A Novel Laterally Resolving Method for Characterizing the Defect Structure in Thermal-Sprayed Coatings

In the present work a new approach based on an electrochemical scanning cell was investigated, allowing fast, quasi-nondestructive, and quantitative measurements on thermal-sprayed, metallic coatings. A wide range of microstructures was produced by atmospheric and vacuum plasma spraying and flame spraying of nickel-based powders (Ni, NiCr, NiCrAlY) on mild steel substrates. Measurements on thermal-sprayed nickel-based coatings performed by the scanning technique yielded good agreement and predictability of the results gained by the salt spray (fog) test on the same set of samples. A lateral resolution in the range of the coating thickness turned out to be appropriate to investigate the significant defect structures of the entire range of microstructures. A quantitative characterization was possible within a few tens of minutes, allowing for the fast comparison of different systems, regardless of their surface topography and geometry. A comparison of the electrochemical measurement to the 700-h salt spray test showed an agreement of the two techniques. The investigation showed the influence of the spraying technique and the composition of the powder. A small scanning area of 18 mm by 18 mm seems to be statistically representative for thermally sprayed nickel coatings in terms of predictability of performance during salt spray testing. In the salt spray tests high-density vacuum plasma-sprayed NiCrAlY coatings with lower anodic current peaks were superior to coatings with high local current densities such as flame-sprayed Ni.