C ORROSION OF METALS

LECTURE 4

MSc: Amir N.Saud

Al-Mustaqbal university college

W HAT I S C ORROSION ? A N W HY D OES I T

O CCUR ? Corrosion is the deterioration of a metal as a result of chemical reactions between it and the surrounding environment. is a natural process, which converts a refined metal to a more chemically-stable form, such as its oxide, hydroxide, or sulfide. Rusting, the formation of iron oxides, is a well- known example of electrochemical corrosion. This type of damage typically produces oxide(s) or salt(s) of the original metal, and results in a distinctive orange colouration Corrosion can occur in materials other than metals, such as ceramics or polymers. Corrosion degrades the useful properties of materials and structures including strength, appearance and permeability to liquids and gases.

C ONDITIONS FOR CORROSION

The components necessary for corrosion to occur:

Metals (such as iron)

Oxygen (usually from the atmosphere) An electrolyte (usually water)

Factors needed for this reaction to occur:

An anodic process: where the metal reacts with the environment and becomes an oxide while generating electrons

A cathodic process: where the electrons generated by the anodic process are consumed in order to reduce one or more environmental species.

An electrolyte: this is the solution where cathode and anode are immersed, it could be a drop of water from rain or a liquid stored in a tank.

Current in the metal: similar to what happens in the electrolyte, electrons move from the anode to the cathode through the metal, closing the circuit and allowing the corrosion reaction to occur.

C ORROSION AS AN

E LECTROCHEMICAL P ROCESS Corrosion happens through a series of reduction-oxidation reactions, similar to those of a battery. The metal being corroded acts as the anode; the metal is oxidized, forming metal ions and free electrons. The free electrons reduce the oxygen, often times forming hydroxide, and providing a complimentary cathodic reaction. The dissolution of the metal at the anode has two possible outcomes; the metal ions can go into solution, becoming hydrated, or the metal ions can form a solid compound that collects on the surface. In the former case, further oxidation of the metal ions can occur and an open pit can form. In the latter case, a protective barrier may be formed and the collection of solid metal ions will inhibit further corrosion.

T YPES OF CORROSIONS

UNIFORM CORROSION:

Uniform corrosion is considered an even attack across

the surface of a material and is the most common type

of corrosion. However, uniform corrosion is relatively

easily measured and predicted, making disastrous

failures relatively rare. In many cases, it is

objectionable only from an appearance standpoint. As

corrosion occurs uniformly over the entire surface of

the metal component, it can be practically led control

by cathodic protection, use of coatings or paints, or

simply by specifying a corrosion allowance. In other

cases uniform corrosion adds colour and appeal to a

surface.

T YPES OF CORROSIONS

PITTING CORROSION:

Pitting is one of the most destructive types of corrosion, as it can be hard to predict, detect and characterize. Pitting is a localized form of corrosion, in which either a local anodic point, or more commonly a cathodic point, forms a small corrosion cell with the surrounding normal surface. Once a pit has initiated, it grows into a “hole” or “cavity” that takes on one of a variety of different shapes. Pits typically penetrate from the surface downward in a vertical direction. Pitting corrosion can be caused by a local break or damage to the protective oxide film or a protective coating; it can also be caused by non-uniformities in the metal structure itself. Pitting is dangerous because it can lead to failure of the structure with a relatively low overall loss of metal.

T YPES OF CORROSION

Galvanic Corrosion :

refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. It occurs when two (or more) dissimilar metals are brought into

electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone.

T YPES OF CORROSION

CREVICE CORROSION:

Refers to corrosion occurring in confined spaces

to which the access of the working fluid from

the environment is limited. Crevice corrosion

occurs in shielded areas such as those under

washers, bolt heads, gaskets, etc. where oxygen

is restricted. These smaller areas allow for a

corrosive agent to enter but do not allow

enough circulation within, depleting the oxygen

content, which prevents re-passivation. As a

stagnant solution builds, pH shifts away from

neutral. This growing imbalance between the

crevice (microenvironment) and the external

surface (bulk environment) contributes to

higher rates of corrosion.

T YPES OF CORROSION

Intergranular Corrosion:

The microstructure of metals and alloys is made up of grains, separated by grain boundaries. Intergranular corrosion is localized attack along the grain boundaries, or immediately adjacent to grain boundaries, while the bulk of the grains remain largely unaffected. This form of corrosion is usually associated with chemical segregation effects (impurities have a tendency to be

enriched at grain boundaries) or specific phases precipitated on the grain boundaries. Such precipitation can produce zones of reduced corrosion resistance in the immediate vicinity.

