Case study 4: Flaking of the zinc coating - error during hot-dip galvanizing

Flaking of the zinc coating - causes, assessment and remediation of coating defects

Mission and initial situation

A construction company commissioned me to examine several galvanized steel components on which the zinc coating had peeled off over a large area shortly after assembly.
The affected components were part of an external balcony structure that is permanently exposed to the weather. Just a few months after completion, blistering, flaking and corrosion nests appeared on the surface.

This case study describes the typical causes of flaking zinc coatings - a common coating defect in metal construction.

Damage pattern detected

  • large-scale Flaking of the zinc coating, especially on edges and weld seams
  • Infiltration corrosion between steel and zinc
  • Gas bubbles and spalling in the area of weld seams and fillets
  • Irregular layer thickness (partly < 40 µm instead of the required 85 µm)
  • Discoloration and oxidation residues, Indication of insufficient pretreatment

The adhesion of the coating was so poor in several places that parts of the zinc coating could already be removed with a spatula.

Root cause analysis

The metallographic examination and the review of the manufacturing documents revealed several process errors in connection with the hot-dip galvanizing:

  1. Missing or inadequate pre-treatment
    - Insufficient degreasing and pickling before the dipping process, residues of rolling skin and welding spatter.
  2. Incorrect bath temperature or dwell time
    - Overheating led to brittle zinc coating and subsequent flaking under thermal stress.
  3. No suitable follow-up treatment
    - Lack of passivation or re-galvanizing after mechanical processing.
  4. Incorrect base material
    - unsuitable steel grade with silicon content > 0.03 %, which led to the „sandelin effect“ (excessive zinc coating, brittle layer).

Normative evaluation

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

„The zinc coatings must be uniform, firmly adhering and free from detachment, flux residues or uncoated areas.“

The measured layer thickness was 35-60 µm well below the required minimum thickness of 85 µm (component class ≥ 3).
The adhesive strength also did not meet the requirements from DIN EN ISO 2409 (cross-cut test).

The test results thus prove a significant execution error by the coater, as the surface preparation and process parameters did not comply with the recognized rules of technology.

Consequences of the shortage

  • premature Corrosion of the base material
  • Flaking of the zinc coating after only a short period of use
  • Loss of the protective effect → Increased need for maintenance
  • Visual impairment and depreciation
  • for load-bearing components: Endangerment of durability and stability

Recommended measures

  1. Complete removal of the non-adhesive layers (blasting up to SA 2½ according to DIN EN ISO 8501-1).
  2. New galvanizing in compliance with the process specifications according to DIN EN ISO 1461.
  3. Materials testing of the steel for silicon content (Si < 0.03 %) to avoid the sandelin effect.
  4. Post-treatment / passivation to increase the corrosion resistance.
  5. Documentation the bath parameters (temperature, dwell time, layer thickness).
  6. Final inspection by adhesion testing and coating thickness measurement.

Note from the expert

As a publicly appointed and sworn expert for the metalworking trade, I assess zinc coatings, coating defects and corrosion protection systems in accordance with the applicable standards.
Thanks to my many years of experience, I can reliably and objectively identify process errors in galvanizing and coating.

Further information

Overview of the case studies:

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