at the surface, Figure 27. The 50-mil thick specimens were perforated 

 by pitting and creivce corrosion in seawater within 6 months of surface 

 exposure as contrasted to incipient crevice corrosion at the 2,500-foot 

 depth. 



In general, the martensitic stainless steel, AISI Type 410, was 

 more susceptible to corrosion than the ferritic stainless steels. This 

 is attributed to the lower chromium content of the AISI Type 410 stain- 

 less steel. 



Precipitation Hardening Stainless Steels 



The precipitation hardening stainless steels differ from the con- 

 ventional stainless steels (AISI Series 200, 300 and 400) in that they 

 can be hardened to very high strength levels by heating the annealed 

 steels to low temperatures (900-1200°F) and cooling in air. 



The corrosion rates, maximum pit depths, maximum tunnel lengths, 

 maximum depths of crevice corrosion and types of corrosion are given 

 in Table 6 and are shown, except for tunneling, graphically in Figures 

 28 through 38. The predominant modes of corrosive attack were pitting, 

 tunneling and crevice. 



Surface seawater was more aggressive to AISI Type 630-H925 than 

 was seawater at depths of either 2,500 or 6,000 feet as shown in 

 Figure 28. At the surface the specimens were perforated by pitting, 

 tunneling and crevice corrosion with no pitting or crevice corrosion at 

 either depth after 6 and 13 months of exposure. After 13 months of 

 exposure both at the surface and at the 6,000-foot depth the weld beads 

 were perforated. Welding did not adversely affect the corrosion resis- 

 tance of the alloy. The bottom sediments were no more aggressive than 

 was the seawater above them. 



The welded specimens of AISI Type 631-TH1050 were perforated by 

 pitting, tunneling and crevice corrosion after 6 months of exposure 

 both at the surface and at the 2,500-foot depth, Figure 29. The bottom 

 sediments were equally as aggressive as the seawater above them. The 

 specimens with the unrelieved circular welds failed by stress corrosion 

 cracking with the cracks chiefly extending radially across the weld 

 beads after 13 months of exposure at the surface and in the bottom sedi- 

 ment at the 6,000-foot depth. 



The welded AISI Type 631-RH1050 specimens were perforated by pit- 

 ting, tunneling and crevice corrosion after 6 months of exposure in sea- 

 water at the surface, Figure 30. However, the specimens at the 2,500- 

 foot depth were less severely attacked than were those at the surface. 

 The specimens with the unrelieved circular weld bead failed by stress 

 corrosion cracking after 6 months and 1 year of exposure in the seawater 

 at the 2,500-foot depth and after 1 year of exposure at the 6,000-foot 

 depth. Portions of the specimens partially embedded in the bottom sedi- 

 ment at the 6,000-foot depth were corroded away after 1 year of exposure, 

 chiefly because of tunneling corrosion. 



10 