T YPES OF CORROSION

Stress corrosion cracking:

is the cracking induced from the combined

influence of tensile stress and a corrosive

environment. The impact of SCC on a material

usually falls between dry cracking and the fatigue

threshold of that material. The required tensile

stresses may be in the form of directly applied

stresses or in the form of residual stresses, see an

example of SCC of an aircraft component. The

problem itself can be quite complex. The situation

with buried pipelines is a good example of such

complexity.

H OW TO PREVENT CORROSION ? Avoid exposure to Corrosive Agents:

Prevent the deterioration of metals by limiting its contact with corrosive agents. For instance, safeguard the metal materials from rainwater or excessive moisture by properly storing it indoors. Moreover, exposure to high chloride containing substances (such as salt water) must be limited. For instance, treat the feed water inside the water boilers with softener to prevent corrosion.

Proper Monitoring of Metal Surface:

Carefully monitor the metal surface. Look for cracks and crevices. These manufacturing flaws can also lead to corrosion. Moreover, use corrosion resistant products. For instance, if you are buying TMT bars for construction, choose corrosion resistant bars (such as SRMB TMT bars). Corrosion resistant TMT bars ensure the longevity of the structure.

Protect the Metal Surface:

Paints can be used to protect the metal surface from corrosion. The paint forms a protective barrier between the metal surface and the corrosive agent. For instance, coating the outdoor metal units with a coat of paint protects them from exposure to rainwater or snowfall etc. A number of solutions such as galvanised zinc coating, paint or oil sealant can be used to prevent corrosion.

H OW TO PREVENT CORROSION ?

Prevent Galvanic Corrosion: Such type of corrosion is common in gas or oil pipelines, the hull of the ships and boats etc. To prevent Galvanic Corrosion, apply a protective coating on the metal surface. This prevents the electrolytes facilitate ion flow between the metals. Providing Cathodic protection by using a sacrificial metal anode in the electrolytic environment also protects the metals.

Metallic Plating: Metallic plating can be applied to prevent corrosion. Common processes of metallic plating include Electroplating (covering the surface with a layer of tin or nickel), Mechanical plating (applying zinc or cadmium to metal surface), Electroless (coating with cobalt or nickel), or Hot Dipping (immersing the metal in molten zinc).

H OW TO PREVENT CORROSION ? Galvanized Steel:

Galvanized steel takes a long time to rust, but it will

eventually rust. This type is carbon steel that has been

galvanized, or coated, with a thin layer of zinc. The zinc acts as

a barrier preventing oxygen and water from reaching the steel,

so that it is corrosion protected. Even if the zinc coating is

scratched off, it continues to protect nearby areas of the

underlying steel through cathodic protection, as well as by

forming a protective coating of zinc oxide. Like aluminum, zinc

is highly reactive to oxygen in the presence of moisture, and

the coating prevents the iron in the steel from further

oxidation.

The types of corrosion that are pertinent to the currently used alloys are:

The types of corrosion in metallic implants are,

Pitting Crevice Corrosion Fatigue Stress-corrosion Cracking (SCC) Fretting Galvanic Corrosion These corrosion types will be discussed in relation to the specific alloys and their occurrence

M ETAL C ORROSION IN THE H UMAN B ODY

EFFECT OF CORROSION-FAILURE OF IMPLANTS The reaction of the metallic ions that leaches away from the implant due to corrosion in the human body affects several biological parameters. As a material starts to corrode, the dissolution of metal will lead to erosion which in turn will eventually lead to brittleness and fracture of the implant. Once the material fractures, corrosion gets accelerated due to increase in the amount of exposed surface area and loss of protective oxide layer. If the metal fragments are not surgically extracted, further dissolution and fragmentation can occur, which may result in inflammation of the surrounding tissues. . The contents of the Table 1 amply illustrate the possible hazardous effects associated with the corroded implant material. The release of corrosion products will obviously lead to adverse biological reactions in the host, and several authors have reported

increased

concentrations of corroded particles in the tissue near the implants

and other parts of the human body such as kidney, liver etc.

