Controlled stripping of aluminide coatings on nickel superalloys through electrolytic techniques

Abstract

Aluminide diffusion coatings are widely employed to improve the oxidation and/or the corrosion resistance of highly added value turbine components operating in harsh environments at high temperatures. Refurbishment of such components requires appropriate removal of worn coatings and of the corrosion products layer—usually an oxide scale. Stripping is mostly carried out using hazardous chemical baths of limited reliability. In this work, an alternative stripping method based on electrochemical techniques has been carried out at laboratory scale for CVD Al diffusion coatings on a directionally solidified Ni base superalloy. Both the galvanostatic and the potentiostatic modes have been investigated. Prior to them, in situ gas bubbling induced by cathodic polarization seems to be an effective way to remove the superficial oxide scales. Measuring the open circuit potential during the experiments allows easy monitoring of the progress of the selective dissolution of the different layers. Complete removal of the aluminide coatings is indicated by potential values similar to those of the substrate. The correlation between the electrochemical features and the surface state after stripping has been carried out by scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and X-ray diffraction (XRD). The electrochemical approach is a promising means to strip out surfaces in a selective and reliable manner.