About 80 percent of the surfaces of fracture of each broken 

 tensile specimen was black in color and the other 20 percent was 

 gray, in contrast to entirely gray surfaces of fracture for unex- 

 posed specimens. Metallographic examinations of surfaces normal 

 to and at the edge of fracture showed that selective corrosion of 

 an intermetallic constituent had occurred which caused the reduction 

 in the mechanical properties. 



The median curves for the two groups of cast irons and the 

 alloy steels are shown in Figure 18 for sea water and in Figure 19 

 for bottom sediments. These curves (Figure 18) show that in sea 

 water at a depth of 5,500 feet corrosion behavior of these three 

 groups of alloys was the same after 750 days of exposure. There 

 was a slight decrease in the corrosion rates of the three groups 

 of alloys with time at a depth of 2,350 feet and the corrosion rate 

 of each group was lower than that of its companion group at a depth 

 of 5,500 feet. In the bottom sediments the behavior of the alloys 

 was somewhat erratic. The lower corrosion rates after 400 days at a 

 depth of 6,780 feet is attributed to the fact that a greater pro- 

 portion of each specimen was embedded in the bottom sediment than 

 during the other three exposure periods at the nominal depth of 5,500 

 feet. The corrosion rates at 2,350 feet tended to increase slightly 

 with time for the steels and austenitic cast irons while those for 

 the cast irons increased sharply. The type of behavior for the cast 

 and wrought alloys can only be attributed to their proximity to the 

 water-sediment interface or the percent embedment in the bottom sedi- 

 ment. 



SUMMARY AND CONCLUSIONS 



The purpose of this investigation was to determine the effects 

 of deep ocean environments on the corrosion of irons, steels and cast 

 irons. To accomplish this, specimens of 47 different alloys were 

 exposed at nominal depths of 2,350 and 5,500 feet for periods of time 

 varying from 123 to 1,064 days. 



The corrosion rates of all the alloys, both cast and wrought, 

 decreased asymptotically with time and became constant at rates vary- 

 ing between 0.5 and 1.0 MPY after three years of exposure at a nominal 

 depth of 5,500 feet in sea water. These corrosion rates are about 

 one-third those of wrought steels at the surface in the Atlantic 

 Ocean at Harbor Island, North Carolina. The corrosion rates of these 

 same alloys in sea water at a depth of 2,350 feet were lower than 

 those at the 5,500 foot depth and decreased with time. 



13 



