Effect of phosphating temperature and activation pretreatment on zinc phosphate coating formed on medium carbon steel

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Date
2016-06
Authors
Muhammad Qusyairi Roslan
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Abstract
Zinc phosphate coating is widely used for corrosion protection of metallic materials, mainly for steel. In the present investigation, phosphating solution comprising phosphoric acid, zinc oxide, nitric acid and sodium fluoride was used to deposit zinc phosphate coating on medium carbon steel substrates. The zinc phosphate coating was deposited on medium carbon steel surface at different phosphating temperature, in order to study the effect of temperature on the coating growth, formation and corrosion resistance. To enhance the coating deposition and properties, activation pretreatments with silver nitrate solutions at different concentrations were introduced in this study. The corrosion behaviour was evaluated in a 3.5% NaCl solution by immersion test, potentiodynamic polarization and electrochemical impedance studies. It was found that, the best coating coverage and corrosion resistance were obtained at temperature of 55˚C. Surface morphology observations by SEM and EDX revealed that the activation pretreatments by silver nitrate increased the population density and refined the grain size of the zinc phosphate coating. The electrochemical results indicated that the corrosion resistance of the phosphated specimens was significantly improved after activation pretreatments.
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