Why and How Do Metals Corrode in Treated Wood? Chemistry and Design Mysteries Revealed

Scientist Sam Zelinka is the FPL expert on corrosion of fasteners in treated wood. This week we continue our discussion of this subject from Zelinka’s publication Guide for Materials Selection and Design for Metals Used in Contact with Copper-Treated Wood and a related publication, Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives.

In these publications, Zelinka discusses the chemistry of waterborne wood preservatives. These preservatives contain cupric ions (copper molecules) that are thermodynamically unstable in the presence of steel or zinc galvanized fasteners. A chemical reaction, such as the one that will be discussed here, requires a lot of energy to be formed. To be thermodynamically unstable is to be unfavorable and means that the reaction is not spontaneous. It requires energy.

Let’s talk about the mechanism of corrosion in treated wood. This process, according to Zelinka, involves “the transport of cupric ions through the wood to the fastener surface, where the cupric ions are reduced and the fastener (zinc or iron) is oxidized.”

What does that mean?  In chemistry, oxidation is the loss of electrons or an increase in oxidation state by a molecule, atom, or ion. Reduction, on the other hand, is the gain of electrons or a decrease in oxidation state by a molecule, atom, or ion.

How does that work? The following graphic illustrates this point by showing the mechanism of corrosion in treated wood.  We can see how cupric ions migrate through the wood to the metal surface where they are reduced as the fastener is oxidized.

fig_04

Zelinka tells us, “For carbon steel and zinc-galvanized fasteners, the reduction of cupric ions is thermodynamically favorable and will occur. The corrosion of embedded metals is strongly dependent upon moisture content.” This, according to Zelinka, “is a war that Nature will eventually win.”

However, Zelinka also gives the homeowner numerous tips for prolonging the life of their outdoor projects through careful materials selection and design. By understanding the corrosion mechanism, “it is possible to develop strategies for maximizing the life of embedded fasteners.”

One strategy starts with understanding that “When wood is dry, embedded metals do not corrode.” Although your deck will obviously be exposed to rain and snow, Zelinka assures us that sound design principles can help, such as keeping rainwater from seeping in through the end grain and designing roofs and overhangs so they do not drain onto lower structures.

Another strategy Zelinka suggests for making sure that the fasteners holding that new deck might have a long life is using a metal noble to copper.  Noble metals are metallic chemical elements that have outstanding resistance to oxidation, the most common (and affordable) being stainless steel. This means that the nail is hot-dipped in a noble metal, and if these nails or other fasteners are not damaged in construction, the coating can go a long way in protecting against corrosion.

With this knowledge at hand, your outdoor structures can (and will) last a long time.