Sealing Treatment of Aluminum Coating on S235 Steel with Thermal Diffusion of Zinc
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JTTEE5 24:1052–1059 DOI: 10.1007/s11666-015-0277-4 1059-9630/$19.00 ASM International
Sealing Treatment of Aluminum Coating on S235 Steel with Thermal Diffusion of Zinc Yong Wang, Timing Zhang, Weimin Zhao, and Xiuyan Tang (Submitted January 19, 2015; in revised form July 1, 2015) The study introduced a thermal diffusion sealing treatment for arc-sprayed aluminum coating on S235 carbon steel. The sprayed aluminum-zinc duplex coating was heated to 420 °C, so that the low-meltingpoint zinc could diffuse into the pores of the aluminum coating. Optical microscope, microhardness, electron probe microanalysis, and x-ray diffraction were used to evaluate the sealing treatment. The calculated diffusion coefficient for zinc in the arc-sprayed aluminum coating was approximately 7.735 3 1029 cm2/s. The diffused zinc could increase the compactness and microhardness of the aluminum coating. Nevertheless, adverse interface reactions could destroy the coating if the zinc made contact with the steel substrate. FeZn10 could form initially, and then the heat from the exothermic reactions between zinc and iron would initiate the reactions among iron, aluminum, and zinc. FeAl-Znx, FeAl2-Znx, and Fe2Al5-Znx were generated following FeZn10. The defected structures were mainly composed of Fe2Al5-Znx and FeZn10. All of these formed tough, brittle, intermetallics that have a negative effect on the coating performance. Thus, the contact between zinc and the steel substrate should be avoided, and the holding time should be restricted to 8.16 3 106L2 to prevent the generation of intermetallics.
Keywords
aluminum coating, interface reaction mechanism, intermetallics, sealing treatment, thermal diffusion
1. Introduction More offshore oil and gas installations are now required because of the newly discovered energy sources in deep water depths (Ref 1). Considering the fabrication cost, carbon steels are the commonly used materials for marine infrastructures (Ref 2). The offshore steel structures are always used in chlorine-containing environments, where corrosion could cause extensive damage if no protection method is applied (Ref 3). To date, various studies on protection systems have been conducted to delay and/ or reduce corrosion. Anodic and cathodic protection (Ref 4, 5), organic coating (Ref 6), and inhibitors (Ref 7) are the commonly used methods. As for steel immersed in seawater, organic coating combined with cathodic protection is usually adopted (Ref 8). However, steel structures may be sensitive to hydrogen embrittlement if the hydrogen reduction potential is more positive than the applied cathodic potential (Ref 9). Moreover, the service life of organic coating is limited because of corrosion progression from coating defects (Ref 10). Compared with
Yong Wang, Timing Zhang, Weimin Zhao, and Xiuyan Tang, College of Electromechanical Engineering, China University of Petroleum, Qingdao 266580, PeopleÕs Republic of China. Contact e-mails: [email protected], [email protected], and [email protected].
1052—Volume 24(6) Augus
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