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TYPES OF CORROSION AND RATES OF CORROSION

Prepared By:

SADAQAT ALI 13CH26

Chemical Engineering(Mehran University of Engineering And Technology, Sind, Pakistan)

CORROSION

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INTRODUCTIONCorrosion is a process of formation of

the compound of pure metal by the chemical reaction between metallic surface and its environment.

It is an oxidation process. It causes loss of metal.

Hence, disintegration of a metal by its surrounding chemicals through a chemical reaction on the surface of the metal is called corrosion.

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Corrosion comes in many different forms and can be classified by the cause of the chemical deterioration of a metal.

Example: Formation of rust on the surface of iron, formation of green film on the surface of copper.

The responsible factors for the corrosion of a metal are the metal itself, the environmental chemicals, temperature and the design.

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TYEPS OF CORROSIONListed below are 10 common types of

corrosion:I. General Attack CorrosionII. Localized CorrosionIII. Galvanic corrosionIV. Environmental CrakingV. Flow Assisted CorrosionVI. Intergranular (Intercrystalline )

corrosionVII. De-alloyingVIII.Fretting CorrosionIX. High Temperature Corrosion

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1. General Attack Corrosion

Also known as uniform attack corrosion, general attack corrosion is the most common type of corrosion and is caused by a chemical or electrochemical reaction that results in the deterioration of the entire exposed surface of a metal.

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• Ultimately, the metal deteriorates to the point of failure.

• General attack corrosion accounts for the greatest amount of metal destruction by corrosion, but is considered as a safe form of corrosion, due to the fact that it is predictable, manageable and often preventable.

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2. Localized CorrosionUnlike general attack corrosion,

localized corrosion specifically targets one area of the metal structure. Localized corrosion is classified as one of three types:

a) Pitting Corrosion b) Crevice Corrosionc) Filiform Corrosion

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a) Pitting corrosion

Pitting results when a small hole, or cavity, forms in the metal, usually as a result of de-passivation of a small area. This area becomes anodic, while part of the remaining metal becomes cathodic, producing a localized galvanic reaction.

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The deterioration of this small area penetrates the metal and can lead to failure.

This form of corrosion is often difficult to detect due to the fact that it is usually relatively small and may be covered and hidden by corrosion-produced compounds

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b) Crevice CorrosionSimilar to pitting, crevice corrosion occurs

at a specific location. This type of corrosion is often associated with a stagnant micro-environment, like those found under gaskets and washers and clamps. Acidic conditions, or a depletion of oxygen in a crevice can lead to crevice corrosion.

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c) Filiform CorrosionOccurring under

painted or plated surfaces when water breaches the coating, filiform corrosion begins at small defects in the coating and spreads to cause structural weakness

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3. Galvanic CorrosionGalvanic corrosion, or dissimiliar

metal corrosion, occurs when two different metals are located together in a corrosive electrolyte. A galvanic couple forms between the two metals, where one metal becomes the anode and the other the cathode. The anode, or sacrificial metal, corrodes and deteriorates faster than it would alone, while the cathode deteriorates more slowly than it would otherwise.

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Three conditions must exist for galvanic corrosion to occur:

Electrochemically dissimilar metals

must be present

The metals must be in electrical

contact

The metals must be exposed to an

electrolyte

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4. Environmental Cracking

Environmental cracking is a corrosion process that can result from acombination of environmental conditions affecting the metal. Chemical, temperature and stress-related conditions can result in the following types of environmental corrosion:

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a) Stress Corrosion Cracking (SCC)

b) Corrosion fatigue

c) Hydrogen-induced cracking

d) Liquid metal embrittlement

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5. Flow-Assisted Corrosion (FAC)Flow-assisted corrosion, or flow-

accelerated corrosion, results when a protective layer of oxide on a metal surface is dissolved or removed by wind or water, exposing the underlying metal to further corrode and deteriorate.

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6. Transgranular and Intergranular Corrosion

Intergranular corrosion is a chemical or electrochemical attack on the grain boundaries of a metal. This often occurs due to impurities in the metal, which tend to be present in higher contents near grain boundaries. These boundaries can be more vulnerable to corrosion than the bulk of the metal.

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8. De-AlloyingDe-alloying, or selective leaching,

is the selective corrosion of a specific element in an alloy. The most common type of de-alloying is de-zincification of unstabilized brass. The result of corrosion in such cases is a deteriorated and porous copper.

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9. Fretting Corrosion:Fretting corrosion occurs as a result of

repeated wearing, weight and/or vibration on an uneven, rough surface. Corrosion, resulting in pits and grooves, occurs on the surface. Fretting corrosion is often found in rotation and impact machinery, bolted assemblies and bearings, as well as to surfaces exposed to vibration during transportation.

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10. High-Temperature Corrosion

Fuels used in gas turbines, diesel engines and other machinery, which contain vanadium or sulfates can, during combustion, form compounds with a low melting point. These compounds are very corrosive towards metal alloys normally resistant to high temperatures and corrosion, including stainless steel.

High temperature corrosion can also be caused by high temperature oxidization, sulfidation and carbonization.

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RATES OF CORROSION

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The rate of corrosion is the speed at which a metal deteriorates in a specific environment. The rate, or speed, is dependent upon environmental conditions as well as the type, and condition, of the metal.

In order to calculate the rate of corrosion, the following information must be collected:

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a) Weight loss (the decrease in

metal weight during the reference

time period)

b) Density (density of the metal)

c) Area (total initial surface area of

the metal piece)

d) Time (the length of the reference

time period)

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Electrochemical methods provide an alternative to traditional

methods used to determine the rate of corrosion. Direct and

quantitative determination of corrosion rates can be

determined from simple electrochemical measurement like a

linear sweep voltammetry (LSV).

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The corrosion rate depends on the kinetics of both anodic (oxidation) and cathodic (reduction) reactions. According to Faraday's law, there is a linear relationship between the metal dissolution rate or corrosion rate, RM, and the corrosion current icorr

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where M is the atomic weight of the metal, ρ is the density, n is the charge number which indicates the number of electrons exchanged in the dissolution reaction and F is the Faraday constant, (96.485 C/mol). The ratio M/n is also sometime referred to as equivalent weight.

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Calculation of corrosion rates requires the determination of corrosion currents. When reaction mechanisms for the corrosion reaction are known, the corrosion currents can be calculated using Tafel Slope Analysis.

The NOVA software provides a convenient interface for making Tafel plots, calculating Tafel slopes and corrosion rates

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Selecting the Corrosion Rate, Tafel Slope analysis tool from the Analysis menu, a Tafel plot is added as shown in Figure.