Electrochemical corrosion testing and characterization of potential assisted passive layer on HVOF Inconel 625 coating

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 A cm(-2) for the coated sample. The charge transfer resistance for bulk sample was 1.80 x 10(4) and 1.70 x 10(3) O 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.