seawater, but the slope of the line was very small (1 

 to 25). However, such relationships were not found 

 for the maximum depths of pitting and crevice cor- 

 rosion. The pit depths were a maximum at the highest 

 oxygen concentration, and the maximum depth of 

 crevice corrosion was at the intermediate oxygen con- 

 centration. 



The corrosion rates of alloy 5456-H321 decreased 

 linearly with increasing concentration of oxygen in 

 seawater, but the slope of the line was very small (1 

 to 10). However, no correlations were possible 

 between maximum depth of pitting and crevice cor- 

 rosion. 



The corrosion rates and changes in the maximum 

 depths of pits and crevice corrosion of the other 5000 

 Series aluminum alloys were erratic and inconsistent 

 with respect to changes in the concentration of 

 oxygen in seawater. Changes in the concentration of 

 oxygen in seawater did not exert a constant or uni- 

 form influence on the corrosion behavior of the 5000 

 Series aluminum alloys. This behavior, like that of the 

 stainless steels and some nickel alloys, can be 

 attributed to the dual role oxygen can play with 

 regard to alloys which depend upon passive films for 

 their corrosion resistance. 



6.4.7. Corrosion Products 



Corrosion products from alloy 5086 were 

 analyzed by X-ray diffraction, spectrographic 

 analysis, quantitative chemical analysis, and infra-red 

 spectrophotometry. The qualitative results were: 

 amorphous A1 2 3 -XH 2 0, NaCl, Si0 2 , Al, Na, Mg, 

 Cu, Fe, Si, Ti, 5.8% chloride ion, 26.2% sulfate ion, 

 and considerable phosphate ion. 



6.4.8. Mechanical Properties 



The effects of exposure on the mechanical pro- 

 perties of the 5000 Series aluminum alloys are given 

 in Table 72. The mechanical properties of the follow- 

 ing alloys were adversely affected by exposure: 

 5456-H321 after 123 days of exposure at the 

 6,000-foot depth; 5052-H32, 5083-H113, and 

 5456-H34 after 403 days of exposure at the 

 6,000-foot depth; and 5456-H321 and 5456-H34 

 after 751 days of exposure at the 6,000-foot depth. 

 The mechanical properties of the above alloys after 

 exposures for different times at different depths and 

 of the other alloys were not adversely affected by 

 exposure at depth in the seawater. 



6.4.4. Stress Corrosion 



Some 5000 Series aluminum alloys were exposed 

 at the depths and for the times given in Table 71 

 when stressed at values equivalent to 30, 50, or 75% 

 of their respective yield strengths to determine their 

 susceptibilities to stress corrosion. They were not sus- 

 ceptible to stress corrosion under the test conditions. 



6.4.5. Other Types of Corrosion 



Alloys 5052-H32 and 5456-H34 were attacked by 

 the exfoliation type of corrosion. Alloys 5083-H113, 

 5086-H32, and 5086-H34 were attacked by inter- 

 granular corrosion. 



6.4.6. Welding 



Welding did not affect the corrosion behavior of 

 alloys 5083-H113, 5086-H34, and 5454-H32. 



6.5. 6000 SERIES ALUMINUM ALLOYS 

 (ALUMINUM-MAGNESIUM-SILICON ALLOYS) 



The chemical compositions of the 6000 Series 

 aluminum alloys are given in Table 73, their corrosion 

 rates and types of corrosion in Table 74, their stress 

 corrosion behavior in Table 75, and the effects of 

 exposure on their mechanical properties in Table 76. 



The aluminum-magnesium-silicon system is the 

 basis for a major class of heat-treatable aluminum- 

 base alloys. They combine many desirable characteris- 

 tics, including moderately high strength and good 

 resistance to corrosion. 



There was only one 6000 Series alloy (6061) in 

 this program. Alloy 6061 corroded chiefly by the 

 crevice and pitting types of localized corrosion. Also, 

 there was some intergranular corrosion. 



6.5.1. Duration of Exposure 



The corrosion rates of 6061 at the surface and at 

 the 6,000-foot depth decreased with duration of 



189 



