Forms of Corrosion

Dr Rakesh Barik

CSIR-Central Electrochemical Research InstituteKaraikudi, Tamil Nadu-630006

Corrosion Science

Examples

Examples

Examples

Fe → Fe2+ + 2e–

O2 + 2H2O + 4e– → 4OH– (oxygen reduction reaction).

2H+ + 2e– → H2 (hydrogen evolution reaction)

Anodic reaction :

Cathodic reaction :

CorrosionCorrosion is defined as the process (or result) of unwanted attack on a metal by its environment.

Driving Force

0<−=∆ οcellcell nFEG

0>οcellE 0>− anode

ecathodee EE

An appreciation of equilibrium thermodynamics is essential both to understand corrosion and to determine whether corrosion can occur in a particular circumstance.

If a corrosion reaction is to be spontaneous, the Gibbs free energy associated with the cell reaction must be negative (∆Gcell < 0). These free energy changes maybe related to the relevant equilibrium potentials:

i.e.: and:

The cathode process must have a more positive equilibrium potential thanthat of the anode process for corrosion to occur.

Electrochemical SeriesElectrode reaction (red = ox + ne -) / V(SHE)

Au 3+ + 3e– → Au +1.50

O2 + 4H+ + 4e– → 2H2O +1.23

Pt2+ + 2e– → Pt +1.20

Ag + + e– → Ag +0.80

Fe3+ + e– → Fe2+ +0.77

O2 + 2H2O + 4e– → 4OH- +0.40

Cu2+ + 2e– → Cu +0.34

Sn4+ + 4e– → Sn2+ +0.15

2H+ + 2e– → H2 +0.00

Pb2+ + 2e– → Pb -0.13

Sn2+ + 2e– → Sn -0.14

Ni2+ + 2e– → Ni -0.23

Co2+ + 2e- → Co -0.28

Cd2+ + 2e– → Cd -0.40

Fe2+ + 2e– → Fe -0.44

Cr3+ + 3e– → Cr -0.74

Zn2+ + 2e– → Zn -0.76

Ti2+ + 2e– → Ti -1.63

Al 3+ + 3e– → Al -1.67

Mg2+ + 2e– → Mg -2.36

Na+ + e– → Na -2.71

K+ + e– → K -2.92

More cathodic (noble )Less tendency to corrode

More anodic (active )Greater tendency to corrode

/ VCathode: 2H+ + 2e– → H2 -0.00Anode Cu2+ + 2e– ← Cu -0.34

Cell Cu + 2H+ → Cu2+ + H2 -0.34

∆Gcell = +66 kJ mol–1 Cu

ο

eE

/ V

Cathode: 2H+ + 2e– → H2 -0.00Anode Fe2+ + 2e– ← Fe -0.44

Cell Fe + 2H+ → Fe2+ + H2 +0.44

∆Gcell = -85 kJ mol–1 Fe

ο

eE

Copper Corrosion - Feasible or Not ?

Iron Corrosion -Feasible or Not ?

Galvanic SeriesLeast likely to corrode:

Platinum Gold Graphite Titanium Silver Hastelloy C – passive (62%Ni, 18%Cr, 15%Mo) 316 stainless steel - passive (18%Cr, 8%Ni, 3%Mo) 304 stainless steel - passive (18%Cr, 8%Ni) Inconel – passive (80%Ni, 13%Cr, 7%Fe) Nickel - passive Nickel aluminium bronze Monel (66% Ni, 32% Cu, 1% Fe) Copper-nickel Bronzes (Cu-Sn) Copper Brasses (Cu-Zn) Hastelloy (B) (65%Ni, 30%Mo, 5%Fe) Hastelloy (A) (60%Ni, 20%Mo, 20%Fe) Inconel - active Nickel - active Tin Lead Hastelloy (C) – active Cast iron Steel or iron Aluminium alloys Cadmium Zinc Magnesium alloys

Most likely to corrode:

Ecorr increasingly more negative

The galvanic series helps in the choiceof metals for protection. The series showa galvanic series of pure metals andalloys based on practical observations ina specific environment such asseawater.

Galvanic series for a number of metals and alloys in flowing seawater (conditions: 2.5 to 4.0 m s–1 and 10 to 25 °C).

• Uniform• Galvanic• Localized• Intergranular• SCC

Forms of Corrosion

• Hydrogen Damage• Corrosion Fatigue• Fretting• Erosion-Corrosion • Cavitation• Microbial Induced

Corrosion

• Characterised by corrosion occurring uniformly over the surface.

• Uniform corrosion is usually manifested in the progressive thinning of a metal component until it virtually dissolves away or becomes a delicate lace-like structure

• Corrosion does not penetrate verydeep inside.

Uniform corrosion

Example: Rusting of steel in air

• Dry atmosphere

• Damp atmosphere

• Wet atmosphere

• Acids (HCl, HCIO4, H3PO4)

• Atmospheric contaminants.

• Process water containing hydrogen sulfide

• Brines.

• Industrial atmosphere

• Hydrocarbon containing wet hydrogen sulfide

Environment

Fe2+(aq) + 2OH- (aq) = Fe(OH)2(s)

4Fe(OH) 2 + O2 + 2H2O = 4Fe(OH) 3

4Fe(OH) 2 + O2 = 2Fe2O3 • H2O + 2H2O

Mechanism

MechanismCorrosion of steel in different environment

• Slow down or stop the movement of electrons(a) Coat the surface with a non-conducting medium such as paint,

lacquer or oil(b) Reduce the conductivity of the solution in contact with the metal an

extreme case being to keep it dry. Wash away conductive pollutants regularly.(c) Apply a current to the material (see cathodic protection)

• Slow down or stop oxygen from reaching the surface. Difficult to do completely but coatings can help.

