Galvanic Corrosion and Pressure Treated Wood

© 2005, Steven J. O’Neill, Attorney at Law

Out of the Frying Pan and Into the Fire

On February 12, 2002, former Environmental Protection Agency (EPA) Administrator Christie Whitman announced a voluntary decision by the wood treatment industry to stop production of arsenic based pressure treated wood for consumer use in favor of new alternative wood preservatives.  Effective December 31, 2003, Chromated Copper Arsenate (CCA) treated lumber may no longer be manufactured for residential or consumer uses in the U.S.  According to the EPA, arsenic is a known human carcinogen.  Beginning in the 1990s, a great deal of public concern began to be focused on the possible risks of human contact with CCA or “green treated” wood.  In its announcement, the EPA stated that it, “has not concluded that CCA-treated wood poses unreasonable risks to the public for existing CCA-treated wood being used around or near their homes or from wood that remains available in stores.”

The wood preservative treatment industry and building materials suppliers have been under legal attack by consumers alleging injuries from exposure to the arsenic contained in CCA treated wood.  However, plaintiffs’ attempts to achieve class action status for their claims against manufacturers and suppliers have been rebuffed.  In 2004, Judge Sparks of the U.S. District Court for the Western District of Texas held that the plaintiff had not satisfied the commonality, typicality and adequacy of representation requirements for class certification against Home Depot. (“Although not disclosed by their briefing to this Court, Plaintiffs' motion is the fourth attempt to certify a class of persons who own wood that is pressure treated with various forms of CCA. The prior three attempts-two of which were by the same plaintiffs' lawyers as in this case-were rejected. Ardoin v. Stine Lumber Co., 220 F.R.D. 459 (W.D.La.2004); Jacobs v. Osmose, et al., 213 F.R.D. 607 (S.D.Fla.2003) (‘Jacobs I’); Jacobs v. Home Depot U.S.A., Inc., 219 F.R.D. 549 (S.D.Fla.2003) (‘Jacobs II’)”), Martin v. Home Depot U.S.A., Inc.,  225 F.R.D. 198, 199 (W.D.Tex.,2004).  The Wood Preservative Science Council continues to defend CCA treated wood, stating that it is safe for people, plants and animals when used as recommended.

CCA treated wood has been in use since the 1930s and it is estimated that it constituted approximately 90% of all pressure treated lumber used in the United States before 2004. Roofing Contractor, Vol. 24, No. 11, Nov. 2004.  The EPA estimated that the phaseout would convert the annual production of some 3 billion board feet of arsenic treated lumber.  This widespread distribution and use in residential, commercial, industrial and agricultural construction guarantees that despite the voluntary substitution of non-arsenic based preservative by most wood treatment manufacturers, the competing legal concerns over injury and liability will be with us for years to come.  These issues are wider than the precipitating concerns over residential decks and playground structures.  Other issues such as occupational exposure are still on the front burner.  For example, the EPA continues to study worker exposure to arsenic and chromium treated wood. On March 19, 2004 the EPA issued its Preliminary Risk Assessment for Wood Preservatives for Workers Who Contact Wood Preservatives Containing Arsenic and/or Chromium.  This assessment study concerned both workers in the preservative treatment plants and those individuals who are exposed to CCA and related pesticides through postapplication activities such as commercial or institutional outdoor settings where the wood is fabricated into structures and professionally installed—that is, the construction industry.

The voluntary discontinued uses of CCA treated wood include dimensional lumber and wood used in play structures, decks, picnic tables and outdoor furniture.  There are notable exceptions in the phaseout for such products as marine lumber, utility poles, large timbers, structural glulams & LVL, and plywood.  Notwithstanding the focus on residential uses, as a consequence of the phaseout, an unscientific poll indicates that most lumber dealers no longer stock CCA treated dimensional lumber even for non-residential uses despite the cost increase of 20-30% for the substitutes.  The voluntary switch to non-CCA wood preservative treatments based mainly on concerns related to human exposures in residential and consumer situations has practically resulted in the unavailability of CCA treated lumber for all uses including commercial construction and wood truss manufacturing.

With limited time to adjust to the phaseout, the wood treatment industry quickly moved to standardize arsenic-free alternatives and move them to market.  Standardization requires a series of rigorous tests  under the auspices of the American Wood Preservers Association (AWPA) to ensure the durability of the wood.  Several arsenic-free preservative formulations have been Standardized by AWPA for uses previously dominated by CCA treated wood. According to the U.S. Forest Products Laboratory, nearly all of the alternatives rely heavily on copper as their primary ingredient.  In general, it is reported that the resistance to decay and insects of the copper-based substitute treatments is equivalent to CCA treatment for most applications.

