corrosion were much greater for 11 of the 16 alloys exposed for 6 

 months at the surface than at either 2,500 or 6,000 feet. This can be 

 attributed to the higher oxygen concentration and the attachment of sea 

 life to the specimens at the surface. 



In general the degree of corrosion appeared to decrease as the 

 chromium and nickel contents of the 300 Series stainless steels in- 

 creased. For example, the incidence and depth of pitting, tunneling 

 and crevice corrosion were less on the Types 309 , 310 and 311 steels 

 than on the Types 304 and 316 steels. 



Sensitization of AISI 304 and 316 stainless steels decreased their 

 corrosion resistance. 



AISI Type 400 Series Stainless Steels 



The AISI Type 400 stainless steels are those which contain a mini- 

 mum of 12 percent chromium. Steels containing up to 18 percent chro- 

 mium are hardenable by heat treatment and are classed as martensitic, 

 i.e., Type 410. Those with chromium above 18 percent and the normal 

 amounts of carbon are non-hardenable by heat treatment and are classed 

 ferritic; ferritic stainless steels in this investigation were Types 

 405, 430 and 446. 



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

 maximum depths of crevice corrosion and types of corrosion are given in 

 Table 5 and are shown, except for tunnel corrosion, graphically in 

 Figures 24 through 27. 



The corrosion of the martensitic stainless steel AISI Type 410 was 

 chiefly of the pitting, tunneling and crevice types, Figures 24 and 

 Table 5. The 50-mil thick specimens were perforated and tunnels nearly 

 3 inches long had developed within 181 days at the surface and 123 days 

 at a depth of 6,000 feet. 



The corrosion of the ferritic stainless steel AISI Type 405 was 

 chiefly of the tunneling type for the shorter periods of exposure (6 

 months) but was perforated by pitting corrosion after 751 days of expo- 

 sure at a depth of 6,000 feet, Figure 25. After 1,064 days of exposure 

 at the 6,000-foot depth a tunnel 12 inches long had developed which 

 nearly severed the panel longitudinally. On the basis of maximum pit 

 depths and the maximum length of tunnels, Type 405 was less susceptible 

 than Type 410; the 50-mil thick specimens of Type 410 were perforated 

 after 123 and 197 days at depth whereas the maximum pit depth in sea- 

 water of Type 405 was 40 mils after 402 days of exposure at depth. 



Ferritic stainless steel AISI Type 430 was perforated by pitting 

 and crevice corrosion after 6 months of exposure at the surface and at 

 the 2,500-foot depth but was less severe at the 6,000-foot depth, 

 Figure 26. The alloy, also, was perforated by tunnels 2.375 and 1 inch 

 long within 6 months of exposure at the 2,500- and 6,000-foot depths, 

 respectively. 



The high chromium (30 percent) ferritic stainless steel, AISI Type 

 446, was much less corroded at either depth (2,500 and 6,000 feet) than 



