5.3. AISI 400 SERIES STAINLESS STEELS 



The chamical compositions of the AISI 400 Series 

 stainless steels are given in Table 44, their corrosion 

 rates and type of corrosion in Table 45, their stress 

 corrosion behavior in Table 46, and the effect of 

 exposure on their mechanical properties in Table 47. 



The AISI 400 Series stainless steels are those 

 which nominally contain 11 to 27% chromium. The 

 400 Series stainless steels are further divided into 

 ferritic and martensitic steels. The ferritic steels are 

 nonhardenable by heat treatment; those in this 

 category in this program were AISI 405, 430, and 

 446. The martensitic steels are hardenable by heat 

 treatment, and the one in this category in this pro- 

 gram was AISI 410. 



The corrosion of the AISI 400 Series stainless 

 steels was erratic and was characterized by the 

 localized types of corrosion (crevice, pitting, and 

 tunnel). The intensities of these types varied from 

 none to complete perforation of the thickness of the 

 specimens for the crevice and pitting types and tunnel 

 corrosion extending laterally the entire 12-inch 

 (12,000 mils) length of specimens. There was no 

 correlation between the intensities of these types of 

 localized corrosion and the corresponding corrosion 

 rates calculated from weight losses. The frequencies 

 and intensities of these types of corrosion were also 

 greater for the AISI 400 Series stainless steels than 

 for the AISI 300 Series stainless steels. 



5.3.1. Duration of Exposure 



The data in Table 45 show no correlation 

 between either intensities of the localized types of 

 corrosion or corrosion rates and duration of expo- 

 sure. Neither one decreased or increased continuously 

 with increasing duration of exposure. 



5.3.2. Effect of Depth 



Depth had no uniform or gradual effect on the 

 corrosion rates of the AISI 400 Series stainless steels, 

 although the rates were lower at depth than at the 

 surface. However, these corrosion rates did not 

 decrease with increasing depth, i.e., they were lower 

 at the 2,500-foot depth than at the 6,000-foot depth 

 for two of the four steels. The intensities of the 



localized types of corrosion were either the same or 

 greater at the surface as at depth. 



Depth had no definite influence on the corrosion 

 of the AISI 400 Series stainless steels. 



5.3.3. Effect of Concentration of Oxygen 



In general, the corrosion rates of the AISI 400 

 Series stainless steels were higher at the highest con- 

 centration of oxygen (at the surface) than at the 

 lower concentrations. However, the increases were 

 not proportional to the increase in the oxygen con- 

 centration except for AISI 410 after 1 year of 

 exposure. The intensities of the localized types of 

 corrosion were not influenced by changes in the 

 concentration of oxygen in the seawater. 



In general, changes in the concentration of 

 oxygen in seawater did not exert a major influence on 

 the corrosion of the AISI 400 Series stainless steels. 



5.3.4. Stress Corrosion 



The AISI 400 Series stainless steels were stressed 

 at values ranging from 30 to 75% of their respective 

 yield strengths. They were exposed in seawater at the 

 2,500- and 6,000-foot depths for various periods of 

 time to determine their susceptibilities to stress cor- 

 rosion cracking. These data are given in Table 46. 



None of the AISI 400 Series stainless steels were 

 susceptible to stress corrosion under the conditions of 

 these tests. 



5.3.5. Mechanical Properties 



The effects of exposure on the mechanical pro- 

 perties of the AISI 400 Series stainless steels are given 

 in Table 47. 



In only two cases were the mechanical properties 

 seriously impaired: (1) After 403 days of exposure at 

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

 and the elongation of AISI 405 were seriously 

 reduced. (2) After 751 days of exposure at the 

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

 the elongation of AISI 430 were completely 

 destroyed. 



In all other exposures and for the other steels 

 there was no impairment of the mechanical pro- 

 perties. 



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