A. Fossati

Glow-discharge nitriding and post-oxidising treatments of AISI H11 steel

Abstract Post-oxidising treatments can be performed to improve the corrosion resistance of nitrided steel components. The glow-discharge process appears particularly attractive, since it allows to carry out both nitriding and post-oxidising treatments in a single technological operation, simply varying the treatment atmosphere and the working conditions. In the present paper the effects of using air as the treatment atmosphere of glow-discharge post-oxidising treatment have been evaluated. Nitriding treatments were performed at 773 K for 5 h, and post-oxidising treatments were carried out at 623 and 773 K for 1 h. The nitriding and nitriding+post-oxidising treatments produce modified surface layers consisting of an outer compound layer and an inner diffusion layer. When the post-oxidising treatments are performed, the compound layer consists of an oxide layer on the top of the nitride layer. With a 623 K post-oxidising treatment the oxide layer consists mainly of magnetite, Fe 3 O 4 , while when post-oxidising is carried out at 773 K it consists essentially of hematite, Fe 2 O 3 . In the inner nitride layer iron (γ′-Fe 4 N, e-Fe 2–3 N) and chromium (CrN) nitrides are present. In all the treated samples high hardness values are obtained in the modified layers. The hardness profiles of the post-oxidised samples are only slightly affected by the 773 K post-oxidising treatment. The corrosion resistance of the treated samples was tested and the results show that the nitriding+623 K post-oxidising treatment is able to significantly improve the corrosion resistance of the untreated and treated AISI H11 samples.

Corrosion resistance properties of plasma nitrided Ti–6Al–4V alloy in hydrochloric acid solutions

Abstract The corrosion properties of TiN layers obtained by glow discharge on Ti–6Al–4V samples were investigated by polarisation curves, cyclic voltammetry and loss weight tests, in different HCl solutions. The results were compared with those obtained on the untreated alloy tested in the same conditions. The corrosion resistance of nitrided samples depends on the quality of the film and increases as the thickness increases. At potential lower than 1000 mV (SSE) nitrided samples show passive behaviour even in concentrated HCl solution (4M). If the potential is higher than a threshold, which depends on the acid concentration, it is observed a high anodic current related to the TiN oxidation. This enhanced oxide growth promotes cracking of the oxide layer and the exposure of fresh TiN surface.

Improvement of corrosion resistance of austenitic stainless steels by means of glow-discharge nitriding

Abstract Austenitic stainless steels are employed in many industrial fields due to their very good resistance to general corrosion in several environments. However, their use is limited due to the fact that they suffer localised corrosion in specific environments, particularly in chloride-ion rich solutions. Low temperature nitriding treatments can improve both corrosion resistance in chloride-ion containing media and surface hardness, due to the formation of a metastable phase known as expanded austenite or S phase; this phase can be outlined as a supersaturated interstitial solid solution of nitrogen in the expanded and distorted γ-Fe f.c.c. lattice. We review the main experimental results obtained in our research on low temperature glow-discharge nitriding of austenitic stainless steels. By means of proper treatment parameters, low temperature glow-discharge nitriding is able to markedly improve the corrosion resistance of austenitic stainless steels, such as AISI 316L and AISI 202 in chloride-ion rich solutions in comparison with the untreated alloys.

Ionic liquids: Electrochemical investigation on corrosion activity of ethyl-dimethyl-propylammonium bis(trifluoromethylsulfonyl)imide at high temperature

In the present work we report on our investigation on the corrosion properties of the ethyl-dimethyl-propylammonium bis(trifluoromethylsulphonyl)imide at temperatures up to 473 K. The tests were performed both for commercially pure iron alloys and for pure copper. The electrochemical measurements showed that the metals corrosion rates can be dramatically reduced by purging the ionic liquid with inert gases to remove the dissolved oxygen.