 48.

56. S. S. Butcher, R. J. Charlson, G. H. Orians and G. V. Wolfe., Global Biogeochemical

Cycles, Academic press, Inc., London, (1992).

57. T. Falk, J. E. Svensson and L. G. Johansson., J. Electrochem. Soc., 39(1988) 145.

58. R. Lindstrom, J. E. Svensson and L. G. Johansson., J. Electro. Soc., 147 (2000) 1751.

59. I. L. Rozenfeld; Atmospheric corrosion of metals, National Association of Corrosion

Engineers, Houston (1973) 115.

60. H. Kaesche; Werkst. Korros., 15 (1964) 378.

61. B. D. Blucher, R. Lindstrom, J. E. Svensson and L. G. Johansson., J. Electrochem.

Soc., B 127 (2001) 148.

62. V. Kucera and E. Mattsson In: F. Mansfeld, (Ed.), Corrosion Mechanism, Chemical

Industries Series, Vol. 28, Corros. Sci., 39 (1997) 823-833.

63. ISO/DP 9223, Corrosion of Metals and Alloys, Classification of Corrosivity

Categories of Atmospheres (1986).

64. B. Bonnarens and A. Bragard; Recherche Collective Sur la Corrosion Atmospherique

des Aciers, Rep. EUR 7400, Commission of the European Communities, Brussels

(1981).

65. L. Atteras et al.; In Proceeding of the 8th Scandinavian Corrosion Congress, Helsinki

University of Technology, Helsinki Finland (1978).

66. H. R. Ambler and A. A. Bain; “Corrosion of Metals in the Tropics,” J. Appl. Chem.,

5 (1955) 437-467.

66

67. J. Morales, S. Martin-Krijer, F. Diaz, J. Hemandez-Borges and S. Gonzalez,

Corros. Sci., 47 (2005) 2005-2019

68. W. Feitknecht; Helv. Chim. Acta, 29 (1946) 1801.

69. K. Barton and Z. Bartonova, Proc. 3rd Int. Congr. Metallic Corros.,4 (1969) 403.

70. W. Vernon; Trans. Faraday Soc., 23 (1927) 113.

71. S. G. Fishman and C. R. Crowe; the application of potentiostatic polarization

techniques to corrosion under thin condensed moisture layers, Corros. Sci., 17 (1977)

27-37.

72. G. Schikorr; Werkst. Korr., 15 (1964) 457.

73. V. V. Skorchelletti and S. E. Tukachinsky; J. Appl. Chem. (USSR), 28 (1995) 615.

74. K. Barton and Z. Bartonova; Werkst. Korrs., 21(2) (1970) 85.

75. G. Schikorr; Werkst. Korrs., 18 (1967) 514.

76. H. Schwarz; Werkst. Korros., 16 (1965) 93.

77. H. Schwarz; Werkst. Korros., 16 (1965) 93.

78. U. R. Evans; “The Corrosion and Oxidation of Metals” (Edaward Arnold, London),

(1960) 482

79. U. R. Evans; “Metallic Corrosion, Passivity and Protection”(Edward Arnold, London),

(1948) 157-159.

80. B. Sanyal and D. V. Bhadwar; J. Sci. Indust. Res., 67 (1959) 184.

81. N. D. Tomashov; Theory of Corrosion and Protection of Metals, Macmillan, New

York (1996).

82. D. W. Broad and A. G. Foster; J. Chem. Soc., 446 (1946).

83. D. Knotkova-Cermakova, J. Vickova and Kuchynka: werkst. Korros., 24 (1973) 684.

84. K. Barton; Schutz gegen atmospharische corrosion, Verlag Chemie, Weinheim (1972).

85. A. Portevin and E. Herzog; Comp. Rend, 199 (1934) 789.

86. L. Whitby; Trans. Faraday Soc., 29 (1933) 527, 844.

87. G. Schikorr; Werkst. Korros., 14 (1983) 69.

88. P. V. Strekalov, V. V. Agatonov and Y.N. Mikhailovskii; Zasch. Met., 8 (1972) 577.

89. K. Barton, Z. Bartonova and E. Beranek; Werkst. Korros., 25 (1974) 659.

90. G. K. Berutshtis and G. B. Klark; Corrosion of Metals and Alloys, Part 5-7, Izd.

AN SSSR, Moscow (In Russian) (1965).