Ironically, in the rush to move away from the perceived or actual harms of arsenic laden lumber in wide use by consumers, the construction industry has unwittingly cast itself headlong into an unintended collision with a new set of legal exposures and liabilities—the accelerated and inevitable corrosion of most metal connectors and fasteners by copper-based pressure treated woods due to galvanic or electrochemical reactions in moist conditions.  Lumber suppliers, home centers, fastener manufacturers, truss manufacturers, code officials, testing agencies, builders, metal connector (e.g. joist hanger) manufacturers and others were all slow to coordinate the universal roll-out of copper saturated wood with the necessary information and supply of adequate corrosion resistant connectors and fasteners.  As will be detailed below, metal connectors and fasteners that had been perfectly serviceable when used with CCA treated wood will corrode and fail when used with copper-based pressure treated wood.  The failures may reduce service life, lead to leaks in the building envelope or lead to structural failures.  Unfortunately, we may have traded one set of legal liabilities for another.

Unintended, but Foreseeable Consequences

Although the CCA treatment contains copper, it also has other chemical constituents that serve to inhibit corrosion.  There are a variety of  substitute water-based preservative treatments now widely available including: alkaline copper quat (ACQ-C, ACQ-D, ACQ-D Carbonate), copper azole (CB-A and CA-B), and Sodium Borate (SBX).  The most popular treatments are given trade names such as: Wolmanized Natural Select™ by Arch Wood Protection (Copper Azole), Preserve® by Chemical Specialties, Naturewood® by Osmose (ACQ) and Advance Guard® (Borate).  The actual chemical makeup and retention level of a particular wood treatment can only be determined by reference to the specific wood preservative treatment manufacturer.  Such technical information is relevant to the proper selection of fasteners and framing aids.  A careful specifier should now require the verification of the compatibility of all metals in contact with copper-based pressure treated woods.

Although no standardized testing protocol is yet available to determine the effect of high copper content treatment on metal connector and fastener corrosion, a broad search of industry literature indicates that it is two or more times more corrosive than CCA treated wood.  Sodium Borate treated wood is actually less corrosive than CCA treated wood but has other shortcomings when used in potentially moist environments such as sill plates.

This should not have been news in the construction industry at the beginning of 2004.  Some substitute treatments were already in use years before the announcement of the voluntary phaseout in 2002.  According to a U.S Forest Products Laboratory report in July 2000, acceptable treated wood options at that time (in addition to CCA) were listed to include: ammoniacal copper zinc arsenate (ACZA),  ACQ-B, ACQ-D, ammoniacal copper citrate (CC), CBA, copper dimethyldithiocarbamate (CDDC) and Borates.  The same Forest Products Laboratory report warned that, “water-based wood preservatives can increase susceptibility to corrosion, so all metal fasteners used with the treated wood should be hot-dipped galvanized or made of stainless steel.”

Indeed, this generalized advice is consistent with various national codes as adopted throughout the United States.  For instance, the Massachusetts  Code concerning fasteners for pressure treated has not been changed since before the phaseout.  It states: “Fasteners for preservative-treated wood shall be of hot-dipped, zinc coated, galvanized, stainless steel, silicon bronze, copper or other corrosion resistant materials.” 780 CMR 2311.3.3.  See, International Residential Code R319.3; International Building Code 2303.1.8.5 (hot-dipped galvanized steel or stainless steel, silicon bronze or copper) & U.B.C 2304.3.

With the copper-based wood treatments already in limited use in 2000 and the national building code organizations and the Forest Products Laboratory already requiring fasteners for preservative treated wood to be hot-dipped galvanized steel, stainless steel, et cetera, it would seem that the voluntary transitional phaseout off CCA treated wood to copper-based treatments required no change in thinking regarding the galvanic corrosion of metal connectors and fasteners.  In practice, the matching of fasteners was not seen as a critical issue.  Who hadn’t seen a shear wall nailed off into a pressure treated wood sill plate with “coated sinkers” instead of hot-dipped galvanized nails?  Surely, as the transition began in January of 2004, many of us witnessed loads of “new” copper-based pressure treated dimensional lumber purchased and shipped with the old “G60” coated joist hangers and single dipped galvanized nails.  In home centers, as the new G185 metal connectors were slowly phased in, the old G60 stocks were prominently displayed on discount racks in the same aisles as the copper-based pressure treated wood, in many cases without any notice of warning. (G60 designates 0.6 oz. zinc per sq. ft. of sheet steel).

