High-Temperature Corrosion of Hot-Dipped Aluminizing on Carbon Steel in Dry and Wet Environments

Abstract

Carbon steel can easily be corroded in marine environments, particularly at high temperature and wet environments. The oxide layer that formed on the carbon steel surface behaves as a deposit non-protective layer when the surface is exposed to high temperature. Thus, the surface of the carbon steel must be coated with a metal that can be formed oxide surface and intermetallic layer that can protect the carbon steel from high-temperature corrosion. The objective of this study is to determine the corrosion characteristics, microstructures and surface morphologies of hot-dipped aluminizing coating material at 750 °C under atmospheric air, dry oxygen (Ar–20%O₂) and wet oxygen (Ar–20%O₂–20%H₂O) flows. The optimum thickness coating was acquired by carrying out the hot dipping coating technique using pure Al at different molten temperatures and dipping time. The coating specimens were exposed to atmospheric air, dry and wet oxygen environments all at temperature of 750 °C. The pre- and post-exposing hot-dipped aluminizing coatings were analysed using optical microscope, SEM and EDAX to study its surface morphologies, coating cross sections and elemental compositions, whereas phase compositions was determined using XRD. The strengths of the coatings were measured according to microhardness index values. The hot dipping technique caused two layers to be formed on the carbon steel surface, the outer Al layer and the inner aluminide layer (Fe–Al). Results from the high-temperature experiments indicates that the optimum lanthanum content in Al is 0.3%wt that gives the lowest corrosion rate with a kinetic constant of 3.45 × 10⁻¹¹g²cm⁻⁴ s⁻¹. This is because the lanthnum content in the aluminizing improve the plasticity of aluminized layer and the adhesion ability of Al₂O₃ film, caused promoting a form of protective oxide film and lead to reduce the size and numbers of void formation at the aluminide layer. The presence of water vapour in the oxidant gas atmospheres lead to the rapid degradation of aluminide layer caused by formation of voids and cracks that led to oxidation occurs at the aluminide layer compared with dry oxygen. In conclusion, in addition of lanthanum in aluminizing up to 0.3%wt can retard the corrosion rate of carbon steel at high temperature in flowing of atmospheric air, dry and wet oxygen gases.