Zinc Layer Thickness Passed But Zinc Powder Peels Off? Sandelin Effect Is The Hidden Trap

Release time:

2026-09-03

Many buyers encounter a confusing situation: galvanized wire passes zinc‑thickness test, yet zinc powder flakes off when bent or knocked. This risk comes from the Sandelin Effect.

Many buyers encounter a confusing situation: galvanized wire passes zinc‑thickness test, yet zinc powder flakes off when bent or knocked. This risk comes from the Sandelin Effect.

When steel wire rod contains silicon within the dangerous range, Fe‑Zn alloy layer grows uncontrollably during hot‑dip galvanizing. Though thickness reading meets standard, most coating becomes brittle Fe‑Zn alloy instead of ductile pure zinc layer. Recycled steel rod often has unstable silicon‑phosphorus content, easily triggering this defect, which cannot be detected only by thickness gauge.

Quick On‑site Inspection

Bend test: Repeated 90° bending. Powder peeling indicates poor adhesion.
Visual check: Uneven color, rough spotty surface across batches.
Tap test: Light hammer tap causes zinc flaking.

Procurement Tips

Avoid unknown recycled‑material wire rod for long‑term outdoor projects.
Add bending & adhesion requirement in contract, not only zinc thickness.
For solar‑mounting, marine cage, desert projects, adhesion sampling inspection is mandatory.
Wires with Sandelin‑effect defect shall not be used for weaving or long‑term field service.