Localised forms of corrosion
Carbon steels suffer from 'general' corrosion, where large areas of the surface are affected. Stainless steels in the passive state are normally protected against this form of attack, however, localised forms of attack can occur and result in corrosion problems.
The assessment of corrosion resistance in any particular environment, therefore, usually involves a consideration of specific corrosion mechanisms. These mechanisms are principally:
- Crevice corrosion
- Pitting corrosion
- Intergranular (or intercrystalline) corrosion (IC)
- Stress corrosion cracking (SCC)
- Bimetallic (galvanic) corrosion
Other related mechanism can also occur, which include:
- Erosion - corrosion
- Corrosion fatigue
Localised corrosion is often associated with chloride ions in aqueous environments. Acidic conditions (low pH) and increases in temperature all contribute to localised mechanisms of crevice and pitting corrosion. The addition of tensile stresses, whether applied by loading or from residual stresses, provides the conditions for stress corrosion cracking (SCC). These mechanisms are all associated with a localised breakdown of the passive layer. A good supply of oxygen to all surfaces of the steel is essential to maintaining the passive layer but higher levels of chromium, nickel, molybdenum & nitrogen all help in their individual ways to prevent these forms of attack.
Resistance to localised forms of corrosion
As a general rule increased corrosion resistance can be expected by moving through the grades:
| 1.4512 to 1.4016 | 409 to 430 | increasing chromium from 11 to 17% |
| 1.4301 | 304 | adding nickel which aids the reformation of the passive layer if it is disturbed |
| 1.4401 | 316 | adding molybdenum reduces the effectiveness of chloride ions in locally breaking down the passive layer |
| 1.4539 and 1.4547 | 904L and 6% molybdenum grades | further increases in chromium, nickel and molybdenum result in overall improved localised corrosion resistance |
Duplex grades such as 2205 (1.4462/S31803) are specifically designed to combat SCC by 'balancing' the structure to increase its strength, but additionally molybdenum and nitrogen enhance the pitting resistance, which in turn has the additional benefit in improving their SCC resistance.
Source: http://www.bssa.org.uk/
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