Under Deposit Pitting Corrosion in Crude Distillation Unit Overhead Systems
Causes
General or localized corrosion, often pitting, normally occurs under ammonium chloride or amine salt deposits, often in the absence of a free water phase. Ammonium chloride salt is formed and deposited by the reaction of HCl gas and ammonia gas: NH3 + HCl NH4Cl
Ammonium chloride readily adheres to metal surfaces. It is hygroscopic and can absorb moisture from steam at temperatures above the dew point. A small amount of water can lead to very aggressive corrosion, for example >100 mpy (>2.5 mm/y). In the presence of heat and a small amount of water:
NH4Cl (H2O) NH4+↑ + H+Cl-
The hydrochloric acid then attacks the metal surface
2HCl + Fe (H2O) FeCl2 + H2↑
Corrosion rates increase with increasing temperature. The concentration of NH3, HCl, H20 or amine salts and water availability are also critical factors. Ammonium chloride salts 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 dew point of 300oF (149oC).
Because the pitting is under the salt deposits, it is difficult to detect with corrosion probes and coupons. The best chances of detection, particularly in air coolers is by profile or otherwise called wall shot radiography
It is possible to decrease these ammonium chloride salt deposits by decreasing the amount of Cl+ present. It is also possible to decrease salt deposits by decreasing the amount ammonia present.
The amount of ammonia can be controlled by improved sour water stripper operation or reducing the nitrogen content by alternative crude selections.
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