Case study 9: Zinc cracks after hot-dip galvanizing - causes, analysis and evaluation

Mission and initial situation

An industrial company reported after the hot-dip galvanizing of several steel frames Fine, net-like cracks in the zinc coating.
The components were load-bearing supports for a carport system, made of structural steel S235 JR and hot-dip galvanized in accordance with DIN EN ISO 1461.
I was commissioned to investigate the cause of the damage and assess the zinc cracks.

Damage pattern detected

  • fine, net-like Cracks and bursts of the zinc coating in the area of edges and weld seams
  • Infiltration corrosion on open crack flanks
  • Discolored, brittle surface, typical indication of thermal stress cracks
  • Cracks run parallel to the heat-affected zones

The cracks appeared immediately after galvanizing and were detected visually during the final inspection (VT).

Test procedure and methodology

The crack analysis was carried out with Visual inspection (VT2) to DIN EN ISO 17637 and additionally by means of Dye penetrant testing (PT2) to DIN EN ISO 3452-1.
The PT test confirmed a large number of linear indications in the area of the fillet welds and edge transitions.

For further clarification, a metallographic cross-section (according to DIN EN ISO 1463).
Under the microscope, a intermetallic layer formation from iron-zinc phases (δ-, ζ-phase) with Brittle fracture structures - typical for zinc cracking.

Root cause analysis

The cracking was caused by the interaction of several factors:

  1. High silicon and phosphorus content in the steel (> 0.03 %) led to the so-called. Sandelin effect → Excessive reaction between iron and zinc.
  2. High bath temperature (> 460 °C) and excessively long dwell time in the zinc bath → strong intermetallic layers, brittle structure.
  3. Stresses from the welding process → Crack initiation during cooling.
  4. Unsuitable edge shape → local stress concentrations.
  5. Lack of follow-up treatment (no stress-relief heat treatment, no passivation).

Normative evaluation

The requirements for hot-dip galvanizing are set out in DIN EN ISO 1461 and DIN EN ISO 14713-1 regulated.

„The zinc coatings must be adherent, crack-free and without flaking.“

According to these regulations, cracks in the coating are regarded as not permitted, as they impair the protective effect.

The cause of the crack was determined after DIN EN ISO 14713-2 (Corrosion protection by galvanizing - Material selection) the category „Material-related cracking due to excessive reactivity of the steel“ assigned.

Consequences of the shortage

  • Loss of the corrosion protection effect through open cracks
  • Infiltration corrosion on crack flanks → Rust escaping
  • Reduced adhesive strength of the coating
  • Visual defects and Impairment
  • for load-bearing parts: Reduced fatigue strength due to thermal-mechanical stresses

Recommended measures

  1. Exchange severely cracked components or Sanding / re-galvanizing of the affected areas.
  2. Verification of the steel analysis certificate for Si and P content (< 0.03 % recommended).
  3. Adjustment of the galvanizing parameters (bath temperature ≤ 450 °C, optimized dwell time).
  4. Constructive stress reduction (larger radii, uniform material thickness).
  5. Post-treatment by passivation or duplex system (zinc + coating).
  6. Final inspection using VT2/PT2 and, if necessary, coating thickness measurement in accordance with DIN EN ISO 1463.

Summary

Zinc cracks are caused by Thermal-metallurgical stresses for hot-dip galvanizing of reactive steels.
According to the DIN standard, they are material defect, as they significantly impair corrosion protection.
Only a coordinated combination of Material selection, Galvanizing parameters and Quality control (VT2/PT2) ensures a durable protective layer.

Note from the expert

As a publicly appointed and sworn expert and certified VT2/PT2 inspector, I assess zinc cracks, coating defects and thermally induced stress cracks in accordance with the applicable standards.
My inspections are non-destructive, documented and court-proof.

Further information

Overview of the case studies:

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