Fastener Corrosion

Fasteners consist of nails, screws, bolts, staples, etc and are widely used in the construction industry to connect lumber, sheathing, shingles, and other wood-based products.  Nail guns allow for nails to inserted at a rapid fire pace.  Chromated copper arsenate (CCA) was the primary preservative used for residential uses. 

CCA has been used for timber treatment since the mid-1930s. It is a mix of chromiumcopper and arsenic (as Copper(II) arsenate) formulated as oxides or salts, and is recognizable for the greenish tint it imparts to timber.

In February 2002, the U.S. Environmental Protection Agency (EPA) announced a voluntary phase-out by industry of most residential uses of this arsenic-based wood preservative. The agreement states that beginning in January 2004, CCA-treated wood can no longer be manufactured for decks and patios, picnic tables, playground equipment, walkways/boardwalks, landscaping timbers, or fencing.

Some of the water-borne alternatives to CCA for residential use include  alkaline copper quaternary (ACQ) compounds, copper azole (CuAz), copper citrate, inorganic boron (SBX), copper HDO (CuHDO) and others.  You will note that many of these biocides contain copper, which is an excellent fungicide and also a good conductor.  After the voluntary phase-out of CCA lumber for residential use a significant amount of treated lumber was produced that led to rapid corrosion of the fasteners.  A galvanic cell can occur and promote corrosion “rust” on the fastener.  Over time the integrity of the fastener can be compromised and the effective diameter of the fastener is decreased.  The solution is to always use galvanized fasteners and perform regular inspections of your fasteners for corrosion and your wood for early stages of decay.   Preservative treated lumber has been modified in recent years and the risk of fastener corrosion is now very minimal.  It is still advisable to use galvanized or stainless steel fasteners for any project that will be subject to moisture.  This is true for projects with untreated wood as well as treated wood.

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Wood Identification

The first step to properly utilize wood is to correctly identify the species.  Wood species have unique anatomical characteristics that allow a trained eye to identify the species.  These unique anatomical characteristics are also important in determining many of the properties of the species, such as strength, dimensional stability, acoustical, conductivity, ease of pulping for paper and other applications, and energy yield.    Some anatomical properties can be modified by growth rate. 

For example, we all know that faster grown pine trees have wider growth rings but did you know that they also have shorter fiber lengths and higher microfibril angles (MFA) on the S-2 wall.  The MFA is the angle of the cellulose chains on the thickest component of the cell wall and a high angle generally leads to less desirable wood properties.  Be careful when relying upon color or odor to make species determinations.  They can be helpful but also tricky.  For example, I have seen hybrid walnut that did not have the traditional chocolate brown color.  Also, aromatic species will lose their scent over time. 

One final warning, if you see an old “pine” board with tight growth rings, don’t always assume that it is old growth southern yellow pine.  If you see little white dots visible on the latewood (the darker bands) then you are seeing the resin canals, and it is a pine.  If not, it is likely cypress. 

I have served as an expert witness on several cases in which species identification was critical.   The most recent involved a family purchasing very expensive oak flooring and the company represented the flooring as oak.  I was asked to identify the wood species and issue a report of my findings.  It was not oak.  It was a much cheaper wood that was stained with an oak finish.  If you need wood identified for a legal matter, hobby, or whatever, please email me at tfshupe@gmail.com.

In Service Product Failure

One of the most common problems that I have addressed over the years is in service product failure.  The problems range from buckling/cracking of wood flooring to failure of utility poles, chairs, decks, fences, bleachers, roofs, deer stands, ladders, etc.   The first question we must determine is a big one.  Is this a manufacturing problem or improper use/application problem.  I have seen instances when it is clearly one or the other and in some cases both.   As a consumer/user we can’t do much for products that are improperly designed and/or manufactured.  However, we can educate ourselves to try to buy the right product for the job.

The second question we must determine is also a biggie.  Was the product inspected and/or maintained over time?  If so, who did it?, What did they do?  What were their credentials?  Was a standard operating procedure being used to determine the nature and frequency of the inspection?  What documentation do we have? 

One example of improper design was a bar stool that was very tall with three small legs that were connected with glue.  An insufficient amount of glue was applied creating what is known as a starved glue joint.  It was inventible that the chair would eventually fail and it did. 

Photo of glue joint – courtesy USDA Forest Service

The interaction of wood and glue is important to develop a good bone and avoid product failure.

Poles that fail in service are carefully examined to determine if it was properly manufactured, type of preservative used, wood species, name of treater, year of last inspection, and pole class.  Then records are evaluated to determine the frequency and scope of inspections.  (Courtesy Dr. H.M. Barnes, Mississippi State University)

Why Did My Deer Stand Deteriorate So Quickly?

Deer stands should be made with treated lumber and plywood or OSB.

