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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:
Case 7.1
Case 7.2
Case 7.3
Case 7.4
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.
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.
| Inside the Crevice | Outside the Crevice |
| O2 depleted | O2 readily available |
| Anodic reaction | cathodic reaction |
Stage 3
| Inside the Crevice | Outside the Crevice |
| O2 depleted | O2 readily available |
| Anodic reaction | cathodic reaction |
| High [Mn+] concentration | High [OH-] concentration |
| Overall positive | Overall negative |
| High [Cl-] concentration | Normal [Cl-] concentration |
Cl- and OH- diffuse into the crevice to maintain a minimum potential energy.
=>Metal chloride is formed. Hydrolysis of metal chloride lowers pH=>
MCln + nH2O = M(OH)n + nHCl
Stage 4
| Inside the Crevice | Outside the Crevice |
| Low pH | Normal pH |
| High [Cl-] concentration | Normal [Cl-] concentration |
| High [Mn+] concentration | Low [Mn+] concentration |
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+
Definition:
Extremely localised attack that results in holes in metals.
(1) difficult to detect
(2) difficult to measure/compare
(3) difficult to predict
(4) vicious
(5) shape of pits
(6) growth of pits
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
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
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 creive is essentially the same as the propagation of pitting. Stainless steels and alloys are particularly susceptible to pitting and crevice corrosion.
To reinforce learnings in this lecture read pages 162-172
(textbook)
To prepare yourself for the next lecture
read pages 172-177 (textbook)
