cast irons was very close to that of the oxygen curve and shows that 

 the corrosion of the alloy cast irons is not depth dependent. The 

 shapes of the curves for gray cast iron, the austenitic cast irons, and 

 the silicon and silicon-molybdenum cast irons show that depth is not an 

 important variable in their corrosion behavior. 



The effect of the concentration of oxygen in seawater on the cor- 

 rosion rates of cast irons is shown in Figure 24. The corrosion rates 

 of gray cast iron and the alloy cast irons decreased practically line- 

 arly with the concentration of oxygen in seawater. The corrosion rates 

 of the austenitic cast irons decreased with the concentration of oxygen 

 in seawater while the silicon and silicon-molybdenum cast irons were 

 uncorroded; hence were insensitive to the concentration of oxygen. 



All the cast irons corroded uniformly except the silicon and 

 silicon-molybdenum cast irons which were uncorroded. 



The effect of time of exposure on the corrosion of cast irons 

 during surface exposure in seawater is shown in Figure 25. Data were 

 available for only two austenitic cast irons and their corrosion rates 

 decreased asymptotically with increasing time of exposure. Their cor- 

 rosion rates became practically constant at between 2 and 3 mils per 

 year after about two years of exposure. 



STAINLESS STEELS 



The chemical compositions of the stainless steels are given in 

 Table 12 and their corrosion rates and types of corrosion in Tables 13 

 through 17. The effect of depth and the concentration of oxygen in 

 seawater on the corrosion rates of stainless steels are shown graphi- 

 cally in Figures 26 through 31 . 



In general, stainless steels corrode chiefly by pitting and crevice 

 corrosion in seawater. In these types of localized attack the majority 

 of the surface area is unattacked so that corrosion rates calculated 

 from weight losses are very misleading because they reflect a uniform 

 thinning of the material. However, in spite of this, the corrosion 

 rates of a number of the stainless steels were plotted versus depth 

 and the concentration of oxygen in seawater to see if any information 

 of value could be obtained. 



The corrosion rates of the 200 and 400 Series stainless steels as 

 affected by depth are shown in Figure 26. The corrosion rates of AISI 

 430 and 18Cr-14Mn-0.5N stainless steels decreased with increasing depth. 

 The corrosion rates of AISI 201, 202, 410 and 446 were lower at depth 

 than at the surface, but they did not decrease progressively with in- 

 creasing depth. 



The effects of changes in the oxygen concentration of seawater on 

 the corrosion rates of the 200 and 400 Series stainless steels are 

 shown in Figure 27. The corrosion rates of AISI 410 decreased linearly 

 with the oxygen content while those for AISI 201, 202 and 446 were not 