I love to deer hunt and have many friends that do also.  In the US South we are in the harshest decay zone due to our high temperatures and humidity.   Therefore, any wood in contact with the ground must be naturally decay-resistant or preservative-treated for ground contact.  Second, wood composites such as particleboard are a bad idea.  The panels will swell and fail very quickly.  A better option is treated OSB or plywood.  Make sure your ladder to get to your stand is in good condition.  There are far too many preventable human deaths each year in deer stands.  Be safe!

Why Are My Nails So Rusty In Treated Wood?

Rusted fasteners have a thinner actual shaft diameter than a non-rusted fastener and are weaker as a result.

Damp wood can cause metals to corrode.  This is partly because wood is slightly acidic.  When a metal fastener is embedded in wet wood, conditions are created that can accelerate the corrosion of the metal. The corrosion products often result in slow deterioration of the wood surrounding the metal. Corrosion of the fastener combined with deterioration of the adjacent wood causes loss of strength of the joint and weakening of the structural integrity of the assembly. The exposed end of a steel fastener in damp wood quickly shows evidence of hydroxyl ion (OH-) formation. The exposed head of a metal fastener can become the cathode and the shank is the anode of a galvanic corrosion cell.

Dissimilar metals in contact with one another in a corrosive environment form a galvanic cell that results in accelerated corrosion of the less corrosion-resistant metal and very little attack of the more resistant metal. Thus, with dissimilar metals in electrical contact, the more noble metal becomes the cathode and the less noble one becomes the anode. Also, if the anode is iron, then ferrous ions can initiate degradation of the adjacent wood.

Corrosion of certain metals could be a problem in waterborne salt-treated wood in contact with soil when the moisture level of the wood is high.  This led to a resurgence in galvanizing and other coatings for fasteners and a reformulation of the copper in many copper-based wood preservatives.

REFERENCE: Baker, A. J., ‘‘Corrosion of Metal In Wood Products,” Durability of Building Materials and Components. ASTM STP 691. P. J. Sereda and G. G. Litvan, Eds., American Society for Testing and Materials, 1980, pp. 981-993. ABSTRACT: A description is given of the source of metal corrosion products that caus

Treated Wood Facts

Treated poles coming out of the cylinder and ready for distribution and installation. This photo is courtesy of: Mike Freeman

Want to know the secret for wood to last forever in an exterior application?   Many years ago, a colleague told me (tongue in cheek) that there are two ways to achieve this – (1) keep the wood dry and (2) don’t let the wood get wet.  However, since that is not possible in exterior applications, and only a small handful of wood species have natural decay resistance, we need to use preservative-treated wood in exterior applications.  It is important to realize the treated wood is decay and insect resistant and not decay and insect proof.   By the way, the same can be said of fire retardant treated wood.  The key for longer lasting treated wood is inspection and remedial treatments.  This is true for the homeowner with treated decks or the utility company with treated poles.  The inspection will identify the problem in its early stages and the remedial treatment will introduce an on-site preservative to stop the problem.   The inspection must be performed by trained personnel to identify early stages of decay, if present, and then assess if the damage has compromised the structural integrity if advanced decay is present.  Remedial treatments are excellent means of extending the service life of industrial products such as poles, piles, and cross ties.    Homeowners can also use them for millwork, columns, decks, etc.   A homeowner that is interested in remedial treatments should consider BoraCare (boron-based and very safe) or copper napthenate (very effective but has a strong odor).   Homeowners can also purchase Bor8 Rods and Cobra rods.    To use these products, you must first drill a hole in your wood, insert the rod, plug the hole.  Bor8 rods contain boron and Cobra rods contain boron and copper.  All of these products are non-restricted used pesticides available to the general public over the internet.   

Solar Dry Kiln

A common question among folks that have small portable sawmills is – how can I dry small loads of lumber?   Kiln drying lumber is an excellent means to add value to the lumber and make a small sawmill more profitable.  Many people that are interested in lumber drying don’t want to buy and operate a boiler and don’t feel that their operation is large enough to justify a dehumidification kiln.   Dehumidification drying is an excellent method of drying small to medium quantities of lumber.

Another alternative is solar drying.  A solar dry kiln is simple to construct and operate.  The USDA Forest Products Lab has already done all the research needed on how to build and operate the kiln.  The operator just needs to be aware of a few drawbacks.  First, there will be little to no drying on cloudy days so your total drying time may take longer than other drying methods.  Second, on sunny days you may have excessive drying so it is important to monitor the drying rate and use your fans and vents appropriately.  Solar lumber drying is no different than any other method in that you will need (1) moisture meter, (2) sample boards to monitor moisture drying, (3) scale or balance to obtain weights of your sample boards during drying, (4) a drying schedule.   Below is an excellent publication that summarizes how to build and operate a solar dry kiln.   Let me leave you with a few final thoughts on solar drying.  It is very important to understand that lumber can dry very fast or very slow in a solar kiln.  It can dry so fast that drying defects such as warp and case hardening develop.  Green wood that is put into a solar kiln has the potential to develop surface mold if the initial drying rate is too slow.  

DESIGN AND OPERATION OF A SOLAR-HEATED DRY KILN FOR TROPICAL LATITUDES