English
For silicon‑containing steel, at the Sandelin peak ($w_{Si}≈0.1\%$) and $w_{Si}＞0.3\%$, local explosive growth tends to occur, forming coarse ζ‑phase, which deteriorates coating appearance, degrades mechanical properties and increases zinc consumption. Sub‑surface oxidation, surface roughness, residual stress, grain orientation and metallographic structure of steel jointly affect coating reactivity.
1. Sub‑surface Oxidation
(1) The existing form of silicon in steel determines reactivity: solid‑solution silicon improves galvanizing reactivity; silicon in the SiO₂ oxidized state shows no reactivity. During hot rolling, silicon is preferentially oxidized to form silicon oxides at the sub‑surface, inhibiting iron‑zinc reactions.
(2) Pretreatments such as pickling and sandblasting destroy the sub‑surface oxide layer and re‑expose solid‑solution silicon, which raises coating reactivity, increases coating thickness and transforms the coating morphology from lamellar to dispersed.
(3) Oxidizing atmosphere promotes silicon oxide formation and reduces reactivity, while non‑oxidizing atmosphere produces no such effect. The degree of surface silicon enrichment is jointly controlled by heating and pickling corrosion intensity.
2. Surface Roughness
(1) Surface uneven morphology alters alloy‑layer growth: ζ‑phase grows rapidly at protrusions to form whisker‑like and dendritic microstructures; depressions are diffusion‑controlled and develop compact coatings.
(2) For identical roughness values, different surface morphologies lead to significant differences in coating stability. Roughness values alone cannot judge whether reactive coatings will form.
(3) Different pretreatments including sandblasting and pickling, combined with the silicon content of steel, modify coating thickness and phase structure. Coating instability occurs once silicon content reaches the critical value.
3. Residual Stress
(1) Residual stress and strain energy introduced by processing are thought by some to accelerate iron diffusion and induce local reactivity as well as strip‑shaped coating defects.
(2) Nevertheless, numerous tests demonstrate that after eliminating interfering factors such as oxidation and sub‑surface oxide layers, residual stress itself exerts little effect on coating reactivity. Most observed reactivity phenomena stem from the destruction of surface oxide layers rather than direct stress action.
4. Surface Grain Orientation
(1) Grain orientation affects coating growth mechanisms, and the (111) crystal plane of iron possesses the highest reactivity toward zinc reactions.
(2) Dominance of the Sandelin effect: when silicon content approaches the critical value of 0.1 %, coating instability occurs regardless of grain orientation. The grain‑orientation effect manifests only under low‑silicon conditions.
(3) Solid‑solution silicon facilitates the formation of highly reactive grain orientations, which is meanwhile restricted by other steel components and heat‑treatment processes.
5. Surface Metallographic Structure
(1) Fine‑grained steel features abundant grain boundaries and relatively high galvanizing reactivity, though this effect is not prominent.
(2) The distribution position of cementite influences reactivity: steel exhibits higher galvanizing reactivity when cementite resides inside grains than when cementite distributes along grain boundaries.
Improvement Strategies
Pre‑plate pure iron or nickel onto workpiece surfaces prior to galvanizing to form a barrier layer. This isolates direct contact between zinc melt and reactive steel substrate, suppresses initial iron‑zinc reactions, lowers coating reactivity and mitigates defects induced by the Sandelin effect.