Hot-dip galvanizing has been applied in steel industries due to its effectiveness to produce a Zn coating with both barrier and cathodic protections again corrosion [6]. Recently, hot-dip galvanized coatings have been studied to evaluate the protection on steels during hot stamping and the corrosion resistance of the resultant coating after hot stamping. The critical issues of the galvanized steels for hot stamping are oxidation and liquid metal embrittlement [7; 8]. The former deteriorates the resistance spot weldability and paintability, whereas the later has a negative effect on formability [9].
Silane, a kind of Si-based coating on light metals, has been used in galvanized steels to offer barrier protection against corrosion [10]. In this study, silane coating was applied on galvanized (GI) and galvannealed (GA) steels to elevate the practicability for the hot stamping application. The GI and GA steel sheets were received from China Steel Co., Taiwan, with coating thickness of approximately 10µm. Silane coating film was formed by a laboratory rolling method. The treating solution was prepared by mixing tetraethyl orthosilicate (TEOS), acidified water (pH= 1.0, adjusted by HNO3), γ-glycidoxypropyltrimethoxysilane (GPTMS), followed by stirring for 2 h at room temperature. The GI and GA steel sheets with and without the silane coating were isothermally held at 900℃ under the atmosphere for 5 min to simulate austenitization prior to hot stamping. XRD, SEM, TEM, and XPS were employed to characterize the surface microstructure of the various steels. Corrosion behaviors were tested by electrochemical stripping and potentiodynamic polarization measurements.
The GI steel displayed a gray color after austenitization. In contrast, the GI steel with the silane coating still exhibited a metallic color, suggesting the silane coating is effective in preventing the GI steel from oxidation at 900℃ (Fig(a)). This is consistent with the XRD pattern showing the absence of the peaks resulting from ZnO for the GI steel with the silane coating (Fig.(b)). However, the silane coating offered a less degree of protection on the GA steel. The cross section of the silane-coated GI and (b) GA steels was further characterized by the SEM. Zn oxides were readily observed on the GA steel; conversely, Zn oxide scale was hardly observed on the GI steel.
After austenitization, the silane-coated GI coating transformed to a mixture of Γ phase and Zn saturated α-Fe, which retained the cathodic protection over the steel substrate. However, the alloy layer of the silane-coated GA steel was solely composed of Zn saturated α-Fe and the cathodic protection was largely reduced after austenitization.
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