Zinc and Other Products
When placed in contact with zinc, certain products can have detrimental effects on the appearance and/or structural integrity of the zinc. Acidic products and products that can generate a galvanic reaction must not be used with zinc. Run-off from non compatible products onto zinc also must be avoided.
There are no issues with zinc in combination with metals other than copper and mild steel. When zinc contacts copper in the presence of an electrolyte (such as water) a galvanic reaction will lead to corrosion of the zinc and subsequent failure of the roof or wall cladding. Run-off from a copper surface to a zinc surface must be avoided under all circumstances. Zinc in contact with mild steel is not desirable, either, due to similar electron transfers between the metal that will result in zinc corrosion and deterioration.
In general, water should not be allowed to run from a higher potential metal to a metal with a lower one. Install metals in the following order (from top to bottom)
Zinc can be installed adjacent to limestone. Limestone buildings in Paris have had numerous zinc protective flashings between floors for decades. The run off from limestone onto zinc material is acceptable. However, limestone dust and gypsum dust generated during cutting operations can react with zinc in the presence of water and form a superficial layer of white rust. The zinc surface must be cleaned of any limestone and gypsum dust. No dust should be in contact with unprotected zinc. To prevent white rust, good construction practices should be use to limit the amount of dust that comes in contact with the zinc.
Acceptable Contact Products for Zinc
- Aluminum (painted, anodized, or bare)
- Galvanized Steel
- Stainless Steel
- Compatible Woods: pine, spruce, Scots pine, poplar.
Unacceptable Contact Products and Run-off for Zinc – This list is not exhaustive
- Steel (Non galvanized)
- Gypsum dust/ Lime stone dust
- Non-compatible woods: larch, oak, chestnut, red cedar, Douglas fir, white cedar, all woods with a pH < 5.
- Rosin paper
- Bituminous membranes
- Products with fire retardant and preservation treatments
- Acidic cleaners (brick cleaner etc…)
Prevention of White Rust
When zinc is in contact with water, or humidity in the absence of carbon dioxide, zinc oxide or hydroxide is formed instead of the protective patina. This substance has a white aspect and hence the name “white rust.” Adherence to the surface is low and the substance offers no protection to the zinc.
To prevent white rust from forming before installation, care must be taken in transporting and storing the zinc. Water between sheets on a pallet or trapped within the coil could cause white rust. It is important to store the zinc in dry places and to use packaging that allows for fast evaporation of water. During transportation avoid large changes in temperature or humidity.
Zinc has been successfully used in Europe on well-ventilated, compatible wood substrates for more than 160 years. The wood support absorbs any trapped water and the ventilation allows the wood to dry. Ventilation is a good practice not only for zinc but also for the building as a whole to keep moisture accumulating. Non-compatible wood is used in North America, therefore all zinc for roofing and flashing must have a back side coating (PLUS). While VMZINC PLUS offers a good level of protection against white rust, it will not protect zinc when it comes in contact with water. Best practices must be used to avoid any water getting trapped between the zinc and the substrate.