Other types of Corrosion

Question 1

What is crevice corrosion?

Answer 1

-Crevice Corrosion refers to the localized attack on a metal surface at, or immediately adjacent to the crevice between two joining surfaces, which usually occurs with a stagnant (not flowing or free moving) solution on a micro-environmental level. The gap or crevice can be formed between two metals or a metal and non-metallic material.

Question 2

Typical places for a stagnant microenvironment

Answer 2

Crevices (small shielded areas) that form under:
• gaskets
• washers
• insulation material
• fastener heads
• surface deposits
• disbonded coatings
• threads
• lap joints
• clamps

Question 3

What is Pitting Corrosion?

Answer 3

Pitting corrosion

• localised form of corrosive attack
• produces holes or small pits in a metal
• bulk of the surface remains unattacked
• based on low oxygen concentration at the bottom of the pit
• often found in situations where resistance against general corrosion is conferred by passive surface films
• localised pitting attack is found where the passive films have broke down

Question 4

Why is pitting corrosion dangerous?

Answer 4

• difficult to anticipate, prevent and detect
• occurs rapidly
• penetrates the metal without loss of significant weight

Question 5

What is intergranular Corrosion

Answer 5

• Intergranular corrosion refers to preferential (localised) corrosion along the grain boundaries or immediately adjacent grain boundaries while the bulk of the grains remain largely unaffected
• associated with chemical segregation effects (impurities have a tendency to be enriched at grain boundaries)
• may lead to dislodgement of grains

Question 6

What is stress corrosion cracking?

Answer 6

Spontaneous corrosion induced cracking of a material under static tensile stress.
Environmental embittlement assisted
cracking forms

Question 7

What is the Crevice Corrosion Mechanism? (stage 1 and 2)

Answer 7

Stage 1 - At time zero, the oxygen content of the
water in a crevice is equal to the
amount of soluble oxygen available
Stage 2 - Due to the difficult access to fresh air caused by the crevice geometry, oxygen consumed by normal uniform corrosion is very soon depleted in the crevice. The corrosion reactions are now contained in the crevice (anodic) and on the open surface (cathodic).

Question 8

What is the Crevice Corrosion Mechanism? (stage 3)

Answer 8

Stage 3 - The crevice development a few more accelerating factors fully develop:

1. The metal ions produced by the anodic corrosion reaction readily hydrolyse 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.
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.