Structural Evolution Mechanisms of Anticorrosion Coatings on Steel Using Raman Mapping Microscopy

Micron-scale coatings are widely used to protect steel plates from oxidation during hot stamping, which is a particularly important manufacturing protocol for the automotive industry. These coatings effectively protect the steel, but they also melt at the necessary temperatures for steel processing. The high-temperature liquids can then transfer to furnace components and damage the manufacturing line, incurring high costs through maintenance and production line downtime. Once melted, however, these coatings are also known as leech Fe from the steel to form intermetallic phases. This leeching leads to a change in composition, inducing a complex multi-step chain of reactions that ultimately yield a solid coating. The identity of these intermetallic phases and the series of phase evolutions have been studied extensively by electron microscopy and energy dispersive X-ray spectroscopy. We introduce Raman microscopic mapping as a simple, rapid, and complementary technique for phase transition analysis. Samples were prepared by heating a series of 22MnB5 steel coupons to set temperatures between 570 to 900°C to simulate the industrial hot stamping conditions. Raman microscopic mapping of cross-sections of each sample reveals the phase transitions with increasing temperatures. This analysis reveals a structural evolution mechanism slightly different than that suggested by electron microscopy. New insights gained through Raman microscopic mapping show that it is a powerful tool to identify the underlying chemical problems that lead to the destruction of industrial infrastructure, and to test the viability of strategies to overcome these problems.