Introduction
St-37 is one of the most popular types of steel, which hasbeen widely used in many industrial applications due to its propermechanical properties. However, the use of this type of steel hasbeen restricted for many applications due to its low corrosion resis-tance [1,2]. Many different methods have been utilized in order toprotect the steel structures against corrosion. Among them, con-version coatings and organic paints have been widely utilized toincrease 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 onthe steel surface led to the increase of the corrosion resistance of∗Corresponding author. Tel.: +98 2122969777; fax: +98 2122947537.E-mail address: ramezanzadeh-bh@icrc.ac.ir (B. Ramezanzadeh).steel and the increase of the organic coatings’ adhesion to the metalsurface.Surface treatment of steel bodies by zinc phosphate conver-sion coatings has been considered as a useful surface preparationmethod 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 providesome protection against under-paint corrosion [10–13] However,the phosphate coating is porous and contains some micro cracksand because of these it cannot provide high corrosion resistancefor 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 [15,16