What the construction industry failed to recognize before the phaseout was that little compatibility testing had been done, that the available information on corrosion needed to be disseminated to actual end users, and that the existing standards for hot-dipped galvanization of metal connectors and fasteners were insufficient to protect against galvanic corrosion for any reasonable service life.  These failures of science and communication were not just evident in the hapless homeowners and builders who carted out their copper treated lumber with G60 or G90 joist hangers and lightly galvanized gun nails in 2004.  These shortcomings during the transition were identified by one of the most sophisticated and regulated components of the building industry.  In a May 14, 2004 letter providing public comment to the EPA, both the President and the Executive Director of the Wood Truss Council of America (WTCA) wrote that the typical G60 galvanized connector plates, “that are in general use today for interior applications and have been used with CCA treated lumber, in both exterior and interior applications, can no longer be used with the new copper based preservative treatment due to the corrosive nature of the treatments.”  The letter further stated, that to assure public safety: “WTCA recommends that CCA be re-registered as a valid preservative treatment for metal plate connected wood truss use.” EPA Docket Number: OPP-2003-0250.

In addition to the various building codes’ long-standing recognition that metal can corrode when in contact with preservative treated wood, the codes also recognized that the moisture content of the treated lumber could influence the corrosion of metal connectors and fasteners.  For instance, the (unchanged) Massachusetts code holds that: “Where wood that is pressure treated with a water-borne preservative is used in locations where drying in service cannot readily occur, such wood shall be at a moisture content of 19% or less before being covered with insulation, interior wall finish, floor covering or other material.” 780 CMR 2311.3.2.  The Truss Plate Institute (TPI) also recognizes that a high moisture content from condensation or other sources will affect corrosion: “Although the preservative treatments protect the lumber from decay in such moist environments, the presence of moisture in preservative-treated wood can substantially increase the corrosion rate of metals in contact with the treated wood.” Kelly Gutting, TPI Technical Director, at www.tpinst.org (accessed 2003).  It should be noted that the non-standardized tests conducted for fasteners (e.g., AWPA E12) and currently relied upon for the predictions of corrosion in the industry, do not subject the samples to a relative humidity of higher than 90%.  It is reasonable to ask what the corrosive properties of copper-based wood treatments are in high humidity environments such as raised wood floor systems, where a building envelope has failed, coastal locations, agricultural confinement facilities and naturally, for decks.

The presence of moisture (and especially of high moisture levels) provides the third element necessary to enable the process of galvanic corrosion in dissimilar metals.  To a chemist, pumping wood full of copper (some treatments consist of over 90% copper) and then placing a zinc or steel nail inside it in the presence of water is an obvious and foreseeable invitation to corrosion.  Somebody should have “connected the dots” before the phaseout began.  Without getting scientific, all it takes to witness galvanic corrosion is to drive an uncoated nail into a scrap of ACQ lumber and leave it in a damp location.

The Science of Galvanic Corrosion

The science of galvanic corrosion is not new and not that complicated.  A basic non-scientific description points out that one of the metals joined in the “galvanic couple” actually corrodes faster than it would naturally in the same environment:

Galvanic corrosion refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. It occurs when two (or more) dissimilar metals are brought into electrical contact under water. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. (Update, www.corrosion-doctors.org link no longer functions.)

In the 1780s Italian biologist and physician Luigi Galvani discovered that electrical charges could be used to stimulate muscle movement in frog specimens and stumbled upon a phenomenon where the use of two dissimilar metal probes resulted in muscle movement without the application of any outside electrical charge.  His colleague, Alessandro Volta, built upon this work and found that the electrical charge was caused, not by animal energy as Galvani had thought, but the presence of the two dissimilar metals in an aqueous environment.  In 1800, Volta used this information to invent the voltaic pile, or battery.  Quite relevant to current concerns about the “coupling” of galvanized (zinc) fasteners with copper treated wood, Volta’s battery was constructed by stacking alternating disks of 1) copper, 2) cardboard soaked with acid or salts, and 3) zinc!

Volta’s electrochemical battery involves the same scientific principles as galvanic corrosion:

Corrosion is the disintegration of metal through an unintentional chemical or electrochemical action, starting at its surface. All metals exhibit a tendency to be oxidized, some more easily than others. A tabulation of the relative strength of this tendency is called the galvanic series. Knowledge of a metal's location in the series is an important piece of information to have in making decisions about its potential usefulness for structural and other applications. (Update, www.corrosion-doctors.org link no longer works.)

