The Anti-corrosion Modification Of Thioacetamide On Phytic Acid Conversion Coating And Cerium-salt Conversion Coating On Galvanized Steel

Galvanized steel is widely used in automotive,construction,electricity,traffic and daily civilian fields.In order to prevent its corrosion,the cost-effective chromate conversion technology is often used in the industry.Due to the carcinogenesis of hexavalent chromium,the world has prohibited the use of chromate.Therefore,green environmental chromium-free conversion technology on Galvanized steel is in urgently needed.In this paper,the anti-corrosion modification of thioacetamide on phytic acid conversion coating and cerium-salt conversion coating on galvanized steel is researched.Because of environment-friendly,wide range of sources and excellent corrosion resistance,phytic acid was often applied in the metal surface anti-corrosion studies.Under the conditions of phytic acid 2m L/L(the mass fraction is 70%),p H 4.5,65?and 10min,phytic acid conversion coating was prepared on the galvanized steel surface.After modification with thioacetamide,the salt spray time of phytic acid conversion coatings increased from the 28h before modification to 48h after modification.Scanning electron microscopy(SEM),neutral salt spray(NSS),X-ray fluorescence spectrometry(XRF),Tafel and EIS test results showed that the phytic acid conversion coatings on galvanized steel was wheat granular arrangement.After modification by thioacetamide,the coatings surface was more smooth and dense,and P content in the coating had significantly improved.With phytic acid conversion solution treatment,the corrosion potential of galvanized steel was shifted by 181 m V positively,the corrosion current density was reduced by 43%;The corrosion potential of phytic acid conversion coating modified by thioacetamide is shifted by 8m V positively,the corrosion current density is reduced by about 3%.The corrosion resistance of the phytic acid conversion coating modified by thioacetamide on galvanized steel was certain improved.The cerium-salt conversion coating on galvanized steel has some imperfections:with short reaction time,the coating is thin;with long reaction time,the coating is easy to crack and falling off,their corrosion resistance is not ideal.Aiming at this problem,based on the previous research of cerium-salt conversion coating on galvanized steel by our group,the anti-corrosion modification of thioacetamide on cerium-salt conversion coating on galvanized steel was studied.Under the condition of thioacetamide 0.3g/L,p H 1.3,55?,15min,salt spray time of cerium conversion coating increased from the 24h before modification to 288h after modification.Scanning electron microscopy(SEM),energy dispersive spectrometer(EDS),neutral salt spray(NSS),Tafel and EIS test results showed that coating thickness is increased to 15?m after modification.Before modification,the coating is too thin to observe by scanning electron microscope(SEM).After modification,the mass fraction of anti-corrosion elements O,P,Ce increased by 22.67%,13.4%and 13.73%,respectively.The corrosion potential of cerium salt conversion coating increased from-1138m V before modification to-1301m V after modification,it moved about 163m V more positively;And the corrosion current density decreased from 1.155×10^-5A·cm^-2to7.575×10^-6A·cm^-2,it decreased by about 40%.The corrosion resistance of galvanized steel was greatly improved.