Introduction St-37 is one of the most popular types of steel, which has been widely used in many industrial applications due to its proper mechanical properties. However, the use of this type of steel has been restricted for many applications due to its low corrosion resistance [1,2]. Many different methods have been utilized in order toprotect the steel structures against corrosion. Among them, conversion coatings and organic paints have been widely utilized to increase the corrosion resistance of the metal substrates [3–5].There are different kinds of conversion coatings which havebeen applied on the steel substrates to protect them against corro-sion and enhance the organic coatings’ adhesion properties. Amongthem, phosphate based conversion coatings are the most commontype of chemical treatments which have been applied on the steelsubstrates [6–8]. Applying these types of chemical treatments on the steel surface led to the increase of the corrosion resistance of steel and the increase of the organic coatings’ adhesion to the metal surface.Surface treatment of steel bodies by zinc phosphate conversion coatings has been considered as a useful surface preparation method to increase the corrosion resistance of the steel [9]. Also,they are useful adhesion promoters for paint, varnish and lacquer.It has been also shown that the phosphate coatings can also provide some protection against under-paint corrosion [10–13] However,the phosphate coating is porous and contains some micro cracks and because of these it cannot provide high corrosion resistance for the steel substrate. Chromates have been used as sealing agentfor phosphate layers to enhance its corrosion resistance [14]. There-fore, dipping phosphates samples in the chemical bath containinghexavalent chromium has been a common and promising methodto obtain a surface with less porosity and higher corrosion resis-tance. Also, the under-paint corrosion protection of the phosphatecoatings can be significantly enhanced when the phosphate coatedmetal is post-treated with a dilute acid chromate rinse solutionbefore paint application. However, due to toxic and carcinogenicnature of the chromate based conversion coatings, the use of thesecoatings has been extremely restricted in last decades