Ivan Juraga

Influence of thermal oxides on pitting corrosion of stainless steel in chloride solutions

Abstract X-ray diffraction analysis, Auger electron spectroscopy, X-ray photoelectron spectrometry, optical micrography, and spot potential measurements have been used to characterise AISI 316L stainless steel specimens with oxide films formed by heating in air, either isothermally (at 200–1000°C) or in a temperature gradient. The pitting corrosion propensities of such specimens were assessed by exposure to FeCl3 solution and the evaluation of nine topographical criteria. Testing was also performed on previously heated specimens after removal of the thermal oxides by pickling. It is demonstrated that thermal oxides facilitate the nucleation of corrosion pits but do not influence their propagation. The effect on nucleation is caused by stresses and lattice defects in oxide film and metal substrate. The maximum susceptibility to pitting liability occurs after heating at 600°C. The removal of thermal oxides formed at 200–800°C by pickling improves the pitting resistance but the initial stability is never restored because of residual stresses and lattice defects on the cleaned metal surface. Indeed, after heating at 1000°C, pitting tendencies are increased by pickling.

Experimental study of imidazoline as an inhibitor of carbon steel in acid media

Abstract The electrochemical behaviour of a carbon steel in deaerated acid media with concentrations of 20 ppm, 50 ppm, 100 ppm and 200 ppm imidazoline at different temperatures and with different stirring rates has been evaluated by using anodic and cathodic polarization curves (Tafel) and electrochemical impedance spectroscopy (EIS) to compare the corrosion inhibition efficiency. The experimental results suggest that imidazoline is a good corrosion inhibitor at 50 ppm concentration.

Pitting Liability of Thermally Oxidised Austenitic Steel

Abstract Effect of thermal oxidation on the behaviour of stainless steel (SS) in an ferrichlorid solution was investigated. Sheet specimens of austenitic AISI 316L with the observed area S = 90 cm2 were immersed into the solution in the initial state, after heating in air isothermally at 200, 400, 600, 800 and 1000°C respectively or in temperature gradient, and after heating and subsequent cleaning of specimens in an nitric acid/hydrofluoride acid mixture. Spot potential profiles of specimens were taken up using a simple device with graphite counter-electrode i.e. by measuring local potential differences between specimen and graphite. The presence of thermal oxides enhances the non-uniformity of profile, which decreases after the removal of oxides by cleaning in the nitric acid/hydrofluoride acid mixture. However, the initial degree of uniformity is not re-established. Prolonged exposure to ferrichlorid solution provokes pitting corrosion of specimens heated isothermally at 400° C and above it or heated in temperature gradient. On such specimens pits number np, their depths hp and orifice areas Sp.

Influence of thermal oxides on pitting corrosion of austenitic and duplex steels

Abstract The influence of thermal oxides on the pitting corrosion of austenitic and duplex (austenitic-ferrite) steels has been studied. Austenitic steel specimens AISI 316L were subjected to heating isothermally (200 to 1000 °C) in order to simulate conditions of the application in production. Duplex steel specimens AISI S31803 were heated isothermally at (1100 to 1300 °C) in order to simulate welding parameters. The characterization of thermal oxides formed by heating isothermally on austenitic steel was performed by AES, and XPS analyses. The microstructures and thickness of thermal oxides on austenitic specimens and microstructures on duplex specimens was determined. Pitting susceptibility of such specimens was tested by exposure to FeCl3 solution at 22 ±2 °C during 72 h (ASTM G 48-99a). Pitting criteria show that oxides developed at 600 °C cause the highest susceptibility to pitting corrosion for austenitic steel and 1300 °C for duplex steel.