Ammonium Chloride Corrosion and Damage in Hydrotreating Units
Damage of ammonium chloride (NH4Cl) corrosion
Ammonium salt, ammonium chloride corrosion is characterized by general or localized corrosion, often pitting corrosion, normally occurring under ammonium chloride or amine salt deposits, and also often in the absence of a free water phase.
Corrosion due to deposition of ammonium chloride salts is only a problem when it occurs at temperatures above the dewpoint. The deposits will be washed away by the condensed water at temperatures below the dewpoint.
- Ammonium chloride may precipitate from high temperature streams as they are cooled, depending upon the concentration of NH3 and HCl, and may corrode piping and equipment at temperatures well above the water dewpoint [300 ºF (149 ºC)].
- Concentration (NH3, HCl, H20 or amine salts), temperature and water availability are the critical factors.
- Ammonium chloride are hygroscopic, and readily absorb water. A small amount of water can lead to very aggressive corrosion ~360 mpy, depending on the temperature.
- Ammonium chloride and amine hydrochloride are highly water soluble, highly corrosive and form an acidic solution when mixed with water. Some neutralizing amines react with chlorides to form amine hydrochlorides that can act in a similar fashion.
- All commonly used materials are susceptible, in order of increasing resistance: carbon steel, low alloy steels, 300 series stainless steels, Alloys 400, duplex stainless steels, 800, and 825, Alloys 625 and C276 and titanium.
- The 300 series stainless steels can also experience chloride stress corrosion cracking in contact with ammonium chloride deposits.
- Reactor effluent streams are subject to ammonium chloride salt fouling and corrosion at temperatures up to 300F (149 ºC).
- Corrosion due to ammonium chloride is more difficult to predict than with ammonium bisulfide due to the higher temperature limits and unpredictability of deposition location, based on temperature and HCL or ammonia partial pressures.
- Inspecting for ammonium chloride corrosion is best done based on prior inspection observations, process stream sampling, input from process engineers and industry experience.
- If the inlet feed has negligible nitrogen content, then ammonium chloride corrosion may not be active. The attached Refincor references document the experience of other refiners with proven and suspected ammonium chloride salt corrosion.
- Locations of detected ammonium chloride salt corrosion included HDS effluent exchangers and a hydrotreater desulfurizer prefractionator feed or bottoms exchanger.