INFLUENCE OF pH ON CORROSION CONTROL OF CARBON STEELBY PEROXIDE INJECTION IN SOUR WATER

摘要

Sour hydrogen damage is considered the most important corrosive process in the light-endsrecovery section of Fluid Catalytic Cracking Units (FCCU). Corrosion in this condition is due toheavy gasoil that originates great amount of contaminants, such as H2S, NH3 and HCN.Hydrogen absorption is promoted by the presence of free cyanides in the environment. Theattenuation of this process requires the use of some inhibitors, such as oxygen, hydrogen peroxide(H2O2) or commercial polysulfides. The effect of these compounds is to neutralize free cyanides(CN-) into thyosulfides (SCN-). When peroxide injection is selected, cyanide concentration insour water has been used as key parameter to start the peroxide introduction. However, theimportance of pH in this system has been pointed out by many authors. The aim of this work is toinvestigate the influence of pH when peroxide injection is carried out in less alkaline conditionsof sour water. Electrochemical techniques - like anodic polarization and hydrogen permeationtests - and weight loss measurements were used to evaluate the effectiveness of corrosion controlof carbon steel. It was concluded that at pH 7.5 peroxide injection can drive to an increment ofthe corrosion rate. Besides that, it was concluded that hydrogen permeation into the metal ispromoted. Both detrimental effects were due to elemental sulfur generation in this pH range. Theadoption of pH as a key parameter for peroxide injection is then suggested.