Conventional plasma nitriding of stainless steels is carried out at about 550°C which leads to an increase of hardness, but also to a decrease of the corrosion resistance. The latter can be avoided by low-temperature nitriding at 380-450°C. Anodic behavior of low-temperature nitrided 304L steel was examined here in 0.1 M Na 2 SO 4 at pH 3, 6, and 9, and in 0.1 M Na 2 SO 4 ϩ 0.4 M NaCl at pH 3. The corrosion resistance of nitrided steel was in the chloride-free solution at pH 3 lower than that of unnitrided steel, whereas in the near-neutral solutions the resistance was higher. In the chloride-containing solution, nitrided steel exhibited much higher resistance to pitting corrosion than unnitrided steel. Nitrogen accelerated anodic oxidation in the initial stages of polarization, and promoted the subsequent passivation. X-ray photoelectron spectrometry analysis indicated the presence of nitrogen-rich species and large amounts of hydrated Fe 3ϩ /Cr 3ϩ oxides and of Fe 3 O 4 /Fe 2 O 3 on corroded nitrided steel. It is proposed that the improved resistance of nitrided steel to pitting corrosion can be related to the enhanced anodic reactivity of nitrogen-containing steel which results in the formation of oxide deposits and also in an accumulation of protective nitrogen-rich species.

Impedance of Cr18Ni10 stainless steel in sulphate solutions after a low-temperature plasma nitriding Impedanz eines nichtrostenden Cr18Ni10 Stahls in Sulfatlö sungen nach einer Plasmanitrierung bei niedriger Temperatur J. Flis* and M. Kuczynska Dedicated to PD Dr. J. Gö llner on the occasion of his 60th birthday The 00Cr18Ni10 stainless steel was examined before and after plasma nitriding in a 80%N 2 /20%H 2 gas mixture at 425 8C for 30 h, resulting in the formation of a supersaturated solid solution of nitrogen in austenite (S phase) with 12.9 wt.% N at the surface. Electrochemical measurements were made on the as-nitrided surface and after abrasion to various depths. Polarisation curves and impedance were measured in 0.1 M Na 2 SO 4 acidified to pH 3.0; some measurements were also made at pH 6 and 9. It was found that nitrogen decreased the corrosion resistance of the steel in the acidified solution (pH 3.0), but increased it in the neutral and near-neutral solutions (pH 6.0 and 9.0). The deleterious effect of nitrogen at pH 3.0 was at potentials of the active state much stronger and more dependent on the nitrogen concentration than in the passive state. It was suggested that the increased anodic reactivity of the nitrided steel in the active state is inherent for the lower thermodynamic stability of the supersaturated solid solution of nitrogen, whereas the increased corrosion resistance in the neutral and near-neutral sulphate solutions is associated with the effect of nitrogen on anodic films.Chrom-Nickel-Stahl Cr18Ni10 wurde vor und nach einer Plasmanitrierung (30 h bei 425 8C in 80%N 2 /20%H 2 ) untersucht. Das Ergebnis war eine übersättigte feste Lösung mit 12,9 Gew% Stickstoff in der Austenitphase auf der Oberfläche des Chrom-NickelStahls. Elektrochemische Messungen wurden an der nitrierten Oberfläche und nach dem Abschleifen auf unterschiedliche Tiefen durchgeführt. Polarisationskurven und Impedanzmessungen erfolgten in 0,1 molarer Natriumsulfatlösung bei pH 3, pH 6 und pH 9.Es stellte sich heaus, dass Stickstoff den Korrosionswiderstand in der sauren Lösung (pH 3) herabsetzte, in der neutralen und leicht basischen Lösung (pH 6 und pH 9) jedoch erhöhte. Der nachteilige Einfluss des Stickstoffs bei pH 3 war bei Potentialen im aktiven Zustand viel stärker ausgeprägt und auch wesentlich abhängiger von der Stickstoffkonzentration als dies im passiven Zustand der Fall war. Es wird angenommen, dass die erhöhte anodische Reaktivität des nitrierten Stahls im aktiven Zustand von der geringen thermodynamischen Stabilität einer übersättigten Stickstofflösung ausgeht, während der erhöhte Korrosionswiderstand in der neutralen und leicht basischen Sulfatlösung mit dem Einfluss des Stickstoffs auf anodische Deckschichten verbunden ist.