%E2%80%9CThe most important variables that affect and differentiate the various forms of wet H2S damage are environmental conditions (pH, H2S level, contaminants, temperature), material properties (hardness, microstructure, strength) and tensile stress level (applied or residual). These factors are outlined below. b) All of these damage mechanisms are related to the absorption and permeation of hydrogen in steels. i) pH %E2%80%A2 Hydrogen permeation or diffusion rates have been found to be minimal at pH 7 and increase at both higher and lower pH%E2%80™s. The presence of hydrogen cyanide (HCN) in the water phase significantly increases permeation in alkaline (high pH) sour water. %E2%80%A2 Conditions which are known to promote blistering, HIC, SOHIC and SSC are those containing free water (in liquid phase) and: %E2%80%A2 >50 wppm dissolved H2S in the free water, or %E2%80%A2 free water with pH <4 and some dissolved H2S present, or %E2%80%A2 free water with pH >7.6 and 20 wppm dissolved hydrogen cyanide (HCN) in the water and some dissolved H2S present, or %E2%80%A2 >0.0003 MPa (0.05 psia) partial pressure of H2S in the gas phase. %E2%80%A2 Increasing levels of ammonia may push the pH higher into the range where cracking can occur. ii) H2S %E2%80%A2 Hydrogen permeation increases with increasing H2S partial pressure due to a concurrent increase in the H2S concentration in the water phase. %E2%80%A2 An arbitrary value of 50 wppm H2S in the water phase is often used as the defining concentration where wet H2S damage becomes a problem. However, there are cases where cracking has occurred at lower concentrations or during upset conditions where wet H2S was not ordinarily anticipated. The presence of as little as 1 wppm of H2S in the water has been found to be sufficient to cause hydrogen charging of the steel. %E2%80%A2 Susceptibility to SSC increases with increasing H2S partial pressures above about 0.05 psi (0.0003 mpa) H2S in steels with a tensile strength above about 90 ksi or in steels with localized zones of weld or weld HAZ hardness above 237 HB. iii) Temperature %E2%80%A2 Blistering, HIC, and SOHIC damage have been found to occur between ambient and 300oF (150oC) or higher. %E2%80%A2 SSC generally occurs below about 180oF (82oC). iv) Hardness %E2%80%A2 Hardness is primarily an issue with SSC. Typical low-strength carbon steels used in refinery applications should be controlled to produce weld hardness <200 HB in accordance with NACE RP0472. These steels are not generally susceptible to SSC unless localized zones of hardness above 237 HB are present. %E2%80%A2 Blistering, HIC and SOHIC damage are not related to steel hardness. v) Steelmaking %E2%80%A2 Blistering and HIC damage are strongly affected by the presence of inclusions and laminations which provide sites for diffusing hydrogen to accumulate. %E2%80%A2 Steel chemistry and manufacturing methods also affect susceptibility and can be tailored to produce the HIC resistant steels outlined in NACE Publication 8X194. %E2%80%A2 Improving steel cleanliness and processing to minimize blistering and HIC damage may still leave the steel susceptible to SOHIC. %E2%80%A2 The disadvantage is that an absence of visual blistering may leave a false sense of security that H2S damage is not active yet subsurface SOHIC damage may be present. %E2%80%A2 HIC is often found in so-called %E2%80%9Cdirty%E2%80%9D steels with high levels of inclusions or other internal discontinuities from the steel-making process. vi) PWHT %E2%80%A2 Blistering and HIC damage develop without applied or residual stress so that PWHT will not prevent them from occuring. %E2%80%A2 High local stresses or notch-like discontinuities such as shallow sulfide stress cracks can serve as initiation sites for SOHIC. PWHT is highly effective in preventing or eliminating SSC by reduction of both hardness and residual stress. %E2%80%A2 SOHIC is driven by localized stresses so that PWHT is also somewhat effective in reducing SOHIC damage.%E2%80%9D Damage Mechanisms Affecting Fixed Equipment in the Refining Industry RECOMMENDED PRACTICE 571 FIRST EDITION, DECEMBER 2003 Hope it helps Luis Marques