Study On The Effect Of Sodium Molybdate As Corrosion Inhibitor On The Properties Of Chromium-free Zinc-aluminum Coating

As a renewal product of Dacromet coating,chromium-free zinc-aluminum coating not only has good corrosion resistance,but also abandons the use of chromium anhydride in traditional Dacromet coating and achieves real green surface treatment.In this paper,sodium molybdate was selected as corrosion inhibitor for chromium-free zinc-aluminum coating.At the same time,by changing the content of sodium molybdate,different analytical and testing methods were used to study the effect of the content of sodium molybdate on the properties of chromium-free zinc-aluminum coating.In this paper,chromium-free zinc-aluminum coatings with 0³.0wt.%sodium molybdate content were prepared.Morphology observation,composition analysis,phase analysis,basic performance test and corrosion resistance test of coatings with different sodium molybdate content were carried out.Finally,the effect of sodium molybdate content on the properties of coatings was obtained,and the influence mechanism was preliminarily explored.The results of morphology,composition and phase analysis are as follows.With the increased of sodium molybdate content,the morphology,composition and phase of the coatings are similar.The coatings were smooth,compact and had good physical shielding effect.The coatings were mainly composed of Zn,Al,C,O,Si and Mo,and the main phases were Zn,Al and ZnO.The results of basic performance test are as follows.With the increased of sodium molybdate content,the porosity of the coatings varied from 4.90%to 6.05%,all of the coatings were compact.With the increased of sodium molybdate content,the roughness of the coatings decreased slowly at first and then increased sharply.When the content of sodium molybdate was 0.9¹.8wt.%,the roughness was smaller,remained at about 6.15um.With the increased of sodium molybdate content,the hardness of the coatings was basically unchanged at first and then decreased.When the content of sodium molybdate was0².2wt.%,the hardness of the coatings was relatively high,the hardness value changed in the range of 111¹14HV.With the increased of sodium molybdate content,the adhesion level of the coatings decreased.When the content of sodium molybdate was 0².2wt.%,the adhesion level of the coatings was ISO-1,the adhesion was good.With the increased of sodium molybdate content,the coatings had good crack resistance at alternating extreme cold and extreme heat temperatures.The results of corrosion resistance test are as follows.With the increased of sodium molybdate content,the time when the coating immersed in ammonium nitrate solution began to appear red rust was prolonged first and then shortened.When the content of sodium molybdate was 1.8wt.%,it took the longest time to appeared red rust,reaching 180 minutes,the corrosion resistance was the best.With the increased of sodium molybdate content,the potential of the coatings was lower than the matrix,and there was little difference in potential difference between matrix and eight coatings.That is to say,all of the coatings could provide good cathodic protection for the matrix and provided the same driving force for cathodic protection.With the increased of sodium molybdate content,the self-corrosion current density of the coatings first decreased and then increased,and the R_f+R_p value of the coatings first increased and then decreased.When the content of sodium molybdate was 1.8wt.%,the corrosion resistance of the coating was the best,the self-corrosion current density was the smallest,only 9.141×10^-6A/cm²,and the R_f+R_p value was the largest,reaching 667.60?·cm².According to all the test results,when the content of sodium molybdate was 1.8wt.%,the comprehensive performance of the coating was the best.The coating was uniform and compact,with low porosity,small roughness,relatively high hardness,good adhesion and best corrosion resistance.