Effect of Selective Oxidation and Interfacial Reactions on the Reactive Wetting of Advanced Steels by the Continuous Galvanizing Bath

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The use of advanced high strength steels in automotive structures is an enabling technology for vehicle weight and emissions reductions, including large-scale vehicle electrification, while maintaining passenger safety. In the case of advanced steels, this weight reduction is achieved using thinner component cross sections, which necessitates that they be protected from corrosion damage in order to maintain their properties and satisfy consumer longevity demands. The continuous galvanizing process, in which steels are coated with a thin sacrificial Zn coating, is the most cost-effective process for achieving this aim. However, the alloying elements (e.g. manganese, silicon and chromium) used to produce the high strengths and other desirable properties of advanced steels also selectively oxidize in the continuous galvanizing process, creating surfaces to which the zinc cannot adhere via a process known as reactive wetting. Fortunately, it has been found by producers and other researchers that proper manipulation of selected galvanizing process variables and alloy chemistry can produce surfaces for which reactive wetting is promoted, resulting in high quality galvanized coatings. However, at this point in time no systematic study has been undertaken to link the factors of alloy chemistry, continuous galvanizing process variables and the process atmosphere composition to the reactive wetting process. This proposal will address two important topics linked to developing this understanding within the context of the continuous galvanizing process: (i) developing kinetic models for the aluminothermic reduction of manganese oxides and (ii) develop quantitative relationships between alloy Si/Mn ratio, annealing temperature and time, process atmosphere composition and reactive wetting. A complete understanding of these two processes will enable producers to manipulate their process variables and alloy designs in a systematic fashion to enable present and future compositions of advanced steels to be successfully galvanized, thereby promoting future reductions in vehicle emissions through increased vehicle light-weighting.