AES Analysis of Surface Films Formed on Mild Steel by Inhibitors Used in Cooling Water Treatment

Auger electron spectroscopy (AES) analysis was performed on the surface films formed on mild steel during indoor simulation of open recirculating cooling water treatment process steps, namely, I) initial treatment, II) intermediate treatment, and III) maintenance treatment. It was found that the basic structure of the films was formed by polyphosphate/zinc-based initial treatment (step I), and the films comprised four layers, P-rich, Ca-rich, Zn-rich and Fe-oxide from the solution side. The energy shift of the Auger spectra suggest that the main component of the P-rich layer was polyphosphate. The basic structure of the films was not changed with phosphonate-based treatment through steps I to III. It is noteworthy that Fe species were absent from the P-rich layer through all steps of phosphonate-based treatment. It is therefore assumed that the outer P-rich layer replaced polyphosphate with phosphonate from step II to step III. In the case of polymer-based (non-phosphorous) treatment, Fe ions appeared on the surface of the films along with a decrease in the P-rich layer at step II, and the outer layer changed from P-rich to Si-rich at step III. It is assumed that silicates in solution reacted with Fe ions on the surface of the films to form insoluble iron silicates.