Hot-Dip Galvanized Steel in Fire: Linking NIR/MIR Reflection to Early-Stage Heating Delay Under Standard Fire Exposure

Hot-dip galvanizing (HDG) protects steel against corrosion, but its influence on heating during fire exposure remains uncertain. This study combines reference-normalized FTIR measurements for five surface conditions with indicative furnace tests for four conditions on HEA 200 steel elements under the EN 1363-1 heating regime. The time to reach 500 °C increased from 570 s for bare steel to 634 s for top-coated HDG, 727 s for polished steel and 773 s for intact HDG. These trends are consistent with reduced radiative absorption by reflective surfaces, while convection continues to supply heat. A section-factor-matched lumped model describes this behaviour through an energy balance combining convective heat transfer and Stefan–Boltzmann radiation. Sensitivity analysis shows that assuming constant low emissivity overpredicts heating delay relative to scenarios in which emissivity increases during exposure. Despite similar initial reflected signals, intact HDG and polished steel exhibited different heating. Thus, from a fire-design perspective, the variability of HDG surface conditions and the resulting uncertainty in its radiative properties may make it difficult to treat galvanized steel as having sufficiently repeatable physical for use in fire safety engineering design or for classification as a certified passive fire-protection system.