SECTION 4 

 NICKEL ALLOYS 



Nickel and its alloys are passive in moving sea- 

 water, but are subject to pitting and concentration 

 cell (crevice) corrosion in stagnant seawater. Their 

 passivity is due to the presence of an impervious 

 oxide layer on their surfaces which breaks down 

 under certain conditions. Fouling organisms, deposits, 

 and crevices which restrict the availability of oxygen 

 to localized areas cause such breakdowns. Where 

 sufficient oxygen is not available to repair the breaks 

 in the protective film, pitting and crevice (concentra- 

 tion cell) corrosion occur. Thus, in seawater, pitting 

 and crevice corrosion are the most prevalent modes of 

 attack. 



Corrosion rates calculated from weight losses due 

 to localized corrosion are meaningless because they 

 present an untrue picture of the corrosion behavior of 

 the alloy. Corrosion rates such as mils-per-year 

 connote a uniform thickness of metal lost over a 

 period of time, assuming uniform corrosion. Hence, a 

 very low corrosion rate resulting from a few deep pits 

 or crevice corrosion in one area will present a very 

 misleading picture of the corrosion behavior of an 

 alloy in that particular environment. 



The data on the nickel alloys were obtained from 

 the reports given in References 3 through 19 and 23. 

 They are separated into different groups (nickels, 

 nickel-copper-alloys, and nickel alloys) for 

 comparison and discussion purposes. 



The chemical compositions, corrosion rates and 

 types of corrosion, and changes in mechanical pro- 

 perties due to corrosion of the nickels are given in 

 Tables 26 to 28; those of the nickel-copper alloys in 

 Tables 29 to 31; those of the nickel alloys in Tables 

 32 to 34; and the resistance to stress corrosion in 

 Table 35. 



The effects of depth and the effect of the concen- 

 tration of oxygen in seawater on the corrosion of the 

 nickels, the nickel-copper alloys, and the nickel alloys 

 are shown in Figures 15 and 16. 



4.1. NICKELS 



corrosion in Table 27, and the changes in their 

 mechanical properties due to corrosion in Table 28. 



4.1.1. Duration of Exposure 



The corrosion rates and types of corrosion of the 

 seven nickels (94% minimum nickel) are given in 

 Table 27. Pitting, crevice, and edge (on the sheared 

 ends) localized types of corrosion were responsible 

 for practically all the corrosion. The edge corrosion 

 was caused by microcracks and microcrevices that 

 formed during the shearing operation; this illustrates 

 dramatically the corrosion damage that can be caused 

 by this fabricating procedure. Lateral penetration, 

 initiated at a sheared edge, of as much as an inch 

 during 6 months of exposure was found. To prevent 

 this type of corrosion, all deformed metal created 

 during shearing or punching operations must be 

 removed by machining, grinding, or reaming. 



Because the corrosion of the nickels was local- 

 ized, no definite correlation with duration of expo- 

 sure was possible. However, the severity of pitting 

 and crevice corrosion increased with increasing time 

 of exposure at depth as well as at the surface. Cor- 

 rosion rates increased with duration of exposure at 

 the 6,000-foot depth, although they were neither 

 progressive nor constant. In some cases corrosion 

 rates were considerably higher during shorter times of 

 exposure than after longer times of exposure. Cor- 

 rosion rates at the 2,500-foot depth were constant 

 with increasing time of exposure. 



4.1.2. Effect of Depth 



The severity and frequency of pitting and crevice 

 corrosion were much greater at the surface than at 

 depth. Also, the average corrosion rates were greater 

 at the surface than at depth, although they did not 

 decrease progressively with increasing depth as shown 

 in Figure 16. The curves in Figure 16 are based on 

 average values for each group of alloys. 



The chemical compositions of the nickels are 

 given in Table 26, their corrosion rates and types of 



89 



