But after pressure from health advocates, CCA lumber was banned. The replacement preserved wood products, like CCA of old, still contain copper, because it is a natural anti-microbial, but non-toxic to humans.
Still, challenges remain for preserving wood with copper. Wood treaters are seeking better ways to embed copper, by, dissolving it in an aqueous liquid to be forced into the wood in pressurized vats, a.k.a. pressure treating, or more recently, by grinding the copper (at a microscopic scale) so it can be carried into the wood by just plain water.
Another persistent challenge: Water running across wood that contains copper creates an electrolyte that carries copper ions. When that water contacts untreated or under-treated steel, the copper ions will corrode steel by galvanic action; the steel rusts.
When CCA pressure-treated was used before 2004, the chromate and the arsenate in CCA actually inhibited release of copper ions. But today, the two main replacements for CCA – alkaline copper quaternary (ACQ) and copper azole (CA) – do not offer those protections.
So, retarding galvanic reaction in pressure-treated wood must be done with the fasteners and hangers’ anti-corrosion treatments
To slow or stop this corrosion, manufacturers will coat the steel or offer stainless steel products. Although there is a new hybrid coating on the market, the most common anti-corrosive coating is hot-dipped galvanized zinc coating. (Zinc plating is merely done to keep the steel from rusting during transportation and storage).
Three Classes Of Anti-Corrosion
There are three classes of true anti-corrosion treatments: 1) zinc-coated, 2) stainless steel, and 3) zinc-polymer hybrids.
For hot-dipped, zinc-coated products, the thickness of the zinc determines the protection, and the product’s rating depends on its compliance with ASTM standards.
G90 Rated Zinc Coatings
G90 is not a brand offered by just one company. For G90 labelled products, the “90” in the label means that there’s 0.9 cumulative ounces of zinc applied per square foot of the steel surface. So, each surface of G90 has 0.45 ounces/sq./ft. of zinc.
G185 Rated Zinc Coatings
The next step up in zinc protection is G185, where 1.85 ounces of zinc per square foot, with .925 ounces/sq./ft. on each surface. G185 is also known as “triple zinc,” or TZ.
Hot-Dip Galvanized (HDG)
G90 and G185 products are all “hot-dipped” galvanized, but the term hot-dipped galvanized has come to mean steel fasteners and connectors that are dipped after fabrication. Hot dipped coatings are typically coated with a thickness on par with G-185 coatings.
In highly corrosive environments (sea side, near a pool) consider stepping up to stainless steel for fasteners and connectors, rather than zinc-coated steel. Stainless steel is a “noble metal,” and does not freely “give” ions in reactions with dissimilar metals; so, stainless steel won’t corrode.
Organic Polymer—Zinc Hybrids
If you don’t want to pay for stainless steel, but you want protection that exceeds G185, there is a new class of product, branded as Gold Coat by USP Structural Connectors. It takes a G90 zinc-coated steel product and adds an additional protective organic chemical coating, providing more than G-185 protection. (The protective top-coat layer that is applied to make a Gold Coat product is 10 to 12 microns thick, rich in aluminum, and resistant to both acids and bases).
Finally, for fasteners, always match the fastener metal or anti-corrosive treatment with the metal or anti-corrosive treatment of the connector. Stainless connectors call for stainless fasteners, period.
Todd Grevious, PE, is Engineering Manager at USP Structural Connectors.
Click here for more information on anti-corrosive treatments.