Maxine Ankora is currently working in cooperation with Tata Steel on a four-year PhD research project in the Corrosion Technology and Electrochemistry Group at TU Delft. The focus of this project is the determination of the fundamental mechanisms and kinetics involved in the electrodeposition of Cr(VI)-free coatings on packaging steel. She has a Master in Materials Science and Engineering from TU Delft. Her Master thesis was on the physicochemical characterization of Cr(VI)-free pretreatments for galvanized steel with respect to corrosion inhibition and adhesion. She has a Bachelor’s degree in Chemical Engineering from Lafayette College, PA, USA.
Electrodeposition of mixed chromium metal-carbide-oxide coatings: Trivalent Chromium Coating Technology (TCCT)
Packaging is one major aspect of the myriad uses of steel. Packaging steel is used in most, if not all, areas of industry: cosmetics, food, drugs, oil and gas, aerosols, amongst many others. The wide use of steel in packaging is attributed to its unique capabilities which include formability, strength and magnetic properties which make it easier to handle in production lines as well as to sort for recycling1,2 as compared to aluminium which is another widely used metal for packaging. It is also described as a permanent material in that it can be recycled several times without a significant loss of quality3. As such, it is one of the packaging materials most likely to meet the long-term objectives set forth by the European Commission’s Directive (COM/2015/0596 final - 2015/0276 (COD)) on Packaging and Packaging Waste of the Circular Economy Package4; one of which is that by 31 December 2030, 85% by weight of ferrous metal packaging waste will be prepared for reuse and recycled5. At present, recycling steel cans saves 70% of the energy required to produce new steel and at 79.5%, packaging steel has the highest recycling rate of any packaging material6.
During use, it is necessary to prevent corrosion of packaging steel to ensure that it serves its purpose as a receptacle for the intended period as well as to prevent contamination of the packaged substance by corrosion products. Conventionally, the corrosion prevention method applied to packaging steel is tin-plating (or tinning) in which the steel sheets are thinly coated with tin. It has been found however that despite the various attractive features accessed via this protection method, the FeSn2 alloy phase has the drawback of being susceptible to pitting corrosion. This property owes to the fact that the overvoltage for the hydrogen reaction is much lower on FeSn2 than on pure tin7. This cathodic reaction, in turn, provides the driving force for corrosion in the absence of oxygen8.
A solution that has been developed in recent years is to apply a thin chromium/chromium oxide (Cr/CrOx) layer to the steel instead of tin9. In the process employed at Tata Steel, the steel substrate used is a thin gauge (0.13-0.49 mm) low carbon steel. The duplex coating deposited comprises a base layer of chromium metal on top of which is a chromium oxide layer10,11. Chromium coatings have been found to not only significantly reduce the susceptibility of the substrate to pitting corrosion but also to improve the adhesion of the organic topcoat subsequently applied to the surface 10. This additional organic layer serves the role of further corrosion protection and secondarily, provides a more attractive appearance. In Tata Steel’s trivalent chromium coating technology (TCCT) product, the organic topcoat applied is polyethylene terephthalate (PET).
Traditionally, deposition of chromium on packaging steel is achieved via electrodeposition from aqueous hexavalent chromium baths. However, in addition to the long acknowledged ill effects of hexavalent chromium especially in terms of worker safety and general human health (found to be teratogenic, mutagenic and carcinogenic; the cause of 300 lung cancer deaths annually) and the environment, recently implemented regulations which stringently restrict usage of hexavalent chromium in industry have driven research into alternative processes for chromium electrodeposition5. Suitable substitutes are required to possess similar or better properties to the deposits obtained via the traditional means. One alternative experiencing revived interest is electrolytes based on trivalent chromium8–15. However, this process has been found to be sensitive to numerous parameters including temperature, the complexing agent used, component concentration, pH and line speed16. As such, in this project a fundamental understanding of speciation during the electroplating process is sought for the sake of gaining a fuller grasp of the process and in turn, ways by which to optimize said process.
L. I. Fockaert, M. V. E. Ankora, J. P. B. Van Dam, A. Yilmaz, S. Pletincx, B. Boelen, T. Hauffman, Y. Garcia-Gonzalez, H. Terryn, J. M. C. Mol. Effect of organic additives in fluoacid-based Ti and Zr-treatments for galvanized steel on the stability of a polymer coated interface (submitted)