Contraction in the context of tube and pipe galvanizing refers specifically to the volume reduction that occurs as the galvanized steel article cools after being withdrawn from the molten zinc bath.
Causes and Consequences
This phenomenon is governed by the basic principles of thermodynamics and material science:
Thermal Contraction: The steel pipe is heated significantly by the 450 degree celsius of molten zinc. Once removed, both the base steel and the thick zinc coating begin cooling to ambient temperature. All materials decrease in volume as they cool, but zinc and steel contract at different rates due to their dissimilar coefficients of thermal expansion.
Internal Stress: Because the zinc coating is metallurgically bonded to the steel substrate, the differential contraction rates induce internal residual stresses within the galvanized layer.
Result: This stress can sometimes manifest as minor deformations or warping in thin-walled or asymmetrical structures, or, rarely, lead to flaking or cracking of the zinc coating if the adhesion or steel quality is poor.