What Is Treated Wood? The Copperific Truth Behind Green Wood

Last week we introduced the subject of corrosion in the fasteners used in wood construction. Homeowners have an enormous choice of lumber to use in their projects. We all know the feeling of being in the big box store and looking at the seemingly endless choices of lumber stacked in huge piles. And why is it colored that weird green? Let’s take a few minutes to talk about treated wood.

Many Types of Wood Preservatives Are Now in Use

Going back again to Forest Products Laboratory Researcher Sam Zelinka’s Guide for Materials Selection and Design for Metals Used in Contact with Copper-Treated Wood, “Wood preservatives are chemicals that are injected into the wood to help the wood resist attack by decay fungi, mold, and/or termites. Waterborne wood preservatives are used in most cases where the wood may be in contact with humans or will be painted. While many different formulations of waterborne preservative treatments have been developed, only a few of these have been used commercially. Most of the commercial treatments contain cupric ions [copper molecules], which give treated wood its characteristic greenish-brown coloration.”

As we mentioned last week, 2004 ushered in major changes with treated wood when Environmental Protection Agency regulations restricted the use of chromated copper arsenate (CCA) in the United States. The European Union and Australasia made similar changes in their regulations at about the same time. This was a significant change, as CCA had dominated the U.S. preservative market for many years.

Zelinka tells us that “CCA can still be used in certain situations, specifically wood used in highway construction (excluding pedestrian bridges or hand railings).” Since the regulation change, alternatives to CCA have been introduced, and these alternatives now dominate the market.

FPL’s Stan Lebow has summarized alternatives to CCA in many publications, particularly Alternatives to chromated copper arsenate (CCA) for residential construction and the Wood Preservation chapter of the Wood Handbook.

Several alternatives with different formulas are now available. Zelinka says, “Although the formulations of . . . wood preservatives are different from each other, they all have a higher percentage of copper than CCA.” This is important, as the corrosion mechanism has to do with reducing cupric ions in the preservative. In 2007, Zelinka and others found in Direct current testing to measure corrosiveness of wood preservatives that chromates and arsenates in CCA act as corrosion inhibitors.

Many of the post-2004 preservatives have been standardized by the American Wood Protection Association. Additionally, several commercially important preservatives have been introduced to the market by ICC-ES (ICC Evaluation Services) evaluation reports.

According to Zelinka, “These preservatives include “micronized” formulations . . . which have various trade names. In these formulations, soluble copper is not injected into the wood; rather solid copper, copper oxide, or copper carbonate is ground into submicron particles or “micronized” and suspended in solution prior to injection. Several different formulations of these preservatives are covered by different ICC-ES evaluation reports. These formulations differ in the listed uses, required retentions, and have slight differences in the formulations, but in general require less copper than the nonmicronized counterparts.”

From left to right are examples of different treated wood: micronized copper quaternary (MCQ), didecyldimethylammonium carbonate (DDAC), and alkaline copper quaternary (ACQ-D). Cupric ions from the wood preservative causes the dark coloration of the wood. Excess copper has deposited on the MCQ (green splotches) and the ACQ (along the end grain).

From left to right are examples of different treated wood: micronized copper quaternary (MCQ), didecyldimethylammonium carbonate (DDAC), and alkaline copper quaternary (ACQ-D). Cupric ions from the wood preservative causes the dark coloration of the wood. Excess copper has deposited on the MCQ (green splotches) and the ACQ (along the end grain).

Behold the many choices available to the homeowner. Armed with knowledge, that deck you build next summer can be beautiful and will last a long time.

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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.”

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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.

 

Desk Reference on Fastener Corrosion Created for Engineers

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Samuel Zelinka, FPL research materials engineer

Over the past few years, FPL research materials engineer Samuel Zelinka has investigated the corrosion of fasteners in new wood preservatives. Recently, Zelinka compiled his research findings into a single report on corrosion of metals in wood. The report, titled Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives, was created to serve as a desk reference for engineers to aid in materials selection when building with treated wood.

The research addresses these pertinent questions on designing durable connections with new preservative treatments:

• How rapidly do embedded metals corrode in wood?
• What is the mechanism of corrosion in treated wood?
• Do extractives affect corrosion?
• How can we rapidly determine the service life of metals in wood?
• How can we use corrosion data to predict service life of metals in wood?
• Do suitable non-metallic fasteners for use in wood exist and how durable are they?

This research was conducted as part of the Research, Technology and Education portion of the National Historic Covered Bridge Preservation Program administered by the Federal Highway Administration.

FPL Research Patented

A patent titled “Method and Apparatus for Determining the Surface Area of a Threaded Fastener” was recently awarded to FPL researchers Douglas Rammer and Samuel Zelinka.  The invention is a method for reliably determining the surface areas of threaded fasteners, such as wood screws, drywall screws, or threaded nails.

The method entails acquiring an image of the fastener, separating the image into three regions (thread region, root region, and body surface), determining the surface area of each region, and summing them. The ability to determine this measurement is important for several applications, including studying the corrosion rate of fasteners.