Gent, Belgium — Zinc-rich paints are often used as primers to coat wind turbines, as they provide excellent, long-term corrosion protection in highly corrosive environments. The more severe environments are classified as C3 (medium corrosive), C4 (highly corrosive) or C5 (heavy industrial and marine). Zinc-rich paints are essentially composed of zinc powders (with average particle sizes between three and 10 microns), a small quantity of binder, and solvents necessary for the spray application and to obtain the right viscosity of the paint.
The thickness of a zinc-rich primer layer on a turbine can easily mount up to 60 microns in a C5 environment. Given that zinc primers have a zinc content in the dry paint film of >80% by weight, a lot of zinc powder is needed to protect a turbine tower adequately. For an offshore turbine a typical zinc-rich paint protection system has a total paint thickness of 320 microns.
EU corrosion standards do not govern the purity of the zinc powders used in paint manufacture, which can vary a lot. Purity is classified according to the ASTM D520 standard, into type I, type II and type III grades.
Lead and Cadmium
A primary difference in the purity of zinc powders used in paint manufacture is lead and cadmium content. Types II and III zinc powders have low lead (0.01% and 0.002% max) and low cadmium contents (0.01% and 0.001% max), while the lead and cadmium content of type I zinc powders is much higher and can reach 1500 ppm (0.15%). Unfortunately, much type I zinc powder continues to be used by paint manufacturers, despite the fact that types II and III are readily available.
What is Acceptable?
The PEL (Permissible Exposure Limit) is the maximum daily human exposure to a specific substance allowed in a workroom’s air over an 8-hour shift, as determined by OSHA (the US Occupational Safety and Health Administration). The Action Level is the exposure value at which action should be taken to preserve workers’ safety. Often this value is set at 50% of the PEL value.
Scientists Gary Tinklenberg and Denise Doezema describe acceptable levels of cadmium and lead exposure in their article Health Concerns for Workers Using Zinc-Rich Coatings. Their research demonstrated that when type I zinc powders are used, lead and cadmium levels in the air are well above the PEL, and action levels for lead and cadmium are exceeded during mixing and spray application of zinc-rich paint.
Turbine manufacturers are developing ‘green’ standards for their suppliers. These should include a requirement to use type II and III zinc powders in anti-corrosion primers.
Image: Pavel Ignatov via Shutterstock