Copper and Brass
Copper exhibits relatively high corrosion resistance to most soils found in the U.S. In cases where the soil is unusually corrosive, it may be necessary to use a protective coating, cathodic protection, or an acid neutralizing backfill such as limestone. For the copper listed in Table 12, it has been found that after three years of burial in a range of dry to wet soils, the uniform corrosion rates were found to vary between 0.05 and 0.35 mils (thousandths of an inch) per year (mpy) (ASM, 2005b). No pitting attack was observed. As was expected, the highest corrosion rates were for soils having the highest conductivity. The brass alloys listed in Table 12 are susceptible to dezincification when exposed to corrosive soils (soil resistivity less than 3,000 ohm•cm). Dezincification leads to porosity and loss of strength of the alloy. While these brass alloys are also susceptible to stress corrosion cracking (SCC), it is unlikely to occur in soils unless ammonia or its compounds such as ammonium nitrate are present.
Similar to other metals, the degree of corrosion damage to ductile iron exposed to soils is a function of soil porosity, drainage, and other contaminants. An extensive study done in the 1950s on metals buried in various soils indicated that the penetration depth suffered by gray cast iron ranged from less than 10 mpy to over 80 mpy (Romanoff, 1957). While malleable cast iron (ductile iron) was not studied as extensively as gray cast iron, a visual inspection of the two indicated that they did not differ significantly in terms of the form and extent of their pitting damage. A more reasonable average penetration depth for ductile iron is 10 to 20 mpy.
Type 302 will likely perform well in most soils located away from the ocean. A possibility for pitting or crevice corrosion attack exists only in soils where the chloride concentration exceeds 500 ppm. In marine locations where the chloride concentrations can reach 1,500 ppm, the likelihood of pitting attack in this alloy is significant. This alloy, however, is unlikely to be directly in contact with soils because its function as a spring will be located in the interior of a valve or regulator.
A steel spring directly exposed to soils will quickly corrode, particularly if any moisture is present. It is anticipated that these parts will be shielded from any direct contact with soils.
Alternative Underground Propane Tank Materials, Phase II—Final Report
September 2009 Battelle and Lincoln Composites