The Research On Trivalent Chromium Passivation And Colored Technology On Hot Dip Galvanized Layer

At present the hexavalent chromium passivation holds the dominant position in hot dipgalvanizing passive products. Hexavalent chromium with high toxicity and carcinogenicity,not only is a great threat to workers’ health, but also has great harm to the environment.Many developed countries have issued a ban on the use of hexavalent chromium products.Trivalent chromium is low toxicity and low carcinogenicity, so the trivalent chromiumpassive technology attracted more attentions.In this paper, a layer has been formed with certain thickness and corrosion resistance onthe hot dip galvanized layer surface. A trivalent chromium passivation formula wasdeveloped which suitable for hot dip galvanizing layers through hole sealing shading formbright color uniformity of the passivation membrane. The chromic chloride is the main salt.Oxidant is NaNO3. Ligand agent is citric acid. NiSO4provides metal active ion (Ni2+) forpassive solution. The influence rules of passive film appearance and corrosion time weredetermined through single factor experiment. The final passivation liquid formulation andprocess conditions are: CrCl36g/L, NaNO320g/L, NaF10g/L, NiSO440g/L, Hole sealingcolorant30g/L, some surface active agent, pH=2.7, T=50℃, t=60s. The durable time tosalt spray of passive membrane is200hours. The passive film is sealing by acrylic resinafter passive and the durable time to salt spray is120hours. The mechanism of passivationprocess and hole sealing chromatic were also studied in this article.