5.3.6. Corrosion Products 



The corrosion products taken from one of the 

 corrosion tunnels in AISI Type 430 stainless steel 

 were analyzed by X-ray diffraction, spectrographic 

 analysis, quantitative chemical analysis, and infra-red 

 spectrophotometry and were found to contain 

 amorphous ferric oxide (Fe 7 3 *XH 2 0), Fe, Cr, Mn, 

 Si, trace of Ni, 1.41% chloride ion, 2.12% sulfate ion, 

 and a significant amount of phosphate ion. 



5.4. PRECIPITATION-HARDENING 

 STAINLESS STEEL 



The chemical compositions of the precipitation- 

 hardening stainless steels are given in Table 48, their 

 corrosion rates and types of corrosion in Table 49, 

 their stress corrosion behavior in Tables 50 and 51, 

 and the effect of exposure on their mechanical 

 properties in Table 52. 



The precipitation-hardening stainless steels differ 

 from the conventional stainless steels (AISI Series 

 200, 300, and 400) in that they can be hardened to 

 very high strength levels by heating the annealed 

 steels to temperatures in the 900 to 1,200°F range 

 and then cooling in air. 



The corrosion of the precipitation-hardening 

 stainless steels was erratic and was of the crevice, 

 pitting, and tunnel types of localized corrosion with 

 some edge attack. 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 greater for the precipitation- 

 hardening stainless steels than for the AISI 300 Series 

 stainless steels. 



The 15-7AMV steels corroded at extremely rapid 

 rates by crevice, pitting, tunnel, and edge corrosion. 

 In many instances, large portions of the specimens 

 had been lost due to corrosion; in other cases tunnel 

 corrosion had extended laterally through the speci- 

 mens for distances of 1 1 or 12 inches within a year of 

 exposure. They were considerably more susceptible 

 to corrosion than were the other precipitation- 

 hardening stainless steels. 



Alloy 17Cr-14Ni-Cu-Mo was nearly free of cor- 

 rosion after exposure for 366 days at the surface, 402 

 days at 2,500 feet, and 1,064 days at 6,000 feet. 



There was one case of pitting to a depth of 3 mils 

 after 181 days of exposure at the surface. There was 

 incipient crevice corrosion at the surface and at depth 

 except for pitting to a depth of 3 mils after 1,064 

 days of exposure in the bottom sediment at the 

 6,000-foot depth. 



5.4.1. Duration of Exposure 



The data in Table 49 show that there was no 

 correlation between the intensities of the localized 

 types of corrosion and duration of exposure. Also, 

 there was no correlation between corrosion rates 

 calculated from weight losses and duration of 

 exposure except for the 15-7AMV steels for which 

 the corrosion rates increased with increasing time of 

 exposure. 



5.4.2. Effect of Depth 



In general, depth had no effect on the corrosion 

 of the precipitation-hardening stainless steels. 



5.4.3. Effect of Concentration of Oxygen 



Changes in the concentration of oxygen in sea- 

 water exerted no definite or major influence on the 

 corrosion behavior of the precipitation-hardening 

 stainless steels. 



5.4.4. Effect of Welding 



A 3-inch-diameter circular, unrelieved weld was 

 made in the center of the 6 x 12-inch specimens of 

 some of the alloys to impose residual stresses in the 

 specimens. Others had a 6-inch-long transverse, 

 unrelieved butt weld across the center of the 

 6 x 12-inch specimens. These welds were to deter- 

 mine whether welding affected the corrosion behavior 

 and stress corrosion susceptibilities of the alloys. 



Welding did not affect the corrosion behavior of 

 the precipitation-hardening stainless steels. 



The effects of residual stresses imposed by 

 welding will be discussed under 5.4.5. 



5.4.5. Stress Corrosion 



The precipitation-hardening stainless steels were 

 stressed at values ranging from 35 to 85% of their 



133 



