CORROSION Gradual eating away or disintegration or deterioration of a metal by chemical or electrochemical reaction with its environment. Causes of corrosion: Most metals exist in nature in ores & minerals as compounds such as oxides, sulfides, sulfates and carbonate etc which show that these metallic compounds are thermodynamically more stable than its pure metal. So when metal extracted from these ores then metal will have a natural tendency to revert back to its natural thermodynamically more stable state.

Secondly metals have low E.N values so they have tendency to loose es- to form more stable cation which is initiating step for corrosion. So there are two main causes of corrosion i.e. electrochemical and chemical. Theories of corrosion: 1. Electrochemical corrosion: Mechanism of electrochemical corrosion is based on principles of electrochemical cell. Hence it is a redox reaction which involved cathode, anode, electrolyte and conductive circuit b/w electrodes whereas dissolution of metal occurs at anode and ionic current flow through electrolyte. In metallic structure, different areas comprises different composition or located under different mechanical stress (e.g. at a bend or weld or joint) or under different environmental stress (e.g. different degree of exposure to air or under a loosely adhering paint film or in different soil conditions) behaves as cathode and anode while moist environment which facilitate transport of ions b/w electrodes acts as electrolyte. Whereas metallic structure, itself provides conductive connecting for electrons to flow b/w these electrodic areas. Electrochemical corrosion redox reaction can be split into following half reactions i.e. oxidation and reduction reactions which occurs at separate location on metal. 1.A. Oxidization Reaction (At anodic surface area) At anodic area surface, metal atom undergoes oxidation to form metal cation by loosing electrons. This cation get dissolve in moist environment while these electrons flow through metallic structure to cathodic area. M Mn+ + ne- At Anode So electrochemical dissolution (corrosion) of metal occurs at anodic areas surface. 1.B. Reduction Reaction (at cathodic surface area) These anodic electrons are taken up at cathodic surface area by depolarizers (electron acceptors) present in environment. There are number of reduction reactions possible depends upon nature of environment. 1.B.a. In alkaline or neutral aerated and hydrated environment. O2 + 2H2O + 4e- 4OH- Due to this reaction, deaerated water must be fed into boiler.

1.B.b. In aerated acidic environment 4H+ + O2 + 4e- 2H2O 1.B.c. In de-aerated (absence of O2) acidic environment. 4H+ + 4e- 2H2 (Hydrogen gas evolution) All the metals which are placed below hydrogen in electrochemical series, corrodes in acidic solution by hydrogen evolution mechanism 1.B.d. In presence of other more noble metals cation in environment. Mn+ + ne- M(s) These cations reduced and deposited at cathodic area surface. 1.C. Secondary reaction: After electrochemical corrosion reaction, the insoluble corrosion products are formed by a secondary chemical reaction e.g. 2Fe 2Fe2+ + 4e- At Anode O2 + 2H2O + 4e- 4OH- At Cathode 2Fe + O2 + 2H2O 2Fe2+ + 4OH- Electrochemical corrosion reaction Depending upon availability of oxygen two types of corrosion products are formed by secondary chemical reaction i). In presence of excess oxygen 4Fe2+ + 8OH- + 2H2O + O2 4Fe(OH)3 Fe2O3.nH2O Ferric hydroxide Brown rust Hematite ii). In presence of limited oxygen 6Fe2+ + 12OH- Fe2O3.FeO.6H2O Black rust (Magnetite or Ferrosoferric oxide) In summary, electrochemical corrosion occur at that area of the metal where current leaves it i.e. anodic area b/c as metal involved in electrochemical reaction at this area so it resulted in metal dissolution (corrosion). No corrosion occurs at cathodic area of metal which picked up that current b/c no metal atom involved in electrochemical reaction at that area. Further more it is protected against corrosion by polarization (hydrogen film build up). Whenever this hydrogen film remains around cathodic surface it acts as an insulator and reduces further corrosive current flow.

