On the Mechanism for Improved Passivation by Additions of Tungsten to Austenitic Stainless Steels

In order to elucidate the role of W in improving the passivity of austenitic stainless steels, anodic polarization curves and X-ray photoelectron spectra have been obtained for pure W and W-containing steels passivated in 1 N HCl, 0.1 M NaCl, and in Cl−-containing H3PO4. Similar experiments were also performed on Cr-Ni steels passivated in WO42− solutions. The results showed that pure tungsten by itself does not develop a protective layer and that alloying tungsten inhibits pitting corrosion, as do tungstate ions dissolved in neutral chloride solutions. In the passive layer, tungstate ions were found to be adsorbed without reduction, and in the case of alloying tungsten, this element is present under the + VI oxidation state. Alloying tungsten is thought to pass directly from the metal into the passive film, by interaction with water and formation of insoluble WO3 which enhances the stability of oxide layers.