Dissolution Kinetics of Iron-Based Intermetallic Compounds (τ5c IMCs) in a Commercial Steel Strip Metallic Alloy Coating Process
© 2020, The Minerals, Metals & Materials Society and ASM International. The precipitation of discrete iron-based intermetallic compounds (τ5c IMCs) is an inevitable phenomenon in a steel strip coating pot due to the dissolution of iron from the steel strip surface and temperature perturbations in the pot. These τ5c IMC particles are a primary source of coating inclusions and bottom dross buildup in the coating pot. It has been hypothesized that they will re-dissolve into the pot during the heating cycle in the coating bath, but their dissolution kinetics remains largely unexplored. In this study, three forms of τ5c IMCs were used to investigate their dissolution kinetics: (i) τ5c-IMC disk samples (diameter: 14.88 mm) fabricated using a special approach; (ii) large τ5c IMCs (size: 1 to 10 mm) in the form of industrial mushy bottom dross (MBD); and (iii) fine suspended in-melt τ5c IMCs (size: < 50 μm). The driving force for dissolution is dictated by both the pre-existing iron content in solution in the AM alloy melt and the melt temperature. Experiments suggest that the dissolution kinetics of the τ5c IMCs in the forms of both τ5c-IMC disk and MBD is extremely slow (negligible) in the coating alloy at 595 °C. A mass transfer coefficient (km) of 1.43 × 10−6 m/s is calculated in an iron-unsaturated AM alloy bath at 595 °C. Re-dissolution of fine suspended in-melt τ5c IMC particles (< 50 μm) occurs at the stirring rate of 220 mm/s and their dissolution rate is much faster than that of either the τ5c-IMC disk samples or the mushy bottom dross τ5c IMC particles. However, overall, their dissolution kinetics is still very slow so that their contribution as a source of Fe is practically negligible, considering that the heating time interval during a temperature perturbation cycle in an industrial coating pot is only about 7.5 minutes. This clarifies a long-standing unanswered important issue to the steel strip alloy coating process. Consequently, coating bath temperature control is important in minimizing the precipitation and growth of τ5c IMC particles in an industrial steel strip coating process, in addition to managing the source of iron from the highly active surface-deoxidized moving steel strip.