Will this steel galvanize well? Check the mill certificate.
Silicon and phosphorus decide how steel reacts in the kettle — coating thickness, adhesion, and whether it comes out bright or matte grey. Enter the values straight off the MTC and see the reactivity zone before the steel is committed.
Why chemistry decides the coating
In the kettle, molten zinc reacts with iron to grow the coating. Silicon (and phosphorus) in the steel act as catalysts for that reaction — which is why two visually identical beams can come out of the same bath looking completely different:
- Si ≤ 0.03% (Category A) — normal, self-limiting alloy growth: bright, smooth coating that meets spec with standard immersion.
- Sandelin range, 0.03–0.14% Si (Category C) — runaway linear growth: excessively thick, brittle, dull-grey coatings that can flake on handling.
- Sebisty range, 0.14–0.25% Si (Category B) — moderately elevated but controllable; the accepted second choice (purchase specs commonly write it as 0.15–0.25%).
- Si > 0.25% (Category D) — steadily thickening grey coatings; shortest practical immersion and careful handling.
Phosphorus matters too: Category A requires P below 0.02%, and in the Sebisty range P above 0.035% gives increased reactivity. Aluminium-killed ultra-low-Si steel has the opposite problem — the coating may struggle to reach minimum thickness, which is why ISO 1461:2022 added a specific provision for it. The zone boundaries used here are per ISO 14713-2:2020 Table 1.
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Questions
What is the Sandelin effect?
Runaway zinc-iron alloy growth in steels with roughly 0.03–0.14% silicon (Category C, ISO 14713-2:2020 Table 1). The coating grows linearly instead of slowing down, producing excessively thick, brittle, dull-grey coatings that can flake on handling.
What silicon content is best for galvanizing?
Si ≤ 0.03% with P below 0.02% — Category A in ISO 14713-2:2020 Table 1. Where reactive steel can't be avoided, the Sebisty range 0.14–0.25% (Category B, commonly written 0.15–0.25% in purchase specs) is the accepted second choice.
Why did my galvanizing come out grey?
Most often reactive chemistry — Sandelin-range or high silicon drives the zinc-iron alloy layers all the way to the surface, which reads as matte grey. A grey coating is fully compliant with ASTM A123 / ISO 1461 and protects at least as long as a bright one. See it next to a bright spangled coating in the defect gallery.
Does phosphorus matter?
Yes — P at or above 0.02% takes steel outside Category A even at low silicon, and above 0.035% it gives increased reactivity in the Sebisty range. Table 1 NOTE 1 adds a combined criterion: Si + 2.5P ≤ 0.09% (hot-rolled) / ≤ 0.04% (cold-rolled) is still expected to behave as Category A. Check both values on the MTC, not just silicon.