91. P. R. Roberge., Handbook of Corrosion Engineering, McGraw-Hill, USA. (1999) 58.

67

92. S. Sharp., Mater. Perf., 29 (1990) 43.

93. S. Haagenrud, V. Kucera and L. Atteraas; In Proceeding of the 9th Scandinavian

Corrosion Congress, Danish Corrosion Centre, Copenhagen (1983).

94. Symposium on Atmospheric Corrosion on Non- ferrous Metals, Spec. Tech. Publ. No.

175, American Society for Testing Materials, Philadelphia, Pa. 1955, Also previous

Symposium (1946).

95. V. Kucera and E. Mattson. Corrosion Mechanisms, Marcel Dekker, New york (1987).

96. T. E. Graedel, J. Electrochem. Soc. 136 (1989) 193-203.

97. X. G. Zhang, Corrosion and Electrochemistry of zinc, Plenum press, New york and

London, (1996).

98. F. Mansfeld, Corrosion Mechanisms, Marcel Dekker, New yark (1987).

99. I. Odnevall, C. Leygraf, Corros. Sci., 34 (1993) 1213-1229.

100. N. G. Vannerberg and T. Sydberger; Corros. Sci., 12 (1972) 775.

101. N. G. Vannerberg and T. Sydberger; Corros. Sci., 10 (1970) 43.

102. J. R. Duncan and D. J. Spedding; Corros. Sci., 14 (1974) 241.

103. D. A. Hegg and P. V. Hobbs; Atmos. Environ., 12 (1978) 241.

104. B. Heimler and N. G. Vannerberg; Corros. Sci., 12 (1972) 579.

105. J. E. O. Mayne; J. Appl. Chem., 9 (1959) 675.

106. S. Beilke and G. Gravenhurst; Atmos. Environ., 12 (1978) 231.

107. N. G. Vannerberg and T. Sydberger; Corros. Sci., 10 (1970) 1084.

108. J. R. Duncan; Werkst. Korros., 25 (1974) 420.

109. L. G. Johanson; SO2 Induced Corrosion of Carbon Steel in Various Atmospheres and

Dew point Corrosion in Stack Gases, Thesis, Chalmers Univ. of Technology,

Gothenberg, Sweden (1982).

68

110. L. G. Johanson and N. G. Vannerberg; Corros. Sci., 21 (1981) 863.

111. I. L. Rozenfeld; Atmospheric Corrosion of Metals, National Association of

Corrosion Engineers, Houston (1972).

112. K. Barton, E. Beranek and G. K. Aklimov; Werkst. Korros., 10 (1959) 377.

113. B. S. Skerry, J. B. Johnson and G. C. Wood; Corros. Sci., 28 (1988) 657-719.

114. V. Kucera; Ambio, 5 (1976) 243.

115. K. B. Barton; In Intergava ’70 (Zinc Development Association, Ed.). Industrial

Newspaper Ltd., London (1971) 199.

116. H. Kaesche; Localized Corosion, National Association of Corrosion Engineers,

Houston (1974) 516.

117. B. Valdez, M. Schorr, R .Zlatev et.al ‘’Corrosion Controll in Industry’’ in

Environmental and Industrial corrosion Practical and Theoritical aspects, INTECH,

2012.

118. S. B. Valdez, W. M. Schorr, B. G. Lopez., et al.’’H2S pollution and its effect on

corrosion of electronic components’’. In air Quality New perspective, INTECH,

2012.

119. K. G. Compton, “Outdoor Exposure Testing on Racks and Test Fences”, Symposium

on Conditioning and Weathering, ASTM STP 133 (1952) 87.

120. J. Gullman et al.; Corrosion Resistance of Weathering Steels, Typical Cases of

Damage in Building Context and Their Prevention, Bull. 94, Swedish Corrosion

Institute, Stockholm (1985).

