Abstract
There is a well known performance gap in corrosion resistance between thermally sprayed corrosion resistant coatings and the equivalent bulk materials. Interconnected porosity has an important and well known effect, however there are additional relevant microstructural effects. Previous work has shown that a compositional difference exists between the regions of resolidified and non-melted material that exist in the as-sprayed coatings. The resolidified regions are depleted in oxide forming elements due to formation of oxides during coating deposition. Formation of galvanic cells between these different regions is believed to decrease the corrosion resistance of the coating. In order to increase understanding of the details of this effect, this work uses X-ray photoelectron spectroscopy (XPS) to study the passive films formed on thermally sprayed coatings (HVOF) and bulk Inconel 625, a commercially available corrosion resistant Ni–Cr–Mo–Nb alloy. Passive films produced by potentiodynamic scanning to 400 mV in 0.5 M sulphuric acid were compared with air-formed films. The poorer corrosion performance of the thermally sprayed coatings was attributed to Ni(OH) 2, which forms a loose, non-adherent and therefore non-protective film. The good corrosion resistance of wrought Inconel 625 is due to formation of Cr, Mo and Nb oxides.
Highlights
Inconel 625 is a solid solution strengthened nickel-based corrosion resistant alloy which is widely used as a thermally sprayed coating for corrosion protection of components in thermal and nuclear energy environments as well as other aqueous corrosive environments [1]
Previous work established that the performance gap in corrosion resistance between HVOF coatings and the corresponding bulk alloy was due to localised areas of different compositions formed by partial oxidation during coating formation [6]
The good corrosion resistance of wrought Inconel 625 is due to formation of Cr, Mo and Nb oxides
Summary
Inconel 625 is a solid solution strengthened nickel-based corrosion resistant alloy which is widely used as a thermally sprayed coating for corrosion protection of components in thermal and nuclear energy environments as well as other aqueous corrosive environments [1]. Sprayed corrosion resistant coatings are widely used to enhance corrosion resistance Such coatings do usefully improve corrosion resistance, there is a well established performance gap between the coating and the equivalent bulk material [2,3,4,5]. This is attributed to the characteristic microstructure of the thermally sprayed coating. Sprayed coatings are built up from the successive impacts of partially or fully melted particles. This results in a distinctive lamellar structure which includes some porosity and regions of resolidified material. Oxides may be incorporated into the coating since, depending on the thermal spraying technique used, the hot particles can partially oxidise as they travel between the spray gun and the substrate
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