Stainless steel, nitriding and corrosion resistance – part 2
A “made in TAG” study to demonstrate that even stainless steels can be nitrided.
We concluded the first part of our in-depth study with the question: “Does the ionic nitriding process in plasma compromise the stainless steels?”
The research, development and innovation laboratory of TAG (RD&I) has conducted electrochemical studies in order to answer this question. The samples examined are made of AISI 310S steel in the (non-nitrided) and nitrided (N) in plasma state at a temperature between 500 and 650 ° C.
Summary of the electrochemical part of the study.
The characterization of the corrosive effects on different samples of AISI 310S with and without nitrided layer was studied in aqueous solution of Cl– or SO4-2 at a temperature of 65 ° C at different pH values, using two electrochemical techniques such as the open circuit potential (OCP) and curves polarization (CP).
The open circuit potential (OCP) corresponds to a thermodynamic measurement of an equilibrium established between the metal surface (AISI 310S electrode) and an electrolyte solution. The OCP is defined as the potential generated within an open electrical circuit, that is, the voltage present when the ends of the terminals of a circuit are disconnected and there is no external electric charge. Therefore, the voltage measured in open circuit is representative of the potential of the power source or, in this case, corresponds to the interaction reaction between the surface under study (AISI 310S electrode) and a known reference (Ag/AgCl).
The open circuit potential is, more correctly, the thermodynamic corrosion potential. Hence, corrosion potential is simply the potential an electrode has when no external current is applied. In an electrochemical cell, current will flow between metals that have a lower corrosion potential to materials with a higher corrosion potential. Therefore, a material with a naturally high corrosion potential is likely to be the cell cathode. This first thermodynamic trend would indicate that the materials that show lower OCP (tending to negative values) with respect to a reference (Ag/AgCl), would tend to have a greater resistance to corrosion.
Graphs “1” & “2” show the evolution of the potential over time of AISI 310S steel samples in the presence, respectively, of chloride and sulphate as main electrolyte in solution, measured at different pH values.
These results show that, during the evolution of the OCP in solution at a different pH in the presence of Cl– ions, the equilibrium was reached towards more negative potentials; therefore, the presence of the nitriding layer would show a decrease in surface corrosion, while in the case of non-nitrided AISI 310S (label 310S) the tendency to equilibrium goes towards positive potentials, indicating a depolarization of the cathode or an increase in corrosion. In other words, the nitrided barrier would show a good inhibitory tendency to corrosion phenomena, under the experimental conditions described above.
