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In situ monitoring of zinc phosphate coating formation on mild steel by acoustic emission technique

In the present work, an acoustic emission (AE) technique, along with potential electrode measurements, is used to study the zinc phosphate coating formation obtained on mild steel by cathodic electrochemical treatment (−15 mA cm−2) at ambient temperature. The AE activity recorded during the phosphate coating formation shows evident links between this activity and the coating parameters such as the potential electrode and coating weight. The principal component analysis (PCA) of AE parameters reveals the presence of three well-separated populations denoted A, B and C. These populations (A, B and C) show peak frequencies <180 kHz, between 214 and 260 kHz, and 280 and 320 kHz, respectively. The energy of the B and C populations is lower than that of A population and the rise time of B population is much longer than that recorded for the populations A and C. These events were respectively attributed to the evolution and release of dihydrogen bubbles, the formation and growth of the phosphate layer, and the stress release generated by the dihydrogen bubbles trapped in the coating pores.