How do Wood Extractives Affect Metal Corrosion?

Did you know that metal fasteners corrode in wood? This week we will look further into the work of Sam Zelinka on this subject. Zelinka is the FPL corrosion expert, and our post today borrows from Corrosion of Fasteners in Wood Treated with Newer Wood Preservatives, a compilation of several papers Zelinka has written on the subject in recent years.

Wood may seem like a simple material, but the lumber you build with is actually chemically and physiologically complex. Research has shown that different wood species contain different extractives that may affect the corrosion of embedded metals. This week, we consider the chemical components of wood and the effect of tannins and pH on the corrosion process. Much of today’s post draws from a 2011 study on corrosion of steel in wood extracts.

Wood is comprised of polymers, natural and synthetic compounds of high molecular weight consisting of millions of repeated linked units. In addition to structural polymers (cellulose, hemicellulose, and lignin), wood contains a variety of additional chemical components.

Wood also consists of a variety of organic compounds that are low in molecular weight—extractives. These small molecules get their name because they can be extracted by rinsing with various solvents (including water). Generally, extractives are present in small amounts, and thousands of different extractives are present in wood.

The type and amount of extractives vary widely among wood species. In some naturally durable species (such as locust or white oak), extractives can protect the wood from decay.

Although a single piece of wood can contain over 700 different extractives, only three types have been thought to affect the corrosion of metals in contact with wood or the black liquors of wood pulp: small organic acids, tannins, and phenols.

According to Zelinka, “many researchers have found correlations between the acidity of wood and its corrosiveness, and pH is largely controlled by formation of acetic [essentially vinegar] and formic acid. However pH cannot be the only variable that affects corrosion.”

Zelinka tells us, “Although the pH of wood, a solid material, is not well defined, the water extracts of nearly all wood species are acidic. The reason for this acidity is that in the presence of water, acetyl and formyl groups in the hemicelluloses are hydrolyzed [decompose by reacting with water] to form different kinds of acid. Many researchers have found correlations between the acidity of wood and its corrosiveness, and pH is largely controlled by formation of acetic and formic acid.”

Research has shown that this process is chemical, rather than biological. Previous research on sawblade corrosion suggested that wood tannins accelerated the corrosion of sawblades; however, in general, tannins are known as a corrosion inhibitor. In the 2011 study, Zelinka and Stone showed that tannins in solid wood act as a corrosion inhibitor to the embedded fasteners. In addition to the corrosion rate data, Zelinka and Stone observed a blue-black patina forming on the steel, indicative of the formation of iron-tannate, a stable blue/black corrosion product.

Blue-black precipitate forming on the surface of the steel plug
exposed to synthetic oak extract.

Tannins and other extractives are often mentioned in the literature as compounds that affect corrosion in wood. The different behavior of tannins most likely depends on what application is being studied. For instance, Zelinka and Stone discovered that tannins act as a corrosion inhibitor in wood extracts, which contradicts the earlier sawblade corrosion findings. The difference is most likely due to the friction and heat produced during sawing.

By combining kinetic models in the literature, Zelinka and Stone created an isocorrosion map for wood extracts as a function of pH and tannins. An isocorrosion map is a kind of tool used to recognize high-corrosion situations during the design process of equipment.

“This map,” says Zelinka, “was based on limited data and it does not explain why synthetic extracts behave differently; nevertheless, in the future with additional data such maps may be able to assess the relative effects of these chemicals when developing a new, non-metallic preservative system.”

After all this work and explanation, Zelinka tells us that “the effect of tannins on the corrosion of metals in wood remains unclear.” As Zelinka and his colleagues continue to study corrosion, perhaps this question will have more answers.

 

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.

 

 

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.”

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.

 

Desk Reference on Fastener Corrosion Created for Engineers

zelinka

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.