These eighteenth and nineteenth century scientific principles were already being applied to the construction of the Statue of Liberty in 1885, for example.  The galvanic coupling between the iron superstructure and the copper skin was originally prevented by placing shellac soaked asbestos cloth between the two dissimilar materials.  Over time, the moist marine environment caused the cloth to become soaked with salt water—an ideal conductive electrolyte for galvanic corrosion.

The potential for corrosion between dissimilar metals depends upon where they are located relative to one another in the galvanic series.  The galvanic series is often referenced as the Galvanic Table which is derived from a military specification based on relative electrical measurements in flowing seawater, MIL-STD-889. The seawater galvanic series lists metals based on their tendency to corrode in marine environments.  If any two metals in the series are coupled, the one closer to the active (anodic) end of the series will corrode faster than it would alone, and the one closer to the noble (cathodic) end will suffer less corrosion.  For purposes of illustration a partial list of metal alloys and their relative voltage potentials is listed in the following example of a Galvanic Table:

Alloy Approximate Voltage Range
Reference Electrode
Magnesium -1.60
Zinc -0.98
Aluminum Alloys -0.80
Cast Irons -0.65
Steel -0.65
Aluminum Bronze -0.35
Brass (Red, Yellow, Naval) -0.35
Copper -0.32
Silicon Bronze -0.25
Silver -0.12
300 Series Stainless -0.07
Platinum +0.22


According to Dr. Stephen C. Dexter, Professor of Applied Science and Marine Biology at the University of Delaware, “[t]he two major factors affecting the severity of galvanic corrosion are (1) the voltage difference between the two metals on the Galvanic Series, and (2) the size of the exposed area of cathodic metal relative to that of the anodic metal.” He further writes that, “[c]orrosion of the anodic metal is both more rapid and more damaging as the voltage difference increases and as the cathode area increases relative to the anode area.” Galvanic Corrosion, www.ocean.udel.edu/seagrant/publications (table also excerpted and adapted).

That is, the further apart the two dissimilar metals are in the galvanic series, the greater the electrical potential or voltage to corrode the metal on the more anodic (top) end of the Series.  Based on the Galvanic Table, it is clear that zinc (galvanized coating) is anodic relative to copper and the electrical potential of the combination is 0.66 volts.  As a consequence, whenever a zinc galvanized coating is in contact with copper in the presence of moisture (electrolyte), the zinc will unendingly corrode until it is gone.

In contrast, stainless steel is below copper in the galvanic series.  The combination also has a smaller electrical potential of .25 volts.  In fact, the presence of the copper as an anode relative to the stainless steel results in an electrochemical process which in theory retards the natural corrosion of the stainless steel.  As will be seen in the recommendations by metal connector and fastener manufacturers (as well as the Consumer Product Safety Commission) below, stainless steel alloys are preferable to zinc galvanization for use with copper treated wood.

In our real life example of copper treated wood in contact with zinc galvanized fasteners, another conclusion might be drawn from Dr. Dexter’s work.  A zinc galvanized nail embedded in copper treated wood meets the condition of a large cathode area (copper) relative to the anode area (zinc) resulting in more rapid and damaging corrosion to the anode.  In contrast, the effect of the cathode to anode ratio using stainless steel is the reverse.

This knowledge can be used to validate another recommendation echoed by metal connector and fastener manufacturers: galvanized fasteners should not be used with stainless steel framing aids.  Using the galvanic series table above, it evident that the electrical potential between the large stainless steel cathode (e.g., a joist hanger) and the smaller zinc galvanized anode (hot-dipped galvanized nail) is an invitation to corrosion even in the absence of preservative treated wood.

Although the basic science underpinning galvanic corrosion is more than two centuries old, galvanic corrosion continues to be the subject of rigorous research and testing in marine environments as well as the space program.  The research behind the Galvanic Table was developed by the Army Missile Command.  In 1960 NASA established the predecessor of the Corrosion Technology Testbed at the Kennedy Space Center to conduct and document corrosion research.  The March 2004 Mars Science Laboratory Preliminary Mission Assurance Plan utilizes the same standard reference for galvanic corrosion:

  • 5.2.4 In applications where dissimilar metals will be in intimate contact, the metals shall be compatible with regard to galvanic corrosion to the greatest extent possible. Methods to minimize the potential for galvanic corrosion shall be implemented. MIL-STD-889 shall be used as a guideline for controlling dissimilar metal interactions.

