INNOVATIONS: PRESENT&FUTURE
Chemical Products for Surface Treatment of Hot Dip Galvanised Metal – Past, Present and Future Marco Zavattoni direzione@condoroil.it
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his article summarises the development of chemical According to the World products utilised on hot dip Steel Association, the annual galvanised (HDG) coils, highlighting the production of coils with progressive elimination of hexavalent metallic coating has increased chromium from its formulation. by over 150 million tonnes with According to the World Steel Association, most of this material is HDG.” the annual production of coils with the environment. Its use, storage, handling metallic coating has increased by over and disposal will be, in the best of cases, 150 million tonnes with most of this increasingly more complex and costly, and material is HDG. in the worst of cases, prohibited. Hot dip galvanised surfaces cannot For this reason, companies have started to be utilised immediately, but must first be coated with a chemical product. Depending on the end use of the coils, three families of treatments are primarily used: passivation; pretreatment prior to painting; and lastly, the application of a thin organic layer, otherwise known as Permanent Coating and AntiFinger. Traditionally speaking, chemical products used for the three treatments indicated have a common element: the presence of hexavalent 1 chromium in their formulation. As we can see in Figure Figure 1: Labelling of chromic acid. 1, products containing hexavalent chromium carry a safety label with all pictograms indicating search for alternative formulas that do not safety hazards, except radioactivity! include hexavalent chromium. Hexavalent chromium is extremely When we begin to analyse the most hazardous to both worker health and to commonly used products, i.e., passivating
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N. 47 - SEPTEMBER/OCTOBER 2017 - international PAINT&COATING magazine
agents, we can see how the progressive elimination of hexavalent chromium started at the beginning of this millennium with the introduction of formulations based on trivalent chromium formulations and later on chromium-free formulations. Due to the lack of specific corrosionresistance properties of Chromium VI, the new formulations developed are more complex, and today’s products can guarantee a passivation performance not too different from that typical of products containing Chromium VI. At present, we can expect a corrosion-resistance of over 100 h in a neutral salt spray chamber and this meets the most stringent standards for use. Furthermore, comparing this result with the 120-144 hours of resistance obtainable with formulations based on Chromium 6, we do not note much of a difference. Major improvements were achieved with the development of completely chromium-free formulations. As it stands now, performance is slightly lower when compared with that guaranteed by a chromium product; for instance the expected neutral salt spray resistance is in the range of 50 © Condoroil Chemicals
Condoroil Chemicals, Casale Litta (VA), Italy