ALUMINUM ALLOYS 



The chemical compositions of the aluminum alloys are given in 

 Table 2 and their corrosion rates and types of corrosion in Table 3. 

 The variations of the corrosion rates and maximum pit depths of the 

 alloys with depth and with oxygen content of seawater are shown graphi- 

 cally in Figures 2 through 9 . 



Aluminum alloys corrode chiefly by the pitting and crevice types 

 in seawater, both of which are localized types, which means that the 

 greater portion of the surface area of a specimen is unattacked. There- 

 fore, corrosion rates calculated from weight losses and expressed as 

 mils per year, which indicates uniform thinning of the material, are 

 very misleading because they create an erroneous impression of the be- 

 havior of the material. In order to present a more realistic picture 

 of the behavior of aluminum alloys, the maximum and average pit depths 

 and the maximum depth of crevice corrosion are also reported. 



In Figure 2 the corrosion rates of the aluminum alloys versus 

 depth are shown. The variation of the oxygen content of seawater with 

 depth is also shown in Figure 2. The corrosion rates of aluminum alloys 

 1100-H14, 5083-H113 and 3003-H14 increase progressively with depth. 

 Those of the 6061-T6 and 2219-T81 alloys are greater at depth than at 

 the surface but their increases are not progressive since their rates 

 at the 2,500-foot depth are greater than those at the 6,000-foot depth. 

 The corrosion rate of 2024-0 at the 6,000-foot depth was greater than 

 at the surface, but at the 2,500-foot depth it was less than at the 

 surface. The corrosion rate of 5086-H34 decreased slightly with depth. 

 It is shown in Figure 2 that based on corrosion rates the corrosion of 

 5083-H113, 1100-H14 and 3003-H14 aluminum alloys are depth dependent. 



The corrosion rates of aluminum alloys 2219-T81 and 6061-T6 in- 

 creased with the decreasing concentration of oxygen in seawater while 

 those of 5086-H34 decreased slightly as shown in Figure 3. 



The corrosion rates of aluminum alloys 1100-H14, 3003-H14, 2024-0 

 and 5083-H113 are independent of the concentration of oxygen in sea- 

 water as shown in Figure 4. The corrosion rates of three of these 

 alloys, 1100-H14, 3003-H14 and 5083-H113, were shown to be depth (pres- 

 sure) dependent, Figure 2. 



The maximum depths of pits of aluminum alloys 3003-H14, 2024-0 and 

 5083-H113 increased with depth (pressure), i.e., they were pressure 

 dependent as shown in Figure 5. The maximum depths of pits of alloy 

 5086-H34 decreased with increase in depth. Although those of alloys 

 2219-T81 and 6061-T6 were deeper at a depth of 6,000 feet than at the 

 surface, the depths of pits were at the maximums at the 2,500-foot 

 depth, Figure 5. 



The maximum depths of pits of aluminum alloys 2024-0, 2219-T81 

 and 6061-T6 increased as the concentration of oxygen in seawater de- 

 creased, while those of 5086-H34 decreased with the concentration of 

 oxygen, Figure 6. 



