The corrosion behavior of the 3000 Series alloys 

 was erratic and unpredictable with regard to duration 

 of exposure. 



6.3.2. Effect of Depth 



After 1 year of exposure the corrosion rates and 

 maximum depths of pits increased with increasing 

 depth, but not linearly. Alclad 3003 did not behave 

 in this manner. In other words, the corrosion 

 behavior of alloy 3003 appears to be depth (pressure) 

 dependent in that it increased in severity with 

 increasing depth. 



6.3.3. Effect of Concentration of Oxygen 



The corrosion rates, maximum pit depths, and 

 maximum depths of crevice corrosion on alloys 3003 

 and Alclad 3003 due to changes in the concentration 

 of oxygen in seawater were erratic. 



6.3.4. Stress Corrosion 



Alloy 3003-H14 was not susceptible to stress cor- 

 rosion when stressed at values equivalent to 50 and 

 75% of its yield strength and exposed at the 

 2,500-foot depth for 402 days as given in Table 67. 



6.3.5. Corrosion Products 



Corrosion products from alloy 3003-H14 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, Si, Mg, 

 Fe, Cu, Ca, Mn, 3.58% chloride ion, 18.77% sulfate 

 ion, and considerable phosphate ion. 



6.3.6. Mechanical Properties 



The effects of exposure on the mechanical pro- 

 perties of alloys 3003-H14, Alclad 3003-H12, and 

 Alclad 3003-H14 are given in Table 68. In general, 

 the mechanical properties of alloys 3003-H14 and 

 Alclad 3003-H12 were adversely affected by exposure 

 at depth. 



6.4. 5000 SERIES ALUMINUM ALLOYS 

 (ALUMINUM-MAGNESIUM ALLOYS) 



The chemical compositions of the 5000 Series 

 aluminum alloys are given in Table 69, their corrosion 

 rates and types of corrosion in Table 70, their stress 

 corrosion behavior in Table 71, and the effect of 

 exposure on their mechanical properties in Table 72. 



Aluminum is alloyed with magnesium to form an 

 important class of nonheat-treatable alloys (5000 

 Series). Their utility and importance are based on 

 their resistance to corrosion, high strength without 

 heat treatment, and good weldability. 



The 5 000 Series aluminum alloys corroded 

 chiefly by the crevice and pitting types of localized 

 corrosion. Other types of corrosion found were: 

 blistering, crater, edge, intergranular, line, and 

 exfoliation. 



6.4.1. Duration of Exposure 



The general effect of duration of exposure on the 

 corrosion of the 5000 Series alloys was erratic and 

 nonuniform. The corrosion rates and the maximum 

 depths of pitting or crevice corrosion neither 

 increased nor decreased consistently with increasing 

 duration of exposure; in many cases, the behavior was 

 erratic. 



6.4.2. Effect of Depth 



After 1 year of exposure the average corrosion 

 rates of all the 5000 Series alloys increased with 

 depth, but not linearly. Also, the maximum depths of 

 pits of all the alloys increased linearly with depth. 

 The maximum depth of crevice corrosion of all the 

 alloys increased with depth, but not consistently. The 

 corrosion behavior of the 5000 Series aluminum 

 alloys appears to be more uniformly affected by 

 depth than by duration of exposure or changes in the 

 concentration of oxygen in seawater. 



6.4.3. Effect of Concentration of Oxygen 



The cor.'osion rates of alloy 5086-H34 increased 

 linearly with increasing concentration of oxygen in 



188 



