Lecture 10: Crevice Corrosion and Pitting

• The Nature of Metal/Environment
• Crevice Corrosion
• Mechanism of Crevice Corrosion
• Pitting Corrosion
• Characteristics of Pitting
• Mechanism of Pitting
• Differential Aeration
• Summary
• Reading Assignments

[Previous Lecture] [Next Lecture] [Course Outline] [General Info] [Study Guide]

The Nature of Metal/Environment

• uniform composition of alloy + uniform composition of environment => uniform corrosion
• non-uniform composition of alloy + uniform composition of environment => Galvanic corrosion/Intergranular corrosion
• non-uniform composition of alloy + non-uniform composition of environment=> localised corrosion: crevice/pitting

• Most of practical situations are non-uniform.

[Back to Top]

Crevice corrosion:

Localised corrosion resulting from the formation of a concentration cell in a crevice formed between a metal and nonmetal, or between two metal surfaces.

Two prerequisites:

• a physical crevice between two surfaces
• the formation of concentration cell

In 1981, it was reported that one of the most serious problems in the US nuclear industry was "denting" (localised attack) of Inconel 600 tubing because of the corrosion in crevices between the tubes and carbon steel support plates.

With about 60 steam generators affected, the COST of rectifying the known problems was estimated to be US$6000 million.

[Back to Top]

The Mechanisms of Crevice Corrosion

The Fontana and Greene Model

Stage 1

corrosion occurs as normal both inside and outside the crevice:

anodic reaction: M=>Mⁿ⁺ + ne

cathodic reaction: O₂+2H₂O +4e=4OH^-

The positively charged metallic ions are electrostatically counterbalanced by OH^-

Stage 2

at this stage, the cathodic reaction inside the crevice consumed most of the oxygen available.

Stage 3

Cl- and OH- diffuse into the crevice to maintain a minimum potential energy.
=>Metal chloride is formed. Hydrolysis of metal chloride lowers pH=>

MCl_n + nH₂O = M(OH)_n + nHCl

Stage 4

More Mⁿ⁺ ions attrack more Cl^- leads to lower pH inside crevice
=> accelerate metal dissolution=>More Mⁿ⁺ ions produced =>attrack more Cl^- =>lower pH
=> autocatalytic process

For stainless steel, it is the dissolution and hydrolysis of Cr that leads to the fall in pH:

Cr³⁺ + 3H₂O = Cr(OH)₃ + 3H⁺

[Back to Top]

PITTING CORROSION

Definition:

Extremely localised attack that results in holes in metals.

Characteristics of pitting:

(1) difficult to detect

• small in size
• covered by corr products
• undercut surfaces, damage to sub-surfaces are severe.
• initiation/incubation period uncertain

(2) difficult to measure/compare

• depth of penetration varies
• number of pits varies

(3) difficult to predict

• requires long time to initiate

(4) vicious

• highly localised
• intense attack
• equipment fails with extreme suddenness

(5) shape of pits

• sharply defined holes, no attack on most metal surface.
• rough surface, pits close together
• line in direction of gravity

(6) growth of pits

• requires 0.5-1 year to initiate
• once started penetrate metal fast
• undercut surfaces, sub-surface damage severe.

[Back to Top]

Mechanism of pitting

Propagation mechanism same as crevice corrosion:

concentration cell causes local acidification within pit.

autocatalytic nature

Initiation of pitting is different from crevice:

Crevice requires physical crevice - a gap between two surfaces

Pitting has no geometrical requirement

[Back to Top]

Differential Aeration Caused Pitting

Pitting underneath a water droplet

(a) initiation period:

general/uniform corrosion

Fe = Fe²⁺ + 2e

(b) the center of the water droplet is depleted in O₂.

Concentration cell causes separation of anode and cathode:

centre of droplet: anode

edge of droplet: cathode

[Back to Top]

Summary

Pitting corrosion is one of the most damaging forms of corrosion. It may be initiated by metallurgical factor or environmental factor or both. Once initiated, the propagation is autocatalytic (self-accelerating) in nature. Crevice corrosion is caused by the presence of a physical gap (crevice) between two surfaces that leads to the set up of differential aeration cell. Inside the crevice, anodic reaction is also self-accelerating (autocatalytic) in nature. The processes inside the crevice is essentially the same as the propagation of pitting. Stainless steels and alloys are particularly susceptible to pitting and crevice corrosion.

Reading Assignments

To reinforce learnings in this lecture read pages 162-172 (textbook)
To prepare yourself for the next lecture read pages 172-177 (textbook)

[Back to Top]

Copyright © 1995-2024. All rights reserved.