Mg Content Dependence of EML-PVD Zn-Mg Coating Adhesion on Steel Strip
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INTRODUCTION
ZN and Zn alloy coatings are predominantly used to improve cosmetic and perforation corrosion resistance of steel because the coatings act as a barrier layer and sacrificial anode. Zn and Zn alloy-coated steel is commonly used for automotive parts, home appliances, and in construction applications. The global production of Zn and Zn alloy-coated steel was approximately 120 million metric tons in 2014.[1] In order to extend the lifetime of steel parts, the enhancement of the corrosion resistance of Zn coatings by the addition of alloying elements such as Al, Mg, and Si has been evaluated.[2–5] Zn alloy coatings such as Zn-11Al-3Mg-0.2Si, Zn-6Al-3Mg, Zn-1Al-1Mg, Zn-1.6Al1.6Mg, Zn-3.5Al-3Mg, and Zn-2.6Al-3.2Mg have recently been developed for use in very corrosive environments.[4–10] Two main mechanisms have been proposed to explain the superior corrosion resistance of Mg-alloyed Zn-based coatings: (1) the presence of the Mg in the alloy results in the formation of a dense and insulating passivation layer,[11] and (2) Mg-hydroxide (Mg(OH)2), the corrosion product formed in the initial stages of corrosion, stabilizes the Zn corrosion products such as Zn5Cl(OH)ÆH2O (simonkolleite) by reducing the pH of the boundary layer on the corroding coating surface,[12] resulting in a significantly reduced corrosion rate. WOO SUNG JUNG and CHANG WOOK LEE, Graduate Students, and BRUNO C. DE COOMAN, Professor and Director, are with the Materials Design Laboratory, Graduate Institute of Ferrous Technology, Pohang University of Science and Technology (POSTECH), Pohang, Republic of Korea. Contact e-mail: decooman@ postech.ac.kr TAE YEOB KIM, Senior Research Manager, is with POSCO Technical Research Laboratories, Gwangyang, Republic of Korea. Manuscript submitted February 27, 2016. Article published online July 11, 2016 4594—VOLUME 47A, SEPTEMBER 2016
The Zn-Mg (Al) alloy coatings can be used with a reduced coating weight because of their high corrosion resistance. This coating weight reduction leads to a reduction or prevention of manufacturing problems, such as spalling of the coating during press forming and brass layer formation on resistance spot welding electrodes.[13] It also might increase the productivity of industrial coating lines by increasing the coating production line speed without loss of corrosion protection. These advantages drive the increased use of Mg-added Zn-based alloy coatings. Several studies report that even small additions of Mg, i.e., 1 wt pct, improve the corrosion resistance of Zn-Mg alloy coatings. Zn-Mg alloy coatings have a maximum corrosion resistance for a Mg content of approximately 10 wt pct.[13–17] However, most Mgadded Zn-based alloy coatings contain about 3 pct or less Mg[4–10] due to production limitations imposed by the hot dipping process in a molten Zn bath containing both Mg and Al additions. If the Mg content in the liquid Zn bath is high, dross formation and Mg burning occur during the manufacturing process. Production costs are therefore increased due to the need to use an antio
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