Kinetics Modeling of Parabolic Growth Competing to Linear Declining: A Theoretical Tool to Interpret Corrosion Data in Real World

Corrosion data may be hard to be interpreted in many cases, where the thickening of the oxide scales is simultaneously accompanied by thinning, and the mass gain verse time curves are non-monotone. For example, the thermal growth of the oxide scales (TGO) on metals may compete to basic or acidic fluxing in molten sulphate induced hot corrosion, or to volatilization in water vapor oxidation. We modified the kinetics Tedmon model, which is based on the diffusion controlled parabolic thickening and interface reaction controlled linear thinning of TGO. Both mass gain and scaling rates have been taken into account. It is theoretically deduced that the mass gain of the specimen becomes a maximum (ξ) at a specific time; and then the thickness of TGO (ζ) and the mass gain rate (v0) of the specimen keep constant after a dynamic balance between thickening and thinning of TGO is reached. Formulations have been derived for the determination of the thickening and thinning rate of TGO using measured ξ, ζ and v0.

Water Vapor Oxidation Behavior under High-Temperature and Pressure Conditions of the Nitrided Alloys for Steam Turbine Valve System Parts

Mechanical and corrosion properties were evaluated in the nitrided and non-nitrided specimens of ASTM 355 and SUS422 steels. The oxides formed on the specimen were analyzed using by XRD and the weight change was measured after oxidation test at high temperature, high pressure, and steam conditions. The oxidation behaviors and rates between two alloys and nitrided/non-nitrided specimens were clearly different. XRD analysis showed that the nitrides formed on the nitrided specimen were composed of CrN, Fe4N, and Fe2-3N. Also, it was found that the hardness on the nitrided specimen at 283°C decreased from Hv 1150 of room temperature to Hv 425.