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.