MATERIALS SCIENCE AND ENGINEERING

CORROSION OF LOW-TEMPERATURE-CARBURIZED AUSTENITIC STAINLESS STEELS

Low-temperature carburization, a novel process developed for improving the surface properties of austenitic stainless steels, produces a surface layer ("case") possessing a very high concentration of interstitially dissolve carbon – up to 10⁵ times the equilibrium solubility at room temperature [1-4]. This provides a very high hardness and other outstanding mechanical properties ( wear resistance, fatigue resistance, etc). In addition, and quite unexpected, the colossal supersaturation with carbon vastly improves the corrosion resistance. To understand the underlying physical principle, we use cyclic potentiodynamic polarization measurements to study the relative susceptibility to localized corrosion (pitting and crevice corrosion) in a chloride environment [5,6]. The setup for this measuring consists of a working electrode, a counter electrode, and a reference electrode. After determining the range of potential and the scan rate, the relationship of potential and current density is obtained by cyclic potentiodynamic polarization. The corrosion behavior of carburized austenitic stainless steels with different compositions is compared to the behavior of the material in the non-carburized state.

To produce a potentiodynamic polarization curve, the potential of anodic electrode (relative to a standard reference electrode) made from the metal of interest is varied at a controlled rate over a range of potentials from cathodic to anodic values. The resulting current, which is related to the corrosion rate, and the controlled potential can be used to plot a potentiodynamic polarization curve (Fig. 1). This curve has a certain characteristic features. Criteria for selecting materials for service in corrosive environments include (i) the potential at which the current reaches its maximum, (ii) the current in the region where there is little change in current with potential, and (iii) the potential at which the current begins to increase again. The potential of initial corrosion corresponds to the OCP (open-circuit potential). Pitting corrosion commences where the slope of the increasing curve abruptly changes to a much lower value (Fig. 2). Comparing the open-circuit potential and the pitting potential of carburized and un-carburized potential, we observe a strong effect of the colossal supersaturation with carbon on the corrosion resistance.

Fig. 1. Example of a Cyclic polarization curve, obtained from low-temperature carburized 316 austenitic stainless steel.