2. Chemical Corrosion Due to the direct reaction of metal surfaces with the atmospheric constituents present in the environment e.g. oxygen, carbon dioxides, hydrogen sulphide, sulphur dioxide, nitrogen, chlorine and other halogens. The chemical corrosion is of four types: oxidative chemical corrosion by O2, corrosion by other gases, liquid metal corrosion and acidic corrosion by atmosphere. 2.A. Oxidative chemical corrosion by O2: Oxidative chemical corrosion is caused by the direct reaction of atmospheric O2 with the metals generally in the absence of moisture (water vapour). At the first instance, oxidation takes place at the surface of the metal [M] with the formation of metal ion [Mn+] and electrons (e-). The electrons formed in the oxidation step react with the atmospheric oxygen to form oxide ions [O2-] which react with the metal ions to give a layer of metal oxide [M2On] at the surface of the metal.

When the entire surface of the metal is covered with metal oxide, the further oxidation is restricted because oxide layer acts as barrier between metal and environment. The oxidation of the remaining metal is possible only if metal ions diffused out on the surface of the metal or atmospheric O2 diffuse inside the metal. The diffusion of metal ions towards the surface of metal is much faster as compared to the diffusion of the O2 inside the metal because of smaller size and higher mobility of metal ions as compared to O2. So, it is evident that extent of oxidation reaction is dependent on the type of the metal oxide layer formed on the surface of the metal. 2.B. Acidic Corrosion by atmosphere: Corrosion (rusting) of iron takes place due to its reaction with O2, CO2 and moisture present in the atmosphere.

Fe(s) + O2 + 2CO2 + H2O Fe(HCO3)2 O2 + H+ + Fe(HCO3)2 Fe(OH)CO3 + CO2 + H2O Fe(OH)CO3 + H2O CO2 + Fe(OH)3

In the first step, O2, CO2 and moisture present in the atmosphere react with iron to form soluble ferrous bicarbonate which is oxidized to ferric hydroxy carbonate, which on further reaction with moisture gives, hydrated ferric oxide (ferric hydroxide). 2.C. Corrosion by other gases: These types of corrosion are caused by gases like CO2, H2S, SO2, N2, Cl2, and other halogens. The extent of corrosion depends upon the affinity between the metal and the gas involved and the type of film formed on the surface of the metal i.e. protective or non-protective. (i) The extent of corrosion decreases if the layer formed is protective & non-porous, for

example Attack of Cl2 gas on Ag metal forms a non-porous layer of AgCl which protects further corrosion of the Ag metal.

(ii) The corrosion continues if the layer formed is non-protective or porous, for example, attack of Cl2 gas on Sn(tin) metal forms SnCl4 which is volatile so produces a fresh surface on tin metal for the corrosion to continue. Similarly, H2S attacks steel at high

temperature in petroleum industry forming FeS scale which is porous and corrosion continues.

2.D. Liquid metal corrosion: Liquid metal corrosion is caused due to chemical reaction of solid metal with a flowing liquid metal at high temperature. Such kind of corrosion occurs in industry particularly in chemical process e.g. caustic embrittlement by untreated water in boiler, corrosion in oil fields & refineries by crude oil, corrosion of underground & submarine pipe lines.

CORROSION CONTROL

The secret of effective engineering lies in controlling rather than preventing corrosion, because it is impossible to eliminate corrosion ---------Michael Henthora Corrosion of a metal is a natural spontaneous process, by which a metal is converted into a more stable compound state. Therefore, corrosion control is more realistic than corrosion prevention. Corrosion types are so numerous, the mechanisms of corrosion are so different, and condition under which corrosion takes place are so varied that no single method can be used to control all possible corrosion cases. The choice of a control method depends on factors such as the type of the structure, the nature of prevailing corrosion, the residual stress in the fabricated articles, the nature of the environment and similar other factors. Four important methods commonly used in curtailing corrosion of metals are discussed in the following sections. 1. Proper design and material selection 2. Electrochemical Techniques 3. Barrier films coating 4. Inhibition of corrosive environment 1. Proper design & material selection: The equipment should be so designed as to avoid localized stresses by avoiding sharp bends, baffles and lap joints. A good design should take care of avoiding accumulation of dirt, stagnation of water and allow for free circulation of water. Prevention against corrosion starts at the design step itself. A faulty design may lead to premature failure of the component or expensive maintenance throughout its life. The basic principle behind designing is that even if corrosion occurs, it is uniform rather than the localized corrosion. Important design principles are: (i) Avoid sharp bends and sharp corners: Under the flowing system at sharp bend

impingement takes place. Also at sharp bends coating is difficult.

