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Surface darkening phenomenon of Zn–Mg alloy coated steel exposed to aqueous environment at high temperature

Published online by Cambridge University Press:  09 December 2015

Sung Jin Kim ,
Young Jin Kwak ,
Tae Yeob Kim ,
Woo Sung Jung  and
Kyoo Young Kim
Sung Jin Kim*
Affiliation:
Graduate Institute of Ferrous Technology (GIFT), Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
Young Jin Kwak
Affiliation:
Surface Treatment Research Group, POSCO Technical Research Center Kumho-dong, Gwangyang, Jeonnam 545-090, Republic of Korea
Tae Yeob Kim
Affiliation:
Surface Treatment Research Group, POSCO Technical Research Center Kumho-dong, Gwangyang, Jeonnam 545-090, Republic of Korea
Woo Sung Jung
Affiliation:
Surface Treatment Research Group, POSCO Technical Research Center Kumho-dong, Gwangyang, Jeonnam 545-090, Republic of Korea
Kyoo Young Kim
Affiliation:
Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea
*
a)Address all correspondence to this author. e-mail: [email protected]
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Abstract

Steel coils coated with Zn–Mg alloy containing high Mg content develop dark rust when exposed to an extremely limited amount of aqueous environment. To understand the nature of the dark rust and its formation mechanism, the steel is evaluated by the immersion test and high temperature–humidity test followed by critical evaluation with transmission electron microscopy for cross-sectional observation, field-emission scanning electron microscopy for surface morphology observation, Auger electron spectroscopy and glow discharge spectroscopy for identification of chemical composition as a function of depth. The results indicate that the dark rust is formed by precipitation of Mg-based corrosion product on the outermost surface when the steel is exposed to aqueous environment at high temperature. This is due mainly to preferential dissolution of Mg phases by the galvanic action with MgZn2 and Mg2Zn11 composed of the coating layer, and easy precipitation of Mg2+ ion in a form of Mg(OH)2 in a limited volume of the condensed water film on the surface.

Keywords

coatingzincmagnesium
Type
Articles
Information
Journal of Materials Research , Volume 30 , Issue 23 , 14 December 2015 , pp. 3605 - 3615
Copyright
Copyright © Materials Research Society 2015 

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