Effect of fluoride on corrosion behavior of UNS N08904 stainless steel in polluted phosphoric acid

The influence of fluoride ions on the corrosion behavior of UNS N08904 stainless steel in polluted phosphoric acid solution was investigated using electrochemical measurements and UV-Vis-NIR spectroscopy. The potentiodynamic polarization curves showed that fluoride accelerates the anodic process by increasing the critical and passive current densities, while the transpassive potential modified towards more active values. The EIS exhibited that the interface solution/metal has a passive film character influenced by the fluoride ions addition. In addition, the UV-Vis-NIR spectroscopy of the surface showed that the fluoride ion increases the iron (III) phosphate dissolution, and the metal oxides into the solution. In contrast, the apparitions of large peaks of the maghemite were a proof that the fluoride ion favors the iron oxidation to Fe3+. It is noted also that the fluoride peaks were identified at 2530 nm, 2556 nm, 2613 nm, which can be interpreted as adsorbed fluoride or a metal-F complex formation. On the other hand, the UV-Vis-NIR spectroscopy of the test solutions showed that the nickel was affected but less than iron and chromium. Those results explain the drastic increase of the local corrosion as shown by Scanning Electron Microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX).