HASTELLOY C-4 alloy exhibits high resistance to chloride-induced pitting and crevice attack, forms of corrosion to which the austenitic stainless steels are particularly prone. To assess the resistance of alloys to pitting and crevice attack, it is customary to measure their Critical Pitting Temperatures and Critical Crevice Temperatures in acidified 6 wt.% ferric chloride, in accordance with the procedures defined in ASTM Standard G 48. These values represent the lowest temperatures at which pitting and crevice attack are encountered in this solution, within 72 hours. For comparison, the values for 316L, 254SMO, 625, and C-4 alloys are as follows:
| Alloy |
Critical Pitting Temperature |
Critical Crevice Temperature |
| °F |
°C |
°F |
°C |
| 316L |
59 |
15 |
32 |
0 |
| 254SMO |
140 |
60 |
86 |
30 |
| 625 |
212 |
100 |
104 |
40 |
| C-4 |
212 |
100 |
122 |
50 |
Other chloride-bearing environments, notably Green Death (11.5% H2SO4 + 1.2% HCl + 1% FeCl3 + 1% CuCl2) and Yellow Death (4% NaCl + 0.1% Fe2(SO4)3 + 0.021M HCl), have been used to compare the resistance of various alloys to pitting (using tests of 24 hours duration). In Green Death, the lowest temperature at which pitting has been observed in C-4 alloy is 85°C. In Yellow Death, C-4 alloy has not exhibited pitting at temperatures up to 120°C (the maximum to which C-4 alloy has been exposed).