Cerium conversion layer for improving the corrosion resistance of phosphated NdFeB magnets

This paper investigates the corrosion behavior of a NdFeB magnet with cerium-based conversion coatings obtained under different deposition conditions, on phosphated or non-phosphated magnet. The best performance was obtained with a cerium conversion coating, obtained in a two-step process, here named Ce 1 coating. In the first step the magnets were submitted to a phosphating treatment by immersing in a 10 g L − 1 NaH 2 PO 4 solution, acidified with H 3 PO 4 to pH 3.8. After phosphating, the specimens were immersed in a 3 g L − 1 Ce(NO) 3 ; 0.3 g L − 1 H 2 O 2 ; 0.02 g L − 1 H 3 BO 3 at pH 4.0 and 25–30 °C during 120 min, rinsed thoroughly with deionized water and dried. After conversion coating, the magnets were tested for corrosion. The corrosion tests were carried out in aerated sulphate solution, using electrochemical impedance spectroscopy (EIS), cyclic voltammetry and surface analysis methods (SEM and EDS). It was demonstrated that the corrosion resistance was strongly dependent on the previous treatment and the conversion coating conditions.