(100c) The Study of Surface Phases on Aluminide Coatings Using X-Ray Diffraction

Aluminide coatings are used to protect metals subject to harsh operating conditions at high temperatures. Halide activated pack cementation (HAPC) is an economical way to produce surface phases that are effective against deleterious corrosion conditions. The type of surface phases formed is dependent on various process parameters, e.g., activator and coating temperature. This study will provide insights into the effects of these parameters, providing a basis for the selection of process parameters to achieve desirable surface phases.   In this study, UNS S30400 austenitic stainless steel was coated at 650, 750, and 850°C for 9 hours by the HAPC process. Six different activators – aluminum chloride, ammonium chloride, sodium chloride, sodium fluoride, sodium hexafluoroaluminate (cryolite) and aluminum fluoride – were used to study the effect of pack composition on the surface phases of the coatings. The nature of the surface phases were examined using X-ray diffraction (XRD) as the principal characterization tool. XRD analysis reveals that aluminide coatings typically contain multiple surface phases when coated at 650°C. At 850°C, the multiple phases evolved into a single phase irrespective of the activators. Among the studied activators and coating temperatures, Al₅Fe₂ was the most common surface phase found.  The thermodynamic stability of the activators was found to have an impact on the prominent phase of the coating, e.g. the prominent phase formed using the least stable activator (NH₄Cl), is AlFe₃ at 650°C, Al₅FeNi at 750°C and Al₅Fe₂ at 850°C. However, for the most stable activator (Na₃AlF₆), the prominent phase is AlFe for all coating temperatures.