Hot Dipping of Chromium Low-alloyed Steel in Al and Al-Si Eutectic Molten Baths

Chromium low alloyed steel substrate was subjected to aluminizing by hot dipping in pure aluminium and Al-Si eutectic alloy at 750°C and 650°C respectively, for dipping time up to 45 minutes. The coated samples were subjected for investigation using an optical microscope, scanning electron microscopy (SEM), Energy-dispersive X-ray analyzer (EDX) and X-ray diffraction (XRD) technique. Cyclic thermal oxidation test was carried out at 500°C for 72 hours to study the oxidation behaviour of hot-dipped aluminized steel. Electrochemical corrosion behavior was conducted in 3wt. %NaCl aqueous solution at room temperature. The cyclic thermal oxidation resistance was highly improved for both coating systems because of the formation of a thin protective oxide film in the outermost coating layer. The gain in weight was decreased by 24 times. The corrosion rate was decreased from 0.11 mmpy for uncoated specimen to be 2.9 x10-3 mmpy for Aluminum coated steel and 5.7x 10-3 mmpy for Al-Si eutectic coated specimens. The presence of silicon in hot dipping molten bath inhabit the growth of coating intermetallic layers, decrease the total coating thickness and change the interface boundaries from tongue like shape to be more regular with flatter interface. Two distinct coating layers were observed after hot dipping aluminizing in Al bath, while three distinct layers were observed after hot dipping in Al-Si molten bath.