22.08.2015   Comprehensive nuclear materials

In Caustic Solutions

Nickel shows excellent corrosion resistance in caustic solutions, such as sodium hydroxide and potassium hydroxide. The corrosion rates of nickel and nickel — based alloys in sodium hydroxide are shown in Table 15.7’8’10’31

Table 14 Pitting and crevice corrosion initiation tem­perature of nickel-based alloys

Alloy or steel

Pitting initiation temperature (°C)

Crevice corrosion initiation temperature (° C)

Alloy C-22

>150

102

Alloy C-276

150

80

Alloy 625

90

50

Alloy G-3

75

40

Alloy 825

25

S-5

316L stainless

20

S-5

steel

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Figure 22 Temperature dependence of crevice repassivation potential (ERcrev) in 20% NaCl solution.

Ni shows excellent corrosion resistance to liquid sodium hydroxide at any temperature and at any concentration with water; in particular, it has an unmeasurable corrosion rate at sodium hydroxide con­centrations lower than 50%. Nevertheless, Ni corrodes in sodium hydroxide in the presence of a sulfide such as sodium sulfide. Nickel-chromium-iron alloys have excellent corrosion resistance to sodium hydrox­ide and also high resistance to ammonium hydroxide and sulfide corrosion at high temperatures. The nickel-molybdenum alloys and Alloy C, which is a nickel-chromium-molybdenum alloy, are inferior to nickel-chromium-iron alloys with respect to corrosion in ammonium hydroxide and sulfide.