Different types of corrosion

Overall electrochemical reaction : at least one oxidation and one reduction

reaction

All electrons generated through oxidation must be consumed by reduction.

Oxidation reactionfor metal M

Oxidation reaction

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Reduction reaction• Reduction of hydrogen ions in an acid solution.

• Reduction reaction in an acid solution containing dissolved oxygen

• Reduction reaction in a neutral or basic solution containing dissolved oxygen

• Reduction of a multivalent metal ion to a lower valence state

• Reduction of a metal ion to its electrically neutral atom2

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COMPARISON OF CHEMICAL & ELECTROCHEMICAL CORROSION

S.No Chemical Corrosion Electrochemical Corrosion

1 Occurs in dry conditions Wet conditions

2 Takes place by direct chemical attack of atmos. gases like O2, CO2, SO2 etc.

Electrochemical attack of corrosive environment-setting up of cells

3 Takes place on homogeneous & heterogeneous metal surfaces

Takes place on heterogeneous metal surfaces only

4 Corrosion product is formed at the place of corrosion

Corrosion product is formed near the cathode

5 Corrosion is Uniform Non-uniform, when anodic area is small corrosion takes place more frequently

6 Slow Process Fast Process

7 Involves chemical reaction Involves electrochemical reaction

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Electrochemical Corrosion

1. Galvanic2. Pitting3. Concentration Cell or differential aeration4. Water line5. Crevice corrosion6. Stress corrosion7. Intragranular

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GALVANIC CORROSION(Bimetallic Corrosion)

Takes place when different metals like Zn & Cu are in contact and jointlyexposed to corrosive environment

The metal having lower reduction potential (acts as anode) in electrochemicalseries will undergo oxidation and get corroded

e.g.. Zn and Cu in contact with each other

At Anode: Zn → Zn2⁺ + 2e⁻ oxidation (loss of electrons)

At Cathode: O₂ + 2H₂O + 4e⁻ → 4OH⁻ reduction (oxygen absorption)

2H+ +2e- →H2 (hydrogen evolution) The difference in reduction potential of a metals form the basis of galvanic corrosion Corrosion occurs on anodic part while the cathodic part is protected.

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Electrochemical series compares electrodepotentials of metals and non-metals withhydrogen electrode under standardconditions.

A metal having low std red. potential will actas anode and undergo corrosion whencoupled to another metal having higher stdreduction potential.

e.g.. Zn (-0.76 V) corrodes faster than Cu(+0.34 V), Fe (-0.44V), Ag (+0.80V)

Low potential

Other Examples of Galvanic corrosion-

(i) Tin coating on copper vessel, (ii) Steel pipes connected to copper plumbing.

PlatinumGoldGraphiteTitaniumSilver316 Stainless Steel (passive)Nickel (passive)CopperNickel (active)TinLead316 Stainless Steel (active)Iron/SteelAluminum AlloysCadmiumZincMagnesium

mor

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mor

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High potential

DIFFERENTIAL AERATION CORROSION

Occurs when one part of the metal gets different concentration of oxygen in comparison to another part.

Anode: poorly oxygenated part

Cathode: highly oxygenated part

Conducting medium acts as electrolyte.

Flow of current takes place causing corrosion at anode.

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8

Zn rod is partially dipped in dilute NaCl.

Part of metal below the solution which has poor concentration of O₂ acts as anode.

Part of metal above solution having good concentration of O₂ acts as cathode.

Difference in potential thus created causes the flow of e⁻s.

Anode: Zn → Zn²⁺ + 2e⁻ (oxidation)

Cathode: ½ O₂ +H₂O + 2e⁻ → 2OH⁻ (reduction)

Net reaction: Zn²⁺ + 2OH⁻ → Zn(OH)₂

PITTING CORROSION

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WATER LINE CORROSION

10

Water storage metallic tanks along the level in tank

In Marine Ships

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CREVICE CORROSION

Takes place within crevices (narrow openings) or other shielded areas on metal surfaces such as bolts, nuts, washers and rivets.

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STRESS CORROSION

The metallic substances are subjected to uneven stresses during manufacturing operations such as bending, welding, pressing, hammering.

Parts have not been cooled properly (non-uniform cooling) during fabrication causes-stress

The areas which are more stressed become anode and corroded faster

It occurs due to combined effect of tensile stress & corroding environment

e.g.. Brass undergoes stress corrosion in presence of ammonia

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GRANULAR CORROSION

Metals and alloys have granular structure

Each grain has clearly define boundary that chemically differ from metal within the grain centre