SECTION 2 

 STEEL AND CAST IRONS 



The data discussed in this section were obtained 

 from the reports given in References 3 through 19. 

 The chemical compositions of the alloys are given in 

 Table 3; their surface conditions and heat treatments, 

 if any, are given in Table 4. 



The corrosion rates and types of corrosion of all 

 the alloys are given in Table 5. Inorganic coatings 

 were applied to some steels to evaluate their protec- 

 tive qualities. These coatings and their conditions are 

 given in Table 6. Steels that were exposed in a 

 stressed condition to determine their susceptibility to 

 stress corrosion cracking are given in Table 7. 



The effects of corrosion on the mechanical 

 properties of many of the alloys were determined 

 after various periods of exposure; these results are 

 given in Table 8. 



Water near the surface in the open sea is quite 

 uniform in its composition throughout the oceans 

 [20] ; therefore, the corrosion rates of steels exposed 

 under similar conditions in clean seawater should be 

 comparable. The results of many investigations on the 

 corrosion of structural steels in surface seawater at 

 many locations throughout the world show that after 

 a short period of exposure the corrosion rates are 

 constant and amount to between 3 and 5 mils per 

 year [21,22]. Factors which can cause differences in 

 corrosion rates outside these limits are variations in 

 marine fouling, contamination of the seawater near 

 the shorelines, variations in seawater velocity, and 

 differences in the surface water temperature. 



2.1. IRONS AND STEELS 



The corrosion rates of the irons; mild steels; high- 

 strength low-alloy steels; high-strength steels; other 

 alloy steels; and nickel alloy steels are given in Table 

 5. Analysis of the corrosion rates of these alloys 

 shows that for all practical purposes their corrosion 

 rates were comparable for any one duration of 

 exposure at any one depth or at the surface. There- 

 fore, these data were treated statistically to obtain 

 one median value for each time of exposure and each 



depth. 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. 



2.1.1. Duration of Exposure 



The effects of the duration of exposure on the 

 corrosion of steels in seawater at the surface and at 

 depth are shown in Figure 3. The corrosion rates of 

 the steels exposed in seawater at nominal depths of 

 2,500 and 6,000 feet in the Pacific Ocean decreased 

 with increasing duration of exposure and were con- 

 sistently lower than the surface corrosion rates by a 

 factor of approximately 3. The corrosion rates at the 

 2,500-foot depth also were lower than those at the 

 6,000-foot depth. The corrosion rates decreased 

 asymptotically with increasing duration of exposure 

 both at the surface and at the 6,000-foot depth. 



The performance of the steels when partially 

 embedded in the bottom sediments at the 2,500- and 

 6,000-foot depths is shown in Figure 4. Here, also, 

 the average corrosion rates of the steels at the 

 6,000-foot depth decreased asymptotically with 

 increasing duration of exposure. During the initial 

 exposures the steels corroded at faster rates in sea- 

 water than in the bottom sediments at the 6,000-foot 

 depth, but after approximately 2 years of exposure, 

 their average corrosion rates were approximately the 

 same as shown by comparing the curves in Figures 3 

 and 4. Here, also, the average corrosion rates at the 

 2,500-foot depth were lower than at the 6,000-foot 

 depth, but they increased with increasing duration of 

 exposure. 



2.1.2. Depth 



The effect of depth of exposure in seawater on 

 the average corrosion rates of the steels 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 shape of the curve 



