incipient crevice corrosion on its modified counterpart, alloy 20Cb-3. 

 The cast alloy equivalents (Ni-Cr-Cu-Mo numbers 1 and 2) of the wrought 

 20Cb alloys behaved similarly to the wrought alloy 20Cb . 



The wrought alloy (18Cr-14Mn-0.5N) was severely corroded by the 

 pitting, tunneling and crevice types of corrosion. Its behavior was 

 comparable with that of Type 301 stainless steel. 



The stainless steel wire ropes were also attacked by pitting and 

 crevice corrosion, the degree of severity varying from rust stains 

 and incipient pits to complete parting of some wires with losses in 

 breaking strength equivalent to 97 percent of the original. A metallic 

 coating composed of 90 percent copper-10 percent nickel, 0.3 mil thick, 

 on AISI Type 304 stainless steel wire was completely corroded away 

 within 402 days of exposure at a depth of 2,500 feet, whereas a 0.7 mil 

 thick coating was still green, indicating that some of it was still 

 present after the same period of exposure. However, this rate of 

 removal indicates that permanent protection cannot be expected. 



The 300 and 400 Series stainless steels were not susceptible to 

 stress corrosion cracking in seawater at depth in the Pacific Ocean. 

 However, some of the precipitation hardening stainless steels were 

 susceptible to stress corrosion cracking when stresses equivalent to 

 50 or 75 percent of the alloy's yield strength were imposed on the 

 alloy: AISI Type 631 in the TH1050 and RH1050 conditions, PH14-8Mo- 

 SRH950 and 15-7AMV in the annealed, RH1150 and RH950 conditions. 



Unrelieved internal residual stresses caused by welding circular 

 welds on specimens caused the following alloys to fail by stress cor- 

 rosion cracking: AISI Type 630-H925, AISI Type 631 in the TH1050 and 

 RH1050 conditions, and AISI Type 632-RH1100. Precipitation hardening 

 alloy 15-7AMV in the RH1150 and RH950 conditions failed by stress cor- 

 rosion cracking as a result of internal stresses imposed by drilling 

 holes in the specimens which subsequently were not reamed or deburred. 



CONCLUSIONS 



Based on the intensity of the pitting, tunneling and crevice types 

 of corrosion, seawater at the surface was more aggressive than was 

 either the seawater or the bottom sediments at depths of 2,500 and 

 6,000 feet in the Pacific Ocean. The bottom sediments were no more 

 aggressive than was the seawater immediately above them. 



Because of the propensity of stainless steels to pitting, tunnel- 

 ing and crevice corrosion and because of their unpredictable behavior 

 in seawater, stainless steels, in general, are not recommended for use 

 in seawater. Two cast alloys, Ni-Cr-Mo-Si which did not corrode and 

 RL-35-100 which corroded uniformly in this investigation, could be 

 used with confidence in seawater applications for periods of time as 

 long as 3 years. Four alloys, AISI 317 and 329, 20Cb-3 and cast Ni-Cr- 

 Mo (all molybdenum- containing alloys) showed traces of crevice and 



19 



