The Corrosive Facts for Home Builders

We may be moving into the season of wood-burning stoves and fireplaces, but many a home owner is already planning next year’s construction. FPL engineer Sam Zelinka recently published a desk reference on fastener corrosion created for engineers. Let’s dig into that desk reference and a related publication, Guide for Materials Selection and Design for Metals Used in Contact with Copper-Treated Wood, a bit and see what these publications can tell do-it-yourselfers.

As Zelinka informs us, “Metal fasteners are an essential part of modern timber construction.” Of course—think nails, screws, brackets, and bolts. These metal fasteners, however, are susceptible to corrosion, and when metal corrodes, this may make the joints of the structure weak. “If the wood remains dry,” Zelinka assures us, “the fasteners will not corrode.”

Corrosion is everywhere! The reddish-brown rust is an inorganic ceramic compound formed as part of the oxidation process.

Corrosion is everywhere! The reddish-brown rust is an inorganic ceramic compound formed as part of the oxidation process.

Zelinka further tells us that “Even when wet, the wood of most species is a relatively benign environment for corrosion. However, wood preserva­tives are frequently added to wood used in exterior environ­ments to protect it from wood decay fungi and termites. Al­though wood preservatives increase the service life of wood, in some cases these preservatives increase the corrosiveness of the wood toward metal fasteners.”

Sounds rather ominous. So, what is corrosion?

Zelinka tells us that corrosion is a reaction in which a metal is oxidized. “Once oxidized, the metal ion quickly reacts with the environment to form an inorganic compound; that is, rust.” Corrosion is pretty much inevitable and spontaneous in all metals except for gold and platinum—not likely to show up in that pergola. “Therefore,” stresses Zelinka, “materials selection is not about selecting materials that will not corrode (which is nearly impossible), but rather about selecting materials that will corrode so slowly that that the metal remains functional throughout its service life.”

Regarding materials selection, most construction fasteners are made of carbon, galvanized, and stainless steel. According to Zelinka, “Depending on how the metals are used, the metals are susceptible to several different types of corrosive attack.”

fig_02

Corrosion of a galvanized joist hanger and galvanized nails supporting a wood deck treated with a copper-containing wood preservative.

Our second graphic shows a galvanized joist (which has definitely been attacked) that is held with corroded galvanized nails. Zelinka writes that “The corrosion of the nail shank embedded in the wood depends upon the wood moisture content and chemistry. The inner face of the wood is similar to the embedded fastener but also may exhibit galvanic corrosion if the joist hanger and the fastener are made from different materials.”

The corrosiveness of preservative-treated wood has been studied since the 1920s when the first treatments were being developed for railroad ties. Treated wood has gone through many changes in the ensuing decades, as numerous preservatives have been replaced with more environmentally friendly substances. Most significant was the voluntary withdrawal of arsenic-containing preservatives in 2004. Most treated wood today has higher concentrations of copper, which has proven to be more corrosive to metals than previous preservatives. Therefore, Zelinka’s research into this subject is timely and important to homeowners contemplating building that deck for next summer’s grill outs.