Corrosion resistance of nitrided layers on austenitic steel

The paper presents the results of investigations on the influence of gas nitriding conditions (atmosphere composition and temperature) on the corrosion resistance of the layers produced on austenitic stainless steel. The treatment was made in gas atmosphere in the temperature range of 688-748 K. The microstructure and phase composition of the layers were investigated using scanning and light microscopy and X-ray diffraction. The elements' composition was evaluated by electron probe microanalysis. The corrosion resistance was determined on the basis of the anodic polarisation corrosion tests in 3% NaCl solution. The results revealed that corrosion resistance for layers containing expended austenite is better even than that of austenite. However, the presence of surface defects enables austenitic matrix to come into contact with aggressive environment and corrosion proceeds beneath the layer. The presence of nitrides in the nitrided layers reduces their protective properties. For layers mainly composed of nitrides the corrosion proceeds in the form of gradual decomposition of the layers. (c) 2005 Elsevier B.V. All rights reserved.