Powder coating over hot dip galvanising, often called a duplex system, combines the sacrificial protection of zinc with the colour and barrier protection of powder. Done well, it lasts far longer than either on its own. Done badly, it can flake off in sheets.
Why it goes wrong
Galvanised surfaces are not like bare steel:
- Zinc salts and oxides form on the surface within hours and weaken adhesion if not removed.
- Outgassing: gases trapped in the zinc escape as the part heats up in the oven, causing pinholes and blisters.
- Surface variation: spangle, drips, ash and thick zinc at edges all affect the finish.
The key steps
- Specify for duplex. Tell the galvaniser the steel will be powder coated, so quenching and post-treatments that interfere with adhesion can be avoided.
- Prepare the zinc. Remove high spots and drips, then prepare the surface by sweep blasting or a suitable chemical pretreatment, as the powder manufacturer specifies.
- Deal with outgassing. Pre-heat the parts above the cure temperature before coating, and use a degassing grade of powder where recommended.
- Control cure. Verify the metal temperature and time, not just the oven setting.
- Test adhesion. Include adhesion testing on trial panels and in routine QC.
Standards to know
EN 13438 covers powder coatings on galvanised and sherardised steel for construction, and ISO 1461 covers the hot dip galvanised coating itself. For architectural work, Qualisteelcoat sets additional requirements.
How we help
Our background includes nine years of quality management at a multi-process finisher running powder coating, blast cleaning, zinc metal spraying and chemical pretreatment. We help finishers set up duplex processes, write the SOPs and fix the defects. See process development.