Corrosion protection of AISI 316 stainless steel by ALD alumina/titania nanometric coatings

Stainless steels are used today in a wide range of applications as a result of their combination of high corrosion resistance and good mechanical properties. In some applications, for example, temporary contact biomedical devices or solar water heaters, corrosion resistance may need further improvement, and surface coatings may be applied for enhanced protection. In this study, AISI 316 stainless steel samples with two different standard industrial finishes were coated using atomic layer deposition (ALD) of Al₂O₃/TiO₂ layers. The morphology, composition and corrosion protection was then investigated using different techniques. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to obtain a morphological characterization of coatings and substrates. Glow discharge optical emission spectrometry (GDOES) was used to obtain an in-depth profile of composition. Polarization curves in a 0.2 M NaCl solution were used to evaluate the corrosion protection given by the coatings. The deposited ALD layers were found to be almost flawless. The measured RMS roughness values were compared before and after the ALD, and were around 50 and 370 nm for the two samples. GDOES profiles were strongly influenced by the roughness of the substrate. The corrosion protection obtained on AISI 316 stainless steel by the application of nanometric coatings proved to be very effective in reducing the passive region current density from 10⁻⁷ to less than 10⁻⁹ A/cm² and increasing the passive region potential interval from 0.8 to 1.3 V before breakdown.