The maximum depth of corrosion attack on aluminum alloy 1100 exposed to industrial and seacoast atmosphere is shown in Figure 8 (ASM, 2005a). While the data were not readily available for the cast and wrought aluminum alloys that are used under the dome, the trend for corrosion would be the same and the actual penetration depths would likely be greater for alloy 6061 because it is less corrosion resistant than alloy 1100. Most of the corrosion penetration is seen to occur on alloy 1100 after only five years of service and will be at least 5 to 6 mils deep.
Figure 8. Depth of Atmospheric Corrosion Attack on Aluminum Alloys 1100 (ASM, 2005a)
Copper and Brass
Copper and brass appurtenances are expected to have relatively low atmospheric corrosion rates of less than 0.12 mpy based on published data, though specific data for two of the brass alloys, namely C37700 and C36000 were not available (ASM, 2005b). The expected trend for these alloys exposed to atmospheric corrosion is shown in Figure 9. The average atmospheric metal loss for brass alloys has been found to be proportional to (time)1/3 and for copper, to (time)2/3. In any case, the total thickness loss for copper alloys exposed to aggressive atmospheric conditions will likely be less than 0.79 mil (20 μm) over a period of 20 years.
Alternative Underground Propane Tank Materials, Phase II—Final Report
September 2009 Battelle and Lincoln Composites