The 2000 Series aluminum alloys contain copper 

 as the chief alloying element. Copper is one of the 

 most important alloying metals for aluminum because 

 of its appreciable solubility and its strengthening 

 effect. 



The 2000 Series alloys corroded by pitting, 

 crevice, intergranular, and exfoliation types of cor- 

 rosion. 



6.2.1. Duration of Exposure 



There was no definite or consistent correlation 

 between corrosion rates and types of corrosion of the 

 2000 Series alloys and duration of exposure. 



6.2.2. Effect of Depth 



In general, corrosion rates were greater, and 

 pitting, crevice, and intergranular corrosion were 

 more severe at depth than at the surface after 1 year 

 of exposure. Thus, seawater at depth is more aggres- 

 sive to the 2000 Series aluminum alloys than is sea- 

 water at the surface. 



6.2.3. Effect of Concentration of Oxygen 



The effect of changes in the concentration of 

 oxygen in seawater on the corrosion behavior of the 

 2000 Series alloys was inconsistent and erratic except 

 for alloy 2219-T81. The corrosion rates, maximum 

 depths of pits, and maximum depths of crevice cor- 

 rosion decreased with increasing oxygen concentra- 

 tion, but not linearly, after 1 year of exposure. This 

 behavior of alloy 2219-T81 shows that the concen- 

 tration of oxygen in seawater exerts considerable 

 influence on the corrosion of this alloy. 



6.2.4. Stress Corrosion 



The 2000 Series aluminum alloys were exposed at 

 the depths and for the times shown in Table 63 when 

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

 their respective yield strengths to determine their sus- 

 ceptibilities to stress corrosion. They were not 

 susceptible to stress corrosion under the test condi- 

 tions. 



6.2.5. Other Types of Corrosion 



Alloys 2014-T3, 2014-T6, 2024-T3, 2024-T81, 

 2219-T81, and 2219-T87 were attacked by inter- 

 granular corrosion. Alloys 2014-T3, 2024-T3, 

 2024-T6, 2024-T81, and 2219-T81 were attacked by 

 the exfoliation type of corrosion. 



6.2.6. Welding 



Welding did not affect the corrosion behavior of 

 aluminum alloys 2024-T3 and 2219-T81. 



6.2.7. Mechanical Properties 



The effects of exposure on the mechanical pro- 

 perties of the 2000 Series aluminum alloys are given 

 in Table 64. The mechanical properties of the 2000 

 Series alloys were impaired except for those of alloy 

 Alclad 2024-T3. 



6.3. 3000 SERIES ALUMINUM ALLOYS 

 (ALUMINUM-MANGANESE ALLOYS) 



The chemical compositions of the 3000 Series 

 aluminum alloys are given in Table 65, their corrosion 

 rates and types of corrosion in Table 66, their stress 

 corrosion behavior in Table 67, and the effect of 

 exposure on their mechanical properties in Table 68. 



The chief alloying element of the 3000 Series 

 aluminum alloys is manganese. Manganese is added to 

 aluminum in amounts above 1% to increase its 

 strength. 



The 3000 Series alloys corroded chiefly by the 

 crevice and pitting types of localized corrosion. There 

 was also some blistering of the Alclad 3003 alloy. 



6.3.1. Duration of Exposure 



The corrosion rates of alloys 3003 and Alclad 

 3003 neither increased nor decreased uniformly with 

 increasing duration of exposure, except for Alclad 

 3003 at the 2,500-foot depth. At this depth the 

 corrosion rates decreased with increasing duration of 

 exposure. In general, the severity of pitting and 

 crevice corrosion was greater after the longer times of 

 exposure. 



187 



