Continuous Assessment of the Electrochemical Behavior of Thermal-Sprayed Stainless Steel Coatings

In the present work, ferritic stainless steel (grade 444) coatings were produced by HVOF and HVAF coatings in a broad range of in-flight particle temperatures and velocities measured with DPV-evolution and AccuraSpray diagnostic tools. Their electrochemical response was continuously assessed in NaCl 35g /L water over a period of 3 weeks. Potentiodynamic polarization and impedance spectroscopy techniques were used to determine parameters such as corrosion potential, Tafel slopes, charge transfer resistance, capacitance, etc. All changes were correlated with microstructural features observed in optical microscope prior to each test. A bulk stainless steel of same composition was used as a reference. The results show that most of the electrode properties change within the first week of immersion including a drop in the corrosion potential, a sharp drop in the cathodic tafel slope, an increase in the capacitance and a decrease in the time-constant dispersion. Moreover, the results suggest that irrespective of spray parameters, all coatings generate a “poorly-protective” film whose properties were assessed by cyclic polarization test. The best films were developed in coatings produced with high velocity and low in-flight temperature.