Titanium-based Environmentally Friendly Conversion Coating On Steel

Steel is one of the most widely used metallic materials in our daily life; however, its corrosion imposes a great loss to the world. At presently, the corrosion protection of more than 60% of steel is galvanization followed by chromate conversion treatment. For its carcinogenic property, chromate treatment is facing a forced end now and concludes a history more than a century. RoHS took effects on 1st, March, 2007 in China, which prohibits the application of lead, mercury, Cr(VI), bromide flame retardant and other stipulated hazard pollutants. Thus, the study focused on substitution for traditional chromate conversion treatment is becoming a hot spot in the field of surface protection.Based on their similarities between titanium and chromium and the present conversion treatment technologies, titanium conversion treatment was researched, the experimental results are given in the foliowings:1. For the pretreatment of A₃ steel's surface, 20% HCl and 5.4 g/100mL hexamine is the optimal composition among the various chemical rust-removing methods according to rust-removing performance, corrosion-resistance and safe operation.2. For titanium conversion treatment, the optimal treatment conditions at room temperature are: 1.0 g/L Ti (SO₄)₂, 15% co-solvent, >1.4 mL/L H₂O₂, pH 8-11. The obtained smooth and gray A₃ surface coating can endure salt solution immersion test for 34 min, and slat spray test for 3.5 h without any rust. The optimal treatment conditions for moderate temperature are: 1.0 g/L Ti (SO₄)₂, 15% co-solvent, 1.4 mL/L H₂O₂, pH 9. The obtained smooth and gray A₃ surface coating can exist in salt solution immersion test for 6.5 h, and slat spray test for 11 h without any change.Compared with the results of phosphate conversion process, titanium conversion process is proved to be more corrosion-resistant and no pollution; at higher treatment temperature, corrosion-resistant performance of titanium conversion treatment is almost the same as that of phosphate conversion treatment.In short, titanium conversion process is a promising treatment technology for its non-toxic and environment-friendly advantages because of having no hexavalent chromium, phosphate, heavy metal and fluoride ions.