All of the prolific research on corrosion was available to construction industry testing agencies, wood preservers, code agencies, fastener manufacturers and government agencies prior to the announcement of the “voluntary” switch to copper based alternatives.  Yet, virtually the first industry news of the potential for galvanic corrosion with the substitute lumber came on September 1, 2003 in an article published in Professional Builder. No. 9, Vol. 68; Pg. 30 (“The International Staple, Nail and Tool Association is watching the transition, too, and developing tests for the corrosiveness of different fasteners with the alternative treated wood products.”)  Notably, Simpson Strong-Tie Co. (a major manufacturer of metal connectors, anchors and other framing aids) began to address the issue in its 2003 catalog: “Relative corrosion based on quantitative measurements, visual observations, and calculations indicate(s) the new wood preservatives tested are generally more corrosive than CCA.”  Beyond this, extensive searches in Lexis/Nexis and on the internet combined with interviews of representatives of building material suppliers, fastener suppliers, contractors and building officials failed to unearth references to industry concerns with the issue of galvanic corrosion prior to late 2003.

Out of Phase

Although the CCA based dimensional lumber inventory was effectively phased out of commerce and generally unavailable by December 31, 2003, the availability of adequate metal connectors and fasteners was not significantly phased in until much later.  Presumably because the building codes had always provided for the use of hot dipped galvanized metal connectors and fasteners with preservative treated lumber, there was no apparent problem shipping and using existing stocks of G60 and G90 metal connectors and fasteners with the new copper-based preservative treated wood.  In 2003 Simpson Strong-Tie Co. advertised its ZMAX® G185 coating (i.e., 1.85 oz. zinc per square foot; ASTM Standard A653) as an option but very few of its G185 product offerings were available as stock items until well into 2004.  Based on a document posted on its website, USP, another metal connector manufacturer, appears to have called for a coordinated transition in 2003.  However, according to spot checks of lumberyards carrying the USP brand of metal connectors during the first half of 2004, the availability of G185 connectors offered by USP similarly lagged behind the inventory of copper-based pressure treated wood.

Not only were most types of G185 metal connectors not available well into 2004, but with some exceptions, manufacturers of fasteners for pneumatic nailers were slow to refresh inventory with heavier galvanized coatings. (In November 2003, Paslode announced that its zinc coated strip-fasteners coated with 1.0 oz. zinc per square foot, per ASTM A153, “fully met the requirements for use in pressure treated lumber.”)  There are several different types of strip nails, each designed for a particular class of pneumatic nailers.  While Paslode appears to have been early to market with a galvanized fastener it represents as adequate, fasteners for other popular types of pneumatic nailers were not widely available in New England until mid-2004.

In addition, as late as mid-2004, building officials in Massachusetts were not yet inspecting (or required to inspect) whether galvanized metal connectors used with copper based pressure treated wood were coated to G60, G90 or G185 standards; G185 is now considered the minimum galvanization required.  This lag in inspection may or may not be representative of the country as a whole.

While the stocks of G185 metal connectors and ASTM A153 fasteners appear to have finally caught up with the ubiquitous availability of copper-based pressure treated wood, no effort has been made to document the extent of use of G60 and G90 connectors and lightly galvanized fasteners during the transitional phase.  That phase began, not necessarily on January 1, 2004, but when the first stocks of copper-based pressure treated dimensional lumber were placed in inventory and sold for residential and commercial use in 2003.  The obvious problem is that the coating on such lightly coated metal connectors and fasteners is now universally considered to be inadequate.  The corrosion is very likely to affect the service life of such combinations and possibly lead to structural failure!

Industry, Code and Governmental Positions

In light of initial concerns with the transition publicized in late 2003 and early 2004, manufacturers of metal connectors and fasteners rushed to accommodate the new need.  Like Paslode, numerous other manufacturers emblazoned their packaging and other marketing materials with an “ACQ” stamp of approval.  For example, the 2004 Simpson Strong-Tie catalog carried a chart indicating that its G185 products were recommended for use with ACQ-C, ACQ-D, CBA-A and CA-B.  Remington, a manufacturer of Powder Actuated Tool (PAT) fasteners has guaranteed, “that there will be no issues with customers using PAT fasteners 1” in length and longer with pressure treated material.”  Chemical Specialties, an ACQ treated wood manufacturer, hosts a website listing about 50 fastener manufacturers.  www.treatedwood.com./fastener.pdf  The Information Sheet states that adequate fasteners include hot-dip galvanized, stainless steel and other fasteners and hardware as recommended by the hardware manufacturer.  It further specifies that: “As a minimum requirement for use with treated wood, hot dip galvanized coated fasteners should conform to ASTM Standard A153 and hot dip galvanized coated connectors should conform to ASTM Standard A653 (Class G-185).”

