Effects of Surface Modification by Means of Low-Temperature Plasma Nitriding on Wetting and Corrosion Behavior of Austenitic Stainless Steel

Abstract

Low-temperature nitriding of austenitic stainless steels produces modified surface layers, consisting mainly of the S phase, which improve surface hardness and corrosion resistance. Because of the localized plastic deformations, owing to modified layer formation, and ion bombardment occurring during the process itself, this treatment produces also modifications of surface morphology and roughness, which can affect wettability and corrosion behavior. In this study the effects of plasma nitriding, performed using different treatment conditions, on the surface morphology and roughness, and thus on wettability and corrosion resistance, of AISI 202 specimens with different initial finishings (2D and polished finishing) were investigated. Different probe liquids, having both high (bi-distilled water and solution of 3.5% NaCl) and low (ethanol and rapeseed oil) surface tension, were employed for assessing the wetting behavior with the sessile drop method. The contact angle values for water increased markedly when nitriding was performed on polished samples, while this increase was smaller for 2D samples, and on selected specimens a hydrophobic behavior was observed. Very low contact angle values were registered using low surface tension liquids, suggesting an oleophilic behavior. Corrosion resistance in a 5% NaCl solution was assessed, and it depended on the characteristics of the nitrided specimens.

From Austenitic Stainless Steel to Expanded Austenite-S Phase: Formation, Characteristics and Properties of an Elusive Metastable Phase

“…The highest CA values were registered for samples subjected to a low pressure process, similar to cathodic sputtering performed with a N-containing atmosphere, and CA as high as 110 • was measured on samples with a 2D finishing. In spite of the low hydrophilicity, and even hydrophobicity, shown by all the nitrided samples types, when liquids having a low surface tension, as ethanol and rapeseed oil, were used as probe, very low CA values were registered, suggesting an oleophilic behavior [127].…”

“…Martinesi et al [223] reported an increase of CA from 72.8 • to 82 • after plasma nitriding at 400 • C and 500 Pa, but a decrease to 71 • , when the nitrided samples were subjected to a sterilization procedure with ethanol and incubation with 2.5 mM/mL acetic acid for 24 h, while no significant changes were observed for untreated specimens. An increase of apparent CA values for AISI 202 samples, having a polished or a 2D finishing, was observed after nitriding with different conditions, when 0.5 and 3 µL drops of bi-distilled water or a solution of 3.5% NaCl were used [127]. The highest CA values were registered for samples subjected to a low pressure process, similar to cathodic sputtering performed with a N-containing atmosphere, and CA as high as 110 • was measured on samples with a 2D finishing.…”

“…It is interesting to note that the features caused by plastic deformations are well observable on samples which have been polished before nitriding, while they are not observable on rougher surfaces [127], which are probably able to better accommodate plastic deformations.…”

“…Since low temperature treatments are able to modify the surface morphology and roughness of samples, a change of wetting behavior is expected. Few studies have evaluated the wetting properties of low temperature treated austenitic stainless steels [78,79,117,127,128,177,223,259], and they have mainly regarded nitriding treatments and water wetting properties. Lin et al [128] tested the water wettability of AISI 303 samples subjected to plasma nitriding at 450 • C, which produced modified layers consisting of S phase.…”

Impact of the Samples’ Surface State on the Glow Discharge Stability in the Metals’ Treatment and Welding Processes

“…Known works consider the possibility of application of gas discharges technique in the field of thin films deposition, metals surfacing, treatment and modification of metals before welding [2][3][4][5]. Still, as practice has shown, diffusion welding [6] and thermal and chemical-thermal surface treatment [7] are the most appropriate. This is due to the high technological capabilities of the glow discharge, which in these processes can serve both a processing tool and a source of thermal energy for their implementation simultaneously.…”