The Latest and Greatest: FPL's NewsLine Hits the Web!

FPL researchers are hard at work discovering the amazing possibilities wood presents to make our lives safer and better. You can read all about what they’ve been up to in our quarterly newsletter, NewsLine.

newsline-2015-2-thumbIn this issue, you’ll learn about the importance of fasteners in keeping your deck safe, research on wood bridges, a new demonstration house, recycling preservative-treated wood, the amazing things we can do in our new pressure treatment plant, and much more.

Past issues of NewsLine can be found on FPL’s website.

If you’d like to have NewsLine delivered right to your inbox, simply send an email request to mailroom_forest_products_laboratory@fs.fed.us

We hope you enjoy this issue and wish you a wonderful holiday season.

-The Newsline Team

Choose the Right Fasteners to Avoid Unsightly Iron Stains

The following information is from Forest Products Laboratory’s Wood Handbook, Wood as an Engineering Material.

Iron stains occur from rusting of fasteners or by the reaction of iron with tannins in wood. The appearance is different for each of these reactions.

In wood species that lack tannins, iron merely rusts, giving a brown stain to the wood surrounding the fastener. The iron also causes slight degradation of the wood near it (often referred to as “wood sickness”). This discoloration develops over many months or years of exposure.

istain

Iron stain on newly installed wood siding. Poor quality galvanized nails corrode easily and, like uncoated steel nails, usually cause unsightly staining of the wood.

In wood species that have tannins, a chemical reaction takes place between the iron and the tannins. Tannins are just one of the many chemicals (extractives) in wood. Species such as the cedars, the oaks, and redwood are rich in tannins. Iron reacts immediately with the tannins to give a blue-black discoloration.

Steel fasteners are the most common source of iron, but traces of iron left from cleaning wood with steel wool or wire brushes cause iron stain. Poor quality galvanized nails corrode easily and, like uncoated steel nails, usually cause unsightly staining of the wood.

Using the wrong fastener can be costly—it may become necessary to replace all the siding. Therefore, your best bet is to use corrosion-resistant fasteners, such as stainless steel, rather than risk iron stain, particularly when using natural finishes on wood containing high amounts of tannin (such as western redcedar, redwood, and oak).

If using galvanized fasteners, they must be hot-dipped galvanized fasteners meeting ASTM A 153/A specification. Other galvanized fasteners fail. Unfortunately, contractors and their employees may have difficulty recognizing the difference among galvanized fasteners.

Can iron stain be fixed?

If iron stain is a serious problem on a painted surface, countersink the fastener, caulk, spot prime, and top-coat. This costly and time-consuming process is only possible with opaque finishes. Little can be done to give a permanent fix to iron stains on wood having a natural finish.

Removing fasteners, cleaning the affected areas with oxalic acid solution, and replacing the fasteners may not give a permanent fix because residual iron left behind continues to cause staining. Removing the fasteners often splits the siding.

Iron stain occurring beneath a finish is extremely difficult to fix. The coating must be removed before the iron stain can be removed. Oxalic acid will remove the blue–black discoloration. Apply a saturated solution (0.5 kilogram of oxalic acid per 4 liters of hot water) to the stained surface. Many commercial brighteners contain oxalic acid, and these are usually effective for removing iron stains.

After removing the stain, wash the surface thoroughly with warm water to remove the oxalic acid. If even minute traces of iron remain, the discoloration will recur.

For more information, please see chapter 16 of FPL’s Wood Handbook, Wood as an Engineering Material.

To find out more about iron stain, and what can be done about it, please see this FinishLine.

Fasteners for Cedar Siding: What Kind of Nails and How to Use Them

Installation, Care, and Maintenance of Wood Shake and Shingle Siding gives handy advice to the homeowner who wants to install cedar siding.

Nail placement for cedar shingles up to 10 inches (254 mm) wide requires two corrosion-resistant nails driven 3/4 inches (19 mm) from each edge and 1 inch (25 mm) above the exposure line. For shingles wider than 10 inches (254 mm), drive two additional nails approximately 1 inch (25 mm) apart near the center.

nails-and-screws_101875306-[Converted]

Corrosion-resistant fasteners.

To decrease the chance of splitting the shake or shingle, fasteners should be blunted siding nails and should be ring- or twist-shank to improve holding. A ring-shank nail will have adequate holding power if it penetrates ¾ inch (19 mm) into the wood.

Corrosion-resistant nails are needed to avoid iron stains caused by extractives in the wood and corrosion by acid rain, salt air, etc. Certain preservative- and fire-retardant treatments also may be corrosive. Check with the product supplier for recommendations on proper fasteners for their products. Non-corrosive siding-nails are available in hot-dipped galvanized (as per ASTM A 153/D) Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware and type 304 or 316 stainless steel. Additional information may be available from supplier websites, and some suppliers may have additional requirements.

 

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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.