Nickel maintains its passivating oxide film in fast­flowing seawater, and its corrosion rate is <0.125 mm year-1. However, it becomes corroded under deposits in static seawater due to breaking down of the passive film by organic deposits.59

Nickel-copper alloys also show excellent corro­sion resistance in flowing sea water, with corrosion rates under 0.02 mm year-1. However, they also can be corroded under deposits in static seawater by the same mechanism as for nickel. Both nickel and nickel-copper alloys show higher resistance to cavi­tation and erosion corrosion in seawater than copper — nickel alloys.

Nickel-chromium-iron alloys have even more outstanding corrosion resistance and can be used in water polluted with substances such as carbon dioxide, iron compounds, chloride, and dissolved oxygen. They also exhibit excellent corrosion resis­tance in fast-flowing seawater but are subject to pitting and crevice corrosion in slow-flowing seawater.

Nickel-chromium-molybdenum alloys have excel­lent SCC resistance in sea water and chloride solutions

due to their high nickel content. Table 13 shows SCC initiation times in a boiling magnesium chloride solu — tion,31,34,37 whereas 304 and 316 stainless steels formed cracks within just 1-2 h, and Alloy 825 formed cracks in 46 h. However, SCC was not detected in Alloys 625, C-276, and C-22, even after testing for 1000 h.

SCC was not detected in Alloy G, which also shows excellent resistance to SCC in a boiling magnesium chloride solution. The SCC resistance of Alloy 825 is superior to that of 304 or 316 stainless steels, but inferior to that of Alloy G as shown in Table 13 .

The addition of molybdenum improves the resis­tance of nickel-based alloys to pitting and crevice corrosion. Nickel-chromium-molybdenum alloys have excellent pitting resistance. In addition, they have higher pitting and crevice corrosion initiation temperatures, as shown in Table 14. They also show much better resistance compared to 316 stainless steel in strong oxidizing environments including

— 33,34,37,38

24 300 ppm Cl.

Crevice corrosion resistance is usually evaluated by measuring the crevice repassivation potential. Figure 22 shows the temperature dependence of the crevice repassivation potential for nickel — chromium-molybdenum alloys in a 20% NaCl solution.59 Alloy C-276 shows a higher crevice repassivation potential and a higher crevice corrosion resistance than Alloy 625.

By contrast, the pitting and crevice corrosion resistance of Alloy G are inferior to those of Alloy C-276 and superior to those of 316 stainless steel and Alloy 825, as shown in Table 14.