Understanding the Passive Behaviour of Low-Chromium High-Strength Hybrid Steel in Corrosive Environments

Abstract

Abstract We examined the unusual passive character of Hybrid steel in de-aerated sulfuric acid and aqueous sodium chloride solutions by benchmark corrosion and surface analytical tests in combination with CalPhaD-based thermochemical modelling. The electrochemical potentiodynamic polarization measurements have shown that Hybrid steel possesses all characteristic features of what makes steel stainless, such as passivity, breakdown and pitting, similar to standard low-alloyed stainless steel. Synchrotron hard X-ray photoelectron spectroscopy (HAXPES) analysis revealed that the stainless nature of Hybrid steel is achieved by a dynamically protective nanometre-sized passive film consisting of Fe, Cr, Ni, and Al oxides. The thermodynamic calculations showed that the surface oxide composition is Fe2O3•FeCr2O4•NiO•Al2O3, which changes structure, fraction and existence over electrochemical polarization. It has become understood that the presence of Al and Ni supports Cr in forming a spontaneously passive and hence protective surface, yielding exceptional corrosion resistance in acidic and chloride-containing aqueous solutions. The surface oxide could withstand breakdown and remain passive/repassivated even after transpassing the Cr(III)-to-Cr(VI) redox potential. While Cr is the prime passivating agent, an adverse effect of Cr was seen on grade 304 and 420 stainless steel when Cr(VI) species were released, savaging the passive film due to extensive interfacial pH reduction. However, among all tested stainless steels, Hybrid steel could repassivate due to the remaining and enriching Al and Ni oxides providing superior anodic passivation. Our work demonstrates that delicate alloying and microstructure engineering can design sustainable stainless steel with optimum high-strength properties without needing the well-known Cr threshold concentration of 10.5 per cent.