lecture 4

Question 1

Selective Attack

Answer 1

Where corrosion occurs at specific regions in a materials structure due to physical or compositional variations.

Question 2

Types of selective attack

Answer 2

• Grain Boundary Corrosion
• Intergranular Corrosion
• Selective Leaching

Question 3

Grain Boundary Corrosion

Answer 3

Atoms at the grain boundary are thermodynamically less stable than those in perfect lattice sites due to mismatch of crystal structures

Question 4

Intergranular Corrosion

Answer 4

Grain boundary is attacked due to the presence of precipitates in these regions. Grain boundaries are often preferred sites for precipitation and segregation.

Question 5

Two types of precipitation

Answer 5

• Intermetallic
• Compounds

*can be cathodic or anodic and result in different corrosion

Question 6

Case study - Welding of stainless steel to 600-800C resulted in rapid corrosion of the stainless steel

Answer 6

• Stainless steel covered in protective chromium oxide layer
• Heat energy of the weld allowed diffusion of the chromium and formation of chromium carbide at the grain boundary
• These means areas around the grain boundary are depleted in chromium oxide as chromium is tied up in chromium carbide
• Chromium depleted region rapidly attacked and leads to intergranular/pitting corrosion
• Solution:
  1. Use low carbon steel <0.03%C
  2. Apply post weld heat treatment to dissovle carbides and quench
  3. Add titanium or niobium (type 321 stainless)

Question 7

Selective leaching

Answer 7

• Alloys containing metals of wide ranging galvanic activity

- one metallic element (the more reactive metal) is removed from the structure of the alloy.

Question 8

Prevention of dezincification

Answer 8

Addition of arsenic (0.05%) or tin (1%)

Question 9

Microbial induced corrosion (MIC)

Answer 9

• Can be direct or indirect
• Microbes can release organic acid that lower the pH
• Can reduce sulphates to corrosive sulphides
• can cover surfaces creating very low oxygen environments at surface

Question 10

Prevention of MIC

Answer 10

1. Redesign to prevent biofouling
2. Application of polymer coatings
3. Switch to a polymeric material for the pipework
4. Use of biocides (Cl- could corrode metal also)