ABSTRACT
Controlling corrosion with naturally occurring corrosion scales is potentially a more environmentally sustainable alternative to current approaches, including dosing of organic corrosion inhibitors. We report operando grazing incidence X-ray diffractograms correlated with electrochemical measurements to elucidate the growth and corrosion protection properties of a corrosion scale composed of FeCO3 crystallites, which is encountered in various key energy industry applications. Data, acquired as a function of time from high-purity iron immersed in CO2-saturated deionized H2O at pH 6.8 and T = 80 °C, show that the FeCO3 scale not only prevents corrosion of the covered substrate but also acts as a significant interfacial diffusion barrier for corrosion reagents and/or products once sufficient coverage is achieved. Most notably, from a corrosion engineering perspective, however, it is determined that corrosion occurring in gaps between scale crystallites remains appreciable; this important insight is gained through the analysis of electrochemical impedance spectra to estimate the variation in electrochemically active surface area as scale coverage increases. These results indicate that naturally occurring FeCO3 scales are not a tenable solution for corrosion protection, as even in their intact state they are highly likely to be, at best, semiprotective.
ABSTRACT
316L stainless steel samples have been prepared by selective laser melting (SLM) using a pulsed laser mode and different laser powers and scanning patterns. The as-fabricated samples were found to be dominated by clusters of nano-sized γ needles or cells. TEM imaging shows that these needles contain a high population of dislocations while TEM-EDX analysis reveals high chemical homogeneity throughout the as-fabricated samples as evidenced by the fact that there is even no micro-/nano-segregation at interfaces between neighbouring γ needles. The good chemical homogeneity is attributed to the extremely high cooling rate after SLM (>106 °C/s) and the formation of Si- and Mn-oxides that distribute randomly in the current samples. The laser-processed samples show both superior strength and ductility as compared with conventionally manufactured counterparts. TEM examination on the deformed specimens reveals a significantly high density of dislocations and a great number of twinning within nano-needles, suggesting that the plastic deformation has been governed by both gliding of dislocations and twinning deformation, which is believed to be responsible for the simultaneous acquisition of superior strength and ductility. Finally, laser power shows a much more dominant role than laser scanning pattern in porosity and grain size development for the SLM-processed 316L stainless steel samples.
