Galvanized steel is common in construction and manufacturing, protected by a zinc coating applied, typically through a hot-dip process, to prevent rust and extend its lifespan. A frequent question is whether galvanized steel gets hot when exposed to direct sunlight. The answer depends on the base material’s inherent properties and the zinc coating’s condition, which govern how the metal interacts with solar radiation.
Understanding Steel’s Thermal Response
The underlying steel substrate possesses specific thermal properties that dictate its tendency to heat quickly under solar exposure. Steel has moderate thermal conductivity (15 to 50 W/m·K), meaning heat efficiently moves through the material. This conductivity ensures that once a portion of the metal is heated by the sun, the temperature is rapidly distributed throughout the entire structure.
The low specific heat capacity of steel also contributes to its quick temperature rise. Specific heat is the energy required to raise a material’s temperature by a given amount. Because relatively little solar energy is needed to increase its temperature compared to materials like water or concrete, steel warms up significantly faster than the surrounding air.
How Galvanization Affects Solar Absorption
The zinc coating applied during galvanization directly influences how much solar energy the steel absorbs. When the surface is new, it appears bright and shiny, possessing high solar reflectivity (albedo). This reflectivity reflects a portion of incoming sunlight away, helping keep the surface cooler than bare, darker steel.
The zinc layer also has low emissivity, which is its ability to radiate absorbed heat back into the environment. While the bright surface reflects incoming energy, any absorbed heat is not easily shed through radiation, counteracting the cooling effect. As the galvanized steel ages, the zinc surface oxidizes and dulls, forming a gray patina. This aged surface loses its initial high reflectivity, causing it to absorb more solar radiation and reach higher temperatures.
Real-World Factors and Safety Considerations
Despite the reflective properties of a new zinc coating, galvanized steel can still reach temperatures far exceeding the ambient air temperature in direct sun. Measurements show bare galvanized surfaces can reach up to 180°F (82°C) in full sunlight, hot enough to cause burns upon contact. The final temperature is influenced by external factors like air temperature, wind speed, and the angle of the sun’s incidence.
The integrity of the zinc coating is also affected by high heat, a consideration for items like handrails or roofing. While sunlight exposure will not reach the material’s maximum limits, continuous exposure above 392°F (200°C) can cause the outer zinc layer to peel. This happens because steel and zinc expand at different rates when heated, potentially compromising the coating’s corrosion protection. Users should exercise caution when handling galvanized steel that has been sitting in strong sunlight.