A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription. AbstractIt is commonly observed that there is a performance gap between the corrosion resistance of thermally sprayed coatings and the equivalent bulk material. This is attributed to the significantly modified microstructure of the sprayed coatings. However, currently there is no detailed understanding of which aspects of microstructural modification are primarily responsible for this performance gap. In this work several deliberately microstructurally modified versions of the Ni-based superalloy Inconel 625 were produced. These were subjected to potentiodynamic electrochemical testing in 0.5M H 2 SO 4 to investigate the links between specific microstructural features and electrochemical behaviour. Samples were prepared by high velocity oxy-fuel (HVOF) thermal spraying, laser surface remelting using a high power diode laser and conventional powder sintering. Microstructural features were examined by optical and scanning electron microscopy and X-ray diffraction.Potentiodynamic testing was carried out on the following forms of Inconel 625: wrought sheet; HVOF sprayed coatings; sintered powder compacts; laser melted wrought sheet and HVOF sprayed coatings. Using the corrosion behaviour, i.e. passive current density, of the wrought sheet as a baseline, the performance of different forms of Inconel 625 were compared. It is found that a fine dendritic structure (with associated microsegregation) produced by laser remelting wrought sheet has no significant effect on corrosion performance.Up to 12% porosity in sintered powder samples increases the passive current density by a factor of only around 2. As observed previously, the passive current density of HVOF sprayed coatings is 20 -40 times greater. However, HVOF coatings subjected to laser surface remelting are found to have a passive current density close to that of wrought material. It is concluded that, whilst porosity in coatings produces some decrease in corrosion resistance, the main contributing factor is the galvanic corrosion of localised Cr-depleted regions which are associated with oxide inclusions within HVOF sprayed samples.

No 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 400mV in 0.5M 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.

High velocity oxyfuel (HVOF) spraying process is commonly used to produce superalloy coatings. The coating of engineering components is required to increase the service life and to obtain a combination of properties that are not possible from wrought alloys. The major shortcoming of the HVOF spraying process is that it produces heterogeneities in the coating microstructure. These heterogeneities may cause initiation and progression of the corrosion, causing the malfunction of engineering components. In this study, corrosion testing of the HVOF-sprayed Inconel 625 coating was carried out by direct current polarization and alternating current impedance spectroscopy to see how microstructural heterogeneities contribute in changing electrochemical response. Furthermore, the air-formed and potentiostatically grown passive oxide layers were analyzed by X-ray photoelectron spectroscopy (XPS) to study oxide growth as a function of applied potential. The bulk Inconel 625 was used as a reference alloy to compare the experimental results of HVOF coating. It is established from our results that the oxide layer formed on bulk alloy is uniform as compared with the one formed at HVOF-sprayed coating samples. The passive current density for the bulk alloy sample was 3.61±0.51 and 34.26± 8.34 μA cm-2 for the coated sample. The charge transfer resistance for bulk sample was 1.80×104 and 1.70×103 Ω cm-2 for the coated sample after passivation. The analysis of the passive layer by XPS also revealed that the oxide layer on bulk alloy is more consistent and protective than coating.