Ammonium Chloride Corrosion | Materials And Corrosion Control

·         General or localized corrosion, often pitting, normally occurring under ammonium chloride deposits, often in the absence of a free water phase.

·         Salts have a whitish, greenish or brownish appearance.
Water washing and/or steamout will remove deposits so that evidence of fouling may not be evident during an internal visual inspection.

·         Corrosion underneath the salts is typically very localized and results in pitting, with high corrosion rates (>100 mpy)

·         NH₄Cl precipitation & damage is worse at higher temperatures that NH₄HS, e.g., fouling of heat exchangers at 200°C ⁽¹⁾

·         Concentration (NH₃, HCl, H₂O or amine salts), temperature and water availability are the critical factors.

·         When salt deposits above the water dewpoint, a water wash injection may be required to dissolve the salts.

·         RT or UT thickness monitoring can be used to determine remaining wall thickness

·         Monitoring of the feed streams and effluent waters will give an indication of the amount of ammonia and chlorides present, however process simulation may be required to determine the concentration and dewpoint temperatures.

·         The presence of deposits is often detected when the pressure drop increases or the thermal performance of exchangers has deteriorated.

·         Corrosion probes or coupons can be useful, but the salt must deposit on the corrosion probe element to detect the corrosion.

·         Perform piping inspections at 3-year intervals

·         Regular NDT at vulnerable locations, e.g., heat exchangers

·         ⁽¹⁾ “Acid Salt Corrosion in a Hydrotreatment Plant of a Petroleum Refinery”, Engineering Failure Analysis 15 (2008) 1035–1041