As stated above, the building codes had already allowed hot-dipped galvanized fasteners for CCA treated wood but do not appear to distinguish among the various preservative treatments with respect to metal connectors and fasteners.  Other organizations seem to concur with the code.  For example, the University of Missouri Extension currently recommends that fasteners meeting ASTM A153 are adequate with ACQ, CBA, or CA‑B treated wood.  The U.S. Forest Products Laboratory states: “Hot-dipped galvanized fasteners and connectors (meeting ASTM A153 and ASTM A653 Class G185 sheet, respectively, or better) are recommended for protection against the effects of moisture often present where treated wood is used.”  However, it has been reported that the Consumer Product Safety Commission now recommends only stainless steel metal connectors and fasteners in conjunction with ACQ treated wood.

On the face of things, with almost all voices on the topic in unison regarding the adequacy of hot dip galvanized coatings, one might conclude that the transition is complete and that the only risks and potential liability that might be of concern would be from the transition period in 2004, before the ASTM A153 and G185 material was fully stocked.

Hedging the Risk of Failure

In actuality, a close reading of the available industry literature and some recent news reports and industry publications shows that the issue may still be alive.  Manufacturers of treated wood rely on the recommendations of metal connector and fastener manufacturers and vice versa; while actively recommending hot dip galvanized connectors and fasteners, both groups seek cover and attempt to hedge the issue by stating that stainless steel should be considered.  Along with a warning that it is impossible to predict accurately if or when significant corrosion of connectors, anchors, and fasteners will begin or reach a critical level, the 2003 Simpson Strong-Tie catalog states: “The treated wood industry specifies or recommends stainless steel and post-production hot-dipped products for use with pressure-treated wood.”  Simpson Strong-Tie is widely considered to be the leader in testing and public dissemination of information on this topic.  See, www.strongtie.com.

However, by 2004, the Simpson Strong-Tie catalog recommendation showed even less confidence in galvanized coatings and again looked to the treated wood industry:

Due to the uncertainties, which are out of the specifier’s control, in regard to the chemicals used in pressure treated wood, Simpson recommends the use of stainless steel fasteners, anchors and connectors with treated wood when possible.  At a minimum, customers should use ZMAX™ (G185 HDG per ASTM A653), Batch/Post Hot-Dip Galvanized (per ASTM A123 for connectors and ASTM A153 for fasteners), or mechanically galvanized fasteners (per ASTM B695, Class 55 or greater), product with the newer alternative treated woods.  Due to the many variables involved, many of which are controlled by the chemical supplier and the wood treater, Simpson cannot make an unqualified recommendation of any galvanized or other coating for use with treated wood.  Additionally, because of the many variables involved, Simpson cannot provide estimates on service life of connectors, anchors or fasteners.  We suggest that all users and specifiers obtain recommendations for Batch/Post HDG, G185 HDG, mechanically galvanized, or other coatings from their treated wood supplier. * * *

Recognizing the still-evolving nature of this issue, the 2005 online Simpson Strong-Tie catalog added several significant new clauses to the “Understanding the Issues” section.  It now calls for periodic inspections:

  • “It is also important that regular maintenance and periodic inspections are performed, especially for outdoor applications.”
  • “If significant corrosion is apparent or suspected, then the wood, fasteners, anchors and connectors should be inspected by a professional engineer or general contractor and may need to be replaced.”
  • “Due to the many different pressure treatment formulations, fluctuating retention levels, and because the formulations may vary regionally or change without warning, understanding which connectors, fasteners, and anchors to use with these materials has become a complex task.”

In its 2005 Corrosion Protection News, USP states that its G185 connectors, “can be used with the new treated wood in many types of applications.”  However USP makes the absurd recommendation that, “[c]onsumers purchasing treated lumber should refer to the chemical supplier of the treated lumber for more detailed information.” www.uspconnectors.com/corrosionprotection.shtml.  Here, one is left to wonder how the end user is to consult with the chemical company that supplied the chemicals to the wood treater.  And most significantly, USP states: “Unfortunately, we are unable to predict the service life of particular connectors in selected environments.”

