Construction Of Phytic Acid Conversion Film On Cold Rolled Steel Surface Based On The Synergistic Effect Of Accelerators

As a rare green multi-tooth metal chelator,phytic acid has been widely used in metal corrosion protection owing to the six phosphonic acid groups which are liable to coordinate with metal ions and consequently form a complex on the metal surface.As a corrosion inhibitor,phytic acid can be adsorbed on the surface of steel to form a phytic acid monomolecular layer.After adding metal ions such as Zn2+ and Ca2+to the phytic acid solution,phytic acid can chelate with these metal ions to form phytic acid-metal conversion film on the steel surface and effectively inhibit the corrosion of steel.However,the research on the formation mechanism of phytic acid-based conversion film on the surface of steel is not deep enough.The researches mainly focus on the performance of phytic acid conversion film and its formation mechanism after adding different metal ions,and the formation of conversion film by direct chelation reaction between phytic acid and Fe3+generated from the dissolving steel substrate in phytic acid solution has not been reported.In this dissertation,phytic acid(PA)was selected as the film-forming substance,and the feasibility of fabrication of conversion film based on synergistic effect between phytic acid and accelerator were investigated and the performance of the film was evaluated.Since the phytic acid molecules can only form a single molecular layer on the surface of cold rolled steel with poor corrosion protection performance,hydroxylamine sulfate(HAS)and ammonium molybdate were selected as promoters to promote the formation of phytic acid conversion film with excellent performance on the surface of cold-rolled steel through the synergistic effect between promoters and phytic acid.The main experimental content and research results in this paper are as follows:(1)Construction of phytic acid-hydroxylamine sulfate conversion film on the surface of cold rolled steel and its corrosion resistancePhytic acid was selected as the main film-forming substance and hydroxylamine sulfate was added as the accelerator to construct phytic acid-hydroxylamine sulfate conversion film on the surface of cold rolled steel.The microscopic morphology,film thickness and formation mechanism of the conversion film were investigated by SEM,ellipsometry and XPS characterization techniques;the conditions of preparation process and corrosion protection performance of the conversion film on the cold rolled steel substrate were investigated by EIS and Tafel curves.The results show that the addition of hydroxylamine sulfate could greatly improve the protection efficiency of the phytic acid conversion film on cold rolled steel,and when the concentration of phytic acid and hydroxylamine sulfate was 3.2 g/L and 0.4 g/L respectively,pH value of the film forming solution was 4.5 and film formation time was 15 min,the film could exhibit best corrosion resistance and the corrosion protection efficiency could reach 89.5%.In the process of film formation,hydroxylamine sulfate can accelerate the dissolution of cold rolled steel and form a dense conversion film with phytic acid on the surface of cold rolled steel through electrostatic interaction with phosphoric acid groups in the phytic acid molecule,and the thickness of the film was about 82 nm.(2)Construction of phytic acid-ammonium molybdate conversion film on the surface of cold rolled steel and its corrosion resistanceIn this chapter,ammonium molybdate was selected as the accelerator.The cold rolled steel was modified by ammonium molybdate solution and phytic acid solution separately or by phytate-ammonium molybdate mixed solution.The differences between the two kinds of films were confirmed by electrochemical tests and the mechanism of the promotion function of ammonium molybdate during the film formation process was further investigated by SEM,XPS and other tests.The results showed that when the cold rolled steel was treated by two solution separately,a ferric molybdate passivation film was formed on the surface of cold rolled steel after ammonium molybdate treatment,and the presence of this film layer facilitated the further formation of phytic acid conversion film layer,while a dense film layer was formed on the surface of cold rolled steel by the synergistic effect between phytic acid and ammonium molybdate after immersion in the phytic acid-ammonium molybdate mixture solution.The corrosion resistance of the two kinds of films was tested in 3.5 wt.%NaCl solution,and the latter film performed better than the former.