corrosion. Thus, the integrity of an aluminum alloy 

 structure will be jeopardized if designed solely on the 

 basis of corrosion rates calculated from weight losses 

 rather than on the basis of measured depths of pits 

 and depths of crevice corrosion. Pitting and crevice 

 corrosion can, and do, penetrate aluminum alloys 

 rapidly in seawater, thus rendering them useless in 

 short periods of time. 



Therefore, corrosion rates expressed as mils pene- 

 tration per year calculated from weight losses, 

 maximum pit depths, maximum depths of crevice 

 corrosion and other type of corrosion are tabulated 

 to provide an overall picture of the corrosion of the 

 aluminum alloys. 



6.1. 1000 SERIES ALUMINUM ALLOYS 

 (99.00% MINIMUM ALUMINUM) 



The chemical compositions of the 1000 Series 

 aluminum alloys are given in Table 57, their corrosion 

 rates and type of corrosion in Table 58, their stress 

 corrosion behavior in Table 59, and the effect of 

 exposure on their mechanical properties in Table 60. 



The 1000 Series aluminum alloys contain a mini- 

 mum of 99% aluminum and are considered unalloyed 

 aluminums. 



The 1000 Series aluminum alloys corroded by the 

 localized types of corrosion, pitting, and crevice. 



6.1.2. Effect of Depth 



The corrosion rates of 1100 alloy increased with 

 increasing depth after 1 year of exposure. However, 

 there were no correlations between maximum depths 

 of pitting and crevice corrosion and corrosion rates. 

 In general, pitting and crevice corrosion were more 

 severe at depth than at the surface. 



There was no definite effect of depth on the 

 corrosion of 1000 Series aluminum alloys. 



6.1.3. Effect of Concentration of Oxygen 



Changes in the concentration of oxygen in sea- 

 water had no definite or consistent effect on the cor- 

 rosion of 1100 aluminum alloy. In general, after 1 

 year of exposure the corrosion rates and severity of 

 crevice corrosion were greater at the lower oxygen 

 concentrations, while the severity of pitting corrosion 

 was greatest at the highest oxygen concentration. 



6.1.4. Stress Corrosion 



Alloys 1100 and 1180 were exposed at the 

 2,500-foot depth for 402 days when stressed at values 

 equivalent to 50 and 75% of their respective yield 

 strengths (Table 59) to determine their susceptibili- 

 ties to stress corrosion. They were not susceptible to 

 stress corrosion under the conditions of the test. 



6.1.1. Duration of Exposure 



6.1.5. Mechanical Properties 



The corrosion rates of 1100 alloy decreased with 

 increasing duration of exposure at the surface, in sea- 

 water at the 2,500-foot depth, and in the bottom 

 sediments at the 6,000-foot depth, while the reverse 

 occurred in the bottom sediments at the 2,500-foot 

 depth and in the seawater at the 6,000-foot depth. 

 There was no correlation between the severity of 

 crevice corrosion and duration of exposure. The same 

 was true for the severity of pitting corrosion, except 

 at the surface where the maximum depth of pitting 

 corrosion increased with increasing duration of expo- 

 sure over a period of 1 year. 



The corrosion of 1180 alloy was comparable to 

 that of the 1 100 alloy. 



The effects of exposure on the mechanical pro- 

 perties of 1 100 and 1 180 alloys are given in Table 60. 

 Their mechanical properties were not affected by 

 exposure in seawater at the 2,500-foot depth for 402 

 days. 



6.2. 2000 SERIES ALUMINUM ALLOYS 

 (ALUMINUM-COPPER ALLOYS) 



The chemical compositions of the 2000 Series 

 aluminum alloys are given in Table 61, their corrosion 

 rates and type of corrosion in Table 62, their stress 

 corrosion behavior in Table 63, and the effect of 

 exposure on their mechanical properties in Table 64. 



186 



