2.3.3. Concentration of Oxygen 



The effect of the variation in the concentration of 

 oxygen in seawater on the corrosion of cast irons 

 after 1 year of exposure is shown in Figure 6. The 

 curves for the average corrosion rates of the gray, 

 alloy, and austenitic cast irons versus the concentra- 

 tion of oxygen are essentially straight lines. This 

 indicates that the corrosion of the cast irons in sea- 

 water is proportional to the concentration of oxygen. 

 However, the different slopes of the curves indicate 

 different degrees of influence, the influence being 

 greatest on the alloy cast irons and least on the gray 

 cast irons. Oxygen exerted no influence on the cor- 

 rosion of high silicon or high silicon-molybdenum 

 cast irons. 



2.3.4. Type of Corrosion 



All the cast irons corroded uniformly both in the 

 seawater and in the bottom sediments. The high 

 silicon and high silicon-molybdenum cast irons were 

 uncorroded in any of the environments. 



2.3.5. Mechanical Properties 



The percent changes in the mechanical properties 

 of the cast irons due to exposure in seawater are given 

 in Table 8. The mechanical properties of the Type 4 

 austenitic cast iron were not affected by exposure 

 either at the surface or at the 2,500-foot depth. How- 

 ever, the mechanical properties of the D-2C austenitic 

 cast iron were significantly lowered. About 80% of 

 the surfaces of fracture of the D-2C specimens were 

 black in contrast to the gray surfaces of fracture of 

 unexposed specimens. Metallographic examinations 

 of polished cross sections of the D-2C alloy adjacent 

 to the surfaces of fracture showed that the alloy had 

 been attacked by selective interdendritic corrosion. 

 This selective corrosion was the cause of the decrease 

 in mechanical properties of the alloy. 



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