Study of efficiency of industrial corrosion inhibitors for cooling water systems at oil refining industry

The aim of this work is to study the influence of some industrial corrosion inhibitors in the presence of biocides, biodispersants and scale inhibitors (anti-scaling agents) of four manufacturers on corrosion of mild steel (C1018) in the cooling water of the Oil Refinery. The chemistry of inhibitors investigated is based on phosphates, phosphonates, zinc and magnesium salts. Weight loss method was used for the determination of the efficiency of corrosion inhibitors. The experiments were carried out in agitated cooling water exposed to air at 25°C. The duration of the corrosion experiments was 7 days. Corrosion rate of carbon steel in uninhibited cooling water was 0.32 mm/year. The minimum acceptable value of inhibition efficiency (65%) was determined according to the reduction corrosion rate of mild steel from 0.32 mm/year to the maximum allowable corrosion rate of 0.11 mm/y in the cooling water of the Oil Refinery. It was shown that in most cases, the concentrations of inhibitors for acceptable protection of mild steel do not coincide with the inhibitor concentrations recommended by manufacturers. Some chemicals recommended by manufacturers accelerated general corrosion of mild steel and even resulted in pitting corrosion. In order to compare different corrosion inhibitors, an "Inhibitor Index" (I.I.) was introduced and calculated. This index takes into account a minimum acceptable value which is 65% of inhibition efficiency and the experimental efficiencies measured above 65%, which were reached at particular quantities of inhibitors. Corrosion inhibitor with the highest I.I. = 9.1 was selected and recommended for use in the cooling water system of the Oil Refinery. This inhibitor together with biocides and anti-scaling agent has been successfully used since 1996.