Hydrogen attack can occur in carbon steel and low-alloy Cr – Mo steels at elevated temperatures in process environments that contain a relatively high partial pressure of hydrogen (see Section 500). Hydrogen attack can be very serious, both reducing the strength of the material and simultaneously causing microfissures to develop that eventually grow until failure occurs. Weld heat affected zones can be especially susceptible, and the attack can be more severe at locations of relatively high stress, such as nozzles.
Materials selected for construction of a vessel should not be susceptible to hydrogen attack at the operating temperature and pressure (see Section 500). Safe limits for carbon and low-alloy steels are usually satisfactorily predicted by the Nelson Curves in API 941, with the exception of C-½ Mo low-alloy steel, which is not recommended for preventing hydrogen attack. (API 941 is included in Volume I of the Corrosion Prevention and Metallurgy Manual). Nevertheless, in-service inspection programs should be established to assure that hydrogen attack does not occur in vessels that operate at high temperatures and high pressures, because of the potentially serious consequences of the failure of high pressure equipment.
Highly specialized ultrasonic examination (UT) techniques have been developed that can detect hydrogen attack before the microfissures grow to a size that can cause failure, but the applicability to weld heat affected zones is presently limited. Removing “boat” samples for metallurgical study can reveal hydrogen attack at a very early stage, but doing so will necessitate repair of the vessel and should only be performed after consultation with pressure vessel and materials engineers. Before failure occurs, ultrasonic examination using shear wave procedures can be used to detect cracks that develop from the growth of microfissures, including cracks that may appear in weld heat affected zones. However, microfissures can progress through the entire wall thickness of a heat affected zone before cracks can be detected by conventional ultrasonic shear wave techniques. Focused beam ultrasonic shear wave techniques are being developed that may enable the detection of microfissures in heat affected zones.
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