Chapter 9 - Corrosion | Crevice Corrosion

Question 1

Characteristics of corrosion?

Answer 1

• localized for of corrosion
• Associated with a stagnant solution
• Tends to occur in small openings (threads, washers etc)

Question 2

How many stages are there in crevice corrosion?

Answer 2

4

Question 3

Describe stage 1 of crevice corrosion?

Answer 3

The oxygen content in the water in the crevice is equal to the level of soluble oxygen and is equal everywhere

Question 4

Describe stage 2 of crevice corrosion?

Answer 4

Due to the difficult access caused by the crevice geometry, oxygen consumed by normal uniform corrosion is very soon depleted in the crevice. The corrosion reactions now specialize in
the crevice (anode) and on the open surface (cathode)

Question 5

Describe stage 3 of crevice corrosion?

Answer 5

1. The metal ions produced by the anodic corrosion reaction readily hydrolyze giving off protons (acid) and forming corrosion products. The pH in a crevice can reach very acidic values, sometimes equivalent to pure acids.
2. The acidification of the local environment can produce a serious increase in the corrosion rate of most metals. See, for example, how the corrosion of steel is affected as a function of water pH.
3. The corrosion products seal even further the crevice environment.
4. The accumulation of positive charge in the crevice becomes a strong attractor to negative ions in the environment, such as
   chlorides and sulfates, that can be corrosive in their own right.