Characterization of Films Grown on 316L Stainless Steel by Pulsed Polarization in Sulfuric Acid

Oxide films have been grown on 316L stainless steel in 5 M sulphuric acid at 60°C by square wave potential pulse polarisation. These types of films are of interest for corrosion protection and for functionalisation of stainless steel surfaces. The polarisation conditions provide increased thickness and chromium content compared with the air-formed passive film. Two different types of film have been formed: anodic and cathodic. Anodic growth involves switching between a potential of -400 mV in the active region and a potential of +700 mV in the passive region, whilst cathodic growth involves switching between +1100 mV in the transpassive region and +600 mV in the passive region. The films are characterised using glow discharge optical emission spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Rutherford backscattering spectroscopy (RBS) and nuclear reaction analysis.

The anodic films are approximately 200 nm thick. Their main constituent is chromium, with lesser amounts of iron, manganese, molybdenum and nickel. Chromium and iron are primarily present in the +3 oxidation state. Molybdenum is enriched in the grown films relative to the air-formed film on mechanically-polished stainless steel, and XPS indicates that this is in the +6 oxidation state. In addition to the presence of oxide, the films contain significant amounts of hydroxides, sulphates and bound water, with an atomic ratio of oxygen to metal species of about 4. Sulphur species are present throughout the film, with a maximum atomic ratio of sulphur to metal species of 0.3 as indicated by RBS. The cation density obtained by RBS is low, which is consistent with the porosity of the films observed by SEM.

The cathodic films are thinner than the anodic films by roughly an order of magnitude. The main constituents of the film are the same as for the anodic films, but with a greater amount of oxide and a lower atomic ratio of sulphur to metal species.

The morphology and composition of the films and their potential for corrosion and protection are discussed in relation to the polarisation conditions and mechanism of film formation.