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In recent years, the automotive industry has increasingly turned its attention to galvannealed (GA) steel sheets, prized for their exceptional attributes like weldability, paintability, and corrosion resistance. To further enhance these properties, various coatings on GA have been explored. In this pursuit, an electroplating process, known as flash coating, has been developed to apply a bright and adherent nickel (Ni) plating onto GA steel sheets. This coating aims to cover GA defects and elevate performance metrics such as weldability, frictional behavior, corrosion resistance, and phosphatability. A comprehensive comparative study with bare GA steel sheets was conducted to underscore the improvements achieved.
Key Findings:
The electroplating process, conducted at a maximum current density of 700 A/m², yielded higher cathode current efficiency ranging between 95-98%.
Ni-coated GA steel sheets, with a coating time of 5-7 seconds, exhibited superior spot weldability, lower dynamic coefficient of friction (0.07 in lubricated conditions), and triple the corrosion resistance compared to bare GA steel sheets.
Notably, the phosphate coating on Ni-coated GA steel manifested as plate-like crystals, with a size ranging from 10-25 µm. The primary phase in the phosphate compound was identified as hopeite (63.4 wt.%), accompanied by other phases such as spencerite (28.3 wt.%) and phosphophyllite (8.3 wt.%).
The introduction of Ni-flash-coated galvannealed steel sheets marks a significant advancement in coating technology. These innovative coatings not only address existing GA defects but also substantially enhance crucial performance parameters. With improved spot weldability, reduced friction, and significantly enhanced corrosion resistance, these coated steel sheets offer promising prospects for various automotive and industrial applications.
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Article with all rights reserved, courtesy of harvard.edu