The effects of nitrogen alloying on the passivity and the pitting corrosion resistance of stainless steels are reviewed and discussed. Nitrogen in the solid solution of austenitic stainless steels is enriched at the passive film/metal matrix interface or at the metal side in the film. The effects of nitrogen alloying on the passivating ability estimated by the critical current density (icrit) in the acid is variable; they depend on the alloy composition and the electrolyte conditions. Alloying nitrogen in general seems to lower icrit when the base steel shows a high value of icrit (above ca. 1mA/cm2). Nitrogen enriched on surface of stainless steels helps passivation of a micropit by preventing the lowering of pH in it before the steadily growing condition is established. Duplex stainless steel shows an optimum pitting corrosion resistance at an appropriate α/γ phase ratio determined by the amount of the favorable alloying elements (i.e. Cr, Mo and N). This can be explained by the partitioning of these elements in each phase.
The corrosion resistance and cytocompatibility of two high N-bearing austenitic stainless steels as well as SUS 316L were evaluated in a simulated physiological environment. Potentiodynamic polarization measurements and analysis of released metal ions from alloy specimens immersed in 0.9% NaCl solution were made to evaluate the corrosion resistance. The surface of the alloys was characterized with X-ray photoelectron spectroscopy (XPS). Cell growth of human gingival fibroblasts was examined under culture conditions on these alloy surfaces. The pitting potential of the high N-bearing Fe-Cr-Mn-Mo alloy was higher than that of high N-bearing Fe-Cr-Mn alloy and SUS 316L. CrN formed in the outer region of the passive film and N enriched at the alloy surface under the passive film may be effective in improving pitting corrosion resistance. The present study demonstrated a correlation between corrosion resistance and metal-induced cytotoxity. The amount of released Fe and Cr ions from high N-bearing Fe-Cr-Mn alloy was approximately one order of magnitude higher than that from high N-bearing Fe-Cr-Mn-Mo alloy and SUS 316L. The cell growth rate on the high N-bearing Fe-Cr-Mn alloy was markedly depressed because of the large amount of released metal ions. The characteristics of the high N-bearing Fe-Cr-Mn-Mo austenitic stainless steel such as non-magnetism, high localized corrosion resistance, and excellent cytocompatibility meet the requirements for use as an implant material.
Effect of nitrogen on the corrosion behavior of austenitic stainless steels was investigated in solutions with low pH and a high concentration of chloride simulating the inside solution of corroding crevice. Nitrogen suppressed the anodic dissolution current of the steels over the potential range of 0 to 500mV vs. Ag/AgCl in 5kmol/m3 NaCl-pH 1 solutions. On the extension behavior of the corroding area over the specimen surface, nitrogen decreased considerably the extension rate of corroding area and changed its microscopic morphology. For the 0.02% nitrogen steel, the corroding area extended by pitting in the passive side of the active/passive boundary, whereas for the 0.2% nitrogen steel, no pitting was observed. Nitrogen had almost no effect both on the depassivation pH, and on the anodic dissolution current in severely corrosive solutions where the passivation did not occur. The suppressive effect of nitrogen on the extension rate of the corroding area may have a correlation with the suppression of the growth of pitting type crevice corrosion.
The influence of nitrogen addition on pitting corrosion of the high nitrogen-bearing austenitic stainless steels with and without nickel, that is, Type 304, Type 316 and high-Mn steels is investigated using electrochemical and in-situ Raman spectroscopic analyses. Electrochemical analyses show that nitrogen addition improves pitting corrosion resistance of the austenitic stainless steels. The beneficial influence on the passivity of the Type 304, Type 316 and high-Mn austenitic stainless steels is expressed by the specific coefficients in the pitting resistance equivalent (PRE) formula: PRE=Cr mass%+1×Mo mass %+3×N mass%-0.2×Mn mass%. In-situ micro-probed Raman spectra within an active pit formed by the anodic polarization at various potentials show the presence of NO3- reaction species, though it is difficult to identify the formation of NH4+ at this experiment. A peak of NO3- shifts toward lower wave number, which suggests the presence of adsorbed NO3- species to suppress the active dissolution in a pit.
The effects of alloyed nitrogen and molybdenum on the pitting behavior of austenitic stainless steels were examined comparing with that of corresponding oxyacid salts (i.e. nitrate and molybdate) in a solution. The comparison was made through measurements of pitting potential as a function of temperature up to 423 K and that of pitting temperature as a function of applied potential. Both alloyed nitrogen and nitrate ion in a solution showed inhibitive effect at lower temperatures rather than at high temperatures. Pitting temperatures measured in a solution containing nitrate showed a unique dependency on potential: the nobler the potential, the higher the pitting temperature. However, it could not be revealed that alloyed nitrogen inhibited pitting through formation of nitrate ion because stainless steels alloyed with nitrogen did not show the similar dependency of pitting temperature on applied potential. Both alloyed molybdenum and molybdate in a solution enhanced pitting potential more effectively at higher temperatures, possibly because the film formation reaction in which molybdenum species involved proceeded more rapidly at higher temperatures. However molybdate ion could miss its distinctive effect at high temperatures when a specimen was polarized before heating. Excellent inhibition of pitting was realized at every temperature when nitrate and molybdate ions were mixed into a chloride solution: nitrate worked at nobler potentials even at lower temperatures and molybdate worked at higher temperatures even at less noble potentials. The stainless steel which contained both nitrogen and molybdenum also showed good performance against chloride solutions, uniting the individual advantages.
By using a flow channel method, the effect of the flow velocity on pitting potential of Type 304 stainless steels with different levels of nitrogen has been studied. Pitting potential was measured by a potentiokinetic method in 3.5%NaCl saturated with nitrogen gas at 303K. Distribution of measured values of pitting potential obeyed a normal probability distribution, mean of which was found to increase with increase of the nitrogen content in the steels. The mean value of pitting potential shifted to the noble direction with increase of the flow velocity in laminar region, approaching a constant value in turbulent region. A lower flow velocity dependence of pitting potential in laminar region was observed for the higher nitrogen steels.