exposure, but not uniformly, while those at the 

 2,500-foot depth increased with duration of expo- 

 sure. However, the maximum depths of pitting and 

 crevice corrosion increased with increasing duration 

 of exposure at the surface and at depths of 2,500 and 

 6,000 feet. 



6.5.2. Effect of Depth 



Although the corrosion rates and the maximum 

 depths of pitting and crevice corrosion were greater at 

 depth than at the surface, these increases did not 

 increase uniformly with increasing depth. Depth 

 exerted no uniform influence on the corrosion 

 behavior of alloy 6061. 



6.5.3. Effect of Concentration of Oxygen 



The corrosion rates and maximum depths of 

 pitting and crevice corrosion decreased with 

 increasing concentration of oxygen in seawater. The 

 maximum depths of crevice corrosion decreased 

 linearly with increasing oxygen concentration. The 

 corrosion rates and maximum depths of pitting 

 decreased constantly, but not uniformly, with depth. 



6.5.4. Stress Corrosion 



Alloy 6061-T6 was exposed at the depths and for 

 the times given in Table 75 when stressed at values 

 equivalent to 30 and 75% of its yield strength to 

 determine its susceptibility to stress corrosion. Alloy 

 6061-T6 was not susceptible to stress corrosion under 

 the test conditions. 



6.5.5. Welding 



The corrosion of alloy 6061-T6 was adversely 

 affected by welding. Alloy 6061 was attacked by 

 intergranular corrosion in the "as-welded" condition. 



6.5.6. Mechanical Properties 



The effects of exposure on the mechanical pro- 

 perties of alloy 6061-T6 are given in Table 76. The 

 mechanical properties of 6061-T6 were adversely 

 affected by exposure in seawater. Those specimens 

 which had been welded and which had been attacked 

 by intergranular corrosion were the most seriously 

 affected. 



6.6. 7000 SERIES ALUMINUM ALLOYS 

 (ALUMINUM-ZINC-MAGNESIUM ALLOYS) 



The chemical compositions of the 7000 Series 

 aluminum alloys are given in Table 77, their corrosion 

 rates and types of corrosion in Table 78, their stress 

 corrosion behavior in Table 79, and the effect of 

 exposure on their mechanical properties in Table 80. 



Combinations of zinc and magnesium in 

 aluminum provide a class of heat-treatable alloys, 

 some of which develop the highest strengths presently 

 known for commercial aluminum-base alloys. The 

 addition of copper to the aluminum-zinc-magnesium 

 system, together with small but important amounts 

 of chromium and manganese, results in the highest 

 strength, heat-treatable, aluminum-base alloys 

 commercially available. 



The 7000 Series alloys were attacked by crevice, 

 edge, exfoliation, intergranular, and pitting types of 

 corrosion. Corrosion of the Alclad alloys was by 

 shallow pitting and crevice corrosion, slight blistering, 

 and general corrosion. 



Because of the erratic behavior of the 7000 Series 

 aluminum alloys during exposure in seawater at 

 depth, it was impossible to find any correlation 

 between their corrosion behavior and duration of 

 exposure, effect of depth, or the effect of changes in 

 the concentration of oxygen in seawater. 



A practical case of unusual corrosion on an 

 aluminum alloy was encountered with the Alclad 

 7178-T6 aluminum alloy buoys used in the installa- 

 tion of the STU structures. During the retrieval of 

 STU 1-3 after 123 days of exposure, the buoy, which 

 was 300 feet below the surface, was found to be 

 corroded. White corrosion products on the bottom 

 hemisphere covered areas where the cladding alloy 

 had corroded through to the core material. The top 

 hemisphere was blistered, the blisters being as large as 

 2 inches in diameter and 0.75 inch high with a hole in 

 the top of each blister. The hole in the top of the 

 blister indicates the origin of the failure: originally a 

 pinhole in the cladding alloy existed where seawater 

 gained access to the interface between the cladding 

 alloy and the core alloy. When this blister was 

 sectioned to inspect the corrosion underneath, it was 

 found to be filled with white crystalline aluminum 

 oxide corrosion products. It appeared that seawater 

 penetrated the cladding alloy at a defect, or a pit was 

 initiated at a particle of a cathodic metal (probably 



190 



