corrosion behavior in Table 42, and the effect of 

 exposure on their mechanical properties in Table 43. 



The corrosion of the AISI 300 Series stainless 

 steels was very erratic and unpredictable. They were 

 attacked by crevice, pitting, and tunnel types of cor- 

 rosion, in varying degrees of severity ranging from 

 incipient to perforation of the thickness of the 

 specimens and tunnels extending laterally for a 

 distance of 11 inches (11,000 mils) through the 

 specimen. Comparing the intensities of these types of 

 localized corrosion with the corresponding corrosion 

 rates indicates no definite correlation between them. 



Two alloys, AISI 317 and 329, were attacked 

 only by incipient crevice corrosion during exposures 

 at all three depths (surface, 2,500, and 6,000 feet) in 

 seawater for periods ranging from 366 days at the 

 surface to 1,064 days at the 6,000-foot depth. 



Sensitization (heating for 1 hour at 1,200°F and 

 cooling in air) rendered AISI 304 and 316 stainless 

 steels more susceptible to corrosion than their 

 unsensitized counterparts. 



5.2.1. Duration of Exposure 



Examination of the data in Table 41 shows that 

 there is no definite or consistent correlation between 

 severity of corrosion or corrosion rates and duration 

 of exposure. For example, at the 6,000-foot depth in 

 seawater the intensities of pitting and tunnel cor- 

 rosion were greater after 403 days than after 1,064 

 days of exposure, the intensity of crevice corrosion 

 was greater after 1,064 days than after 403 days of 

 exposure, and the maximum corrosion rate was 

 greater after 1,064 days than after 403 days of 

 exposure. 



5.2.2. Effect of Depth 



The data in Table 41 show, in general, that the 

 intensities of crevice, pitting, and tunnel corrosion 

 were either about the same or greater at the surface 

 than at the depth. The corrosion rates are in agree- 

 ment with this conclusion in that those of most of 

 the alloys were greater at the surface than at depth. 

 Based on these data and the above statements, it is 

 concluded that depth in the ocean exerts no signifi- 

 cant influence on the corrosion of AISI 300 Series 

 stainless steels. 



5.2.3. Effect of Concentration of Oxygen 



There was no definite correlation between the 

 intensities of pitting, tunnel, and crevice corrosion of 

 the AISI 300 Series stainless steels and changes in the 

 concentration of oxygen in seawater after 1 year of 

 exposure. On the basis of corrosion rates for those 

 alloys which had definite weight losses, the rates 

 increased with increasing concentration of oxygen, 

 but not uniformly. 



These data indicate that the corrosion of the AISI 

 300 Series stainless steels is not proportional to 

 changes in the concentration of oxygen in seawater. 

 The dual role oxygen plays in the corrosion of stain- 

 less steels in seawater, as discussed previously, also 

 applies here as an explanation for the erratic behavior 

 of AISI 300 Series stainless steels. 



5.2.4. Stress Corrosion 



Some of the AISI 300 Series stainless steels were 

 stressed at values ranging from 30 to 80% of their 

 respective yield strengths. They were exposed in the 

 seawater at depths of 2,500 and 6,000 feet for 

 various periods of time to determine their suscepti- 

 bility to stress corrosion cracking. These data are 

 given in Table 42. 



None of the steels were susceptible to stress cor- 

 rosion under the conditions of these tests. 



5.2.5. Mechanical Properties 



The effects of exposure on the mechanical 

 properties of some of the 300 Series stainless steels 

 are given in Table 43. 



In only two cases were the mechanical properties 

 adversely affected: (1) After 1,064 days of exposure 

 at the 6,000-foot depth, the tensile and yield 

 strengths and the elongation of AISI 304L were 

 reduced by about 30%. This is attributed to the per- 

 foration of the specimen by both crevice and pitting 

 corrosion, and edge and tunnel corrosion. (2) After 

 402 days of exposure at the 2,500-foot depth, the 

 tensile and yield strengths of welded, sensitized AISI 

 316 were reduced by 45%. These reductions are 

 attributed to the effects of welding. 



131 



