The Research Of Washing-free Trivalent Chromium Black Passivation Process On Electrogalvanized Steel Sheets

Chromate passivation treatment to galvanized steel products is a common means ofindustrial production which improve the corrosion resistance of products. Because of matureprogress and good coating quality, hexavalent chromium passivation technology is widelyused in relevent markets. However, hexavalent chromium is a highly toxic substance, whichare highly toxic and carcinogenic. Therefore, countries in the world have introduced variousmeasures to limit use for its greater harm to Humans and the environment.Compared with hexavalent chromium compounds, trivalent chromium compounds is lesstoxic and non-carcinogenic. Because trivalent chromium passivation film has similar structureand properties to hexavalent chromium passivation film,so trivalent chromium passivationprocess has become the best alternative to hexavalent chromium passivation. Trivalentchromium passivation have blue,multicolor,black passivation three categories. After decadesof research and development, Process stability and overall performance of blue and multicolortrivalent chromium passivation film has reached even more than the hexavalent chromiumpassivation film. Up to now, market share of trivalent passivation has exceeded thehexavalent chromium passivation. Compared with blue and multicolor passivated, blacktrivalent chromium passivation have many drawbacks such as poor process stability, thesolution containing a large amount of toxic metal ions, poor adhesion of the passivation film.the organic polymer must be closed to meet the technical requirements of the application.Furthermore, the organic polymer post-treatment is necessary to achieve the technicalrequirements of the application.In this paper, the black passivation process on electrogalvanized steel sheets was studied.Two-step bath process was employed to passivating treatment, the first step is to blackening the zinc plating samples by trivalent chromium aqueous solution, then process sealingtreatment immediately without washing samples, The composition of two-step aqueoussolution are similar, they are all rich in trivalent chromium ion, but the operating conditions isdifferent. The trivalent black passivation films are completely inorganic layer, the quality andappearance of films can completely satisfy the requirements of related industrial manufacture.In this article, orthogonal experiment and single factor experiment was used to optimizethe formula and operating conditions of two-step black passivation solution. The method oflead acetate titration, neutral salt spray test and electrochemical test were used to evaluate thecorrosion resistance of passivating film producted under different process conditions. Filmadhesion was tested with the eraser according to the corresponding national standard. Thecomposition and microstructure of film was analyzed by SEM/EDS. The results show that theoptimum formula and the best processing condition of first step is Cr3+3g·L-1, Citric acid16.6g·L-1, Co2+0.6g·L-1, H3PO44g·L-1, FeSO4·7H2O4g·L-1, sulfosalicylic acid2g·L-1,PH=1.8~2.0, T=30℃, t=30s. By this process, the black-light film was obtained. But the filmadhesion and corrosion resistance is still not adequate to fully satisfy the requirements ofindustrial manufacture. Through the research, the optimum formula of second step is Zn2+4g·L-1, Cr3+5g·L-1, organic phosphoric acid20g·L-1, thickener5g·L-1, Citric acid26g·L-1,H3PO412g·L-1. After two-step process the film is deep dark and the corrosion resistance issatisfy the industrial production.