121. S. I. Ali and G. C. Wood; Br. Corros. J., 4 (1969) 133.

122. A. I. Brokskil; Zhur. Fiz. Khim., 30 (1956) 676.

123. I. L. Roikh; Ibid, 32 (1958) 1137.

124. I. Matsuskima and T. Ueno; Corros. Sci., 11 (1971) 129.

69

125. U. R. Evans; Trans Inst. Met. Fin., 37 (1960) 1.

126. K. Barton, D. Kuchynka and E. Beranek: Werk. Korros, 29 (1978) 199.

127. K. Barton and Z. Bartonova; Werk. Korros, 20 (1969) 216.

128. G. Becker et al.; Arch. Eisenhuettenwes, 40 (1969) 341.

129. F. A. Cotton and G. Wilkinson; Advanced Inorganic Chemistry (3rd Edn.)

Interscience Press, London (1972) 443.

130. T. E. Graedel; “Corrosion Mechanisms for Zinc Exposed to the Atmosphere”,

J. Electrochem. Soc., 136(4) (1989) 193-203.

131. I. Suzuki; Corrosion-Resistant Coatings, Dekker, Inc., New York, NY (1989).

132. H. Guttman; “Effect of Atmospheric Factors on the Corrosion of Rolled Zinc”, Metal

Corrosion in the Atmosphere, STP 435, ASTM, Philadelphia, PA (1968) 223-239.

133. C. J. Slunder and W. K. Boyd; Zinc; Its Corrosion Resistance, 2nd Ed., International

Lead Zinc Research Zinc Organization, Inc., New York, NY (1986).

134. D. E. Tonini; “Atmospheric Corrosion Test Results for Metallic-Coated Steel Panels

Exposed in 1960”, Atmospheric Corrosion of Metals, STP 767, ASTM, Philadelphia,

PA (1982) 163-185.

135. J. W. Spence, E. O. Edney, F. H. Hatnie, D. C. Stiles, E. W. Corse, M. S. Wheeler

and S. F. Cheek; “Advanced Laboratory and Field Exposure System for Testing

Materials”, Corrosion Testing and Evaluation : Silver Anniversary Volume, STP

1000, ASTM Philadelphia, PA (1990) 191-207.

136. C. E. Bird and F. J. Strauss; Mater. Perform. 15(11) (1976) 27.

137. E. Mattson; Tek. Tidskr., 98 (1968) 767.

138. C. J. Smithells; Metals Reference Book, Butterworth, London (1967).

70

139. “Handbook of Electrochemistry”, Supplement to Transactions of the “SAEST”,

(Karaikudi) 22, No. 4 (1987) 41, 42.

140. T. P. Sastry and V. V. Rao; J. Electrochem. Soc., India, 30(4) (1981) 288-296.

141. G. A. Nelson; “Corrosion Data Survey”, Shell Development Co., Emeryville,

California (1960).

142. R. W. King; Manufacture of Sulfuric Acid, Chap-23, W. W. Duecker and J. R. West

(eds.), Reinhold Publishing Corp., New York (1959).

143. S. C. Chung, A. S. Lin, J. R. Chang, H. C. Shih, Corros. Sci., 42 (2000) 1599-1610.

144. Q. Qu, C. Yan, Y. Wan, C. Cao, Corros. Sci., 44 (2002) 2789-2803.

145. J. Morales, S. Martin-Krijer, F. Diaz, J. Hernandez-Borges, S. Gonzalez, Corros.

Sci., in press. (1987)

146. M. Lagrene´e, B. Mernari, N. Chaibi, M. Traisnel, H. Vezin, F. Bentiss, Corros. Sci.,

(2005) 951.

147. P. C. Okafor , U. J. Ekpe, U. J. IBok, B. O. EKPO, E. E. Ebenso & C. O. Obadimu,

atm. Corr. Of Mild steel in the Nigerdelta Region of Niger, Global Journal of

Environmental Sciences, 8(1) (2009) 9-18.

148. H. K. Kadiya and R. T. Vashi, atmospheric corrosion study of metals in an Industrial

Environment, Asian Journal of chemistry, 22(2) (2010) 1151-1157.

149. F. Corvo, T. Perez, L. R. Dzib, Y. Martin et al., Corros. Sci., 50 (2008) 220-230.

150. D. D. N. Singh, Shyamjeet Yadav, Jayant K.Saha, Corros. Sci., 50 (2008) 93-110.

151. A Juan. Jaen, Josefina I. & Cecillio Hernandez, Analysis of short term corrosion

product s formed in tropical marine environments of Panama, Inter. Jou. of Corr.