(ii) Avoid galvanic coupling: If two metals have to be coupled a. Use insulation between them. b. Try to select the metals which are close to one another in electrochemical series so that

the potential difference between them is less and hence the less corrosion. e.g. when steel pipe connected to a copper plumbing than they are in electrical as well as electrolytical contact with each other, the more reactive metal (iron, placed lower in

the ECS) acts as an anode while Cu (placed higher in ECS) acts as cathode. Due to galvanic corrosion iron or steel pipe suffers corrosion while the copper plumbing is protected.

(iii) Proper drainage: Improper drainage leads to stagnant condition leading to the intense localized corrosion.

(iv) Avoid Crevices: If it can not be avoided it must be filled by fillers to avoid crevices where deposits of water-soluble compounds and moisture can build up and are not accessible for maintenance. Any area where two surfaces are loosely joined, or come into closeness, also qualifies as a crevice site. Joining geometries also lead to various crevice corrosion problems. Examples of such cases are bolting, back-to-back angles, rough welds, weld spatter, sharp edges, corners, discontinuities and irregular welding.

(v) Avoid small anodic area and large cathodic area:

As it gives rise to intense localized corrosion. As we know that

Current Density = J = I/A Where I = Current A = Area For small anodic area, Janode > Jcathode for same current.

Therefore the current density at smaller anodic area is very large and the demand for electrons by large cathodic area (reduction/gain of electrons) can be met by smaller anodic area(oxidation/loss of electron) only by undergoing corrosion more briskly. (vi) Proper support: Avoid hanging joints, choose supporting points in such a way that there is less and diffused stress due to supporting.

The resistance of a metallic material can be improved by changing its composition either by refining or alloying. Refining means decreasing the concentration of impurities, e.g. Lowering of Sulphur, Phosphorous and Carbon in steels increases the corrosion resistance of steels. Alloying is often used to improve the corrosion resistance of the metals e.g. Al, Be, Mg etc. are added to Cu to improve its oxidation resistance, Cr is added to stainless steel as it improves the chemical corrosion resistance by forming a layer of Cr2CO3 and stress corrosion cracking resistance by forming a layer of Cr2O3. Selection of right type of material is the main factor for corrosion control. The choice of metal should be made not only on its cost and structure but also on its chemical properties and its operational environment e.g.

i- Stainless steels containing chromium produce an oxide film which protects steel from further attack so make it best for marine application

ii- Nimonic alloys (Ni-Cr-Mo alloys) used in gas turbines are very resistant to hot gases. iii- Copper-nickel alloys used extensively for bubble trays used in fractionating column in

oil refineries. 2. Electrochemical Techniques: 2.A. Cathodic Protection: Principle: The principle involved in cathodic protection is to enforce the metal to behave like a cathode, since there will not be any anodic area on the metal, corrosion does not occur. Mechanism: As electrochemical corrosion reaction involve dissolution of the metal and evolution of hydrogen gas as follow

Anodic reaction M M+n + ne- Cathodic reaction 2H+ + 2e- H2

So it is obvious that the addition of electron to metallic structure will tend to suppress the metal dissolution and increase rate of hydrogen gas evolution. Methods: Cathodic protection can be achieved by supplying electrons to the metallic structure to be protected. It done by two methods 2.A.i. Sacrificial anode method: In this method, the protected metal structure is converted into a cathode by connecting it to a more active (less noble) metal like Mg, Ca, Zn etc. This active metal acts as an auxiliary anode. Since the anodic metals are sacrificed to protect the metal structure, the method is known as sacrificial anode method. Exhausted sacrificial anodes are replaced by new ones as and when required.

Application / Examples: Magnesium bars are bolted along the sides of ship near the bilge keel for protecting the hulls. Magnesium rods are inserted into domestic water boiler or tanks to prevent the formation of rusty water. Magnesium blocks used to prevent the rusting of pipelines & fuel storage tank. Calcium metals slugs are used to suppress engine corrosion. Cathodic protection is frequently used in conjunction with coatings to reduce the cost and current capacity of system.

