Metals Attacked
Certain austenitic stainless steels and aluminium alloys crack in the presence of chlorides, mild steel cracks in the presence of alkali (boiler cracking) and nitrates, copper alloys crack in ammoniacal solutions (season cracking). This limits the usefulness of austenitic stainless steel for containing water with higher than few ppm content of chlorides at temperatures above 50 °C. Worse still, high-tensile structural steels crack in an unexpectedly brittle manner in a whole variety of aqueous environments, especially containing chlorides. With the possible exception of the latter, which is a special example of hydrogen cracking, all the others display the phenomenon of subcritical crack growth, i.e. small surface flaws propagate (usually smoothly) under conditions where fracture mechanics predicts that failure should not occur. That is, in the presence of a corrodent, cracks develop and propagate well below KIc. In fact, the subcritical value of the stress intensity, designated as KIscc, may be less than 1% of KIc, as the following table shows:
| Alloy | KIc
MN/m3/2 |
SCC environment | KIscc
MN/m3/2 |
|---|---|---|---|
| 13Cr steel | 60 | 3% NaCl | 12 |
| 18Cr-8Ni | 200 | 42% MgCl2 | 10 |
| Cu-30Zn | 200 | NH4OH, pH7 | 1 |
| Al-3Mg-7Zn | 25 | Aqueous halides | 5 |
| Ti-6Al-1V | 60 | 0.6M KCl | 20 |
Read more about this topic: Stress Corrosion Cracking
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