In reciprocal fashion, the wood treatment industry points to the individual fastener manufacturers for specifics.  For example, both the 2004 and 2005 versions of the treatedwood.com Fastener Information Sheet approve hot dip galvanized fasteners (ASTM A153) as a minimum but suggest that stainless steel be considered for “optimum performance and longevity” in treated wood.  The document then bounces the responsibility back to the fastener manufacturers: “Consult individual fastener manufacturer’s recommendations for information about the performance of their products with treated wood.”

Beyond the legal issues of whether such disclaimers are effective to shield the hardware manufacturers, treated wood manufacturers, design professionals, suppliers and contractors from liability, the industry must confront the technical and economic aspects of the corrosion issue before the installed base grows.

Hedging More than a Theoretical Risk

The risks are more than theoretical given the science behind galvanic corrosion.  The 2005 online Simpson Strong-Tie catalog candidly concedes: “The corrosion performance of a hot-dip galvanized product is a function of the amount of zinc on its surface.”  That is, where the conditions for corrosion are present, it will continue unabated until all of the zinc plating is gone and the corrosion reaches the steel substrate, where it will radically accelerate.  Under controlled conditions, scientists can actually measure and predict the thickness of a zinc coating that will corrode away in a given time.  (For reference, a G185 coating is 40 microns thick, or 0.00156 inches.)  It is essential to understand that galvanization is not some magic shield, impervious to its surroundings like a golden dome, it is a sacrificial coating in a corrosive environment!

The industry, code officials and the public have slowly awakened to the risks.  In July 2004 the Massachusetts Board of Building Regulations and Standards began recommending that building contractors be required to indicate treatment type and fastener selections on plans submitted for approval.  In November 2004, the District Attorney for Contra Costa County California, issued a Consumer Alert warning that wood treated with ACQ or Copper Azole, “may result in serious and premature corrosion . . . especially in wet or moist conditions.”  According to commission spokesman Scott Wolfson, the Consumer Product Safety Commission is now recommending that consumers use stainless steel connectors and fasteners with ACQ treated wood. Sacramento Bee, Nov. 10, 2004.  W.R. Grace, marketing a peel-and-stick membrane to isolate copper treated wood from galvanized metal connectors, states: “Under continuously wet laboratory conditions, corrosion of galvanized metal connectors in contact with ACQ, AC-B, and ACZA-treated wood can become visible in a very short period of time—weeks or months rather than years.  * * * Since corrosion begins from the inside of the joist hanger (at the point of contact with the wood), it is difficult to notice until it becomes quite advanced.”  (Source - WR Grace and Company, accessed as www.graceathome.com 2005.)

It is important to reiterate that although arsenic treated wood was taken off the market for residential and consumer use, the substitutes are now in common use for commercial construction, utility trailers, roof blocking, special application trusses and myriad other uses.  For example, Wisconsin DOT found that their use of ACQ signposts caused aluminum road signs to literally disintegrate in half after only 15 months of service.  See, Aluminum Sign Corrosion Investigation Final Report #WI-06-04, September 2004.  As stated above, the Wood Truss Council has expressed concern regarding the ability of metal plate connectors to hold up in corrosive environments.  Also,  aluminum flashing could previously be placed next to CCA treated wood but generally remained intact with some pitting.  Now, placed against copper based treated wood, aluminum flashing will essentially disappear and open the building envelope to moisture.

Several independent sources have warned that white and red rust quickly forms when hot dipped galvanized fasteners are tested with non-arsenate treated wood.  According to an EPA report, “[t]here has been one age-accelerated test conducted by the building industry which indicates that even hardware that advertises the improved resistance to corrosion begins to show signs of rust within 1000 hours of age-accelerated testing (equivalent to 16 years of installed exposure) when used with ACQ-treated wood.”  www.epa.gov/oppad001/safetyprecautions.htm.  This finding was echoed in tests conducted by PT Plating Technology (a fastener manufacturer) using the ASTM B117 salt-spray protocol.  (600 hours to red rust at the heads of fasteners; 1356 hours to red rust on the shank of the nail encapsulated in the ACQ wood).  www.pt2000plus.com/magazine.htm

Senco Products, Inc. (a fastener and construction tool manufacturer) conducted its own tests also using the ASTM B117 protocol.  The tests indicated white and red rust at approximately 300 hours on hot dipped galvanized and electro-galvanized fasteners and further that stainless steel nails, “did not exhibit any corrosion.”  Contrary to the EPA statement correlating test results to service life, Senco stated: “Unfortunately, there is no accepted correlation between the number of hours to red rust in B117 versus the number of hours in real-world performance. See, Pressure Treated Lumber Changes Senco Products, Inc. Recommendations, November 2003. at www.senco.com.  Senco also plainly states as a fact: “The long-term structural impact of non-arsenate based treated lumber on steel fasteners is unknown.” Id. (Emphasis added.)  Senco, clearly aware of the problem, hedges by recommending “first and foremost stainless steel products” for pressure treated lumber, yet permits treatedwood.com to recommend its hot dipped galvanized and Weatherex®3 coated screws for use with ACQ treated wood.