2.A.ii. Impressed (direct) current method: Another method of providing cathodic protection is executed by applying a direct current through battery larger than the corrosion current. Metal to be protected is made cathodic by connecting it to the anode of the external source of current. The cathode of the source is connected to an inert electrode. The metal structure being cathode does not undergo corrosion. Anode being inert remains unaffected. Graphite is widely used as the inert anode in this method. Platinum, silicon and iron are also used as anodes.

Advantages: It can be designed for a wide range of voltage and current. High ampere output is available from single ground bed.

Large area can be protected by single installation. Low maintenance cost. 3. Barrier films coating: The main function of a protective coating is to cordon off the exposure of structural reactive element surface from environmental corrosives. Protective coatings in themselves provide little or no structural strength yet they protect subject metals to protect their strength. A coating must provide a continuous barrier film to a substrate because any imperfection can become the focal point for degradation and corrosion of substrate. Three kinds of coating are used. A. Metallic coating B. Inorganic coating C. Organic coating

3.A. Metallic coating: Metallic coating provides a layer that changes the surface properties of the corroding metal to those of the metal being applied. The coating metal provides a durable and corrosion resistant layer while the core material provides the load-bearing capability. Different methods are used for deposition of metallic coating like electroplating, electro less plating, spraying. Hot dipping, chemical vapour deposition and physical vapour deposition. 3.A.i. Electroplating: Electroplating involves deposition of metallic coating on to an object by putting a negative charge on the object and immersing it into a solution which contains a salt of metal to be deposited while anode of the metal being deposited is used. Thus metal object serves as the cathode in electroplating cell, attracting metal ions from the solution. Metallic cations reach cathodic object and electron flow from object to cation so neutralize it or reduce it in metallic form. At anodic metal, electrons are removed so oxidizing metal into cations which dissolved into solution supplying substitute metallic cations for that which has already been plated. Therefore electrolyte solution concentration retained in the cell. In Electroplating process, coated metal is thicker at focus in direction of anode due to high voltage and more ionic current supply at these dimensions.

3.A..ii. Electroless plating: It is a chemical reduction process, which involves catalytic reduction of metal cations in aqueous solution containing reducing agent and subsequent deposition of metal without the use of electrical energy. Driving force for this process supplied by a chemical reducing agent in the solution. 3.A..iii. Metal spraying: In this method, the molten metal is sprayed on the clean base metal with the help of a spray gun or pistol, which can direct molten metal stream as required. Oxyacetylene or H2 gas use for heating & blowing purpose. By this method coating can be applied to the finished structure of the base metal in place & to any desired spot of base metal.

3.A.iv. Hot dipping: It this process, the metal to be coated is dipped in the molten bath of low M.P metal for sufficient time and than removed out along with adhering film. The process of providing Zinc coating called galvanizing and tin coating called tinning. 3.A.v. Metal cladding: It involves metallurgical bonding of a dense and homogeneous layer of a metal or an alloy with good corrosion resistance to one or both side of a metal object. It is a mill stage process, in which thickness and distribution of cladding is controlled over a wide range by pressing, rolling or extrusion. It is limited to simple shape articles that don not required much mechanical deformation like sheets, plates or tubing e.g. aluminum cladding in air craft industry, lead and cadmium sheeting for cables, lead sheeted sheets for architectural applications and composite extruded tubes for heat exchangers. 3.A.vi. Pack cementation: (chemical vapour deposition) Pack cementation is widely used to confer oxidation resistance on ferrous alloys which include aluminizing, chromizing and siliconzing. Pack cementation involves deposition of a layer of Al/Cr/Si on the metal surface and then diffusion of this layer into metallic structure surface by heat treatment in a furnace for a period of time. Both deposition & diffusion occur simultaneously so it also called diffusion coating. In pack cementation process, a source of metal (Al/Cr/Si) reacts with a chemical activator on heating to form a gaseous compound (e.g. Al reacts with NaF to form AlF). This gas acts as transfer medium which transport the depositing material to the metallic component surface. This gas decomposed at the substrate surface and deposited the metal (Al/Cr/Si) by discharging activator which returns to the pack and reacts with the depositing metal source again. This process continuous until all of the metal in the pack is used or until the process is stopped by cooling. The coating form in few hours at temperature ranging from 700 1000 oC. 3.A.vii. Physical vapour deposition: Both PVD & CVD are basically vaporization coating techniques, involving transfer of material at atomic level. Difference b/w CVD & PVD is in mode of vaporization. In CVD depositing metal is converted into its volatile compound gas generally at atmospheric pressure which then decompose at base metal surface to liberate depositing metal atoms. While in PVD, depositing metal is atomize under vacuum by bombardment of high energy source such as beam of light or ions or laser which dislodged atoms from the surface of metal and vaporized them. These vaporize atoms then mix with active gas such as nitrogen, oxygen or methane and carried out to base metal surface where it deposited by plasma bombardment. It is a line of sight technique i.e. it do not coat under cuts and grooves. It is a sophisticated technique so required skilled operator. It is used in aerospace and automotives. 3.B. Inorganic coating: Inorganic coatings involved inorganic material layer deposition and metal surface chemical conversion type coatings. In chemical conversion type, a surface layer of the metals is converted into a compound, by chemical or electrochemical reactions, which forms a barrier between the underlying metal surface and the corrosion environment. The chemical conversion coatings are different from other types of coating in the sense that, they are the integral part of the metal itself. This type of coatings are formed on the metal surface by chemical dip, spray or by electrolytic methods. In addition to the corrosion resistance, this

