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Only steel and possibly zinc components of appurtenances will require coatings to protect them from atmospheric corrosion. The steel components should have satisfactory service life if they are sufficiently sheltered and protected from the atmosphere. Experience has shown that the integrity of the housing protecting the steel is critical in ensuring satisfactory service life of unprotected steel. Particular scrutiny should be given to the quality of the seals or gaskets of the housing as these will likely degrade with time and allow water vapor from the atmosphere to freely enter the protective housings. Change in ambient temperature will result in alternately wetting and drying of components within sealed structure that has compromised seals.

Coating protection recommended

No coating necessary if sufficient corrosion allowance is used

Coating recommended or dezincification possible

Should perform satisfactorily

Satisfactory in most soils

Protective patina likely to be formed

Coating recommended, corrosion rate as high as 20 mpy

No coating necessary away from marine environments

Expected to perform well in non-marine soils, otherwise susceptible to pitting attack

Satisfactory performance unless exposed to severe industrial environment

Must be sheltered or coated to escape severe corrosion

Must be sheltered or coated to escape severe corrosion

Coating is recommended though performance is good uncoated

Coating is recommended unless sufficient thickness allowance is made

Stainless Steel

Type 302

Steel

Piano Wire

ASTM A228

Zinc

ZA-7

Copper

C12200

Ductile Iron

N/A

Table 18. Summary of Expected Corrosion Performance for Appurtenances in Soil and Under Dome

Atmospheric

Buried

Base Metal

Aluminum

Brass

Alloy Name AA 360 AA 360 AA 6061-T6 C37700 C36000

It is clear from the evaluation undertaken in this task that some alloys used in the appurtenances are at higher risk than others for suffering corrosion damage in service, particularly if they are exposed to soils rather than atmospheric conditions. Only copper and stainless steel do not need to have protective coatings on them in either environment. All other components will need to have a robust coating if they are buried in soils. The use of magnesium anodes to protect appurtenances in soils is probably needed only in the most aggressive soils.

The use of sacrificial anodes for appurtenances that are above ground under the dome will not be effective because a continuous electrical path through an electrolyte, such as moist soil or a water film, needs to be contacting the anode and appurtenance for the anode to provide protection. The steel component, which is a spring, will need to be properly sealed within a housing to ensure satisfactory service life. All the metals of construction for the appurtenances, namely aluminum, brass, copper, ductile iron, stainless steel, and zinc may suffer various degrees of corrosion attack

Alternative Underground Propane Tank Materials, Phase II—Final Report

34

September 2009 Battelle and Lincoln Composites

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