(2012) 1-11.

152. A. Raman, A. Razvan, B. Kuban, K. A. Clement and W. E. Graves; Corrosion,

42 (1986) 447.

71

153. T. Misawa, K. Hashimoto and S. Shimodaira; Corros. Sci., 14 (1974) 131.

154. C. J. Indira, K. V. Prasad, kumari V. S. Syamala, P. N. N. Namboodiri, M. Natesan,

N. Palaniswamy and M. Raghavan, (2000) 10th Nat. Cong. Corros. Cont., Madurii,

India, 148.

155. B. Chen, Y. L. Xu and W. L. Qu, International Journal of Solids and Structures, 42

(2005) 4673-4694.

156. M. Natesan, G. Venkatachari and N. Palaniswamy, Corros. Sci., 48 (11) (2006)

3584-3608.

157. O. Troconis de Rincon et al., Building and Environment 41 (2006) 952-962.

158. P. S. Mohan, M. Sundaram and S. Guruviah, 10th Int. Cong. Metallic Corros.,

Madras, India, (1987) 179.

159. W. pongsaksawad, E. Viyanit, S. Sorachot and T. Shinohara,Corrosion assessment of

steel in Thailand, Journal of Metals,Minerals and Materials, 20(2)(2010), 23-27.

160. C. P. Larrabee; Corrosion., 9 (1953) 259.

161. H. R. Ambler; Nature, 176 (1955) 1082.

162. J. B. Upham; Atmospheric Corrosion Studies in Two Metropolitan Areas,

J. Air Pollution Control Assoc., 17 (1967) 398-402.

163. C. P. Larrabee and O. B. Ellis; Corrosiveness of various atmospheric sites as

measured by specimens of steel and zinc, Pro. ASTM 59 (1959) 183, 201.

164. C. J. Slunder, W. K. Boyd, Zinc : its corrosion resistance, zinc Institute Inc.,

New york (1971).

165. R. A. Legault, V. P. Pearson, in : Atmospheric factors affecting the corrosion of

engineering metals, ASTM STP, Philadelphia (1978) 646.

72

166. J. Morales, F. Diaz, J. Hemandez-Borges, S. Gonzalez and V. Cano., Corros. Sci.,

49 (2007) 526-541.

167. N. W. Ferro, L. Veleva & P. Aguilar, Copper Marine Corrosion, The open corrosion

journal, 2 (2009) 130-138.

168. F. L. Laque; Am. Soc. Testing Mater. Proc., 51 (1951) 495.

169. G. Schikorr and I. Schikorr, Z. Metallk., 35 (1943) 175.

170. G. Schikorr, Schweiz Arch. Angew. Weiss. Tech., 2 (1958) 37.

171. J. C. Hudson Atmospheric Corrosion on Metals, Third Report to the Atmospheric

Corrosion Research Committee. (1987)

172. J. C. Hudson and J. F. Stanners; The effect of climate and atmospheric pollution on

corrosion, J. Appl. Chem., 3 (1953) 86-96.

173. G. Schikorr; Corrosion behaviour of zinc, Vol. I. Behaviour of zinc in the

atmosphere, metalverlag Gmbh, Berlin Grunewald (1964) 72; English edition

published by the American Zinc Institute & Zinc Development Association (1965).

174. E. A. Anderson; The atmospheric corrosion of rolled zinc, ASTM STP 175 (1955)

126-134.

175. S. K. Colburn et al.; “Corrosiveness of various atmospheric test sites as measured by

specimens of steel and zinc”, Metal Corrosion in the Atmosphere, ASTM STP

435 (1968) 360.

176. W. Showak and S. R. Dunbar; “ Effect of 1 percent copper Addition on Atmospheric

Corrosion on Metals, STP 767 ASTM, Philadelphia, PA (1982) 135-162.

177. S. Feliu and M. Morcillo; “Atmospheric Corrosion Testing in Spain”, Atmospheric

Corrosion, W. H. Ailor (ed.), John Wiley and Sons, New York, NY, (1982) 913-921.

