Ammonia at normal atmospheric temperature and pressure is a gas. It can be stored under pressure as a liquid or below it’s atmospheric boiling point of -34 degC. It has been assumed that there is no corrosion risk to stainless steel that are normally considered for the storage and handling of bulk ammonia (ie stainless steel 304 or stainless steel 316 types) although there does not appear to be any published data to substantiate this. The general corrosion resistance of the 304 types should be adequate for installations at most sites, but for coastal or marine sites stainless steel 316 should be considered if the outer-casing or parts are exposed.
Corrosion resistance and cryogenic toughness of stainless steel.
The main issue is the cryogenic toughness of stainless steels at around -40 degC, compared to carbon steel, which can also be considered for ammonia service under these conditions. The austenitic stainless steel are impact tough at ‘cryogenic’ temperatures.
Selection of stainless steel for cryogenic application
Even when tested at the temperature of boiling liquid nitrogen (-196 degC) unless they have been severely cold worked (ie contain martensite) or have high levels of ferrite, the austenitic stainless steels are ‘tough’ and are accepted as suitable cryogenic materials.
Risk of corrosion to insulated storage tanks
If external tank insulation is proposed, then painting or a coating of aluminium foil on the outside face of a stainless steel 304 type can be considered. This is particularly important if there is a risk of the insulation becoming wet and absorbing chlorides. In a marine environments, where normally a type stainless steel 316 would be the more normal choice of grade, this can reduce the material selection costs.
Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)
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