Quantitative assessment of localized corrosion occurring on galvanized steel samples using the scanning vibrating electrode technique

Zinc and zinc alloy aalvanized steel is used increasingly for structural cladding, automotive, and domestic appliance applications. In assessing the different galvanizing coatings, it is important to understand the nature of corrosion reactions occurring on the metal surfaces. To this end, the scanning vibrating electrode technique (SVET) has been used to study the effect of variation in metallic coating on the localization and intensity of corrosion reactions occurring on the bare metal surfaces when immersed in aerated 0.1% sodium chloride (NaCl). The samples used comprised pure zinc and galvanized steel substrates, namely electro-zinc (EZ), hot dip galvanized steel (HDG), iron (9016) zinc intermetallic (IZ), 5% aluminum zinc alloy (Galfan(+)), and 55% aluminum zinc alloy Zalutite(+)). The SVET has the resolution and sensitivity to enable the number and intensity of active anodes to be quantified. Zinc galvanized materials show anodes, which do not deactivate within the 24 h of the test whereas zinc aluminum alloy anodes display typical anode lifetimes of 6 h to 12 h. The SVET data has been calibrated and integrated to provide a total current per scan and subsequently converted to zinc loss using Faraday's law. The total average mass losses obtained from 10-mm by 10-mm exposed areas were measured using the SVET: 1.133, 0.601, 0.432, 0.615, 0.264, and 0.051 mg for zinc, EZ, HDG, IZ, Galfan, and Zalutite, respectively, and these values were confirmed using inductively coupled plasma mass spectrometry (ICP-MS). The SVET data for zinc loss obtained over 24 h has been compared to external weathering data obtained after 2, 6, and 12 months of external exposure. There is an excellent correlation between metal runoff in initial external exposure and 24-h SVET experiments. In longer-term exposure, however, the IZ coating becomes covered in a metal hydr(oxide) layer, reducing runoff, and. penetrative defects to the iron substrate in EZ lead to elevated runoff rates within 12 months.