178. J. Morales, F. Diaz, J. Hemandez-Borges, and S. Gonzalez, Corros. Sci., 48 (2006)

361-371.

179. R. J. Bawden and J. M. Ferguson; Trends in Material Degradation Rates in U. K.

Proceedings of Confe., U.K., Corrosion, 88, Brighton 26-28, October (1987) 383.

180. E. Mattson; Material Performance, 21(7) (1982) 9.

181. Symposium on Atmospheric Corrosion of Non-ferrous metals, Spec. Tech. Publ. No.

175, ASTM, Philadelphia, Pa. (1955).

73

182. V. E. Carter and H. S. Campbell; “The Effect of Initial Weather Condition on the

Atmospheric Corrosion of Aluminium and Its Alloys”, Metal Corrosion in the

Atmosphere, ASTM STP 435 (1968) 39.

183. J. Kobus, Long-term Atmospheric Corrosion Monitoring, Mater. Corros., 51, 2

(2000) Feb., 104 (Deutsch).

184. R. K. Shah; Pollution of Air and Water, University Granth Nirman Board,

Ahmedabad (1989) 150-153.

185. B. Sanyal, A. N. Nandi, A. Natarajan and D. Bhadwar, Atmospheric Corrosion of

Metals, Pt. ll, Corrosion of Metals in Bombay, J. Sci. Indust. Res., 18-A (1959)127.

186. B. Sanyal, G. K. Singhania and S. K. Chakravarti,; Atmospheric Corrosion of

Metals PT. V, Corrosion of Metals at Balasore, Labdev., J. Sci. Tech., 5(2)

(1967)108.

187. R. T. Vashi and H. G. Patel, Atmospheric Corrosion study of Baroda Industrial area,

Electro. chem., 13(8-9) (1997) 343-347.

188. B. Sanyal, B. K. Das Gupta, P. S. V. Krishnamurthy and G. K. Singhania,

Atmospheric Corrosion of Metals, Pt. lll, Corrosion of Metals in Calcutta,

J. Sci. Indust. Res., 20-D (1961)27.

189. B. Sanyal, A. Balkrishnan, G. K. Singhania and U. G. K. Menon, Atmospheric

Corrosion of Metals, Pt. lV, Corrosion of Metals in Cochin, J. Sci. Indust. Res., 21-D

(1962) 185.

190. M. L. Prajapati, G. K. Singhania and B. Sanyal., Atmospheric Corrosion of Metals,

Pt VI, Corrosion of Metals at Jodhpur, Labdev, J. Sci. Tech., 7A (1) (1969) 34.

191. B. Sanyal and G. K. Singhania, Atmospheric Corrosion of Metals, Pt I J. Sci. Indus.

Res., 15-B (1956) 448.

192. K. S. Rajagopalan, M. Sundaram and Annamalai; Corrosion of Metals at Mandapam

Camp, India, Corrosion, 15 (1958) 25.

193. J. D. Talati and B. M. Patel, Atmospheric Corrosion of Metals in Patan (N.G.),

Vidya, X-2 (1967) 182-186.

194. A. L. Nair, G. K. Singhania and B. Sanyal; Atmospheric Corrosion of Metals, pt.

VII, Corrosion of metals of Tezpur, Labdev, J. Sci. Tech., 9-A No. 1 (1971) 58-62.

195. R. T. Vashi and R. N. Patel, Corrosion of aluminium, zinc and mild-steel in an

industrial atmosphere, J. Indian chem. Soc., (2004) 680-682.

74

196. R. T. Vashi, G. M. Malek, V. A. Champaneri and R. N. Patel, Bull. Electrochem.,

18(2) (2002) 91-96.

197. H. K. Kadiya and R. T. Vashi, Corrosion study of metals in an Urban Environment,

Mat. Sci. Res. India, 6(2) (2009) 351-356.

198. H. K. Kadiya and R. T. vashi, Corrosion studies of metals in marine environment,

E. Jou. of Chem, 6(4) (2009) 1240-1246.

199. H. K. Kadiya and R. T. Vashi corrosion studies on metals in Rural atmosphere,

Journal of Environment Research and Development, 3(1) (2008) 244-249.