The Ontario Association of Architects recently issued a practice bulletin stating: “Practices and customs that were adequate with CCA treated lumber may not be appropriate for use with the new products.”  Pressure Treated Wood Alert – Chemicals in Common Use and Corrosion of Metals, Ontario Association of Architects, Practice Bulletin E.3, February 1, 2005, at www.oaa.on.ca.  Relying mostly on the American manufacturer data and ASTM references, the Canadian Bulletin warned of the failure of components:

Architects and their consultants are advised to review their designs, specifications and site review activities in recognition that this increased corrosion can lead to loss of service life or early failure of components that are not sufficiently corrosion resistant including steel hangers and connectors, screws, nails, anchor bolts, and sheet metal flashings and cladding which are in contact with the wood. Id.

Similarly, the Construction Specifications Institute published an online set of recommendations for changes in construction practices in response to the new treated wood properties.  The recommendations, authored by Dennis R. Cook, P.E., for example, cautioned that one should be aware of any reduction of allowable shear values for shear wall sheathing at the sill plate because of a change in fasteners.  He also cautioned that fasteners used to affix studs to a pressure treated sill plate must be compatible with the treated wood.  According to Mr. Cook, G185 metal connectors are recommended as an alternative to stainless steel.  See, Horizons: Wood Preservatives & Corrosion, March 2004, at www.csinet.org.

Risk-Benefit and Externalized Costs

Without any evident oversight or coordination, the transition from CCA treated wood to non-arsenic copper based treatments has been guided by market forces.  Stainless steel connectors and fasteners are substantially more expensive than even the new heavily galvanized coatings.  In addition, stainless steel connectors and fasteners are generally not stock items and are more difficult to obtain.  Stainless steel nails also do not have the holding power and strength of hot-dipped galvanized nails.  The moderate additional cost of G185 connectors together with the unchanged building codes, the lack of standardized tests for corrosion, the prior lack of test data on corrosion, and the hedged statements from the treated wood industry and the metal connector and fastener industries all streamlined the transition.

At the same time, the industry was aware that, “the long-term structural impact of non-arsenate based treated lumber on steel fasteners is unknown.”  In little more than a year we have moved to a product combination where no one is willing or able to predict a service life for an assembly.  Not very long ago G60 metal connectors were considered standard in the U.S. even for CCA treated wood.  A broad search of Lexis/Nexis identified no prior concerns with the corrosion of these assemblies except in composting facilities, sewage treatment plants, agricultural buildings with high ammonia atmospheres and similar extreme conditions.  While manufacturers advertise that they have tripled the galvanized coatings of connectors, they now indicate that if you want your structure to last, you really ought to consider stainless steel connectors and fasteners.

Should we be concerned when metal connector manufacturers for the first time recommend that their connectors should be subject to regular re-inspection?  Since corrosion of galvanized metal connectors logically occurs from the inside out, is it necessary to remove the connectors to actually inspect them?  What if the building assembly is such that it is covered by follow-on work such as strap-tie holdowns nailed to pressure treated sill plates?  Is invasive inspection required?  Who is to bear these costs?

There are potential legal liabilities that have been created by the disconnected manner in which CCA wood has been replaced by non-arsenic copper-based alternatives.  However, the analysis of the various legal claims that might be made by and against different parties related to this evolving issue is beyond the scope of this article.  Fortunately, there have been no news reports linking structural failure or collapse to the new product combination.  The delayed phase-in of the more heavily galvanized connectors and fasteners should motivate anyone (specifier, designer, contractor, owner, etc.) who dealt  with lightly galvanized connectors and fasteners between late-2003 to mid-2004 to seriously consider the recommendation of at least one connector manufacturer to re-inspect and replace if necessary.  The continued grave questions surrounding the long term adequacy of any galvanized metal connectors or fasteners when used with the new copper based treated wood should give pause to anyone who is concerned with service life, especially where there is any likelihood of moisture in the assembly.  With everyone trying to disclaim responsibility, be careful that the liability doesn’t end up on you!

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