Corrosion protection of sintered metal parts by coating deposition. Part II: corrosion and electrochemical characterization

Coated sintered steel parts were subjected to corrosion resistance tests and electrochemical characterization. A wide range of industrially produced coatings, covering all the main proposed treatments, were analyzed. Conventional corrosion tests were employed to compare corrosion resistance, whereas electrochemical test methods were used with the aim of giving a better understanding of the mechanisms which control corrosion processes and the corrosion rate. In particular, electrochemical impedance spectroscopy showed to be very helpful in allowing the monitoring of corrosion behaviour as a function of testing time, without introducing significant electrical perturbation on the examined system. The experimental results of this study showed that sintered steel protection by coating is possible if the process is very well controlled, however, defects in the coating can always be present due to the substrate porosity. With this fact in mind the use of sacrificial coatings is recommendable due to the possibility of a cathodic protection corresponding to coating defects.