International Science and Technology Journal

The Cooling system in refining and petrochemical plants suffers from aqueous corrosion due to the direct contact between the metal and the aqueous solution which can be controlled by many methods. Corrosion inhibitors are the most used water treatments which are found in various types and added depending on the cooling system conditions; e.g. the presence of aggressive ions, working temperature, water movement, etc. Due to the high toxicity of chromate as an anodic corrosion inhibitors, they have been replaced by more friendly corrosion inhibitors such as molybdate which are considered relatively expensive, and inhibition is achieved at high concentrations, thus another inhibitor must be added (e.g. polyphosphate and/ or zinc-phosphonate) to afford the required inhibition with low concentrations of molybdates. In this study, two different inhibitors were used (sodium molybdate and zinc-phosphonate) individually and together in different concentrations. Sodium molybdate, Na2MoO4, a non-toxic anodic, environment-friendly corrosion inhibitor and zinc-phosphonate is a cathodic inhibitor which is also environment-friendly corrosion. The best performed inhibitor concentration with the use of sodium molybdate was 4000 ppm and 20 ppm with the use of zinc-phosphonate. When both inhibitors were used with different concentrations, the best inhibitor efficiency on the mild steel surface was achieved with 50 ppm Zn- phosphonate with 300 ppm sodium molybdate where it reached 96%.