Degradation mechanism of galvanized steel in wet–dry cyclic environment containing chloride ions

• Published in: Corrosion science 2004-02, Vol.46 (2), p.361-376
• Main Authors: Yadav, Amar Prasad, Nishikata, Atsushi, Tsuru, Tooru
• Format: Article
• Language: English
• Subjects: Applied sciences; B. EIS; B. Raman spectroscopy; B. SEM; C. Atmospheric corrosion; Corrosion; Corrosion mechanisms; Exact sciences and technology; Galvanized steel; Metals. Metallurgy
• ISSN: 0010-938X; 1879-0496
• DOI: 10.1016/S0010-938X(03)00153-7

The wet–dry cyclic test of a galvanized steel (GI) and pure zinc (ZN), which simulates marine atmospheric environment, has been conducted to clarify the degradation mechanism of galvanized steel. The samples were exposed to alternate conditions of 1 h-immersion in a 0.05 M NaCl solution and 7 h-drying at 25 °C and 60%RH, and the corrosion was monitored for 10 days (30 cycles) using a two-electrode type probe. Simultaneously, the corrosion potential was measured every three cycles only during the immersed conditions. The reciprocal of polarization resistance R p −1 was taken as an index of the corrosion rate. Several sample plates of GI and ZN were exposed, together with the monitoring probes. They were removed from the test chamber at the end of 1st, 3rd, 9th, 18th, and 30th cycles of exposure and were analyzed for the corrosion products with XRD and laser Raman spectroscopy. Further, their cross sections were analyzed with FESEM–EDS. The FESEM photographs and elemental analysis of cross sections confirmed that the R p −1 value commences to decrease when the corrosion front reaches Zn–Fe alloy layers (boundary layers of zinc coating and steel substrate) due to localized nature of attack. A schematic model of degradation mechanism and the role of galvanic protection have been discussed.