Effect of Sr Addition in the Microstructure and Corrosion Resistance of Hot-Dip Al-Si Coatings for Hot-Press-Formed Steel

Hot-press forming (HPF) steel is a promising lightweight material for automotive applications but suffers from oxidation and reduced corrosion due to high-temperature processing. Aluminized coatings, particularly Al-10Si, are widely used to mitigate this issue. However, HPF heat treatment can create brittle alloy layers with cracks, compromising retention and increasing corrosion risk. This study investigated the effects of Sr addition on the microstructure and corrosion resistance of Al-Si-coated HPF steel. Al-Si and Al-Si-Sr coatings were applied to steel substrates and subjected to heat treatment to produce heat-treated (HT) Al-Si and HT Al-Si-Sr samples. Sr addition refined and spheroidized eutectic Si particles, improved coating homogeneity, and mitigated vertical crack formation in the Al-Fe-Si intermetallic layer. The resulting dense, crack-free alloy layer effectively shielded the Fe substrate from corrosion. After heat treatment, Sr facilitated the formation of a fine lamellar microstructure and a dense, continuous oxide film, enhancing coating retention and sustaining barrier protection. These improvements significantly delayed corrosion propagation into the Fe substrate. Corrosion resistance was evaluated using salt-spray tests (ASTM B117), potentiodynamic polarization, and electrochemical impedance spectroscopy in 3.5 wt.% NaCl solutions. Microstructural analyses revealed that even minimal Sr content (0.05%) considerably enhanced the performance of Al-Si coatings, demonstrating industrial applicability. This study highlights the potential of Sr-added Al-Si coatings in addressing the demand for lightweight and corrosion-resistant materials in the automotive industry, offering a viable solution for high-performance and environmentally sustainable applications.