Even using corrosion rates based on pitting, tunneling or crevice 

 corrosion for design purposes is not satisfactory. To illustrate this, 

 examine the extent of these three types of corrosion reported for Type 

 302 stainless steel in Table 4. During 1,064 days of exposure in sea- 

 water at a depth of 6,000 feet the alloy was completely free of any 

 type of corrosion, and if only these data were considered equipment 

 could be designed upon a 3-year life. However, to design on these data 

 would be unsound practice because this same alloy was perforated (53 

 mils) by both pitting and crevice corrosion and tunnel corrosion ex- 

 tended for a distance of 1-3/8 inches laterally through the sheet with- 

 in only 197 days of exposure at a depth of 2,500 feet; and, it was per- 

 forated by pitting corrosion with tunnel corrosion extending for 0.5 

 inch laterally through the sheet during 181 days of exposure at the 

 surface. The information from this example shows that from an engineer- 

 ing standpoint no reliability could be expected from applications of 

 stainless steels in seawater. 



Generally the rates of corrosion based upon the pitting, tunnel- 

 ing and crevice types of corrosion were greater in surface seawater 

 than in the seawater and in the bottom sediments at depths of 2,500 

 and 6,000 feet for equivalent periods of exposure. 



Cast stainless steel, Ni-Cr-Mo-Si, was the only alloy not attacked 

 by any type of corrosion during exposure for 366 days at the surface, 

 402 days at a depth of 2,500 feet, and 1,064 days of exposure at a 

 depth of 6,000 feet. 



Series 200 Stainless Steels (Cr-Ni-Mn) 



Type 201 was more resistant to corrosion in seawater than was 

 Type 202. There was severe edge corrosion of Type 201 within 366 days 

 of exposure at the surface whereas Type 202 was perforated (50 mils) 

 by both pitting and crevice corrosion within 182 days of exposure at 

 the surface. Also Type 201 was not perforated by pitting corrosion 

 until between 751 and 1,064 days of exposure at the 6,000-foot depth. 



Series 300 Stainless Steels 



Basic Type, 18% Cr-8% Ni , Nos . 301, 302, 304 and 304L . During 

 surface exposure in the seawater Types 301, 302 and 304 were perfor- 

 ated by pitting attack within 6 months and Type 304L was penetrated by 

 pitting to 84 percent of its thickness. Tunneling corrosion was most 

 severe in Type 302, being twice as long as in Type 301 and 10 times as 

 long in Types 304 and 304L. At depth all four alloys were perforated 

 by pitting corrosion within 400 days of exposure while crevice corro- 

 sion was not as severe. Tunnel corrosion at depth varied from none to 

 5.0 inches within the first year of exposure. 



Basic Type Plus Stabilizing Elements, Types 321 and 347 . The ad- 

 dition of Ti (Type 321) and Cb&Ta (347) to stabilize carbide formation 



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