Characteristics of Zinc Phosphate Coating Activated by Different Concentrations of Nickel Acetate Solution
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NTRODUCTION
CARBON steels exhibit a number of desirable properties that make them ideal candidates for many industrial applications. They have a great strength, good hardness, and proper ductility.[1] However, carbon steels undergo corrosion when in contact with an aggressive environment, which limits their applications in some fields.[2–4] Consequently, many surface modifications have been reported to improve the protection characteristics of carbon steels. Among the proposed surface treatments, zinc phosphate conversion coating is one of the most widely used surface treatments for improving the protection ability of steel because of its low cost,[5] good corrosion resistance,[6] rapid coating formation, and suitability for treatment of irregular shapes.[7] It is known that an increase in surface coverage and uniformity of the phosphate coating leads to an increase in the corrosion resistance of steel substrate.[8] For that, recent efforts have been mainly focused on obtaining compact, fine, and uniform phosphate coatings through different strategies such as activation treatments before phosphating[9] and process technologies.[10–12] Activation
KHALID ABDALLA is with the School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia, and also with the Faculty of Engineering, Omar Almukhtar University, 00218 El-Beida, Libya. H. ZUHAILAWATI and A. AZIZAN are with the School of Materials and Mineral Resources Engineering, Engineering Campus, Universiti Sains Malaysia. Contact e-mail: [email protected] AZMI RAHMAT is with the School of Materials Engineering, Universiti Malaysia Perlis, 02600 Jejawi, Perlis, Malaysia. Manuscript submitted July 4, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
pretreatment is helpful for increasing nucleation sites and the population density of the phosphate coating crystals, resulting in compact and uniform coatings with excellent corrosion performance. In many instances leaving out the activation step produces a poor coating coverage with a few large zinc phosphate crystals,[13] particularly on the metal surfaces that exhibit some resistance to the coating formation.[14] For this reason, pretreatment solutions of disodium phosphate containing a small amount of titanium compound are conventionally used to activate phosphate coatings. However, the activation effect of such pretreatments is not stable and decreases with increasing dispersion age, as a result of coagulation and sedimentation of the titanium phosphate colloids.[15,16] Activation treatment with a nickel acetate solution before the phosphating step on steel surface was studied in our previous work.[9] The activation solution of nickel acetate can be used without problems of sedimentation and coagulation which occur in the conventional activation treatment of titanium phosphate colloids. The results revealed that the activation pretreatment by nickel acetate solution had a significant role in nucleation and growth of the zinc phosphate coating on mild
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