Corrosion behaviour of stainless steels after low temperature thermochemical treatment

Abstract

The corrosion behaviour of nitrided, carburised and nitrocarburised samples of the steels X2CrNiMo17‐12‐2 (AISI 316L) and X2CrNiMoN22‐5‐3 (UNS 31803) was tested with electrochemical methods. Generally the current density potential curves of the samples are highly sensitive on the chromium depletion of the matrix due to the precipitation of chromium nitrides at the grain boundaries. It was observed that in this case, especially in 0.05M sulphuric acid, the passive current density in the complete range of the current density potential curve increases considerably. A criterion derived for the beginning of the decomposition of the expanded austenite allows the determination of its stability in dependence on the treatment temperature and time. The expanded carbon austenite has a higher thermal stability compared to the expanded nitrogen austenite. By controlled sequential removal of the samples surfaces the corrosion behaviour into the depth of the surface layers was investigated. Das Korrosionsverhalten nitrierter, carburierter und nitrocarburierter Proben der Stähle X2CrNiMo17‐12‐2 (AISI 316L) und X2CrNiMoN22‐5‐3 (UNS 31803) wurde mit elektrochemischen Methoden untersucht. Die Stromdichte‐Potenzial‐Kurven reagierten sehr empfindlich auf eine Chromverarmung der Matrix durch die Ausscheidung von Chromnitriden an Korngrenzen. Es wurde beobachtet, dass sich besonders bei den in 0,05M Schwefelsäure aufgenommenen Stromdichte‐Potenzial‐Kurven die Passivstromdichte im gesamten Bereich deutlich erhöhte. Ein daraus abgeleitetes Kriterium für den beginnenden Zerfall des expandierten Austenites ermöglicht eine Beschreibung seiner Stabilität in Abhängigkeit von den Behandlungsbedingungen. Der expandierte Kohlenstoffaustenit besitzt eine höhere thermische Stabilität als der expandierte Stickstoffaustenit. Durch kontrollierte Schichtabträge wurde das Korrosionsverhalten der Randschicht mit wachsendem Oberflächenabstand untersucht.

Functional Surface Layers on Stainless Steels Generated by Means of EB Cladding and Nitrocarburizing

“…Due to the nitrocarburizing the corrosion resistance decreased one order of magnitude independent of the treatment temperature. However, the current density, measured at +0.4 V, is still below the tolerance limit of 50 μA cm −2 and the corrosion resistance is maintained …”

“…Plasma‐enhanced thermochemical processes like nitriding, carburizing, or nitrocarburizing are highly efficient surface treatments because they permit the formation of a surface layer with a high nitrogen and/or carbon concentration, known as expanded austenite . Such layers have a high surface hardness and exhibit improved wear behavior, whilst the corrosion resistance is maintained if the precipitation of chromium (Cr) nitrides and/or carbides, which lead to undesired Cr depletion within the substrate, is avoided by means of low temperature and reduced process time …”

Effect of Low-Temperature Plasma Nitriding on Corrosion and Surface Properties of Duplex Stainless Steel UNS S32205

“…The current response is plotted as a function of the potential, when the metal is subjected to the potentiodynamic polarization. Some studies regarding plasma-nitrided DSS have reported polarization curves with a hillside on the passive-to-transpassive transition ( Ref 2,3,[13][14][15][16][17]. This hillside was recognized in all cases by a continuous increase followed by a drop in current density at the end of the passivation region.…”

Effect of martensite transformation on chemical composition, semiconductor property and corrosion resistance of passive film on SAE 304 stainless steel

“…Stainless steel is a widely used metallic material due to excellent mechanical properties and corrosion resistance [1][2][3][4][5][6][7][8]. As a common processing method for stainless steel, cold working can result in the formation of residual stress and a strain-induced martensite due to plastic deformation, which can lead to the deterioration of the corrosion resistance through an increase in the number of surface active sites [9].…”