4.1.3. Effect of Concentration of Oxygen 



The severity and frequency of pitting and crevice 

 corrosion, in general, increased with increasing con- 

 centration of oxygen in seawater. The average cor- 

 rosion rates increased progressively, but not 

 constantly, with increasing concentration of oxygen 

 in seawater as shown in Figure 17. 



4.1.4. Effect of Welding 



The weld beads were preferentially corroded 

 when nickel Ni-200 was welded by manual shielded 

 metal-arc welding using welding electrode 141, and 

 by TIG welding using filler metal 61. The weld beads 

 were severely pitted when welded with electrode 141. 

 The weld beads and heat-affected zones were per- 

 forated when welded with filler metal 61. This 

 preferential attack of the weld bead materials 

 indicates that they were anodic to the parent sheet 

 metal. 



4.1.5. Mechanical Properties 



The effect of exposure on the mechanical pro- 

 perties of nickel Ni-200 is shown in Table 28. The 

 mechanical properties were not affected by exposure 

 at depth for 1,064 days or for 181 days at the sur- 

 face. 



4.2. NICKEL-COPPER ALLOYS 



The chemical compositions of the nickel-copper 

 alloys are given in Table 29, their corrosion rates and 

 types of corrosion in Table 30, and the changes in 

 their mechanical properties due to corrosion in Table 

 31. 



4.2.1. Duration of Exposure 



The corrosion rates and types of corrosion of 

 seven nickel-copper alloys are given in Table 30. 

 Except for the cast alloys 410 and 505, the pre- 

 dominant types of corrosion were pitting and crevice. 

 At the 6,000-foot depth there was an overall 

 tendency for the corrosion rates of the cast alloys to 

 decrease with increasing duration of exposure, but 



this tendency was neither progressive nor constant. 

 Because the corrosion of the other nickel-copper 

 alloys was localized (pitting and crevice corrosion), 

 no definite correlation with duration of exposure was 

 possible either at depth or at the surface. However, 

 the intensity of pitting and crevice corrosion, in 

 general, increased with increasing duration of 

 exposure both at depth and at the surface. 



4.2.2. 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 or constantly with increasing 

 depth, as shown in Figure 16. Although these 

 corrosion rates are unreliable because they are based 

 upon localized corrosion weight losses, they do sub- 

 stantiate the conclusion based upon the frequency 

 and severity of pitting and crevice corrosion. 



4.2.3. Effect of Concentration of Oxygen 



The severity and frequency of pitting and crevice 

 corrosion, in general, increased with increasing con- 

 centration of oxygen in seawater. Even though pitting 

 and crevice corrosion were the predominant types for 

 these alloys in seawater, their average corrosion rates 

 calculated from weight losses increased linearly with 

 increasing concentration of oxygen, as shown in 

 Figure 17. 



4.2.4. Effect of Welding 



When Ni-Cu 400 alloy was welded with filler 

 metal 60 by the TIG welding process, the weld beads 

 were severely pitted both in the seawater and in the 

 bottom sediment after exposure for 402 days at a 

 depth of 2,500 feet, but they were corroded uni- 

 formly after 181 days of exposure at the surface. 

 Butt welds in Ni-Cu 400 alloy made by the manual 

 shielded metal-arc process with electrode 190 were 

 attacked by incipient pitting corrosion both in the 

 seawater and in the bottom sediment after 189 days 

 of exposure at a depth of 5,900 feet and by crater 

 corrosion of the weld bead after 540 days of expo- 

 sure at the surface. Three-inch-diameter, unrelieved, 



90 



