Other Corrosion and Materials Problems

Corrosion problems have also occurred in compressors, sour water valve trims, expansion bellows, and hydrocracker fractionators. Type 4140 (UNS G41400) compressor impellers have sulfide cracked in sour gas when the hardness exceeded 235 Brine)i(e . Hardened 12Cr valve trim has sulfide cracked to prevent this problem, 18Cr-8Ni SS trim in sour water service is commonly used. Sulfide cracking has not occurred significantly in pumps. 

In addition to the potential for SCC from the inside as a result of the process fluids, austenitic SS s can experience SCC from the outside, if the austenitic SS is neither low carbon nor stabilized, intergranular SCC can occur as low as at room temperature as a result of either chlorides or sulfur dioxide in the atmosphere. In the 140 to 220°F (60 to 104°C) range, transgranular SCC of stabilized, low carbon, and regular grades of austenitic SS s has occurred as a result of chlorides from seacoast atmospheres. 

Above 220°F (104 C), moisture needed for SCC does not form on the surface In the absence of hygroscopic salts. SCC from the atmosphere is most common in warm climates. It also can occur under insulation, particularly where a steam leak supplies moisture to leach out chlorides from the insulation or to dissolve salts from the atmosphere. 

The fractionation equipment in a hydrocracker is generally carbon steel however, not all the sulfur has been removed. If the temperature will exceed 500° F (260° C), the sulfur content should be checked to determine whether SS is required. Severe corrosion of carbon steel has occurred in hydrocarbon fractionator furnace tubes and transfer lines as a result of the introduction of residual hydrogen sulfide in recycle gas from an amine unit. 

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