of a stainless steel structure can be jeopardized if 

 designed solely on the basis of corrosion rates calcu- 

 lated from weight losses rather than on the basis of 

 measured depths of pits, lengths of tunnel corrosion, 

 and depths of crevice corrosion. Pitting, tunneling, 

 and crevice corrosion, can and do penetrate stainless 

 steel rapidly, thus rendering it useless in short periods 

 of time. 



Therefore, corrosion rates expressed as mpy 

 calculated from weight losses, maximum pit depths, 

 maximum lengths of tunnel corrosion, maximum 

 depths of crevice corrosion, and types of corrosion 

 are tabulated to provide an overall picture of the 

 corrosion of the stainless steels. 



5.1. AISI 200 SERIES STAINLESS STEELS 



The chemical compositions of the AISI 200 Series 

 stainless steels are given in Table 36, their corrosion 

 rates and types of corrosion in Table 37, their stress 

 corrosion behavior in Table 38, and the effect of 

 exposure on their mechanical properties in Table 39. 



The AISI 200 Series stainless steels are 300 Series 

 stainless steels modified by substituting manganese 

 for about one-half of the nickel. This modification 

 does not adversely affect the corrosion resistance of 

 iron-chromium-nickel alloys in many environments. 



5.1.1. Duration of Exposure 



The AISI 201 and 202 alloys were attacked by 

 crevice and pitting types of corrosion both at the 

 surface and at depths in the seawater. There was a 

 tendency for both alloys to be more severely cor- 

 roded after longer periods of exposure both at the 

 surface and at depth. The bottom sediments were 

 about as corrosive as the seawater above them. 



5.1.2. Effect of Depth 



5.1.3. Effect of Concentration of Oxygen 



The effect of changes in the concentration of 

 oxygen in seawater on the corrosion of both AISI 

 201 and 202 stainless steels was nonuniform. In 

 general, crevice and pitting corrosion were more rapid 

 and severe at the surface than at depth, but there was 

 no definite correlation between corrosion and oxygen 

 concentration. 



As is well known, oxygen can and does play a 

 dual role in the corrosion of stainless steels in electro- 

 lytes (for example, seawater). An oxidizing environ- 

 ment (presence of oxygen or other oxidizers) is 

 necessary for maintaining the passivity of stainless 

 steels. However, this same oxidizing environment is 

 necessary to initiate and maintain pitting in stainless 

 steels. Oxygen often acts as the depolarizer for 

 passive-active cells created by the breakdown of 

 passivity at a specific point or area. The chloride ion 

 (present in abundance in seawater) is singularly 

 efficient in accomplishing this breakdown. Therefore, 

 this dual role of oxygen can be used to explain the 

 inconsistent and erratic corrosion behavior of stain- 

 less steels in seawater. 



5.1.4. Stress Corrosion 



AISI 201 stainless steel was exposed at the depths 

 and for the times shown in Table 38 when stressed at 

 values equivalent to 30 and 75% of its yield strength 

 to determine its susceptibility to stress corrosion. 

 AISI 201 stainless steel was not susceptible to stress 

 corrosion under the above conditions of test. 



5.1.5. Mechanical Properties 



The effects of exposure on the mechanical 

 properties of AISI 201 and 202 stainless steels are 

 given in Table 39. The mechanical properties were 

 not adversely affected. 



The corrosion of AISI 201 was approximately the 

 same at the surface as at depth, while that of AISI 

 202 was less severe at depth than at the surface. How- 

 ever, it was concluded that depth had no definite 

 influence on the corrosion of the AISI 200 Series 

 stainless steels. 



5.2. AISI 300 SERIES STAINLESS STEELS 



The chemical compositions of the AISI 300 Series 

 stainless steels are given in Table 40, their corrosion 

 rates and types of corrosion in Table 41, their stress 



130 



