Study of corrosion resistance and surface properties of carbon steel after a duplex plasma treatment

The aim of this work was the improvement of the corrosion resistance, of the hardness and of the tribological properties of a commercial steel by a cold plasma treatment consisting of a nitriding treatment (N-2/H-2 useful 25% N-2/75% H-2) followed by carbon treatment (CH4/H-2 useful 5% CH4, 95% H-2) in an inductively coupled plasma reactor with a radiofrequency generator (40 MHz). This type of treatment was called duplex treatment. Optical emission spectroscopy (OES) was used to characterize both N-2/H-2 and CH4/H-2 discharges. The modifications of the corrosion resistance characteristics of the steel due to the treatment were investigated by electrochemical tests. Surface analytical techniques (dynamic secondary ion mass spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy and scanning electronic microscopy) were also used to study the modifications of the surface composition and of the superficial layer. It was shown that a nitriding treatment improves the hardness and also the corrosion resistance, yet to a lesser extent than the duplex treatment. A protection rate (7) as high as 99.6%, relatively to the non-treated sample, together with an increase of the hardness from HV1g = 280 to HV1g = 950 for the duplex steel were obtained. The morphological and chemical analysis showed the formation of a uniform structure on the surface with an enrichment in carbon and nitrogen, and the presence of oxygen. (C) 2001 Elsevier Science B.V. All rights reserved.