Why do zinc rich primers sometimes leave a powdery topcoat?
The white powdery residue produced by zinc rich primers is a corrosion product known as white rust, white corrosion or white storage stain. White rust occurs when a zinc coated surface is exposed to moisture (such as rain, dew or condensation) while being stored in an environment with limited airflow.
Zinc-based coatings are a popular form of corrosion protection for metal substrates mainly because of its resilient oxide layer. (Learn why in How Metallic Coatings Protect Metals from Corrosion.) Fresh zinc coated steel reacts with the surrounding atmosphere to form a series of corrosion products that protect the underlying substrate. When exposed to air, zinc reacts with oxygen to form a thin zinc oxide layer. The zinc oxide layer then reacts with moisture in the atmosphere to produce zinc hydroxide. Finally, zinc hydroxide reacts with carbon dioxide to form zinc carbonate. This zinc carbonate layer is chemically stable, resilient, and insoluble in water and acts as a protective barrier.
Zinc surfaces that have not had the time to fully develop the protective carbonate layer are left with a zinc hydroxide top layer that has a white powdery appearance. This type of corrosion is commonly seen when zinc coated items are kept in storage. In most cases, these items are exposed to moisture while being tightly packed or nested together. The stifling of free-flowing air over the zinc coating effectively prevents the zinc hydroxide from undergoing the required chemical reactions to form the desired zinc carbonate.
White rust can be avoided by ensuring proper storage of zinc-coated metals. Methods for preventing white rust include:
- Storing items in a way that permits free airflow over the coated surface
- Allowing water to drain freely from the coated surface
- Avoiding the use of plastics or other coverings that can obstruct airflow and encourage condensation
- Maintaining a dry storage environment with low humidity
- Keeping zinc coated items away from other wet or damp elements such as soil and grass
Light white rusting is typically considered to be superficial and can be remedied by placing the affected metal in a dry area with free airflow. The existing white rust deposits will eventually convert to zinc carbonate.
Heavy white rusting may require removal before placing the material in a dry open area. Removal usually entails using a non-metal bristle brush with a weak acid such as vinegar. If there is any significant loss of the zinc coating after the white rust removal, recoating may be required to achieve the minimum film thickness.