resulted in improved corrosion resistance over the Basic Type alloys, 

 especially on Type 321. The maximum depth of pitting corrosion on 

 Type 321 was 22 mils within 366 days of exposure at the surface and 

 perforation by crevice corrosion within 402 days of exposure at 2,500 

 feet. Type 347 was perforated (50 mils) by both pitting and crevice 

 corrosion within 182 days of exposure at the surface and was perfor- 

 ated by crevice corrosion within 1,064 days of exposure in the bottom 

 sediment at a depth of 6,000 feet. 



Basic Type Plus Molybdenum, Types 316, 316L and 317 . Molybdenum 

 is added to the Basic Type stainless steel to improve its resistance 

 to pitting corrosion. The incidence of pitting, but not the intensity, 

 was decreased during surface seawater exposure in Types 316 and 316L 

 stainless steels which contained 2.5 percent molybdenum. Both steels 

 were perforated by pitting corrosion within 181 days of exposure at 

 the surface. At depth, Type 316 was perforated by pitting corrosion 

 (230 mils) during 402 days of exposure at 2,500 feet while there was 

 no pitting of Type 316L at depth. Also, the incidence of tunnel cor- 

 rosion was less than in the Basic Type steels. 



The addition of 3.5 percent molybdenum, Type 317, resulted in 

 further improvement in resistance to pitting corrosion. Type 317 

 was free from pitting and tunnel corrosion at the surface and at both 

 depths, and there was only incipient crevice corrosion in all three 

 environments . 



Steels with Increased Chromium and Nickel, Types 309, 310, 311, 

 23-25% Cr-13-20% Ni . Increasing the chromium and nickel contents re- 

 sulted in some increase in corrosion resistance, particularly resis- 

 tance to pitting corrosion. Types 309 and 310 were free of pitting 

 corrosion in all three environments while Type 311 was attacked by 

 pitting corrosion in surface seawater. There was less improvement in 

 resistance to crevice corrosion, all three of them being attacked, 

 with Types 310 and 311 being perforated in surface seawater exposure. 



Miscellaneous Type 300 Series, Types 325, 329 and 330 



Type 325 stainless steel which contained 9% Cr-23% Ni corroded 

 chiefly by pitting and general surface corrosion, the general corro- 

 sion being contrary to the behavior of all other 300 Series stainless 

 steels. The reason for this behavior is the low chromium content, 9% 

 in contrast to the minimum of 12% required to impart a completely 

 passive surface for stainless steels. 



Type 329 stainless steel contains 27% Cr-4% Ni-1.4% Mo. The 27% 

 chromium is more than twice as much as is necessary to form a com- 

 pletely passive film in an oxidizing environment and this steel can 

 be classed as an austenitic stainless steel. There was incipient crevice cor- 

 rosion in only three exposures, indicating better corrosion resistance 

 than the basic 18-8 type stainless steels. 



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