The oxide film which inhibits the dissolution of metal ions

is not always stable in the human body and hence a

thorough understanding of the behavior of the oxide film in

in vivo condition is essential to have a better insight of the

corrosion phenomenon. The analysis of the surface oxide

film on various metallic biomaterials is given in Table

C ORROSION OF METAL ALLOYS

stainless steel is susceptible to localized corrosion by

chloride ions and reduced sulfur compounds . The presence of micro organisms on a metal surface often leads to highly localized damages in the concentration of the electrolytic constituents, pH and oxygen levels . Studies on corrosion and electrochemical behavior of 316L SS in the presence of aerobic iron-oxidizing bacteria (IOB) and anaerobic sulfate-reducing bacteria (SRB) reveal that the interactions between the stainless steel surface with the corroded products, bacterial cells and their metabolic products increases the corrosion damage and also accelerates

These localized corrosion attacks and leaching of metallic ions from implants necessitate improvement in the corrosion

resistance of the currently used type 316L SS by bulk alloying or modifying the surfacepitting propagation

Cobalt-based alloys have been widely employed in

orthopaedic implants and biocorrosion of this alloy is one

of the major problems to be dealt with as there is larger

release of metal ions which causes adverse effects. Co-Cr-

Mo alloy is used as a femoral head of joint prostheses in

conjunction with an ultra high molecular weight

polyethylene (UHMWPE) cup because of the high wear

and corrosion resistance of this alloy. The problem with the

metal-on-metal couple is that the release of metal ions is

higher than that of the polymer-on-metal couple in in

vivowhich will, over many years lead to toxicity problem.

it has been well established that titanium is completely inert and immune

to corrosion by all body fluids and tissue and is thus completely

biocompatible . High modulus of elasticity of the conventional alloys

has resulted in the stress shielding effect and the failure of the implant.

The modulus of elasticity of titanium based alloys is much lower and

closer to that of the bone when compared to SS and Co-Cr alloys and

hence they are more preferred for long term applications. As of now, they

are used as implants for joint replacements, bone fixation, dental

implants, heart pacemakers, artificial heart valves, stents and components in

high-speed blood centrifuges because of their high specific strength and

chemical stability . However, these implants such as artificial joints and

bone plates are likely to be damaged mostly due to fatigue . The reason for this

is due to the decrease in fatigue strength, which in turn should arise from the

synergistic effect of the formation of corrosion pits on the surface, which

arise from the dissolution of Ti2+ions in the living body, wearing at

sliding parts and fretting

Although Ti-6Al-4V alloy has got several positive

features, detailed studies have shown that they lead to

long term ill effects such as peripheral neuropathy,

osteomalacia and Alzheimer disease due to the release

of aluminum and vanadium ions from the alloy.

Titanium alloys.—The shape memory alloy, Nitinol, is

composed of near equi-atomic amounts of Nickel

and Titanium. Since the early 1970s it has found

widespread clinical use as an orthodontic material and

more recently as vascular stents due to its exceptional

mechanical characteristics and its high biocompatibility

.Several studies have highlighted the variation in the

corrosion performance of Nitinol depending upon the

surface condition of the test specimens used and the

surface condition given

P ROTECTION OF METAL IMPLANT

heat treatment is involved during the manufacturing

process, the passivation oxide present on Nitinol is

polycrystalline in nature, and has been found to exhibit

severe pitting and crevice corrosion, whereas surface

treatment to form amorphous oxide results in excellent

corrosion resistance. Other surface treatments, such as

electrochemical polishing, has also been found to be a good

surface treatment prior to implantation, resulting in

significantly increased corrosion resistance

Recently carbon-based coatings namely Diamond Like Carbon (DLC) are found to be more promising and the corrosion resistance of

NiTi alloys with this coating has shown tremendous improvement

Ti dental implants are generally surface modified to reduce corrosion, improve

Osseo integration and increase the biocompatibility. To achieve this, surface

treatments, such as surface machining, sandblasting, acid etching, electro-

polishing, anodic oxidation, plasma-spraying and biocompatible/biodegradable

coatings are performed to improve the quality and quantity of the bone-implant

interface of titanium-based implants [101-104]. Unlike the above treatments,

laser-etching technique was introduced in material engineering originally

which resulted in unique microstructures with greatly enhanced hardness,

corrosion resistance, or other useful surface properties [105, 106]. Laser

processing also is now being used in implant applications to produce a high

degree of purity with enough roughness for good Osseo integration [107]. .

In addition to the above, nanoceramic HAP coatings are

used to enhance the osseointegration. Nanostructured

graded metalloceramic coatings have also been tried to

achieve better adhesion between the metal and ceramic

coatings and thus nanoceramic coatings are gradually

receiving greater attention