rates usually were less than 0.1 mils penetration per year; in one 

 case, after 751 days of exposure at a depth of 5,640 feet in sea 

 water the rate was 0.5 mils penetration per year. Because of the 

 localized nature of the corrosion, low corrosion rates are mis- 

 leading if considered by themselves, 



Nickel-Molybdenum Alloy 2 



Alloy 2 was attacked by general corrosion; therefore, corro- 

 sion rates calculated from weight losses are meaningful and signifi- 

 cant. The corrosion rates of alloy 2 are shown in Figure 12. At 

 both depths, 5,500 and 2,350 feet, the corrosion rates increased 

 with increasing duration of exposure. No explanation is offered at 

 this time for the exceptionally high corrosion rate in sea water 

 after 751 days of exposure at a depth of 5,640 feet. The corrosion 

 rates were less in the bottom sediments than in the sea water at 

 both depths. 



Nickel-Silicon Alloy D 



Alloy D was attacked by the crevice and pitting types of cor- 

 rosion. The most severe attack was after 1064 days of exposure in 

 the sea water at a depth of 5,300 feet; crevice corrosion extended 

 to a depth of 42 mils and pitting corrosion to a depth of 38 mils. 



STRESS CORROSION 



Nickel-iron-chromium alloy 825 and nickel-molybdenum-chromium 

 alloy C were exposed in the stressed condition at stresses equiva- 

 lent to 35, 50 and 75 percent of their yield strengths, as shown 

 in Table 10. Alloy 825 was not susceptible to stress corrosion 

 cracking after 402 days of exposure at a depth of 2,370 feet. Alloy 

 C was not susceptible to stress corrosion cracking for periods of 

 exposure as long as 751 days at a depth of 5,640 feet. 



MECHANICAL PROPERTIES 



The changes in the mechanical properties of the nickel alloys 

 are given in Table 11. The mechanical properties of nickel- iron- 

 chromium alloy 825 and nickel-molybdenum- chromium alloy C were not 

 impaired by exposure to sea water or in the bottom sediments for 

 periods of time of 751 and 1064 days at the nominal depth of 5,500 



10 



