iron), and the corrosion was then concentrated at the 

 interface between the two alloys (cladding alloy and 

 core alloy). The thickness of the remaining Alclad 

 layer indicated that it had not been sacrificed to 

 protect the core alloy as was its intended function. 

 On the other hand, the selective corrosion of the 

 Alclad layer on the bottom hemisphere and the 

 uncorroded core material showed that, in this case, 

 the cladding alloy was being sacrificed to protect the 

 core material as intended. 



When an attempt was made to repair these buoys 

 for reuse by grinding off all traces of corrosion prior 

 to painting, it was found that the corrosion had pene- 

 trated along the interface between the cladding alloy 

 and the core alloy for considerable distances from the 

 edges of the blisters and the edges of the holes where 

 the cladding alloy layer had been sacrificed. Polished 

 transverse sections taken from the buoy through 

 these corroded areas corroborated the indications 

 found from grinding operations. Metallurgical 

 examinations showed that the corroded paths were, 

 in fact, entirely in the cladding alloy, with a thin 

 diffusion layer of material between the corrosion 

 path and the core material. 



Blistering of Alclad aluminum alloys such as 

 encountered with these Alclad 7178-T6 spheres was 

 very unusual. Blistering due to corrosion and the 

 rapid rate of sacrifice of Alclad layers had not been 

 encountered previously by the author and other 

 investigators in surface seawater applications. Because 

 of this unique blistering one of the spheres was sent 

 to the Research Laboratories of the Aluminum Com- 

 pany of America where an investigation was made to 

 determine the mechanism of this behavior. 



Wei [15] showed that there was preferential 

 diffusion of zinc over copper from the core alloy into 

 this interfacial zone. The high zinc and low copper 

 contents of this interfacial zone rendered it anodic to 

 both the cladding and core alloys. Selective attack 

 was inevitable once corrosion reached this anodic 

 diffusion zone. 



That this type of blistering has been encountered 

 on buoys at depths from 300 to 6,800 feet 

 emphasizes the fact that there is some factor present 

 which either is more influential at depth or is not 

 present at the surface. The fact that this thin anodic 

 zone is probably present in all Alclad 7178-T6 pro- 

 ducts and, as such, is not blistered during surface 



seawater exposures indicates that the seawater 

 environments at depths of 300 feet and greater differ 

 from the seawater environments at the surface, at 

 least with respect to the corrosion behavior of this 

 alloy. 



6.6.1. Stress Corrosion 



The 7000 Series aluminum alloys were exposed at 

 the depths and for the times given in Table 79 when 

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

 their respective yield strengths to determine their sus- 

 ceptibilities to stress corrosion. Alloys 7075-T6, 

 7079-T6, Alclad 7079-T6, and 7178-T6, failed by 

 stress corrosion cracking. 



6.6.2. Corrosion Products 



Corrosion products from alloy 7079-T6 were 



analyzed by X-ray diffraction, spectographic analysis, 

 quantitative chemical analysis, and infra-red spectro- 

 photometry. The qualitative results were: amor- 

 phorous A1 2 3 "XH 2 0, NaCl, Al metal, Al, Cu, Mg, 

 Mn, Zn, Na, Ca, traces of Ti and Ni, 2.82% chloride 

 ion, 16.7% sulfate ion, and considerable phosphate 

 ion. 



6.6.3. Mechanical Properties 



The effects of exposure on the mechanical pro- 

 perties of the 7000 Series aluminum alloys are given 

 in Table 80. The mechanical properties of alloys 

 7002-T6, 7039-T6, 7075-T6, 7075-T64, 7075-T73, 

 7079-T6, and 7178-T6 were adversely affected. 



191 