• Prevent the metal from giving up electrons by using a more corrosion resistant metal higher in the electrochemical series. Use a sacrificial coating which gives up its electrons more easily than the metal being protected. Apply cathodic protection. Use inhibitors.

• Select a metal that forms an oxide that is protective and stops the reaction.

Control and consideration of environmental and thermal factors is also essential.

Control

Heat Exchanger

SS Screw

Cadmium plated Steel washer

Bimetallic / dissimilar metal or contact corrosion

Galvanic Corrosion

Galvanic SeriesLeast likely to corrode:

Platinum Gold Graphite Titanium Silver Hastelloy C – passive (62%Ni, 18%Cr, 15%Mo) 316 stainless steel - passive (18%Cr, 8%Ni, 3%Mo) 304 stainless steel - passive (18%Cr, 8%Ni) Inconel – passive (80%Ni, 13%Cr, 7%Fe) Nickel - passive Nickel aluminium bronze Monel (66% Ni, 32% Cu, 1% Fe) Copper-nickel Bronzes (Cu-Sn) Copper Brasses (Cu-Zn) Hastelloy (B) (65%Ni, 30%Mo, 5%Fe) Hastelloy (A) (60%Ni, 20%Mo, 20%Fe) Inconel - active Nickel - active Tin Lead Hastelloy (C) – active Cast iron Steel or iron Aluminium alloys Cadmium Zinc Magnesium alloys

Most likely to corrode:

Ecorr increasingly more negative

The galvanic series helps in the choiceof metals for protection. The series showa galvanic series of pure metals andalloys based on practical observations ina specific environment such asseawater.

Galvanic series for a number of metals and alloys in flowing seawater (conditions: 2.5 to 4.0 m s–1 and 10 to 25 °C).

Voltage difference between thetwo metals on the galvanicseries

Factors involved

• the voltage difference between the two metals on the

galvanic series

• the size of the exposed area of cathodic metal relative to

that of the anodic metal

• the amount of dissolved oxygen available at the cathodic

surface

• cathodic efficiency of the noblest metal

• temperature

• the presence of biofouling.

Condition necessary

Effect of AreaFor static conditions, when corrosion rates (CR in mm y–1) were plotted as afunction of cathode/anode area ratios (C/A), a CR ∝ C/A relationship wasobtained. This is due to the cathodic reaction, the reduction of oxygen understatic conditions the mass transport of oxygen is by diffusion, thus the overallcorrosion process is diffusion controlled.

Effect of parameters

Effects of Solution /distance

Solution conductivity varies inversely with the length of the conduction path

Effect of parameters

Low conductivity High conductivity

Effect of Biofouling

Copper alloys are more resistant to biofouling than most metals due to the toxicityof the released copper-ions. When coupled to other metals the release of copper-ions can be greatly reduced.

Biofilms on titanium have been reported to catalyse the cathodic reduction ofoxygen – thus increasing the overall cathodic efficiency

Effect of parameters

Prevention of this problem is based on ensuring that one or more of the three features do not exist.

• Break the electrical contact using plastic insulators or coatings between the metals.

• Select metals close together in the galvanic series. • Prevent ion movement by coating the junction with an impermeable material, or

ensure environment is dry and liquids cannot be trapped.

Control

Cathodic Protection

Crevice /krevis/ A narrow crack or opening; a fissure or cleft.

Metal to MetalMetal to Non metal (under the gasket)Overlapping of surfaces

Flanges, Washers ,Rolled tube ends, Threaded joints, O-rings, Gaskets, Lap joints, Sediment Deposits.

Crevice Corrosion

Mechanism

O2 + 2H2O + 4e– → 4OH–

Cl–

Cl–Low O2 and high M+,CI-,H+concentration

M++ Cl- +H2O → MOH + HCl

• Use welded instead of riveted or bolted joints

• Use nonabsorbing gaskets when possible

• Remove accumulated deposits frequently

• Designing containment vessels to avoid stagnant areas and ensure complete drainage.

Control

Pitting cause

formation of pits or holes

Pitting corrosion

Mechanism

Cl–Cl–Cl–Cl–

MOHMOH

MOHH+

H+H+

H+• Pitting initiation

• Pitting propagation

• Pitting termination.

THROUGH PITSSIDEWAY PITS

Mechanism

Mechanism

Mechanism

• Selecting a resistant material

• Ensuring a high enough flow velocity of fluids in contact with the material or frequent washing

• Control of the chemistry of fluids and use of inhibitors

• Use of a protective coat

• Maintaining the material’s own protective film.

Control

Intergranular Corrosion• Metal consists of grains• Under some circumstances they

corrode preferentially• The material is said to be

sensitised

This is preferential attack of the grain boundaries of the crystals that fothe metal. It is caused by the physical and chemical differences between the centres and edges of the grain.

Intergranular corrosion of a failed aircraft component made of 7075-T6 aluminum (picture width = 500 mm)

Intergranular Corrosion of stainless steels and nickel based alloys

Exposed temperature : 425 oC to 815 oC

Sensitization

This results from the preferential attack of Cr- depleted zone due to precipitation of Cr23C6 at grain boundary.

Mechanism

Metallurgical measures:

Solution annealing : heating the alloy to 1050 C where all Cr- carbides are dissolved, followed by rapid cooling.

Low-carbon alloy modifications : lower the carbon content to below 0.03% for austenitic stainless steels(304L, 316L) or to below 50ppm for ferritic stainless steels.

Stabilization treatment : add strong carbide former(Ti, Nb) in melt: Types 347 and 321 stainless steels.

Environmental measures :

Lower acidity and less oxidizing conditions will generally reduce the susceptibility to IGC.

Control