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1. Background and Sandelin Effect
Silicon is used as a deoxidizer in continuous steel casting to produce silicon‑containing reactive steel. Silicon in steel significantly accelerates the iron‑zinc reaction, which is known as the Sandelin effect. Two reaction peaks occur at silicon contents of 0.07%‑0.1% and above 0.3%, resulting in dull coatings, excessive coating thickness and poor adhesion, and degrading product quality.
Steels are classified into sub‑Sandelin steel, Sandelin steel and super‑Sandelin steel according to silicon content.
Solutions: adjust the zinc bath temperature or add alloying elements to the zinc bath to regulate the iron‑zinc reaction and obtain qualified coatings.
In accordance with ISO standards, steels are divided into four grades A‑D based on silicon and phosphorus contents. As silicon content rises, the coating changes from bright and compact to dull and coarse. The iron‑zinc alloy layer grows toward the coating surface, and coating thickness increases with higher silicon content. Phosphorus also synergistically affects reaction activity.
2. Mechanism of Silicon’s Influence on the Growth of Alloy Phase Layers
(1) Growth kinetics: Silicon modifies coating growth patterns. Coating growth follows a parabolic law for silicon‑free steel. For silicon‑containing steel, growth is linear at the initial stage controlled by the dissolution process and returns to parabolic growth at the later stage under diffusion control. Phase‑layer growth rate is jointly determined by dipping time, temperature and silicon content; the effect of silicon does not manifest under short dipping durations.
(2) Microstructural phase evolution: Silicon participates in iron‑zinc diffusion and forms iron‑silicon compounds. It destabilizes the Γ phase and alters the nucleation and growth behaviours of Γ, Γ₁, δ and ζ phases. Phase equilibrium can be interpreted using the Fe‑Zn‑Si ternary phase diagram. Silicon cannot co‑exist with the ζ phase in a binary system; it can only form iron‑silicon compounds or be released via the δ phase.
(3) Reaction modes under four silicon‑content levels
Sub‑Sandelin steel (low silicon): Heterogeneous nucleation of the ζ phase takes place on the steel substrate, followed by sequential formation of δ and Γ phases. The coating grows parabolically with satisfactory coating quality.
Silicon near the Sandelin peak (0.07%): Homogeneous nucleation of the ζ phase occurs within the liquid phase. Iron can be transported by convection, leading to rapid linear coating growth.
Further increased silicon content: δ phase plus iron‑silicon compounds rapidly form in the liquid phase, the liquid‑phase lifetime shortens, and the coating growth rate decreases.
High‑silicon (super‑Sandelin): Homogeneous nucleation of the ζ phase occurs away from the steel substrate, with rapid growth perpendicular to the substrate and a remarkable increase in coating thickness.
3. Nature of Detrimental Effects
Silicon alters the nucleation and diffusion mechanisms between iron and zinc, disrupts the normal layered coating microstructure, triggers excessive growth of the ζ phase, and induces defects including over‑thick coatings, dull appearance and poor adhesion.