types of coatings also provide increased electrical insulation, and enhanced adherence for paints and other similar organic coatings. Different types of electrolytic baths are used for inorganic coatings depending upon nature & properties of resulted inorganic coat e.g. sulfuric acid, oxalic acid or boric acid bath are used for oxide coating of Al, Zn & Mg. Phosphoric acid bath is used for phosphate coating of steel surfaces while chromic acid is used for chromate coating of different metals. The base metal is made as anode, in a suitable electrolytic bath. Then passage of direct electric current cause dissolution of anodic base metal atoms as ions, forming an anodic Inorganic complex with bath solution which subsequently gets deposited on the surface of the metal. The strength and corrosion resistance of anodized inorganic film can be increased by sealing, which involves heating in boiling water or steam or metal salt solution. This treatment changes porous inorganic coating into its hydrate e.g. alumina (Al2O3) into its monohydrate (Al2O3.H2O), which occupies more volume, thereby the pores are sealed. 3.C. Organic coating:

The functions of organic coatings are of two folds: 1. The coatings serve to keep out air and moisture from the metal surface or serve as a

barrier between the metal surface and the corrosion environment. 2. The pigments (red lead, zinc chromate, etc), or drying oils (linseed oil, Wood oil, etc)

present in the paint often exert an inhibitive action by electrochemical and other means. The requirements of a good organic coating are: 1. It should adhere tenaciously to the metal surface and improve its physical appearance. 2. The film formed should be continuous, uniform and impervious to air and water. 3. Should be chemically inert to the corrosion environment. 4. Should have reasonably long life. 5. Should have proper application methods.

The performance of the paint and lacquer coatings depends to a large extent on the application technique and even slight negligence at any stage may cause failure of the coatings.

4. Inhibition of Corrosive Environment: It is achieved by inhibition of corrosives constitutes and conditions of environment. It involves following sub process. A. Dehumidification: Presence of moisture in air causes water condensation at metal surfaces which is very corrosive condition. So first air passes from dehumidifier (alumina or silica) to remove moisture from air. B. Alkaline conditioning: Corrosion enhances in acidic environment due to presence of HCl, CO2, SO2, H2S etc. which can be neutralized by adding alkaline substances such as NH3, NaOH, lime etc. e.g. alkaline conditioning of crude oil to protect refinery equipments for corrosion. C. Corrosion inhibitors: Corrosion inhibitors are substances, which when added in small concentrations to a corrosive environment decreases the corrosion rate. The inhibitors reduce the corrosion rate either by reducing the probability of its occurrence (deterrent) or by reducing the rate of attack (retardant) or by doing both. They are mostly used to provide protection to systems in which corrosion environment is re-circulated or confined for a longer period such as internal combustion engine, re-circulating cooling water systems and pipe lines for transporting oils, etc.