Unlike paints (which merely cover the surface), hot-dip galvanising modifies the outer molecular structure of the steel. Immersed in liquid zinc, the iron reacts to create extremely hard alloy layers and a pure surface deposit.
The success of the treatment depends on preparation. The fabricated steel element is degreased, pickled in hydrochloric acid to remove every trace of oxide, and fluxed. Finally, immersion takes place in the molten zinc bath at about 450°C. The extreme heat ensures that the zinc penetrates every cavity and internal box section, providing total protection unachievable with spray painting.
The outer zinc layer acts as a first physical barrier against oxygen. However, the real advantage is electrochemical: zinc is a more anodic (less noble) metal than iron. In the event of deep scratches, impacts or cuts that expose bare steel, the surrounding zinc sacrifices itself, corroding in place of the iron and forming protective salts that seal the wound, preventing rust from spreading beneath the coating.
For extreme durability (over 80 years in C4 atmospheres) the duplex system is used: hot-dip galvanising followed by powder coating after preparation (sweep-blasting). It must also be considered that, being a hot bath, the steel undergoes thermal shock: asymmetric designs or markedly dissimilar thicknesses welded together can cause warping and structural distortion during immersion.
Standards
European and international references applicable.
Physical properties
Usage environment
Suitable for all ISO 9223 corrosivity classes from indoor C1 to marine/industrial C5. Being a hot dip, the steel undergoes thermal shock: asymmetric designs or markedly different thicknesses welded together can cause warping during immersion. Tubular components must include vent and drainage holes for the molten zinc.