Quantitative Assessment of Localized Corrosion Occurring on Galvanized Steel Samples Using the Scanning Vibrating Electrode Technique

• Published in: Corrosion (Houston, Tex.) Vol. 60; no. 5; pp. 437 - 447
• Main Authors: Worsley, D.A., McMurray, H.N., Sullivan, J.H., Williams, I.P.
• Format: Journal Article
• Language: English
• Published: Houston, TX NACE International 01.05.2004; NACE
• Subjects: Aeration; Alloys; Aluminium; Aluminum; Aluminum base alloys; Anodes; Applied sciences; Coatings; Corrosion; Corrosion environments; Corrosion mechanisms; Deactivation; Defects; Electrodes; Exact sciences and technology; Exposure; Galvanized steel; Galvanized steels; Heavy metals; Hot dip galvanizing; Hot dipping; Household appliances; Inductively coupled plasma mass spectrometry; Iron; Localization; Localized corrosion; Mass spectrometry; Mass spectroscopy; Metal surfaces; Metals; Metals. Metallurgy; Runoff; Runoff rates; Scanning; Sodium; Sodium chloride; Steel; Substrates; Weight reduction; Zinc; Zinc base alloys; Electrodes; Localized corrosion; Metal coating; scanning vibrating electrode technique; zinc alloy coatings; Galvanized steel
• ISSN: 0010-9312; 1938-159X
• DOI: 10.5006/1.3299239

ABSTRACTZinc and zinc alloy galvanized 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 (9%) 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). TheThe production of galvanized steel is increasing rapidly due to demand from the construction, automotive, and domestic appliance manufacturing sectors industry for high-performance structural materials. The corrosion reactions occurring on galvanized steel materials initiate on the zinc surface as this layer provides sacrificial protection. The principal failure mechanisms when materials are coated organically are due to corrosion occurring at damaged areas or the exposed cut edges of the organically coated steel produced when the material is cut to shape.1 Corrosion on galvanized materials comprises anodic zinc dissolution (Equation [1]) with cathodic oxygen reduction (Equation [2]) localized on the steel if a penetrative defect or exposed cut edge are present.Zn(s ) Zn 2 + (aq ) + 2e­ O2 ( g ) + 2H2O( l ) + 4e­ 4OH­ (aq )