Effects of phosphate species on localised corrosion of steel in NaHCO 3 + NaCl electrolytes

• Published in: Electrochimica acta Vol. 54; no. 18; pp. 4389 - 4396
• Main Authors: Reffass, M., Sabot, R., Jeannin, M., Berziou, C., Refait, Ph
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.07.2009
• Subjects: Electrochemical microprobe; Micro-Raman spectroscopy; Phosphate; Pitting corrosion; Steel; Pitting corrosion; Electrochemical microprobe; Steel; Phosphate; Micro-Raman spectroscopy
• ISSN: 0013-4686; 1873-3859
• DOI: 10.1016/j.electacta.2009.03.014

Pitting corrosion of carbon steel electrodes in 0.1 M NaHCO 3 + 0.02 M NaCl solutions was induced by anodic polarisation. The evolution of the breakdown potential E b with the phosphate concentration was investigated by linear voltammetry. E b increased from −15 ± 5 mV/SCE for [HPO 4 2−] = 0 to 180 ± 40 mV/SCE for [HPO 4 2−] = 0.02 mol L −1. During anodic polarisation ( E = 50 mV/SCE), the behaviour of the whole electrode surface, followed by chronoamperometry, was compared to the behaviour of one single pit, followed via the scanning vibrating electrode technique (SVET). The addition of a Na 2HPO 4 solution after the beginning of the polarisation did not lead to the repassivation of pre-existing well-grown pits. The corrosion products forming in the pits were identified in situ by micro-Raman spectroscopy. They depended on the phosphate concentration. For [HPO 4 2−] = 0.004 mol L −1, siderite FeCO 3 was detected first. It was oxidised later into carbonated green rust GR(CO 3 2−) by dissolved O 2. The beginning of the process is therefore similar to that observed in the absence of phosphate. Finally, GR(CO 3 2−) was oxidised into ferrihydrite, the most poorly ordered form of Fe(III) oxides and oxyhydroxides. Phosphate species, adsorbing on the nuclei of FeOOH, inhibited their growth and crystallisation. For [HPO 4 2−] = 0.02 mol L −1, siderite was accompanied by an amorphous precursor of vivianite, Fe 2(PO 4) 3·8H 2O. This shows that, in any case, phosphate species interact strongly with the iron species produced by the dissolution of steel.