Comprehensive and Detailed Explanation From Exact Extract:
According to API RP 571, ammonium bisulfide (NH₄HS) corrosion typically occurs in high-velocity, wet sour service, such as hydroprocessing reactor effluent systems. The damage is characterized by localized wall thinning, often with highly irregular corrosion profiles that vary with flow regime.
API RP 571 emphasizes that the most effective inspection techniques for NH₄HS corrosion are those capable of:
Detecting localized thinning
Identifying corrosion patterns related to flow velocity
Covering large areas efficiently
Ultrasonic scanning (C-scan) and profile radiography are specifically recommended because they:
Reveal localized and preferential metal loss
Allow comparison between high- and low-velocity zones
Are far superior to spot UT, which may miss localized attack
Referenced Documents (Study Basis):
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