2.1.12. Corrosion Products 



The corrosion products from some of the steels 

 were analyzed by X-ray diffraction, spectrographic 

 analysis, quantitative chemical analysis, and infrared 

 spectrophotometry. The constituents found were: 



Alpha iron oxide — Fe 2 3 ■ H 2 



Iron hydroxide — F3(OH) 2 



Beta iron (III) oxide hydroxide — FeOOH 



Iron oxide hydrate — Fe 2 3 ■ H 2 



Significant amounts of chloride, 

 sulfate, and phosphate ions. 



2.2. ANCHOR CHAINS 



Two types of 3/4-inch-diameter anchor chains, 

 Dilok and welded stud link, were exposed as shown in 

 Table 9. The chain links were covered with layers of 

 loose, flaky rust which varied from thin to thick as 

 the time of exposure increased. Exposure for as long 

 as 751 days did not decrease the breaking loads of the 

 chains as shown in Table 9. In most cases there was 

 rust in the bottoms of the sockets of the Dilok chain, 

 indicating that seawater had penetrated the sockets. 

 This could be a source of additional corrosion and 

 early failure of this type of chain. 



2.3. CAST IRONS 



The corrosion rates of the cast irons are given in 

 Table 5. Analysis of this data shows that for all prac- 

 tical purposes the corrosion rates of the alloy cast 

 irons (nickel, nickel-chromium No. 1 and 2, and 

 ductile irons No. 1 and 2) are comparable. This is also 

 true of the austenitic cast irons. These average data 

 values were used to plot curves to show the general 

 corrosion behavior to be expected from these alloys 

 with regard to duration of exposure, depth in the 

 ocean, and concentration of oxygen in seawater. 



2.3.1. Duration of Exposure 



The effects of duration of exposure on the cor- 

 rosion of cast irons in seawater at the surface and at 

 depth are shown in Figure 3. 



There was no measurable corrosion of the high 

 silicon and the high silicon-molybdenum cast irons in 

 seawater, either at the surface or at depth. 



In all three environments (surface, 2,500-, and 

 6,000-foot depths), the corrosion rates decreased 

 with increasing duration of exposure and were con- 

 sistently lower at depth than at the surface. The 

 corrosion rates at the 2,500-foot depth were lower 

 than those at the 6,000-foot depth. At the surface 

 and at the 6,000-foot depth the corrosion rates 

 decreased asymptotically with increasing duration of 

 exposure. At the 6,000-foot depth the corrosion rates 

 of the austenitic cast irons, for the first 400 days of 

 exposure, were lower than those of the gray and alloy 

 cast irons, but they were comparable after longer 

 periods of exposure, about 1 mpy. However, at the 

 2,500-foot depth, the corrosion rates of the austenitic 

 cast irons were lower than those of the alloy and gray 

 cast irons for exposures of up to 400 days. 



The corrosion of the cast irons when partially 

 embedded in the bottom sediments is shown in 

 Figure 4. Here again, there was no measurable cor- 

 rosion of the high silicon and high silicon- 

 molybdenum cast irons in the bottom sediment at 

 either depth. 



The other cast irons behaved essentially the same 

 as in the seawater except that the alloy cast irons 

 initially corroded at slower rates than in the seawater 

 at the 6,000-foot depth. After 2 years of exposure at 

 the 6,000-foot depth in both the seawater and the 

 bottom sediments, all the steels and cast irons cor- 

 roded at essentially the same rate. 



In the sediments at the 2,500-foot depth the 

 corrosion rates of the austenitic cast irons tended to 

 increase very slightly with increasing duration of 

 exposure, while those of the alloy cast irons increased 

 considerably. 



2.3.2. Depth 



The effect of depth of exposure in seawater on 

 the average corrosion rates of the alloy and austenitic 

 cast irons as well as those of the gray and high silicon 

 cast irons is shown in Figure 5. The variation of the 

 concentration of oxygen in seawater with depth is 

 also shown in Figure 5 for comparison purposes. The 

 shapes of the curves for the cast irons show that the 

 corrosion of the cast irons is not directly affected by 

 depth (pressure), at least to a depth of 6,000 feet for 

 a period of 1 year. 



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