Selection of stainless steels for handling ammonia (NH3)
Ammonia at normal atmospheric temperatures and pressures is a gas.
It can be stored under pressure as a liquid or below it’s atmospheric boiling point of -34 oC.
Corrosion resistance and cryogenic toughness of stainless steels
The main issue is the cryogenic toughness of stainless steels at around -40 oC, compared to carbon steels, which can also be considered for ammonia service under these conditions.
The austenitic stainless steels are impact tough at ‘cryogenic’ temperatures.
stainless steels for cryogenic applications
Even when tested at the temperature of boiling liquid nitrogen, (-196 oC), unless they have been severely cold worked, (i.e. contain martensite), or have high levels of ferrite, the austenitic stainless steels are ‘tough’ and are accepted as suitable cryogenic materials.
It has been assumed that there is no corrosion risk to stainless steels that are normally considered for the storage and handling of bulk ammonia, (i.e. 304 or 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 316 should be considered if the outer-casings or parts are exposed.
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 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 316 type would be the more usual choice of grade, this can reduce the material selection costs .
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