Corrosion of Fasteners and Connectors Used with
“Next Generation” Preserved Wood

Questions have been raised about the corrosive effect of the “next generation” products on fasteners and connectors. The increased levels of copper contained in their preservatives, as well as the azoles and quaternary co-biocides used in the formulations, have increased concern about and awareness of fastener and connector recommendations.

What types of treated wood are replacing CCA?
The common generic names for American Wood-Preservers’ Association (AWPA) listed treatments available today are Copper Azole and ACQ. There are two variations of copper azole, Type A and Type B. These two formulations differ in that the Type B contains no borates. For ACQ, AWPA lists ACQ-A, ACQ-B, ACQ-C, and ACQ-D. These formulations vary by the type and quantity of quaternary compound in the formula as well the use of an amine or ammonia solvent. Most of these quaternary compounds used (BAC, DDAC) contain chloride ions that can be highly corrosive to steel.

Does treated wood have to undergo testing before it is sold?
Corrosion tests are recommended in the AWPA evaluation process to list a preservative system in the AWPA book of Standards. Arch Wood Protection has submitted AWPA E12 Corrosion Tests on both copper azole Type A and B for committee consideration. These tests showed that corrosion rates of Natural SelectÔ (CBA-A and CA-B) treated wood as well as CCA treated wood, in contact with hot dipped galvanized steel, were in the “excellent range” as defined in the “Corrosion Engineering Handbook.” The makers of ACQ have also provided data to the AWPA on 3 of the 4 types of ACQ, Types B, C, and D. Their recent results for the amine formulations are categorized as “good” in the “Corrosion Engineering Handbook.” Corrosion test results on a new quaternary compound known as “carbo-quat” which was introduced for use in ACQ this year have not been submitted to AWPA and are not included in the values above. Typically results from the E12 Corrosion Test are not repeatable enough to be compared statistically from test to test, or, often, within a single test. It should also be noted that results are reported as a corrosion “rate” measured in mils per year, but are based on measurements taken at two points in time. Preliminary data from tests with additional points of measurement indicate that the two-point rates do not accurately reflect ongoing degradation.

Are connector manufacturers’ test results different?
The Simpson Strong-Tie and USP Structural Connector results are reported as “Relative Corrosion Rates.” They compare corrosion rates for copper azole and ACQ-D (specified as carbo-quat by Simpson Strong-Tie) to corrosion rates for CCA treated wood. The relative corrosion rate of copper azole compared to CCA is similar to that in earlier tests Arch submitted to AWPA, which indicates consistency in the copper azole results. Similar results on the ACQ treated wood may be due to the use of the recently approved carbo-quat quaternary compound tha t has less corrosion potential than quats used in the original tests reported to AWPA.

So how do I interpret these results?
Despite the problems with comparing results from test to test and even within a test, looking at the trend of results from multiple tests can still be informative. We believe they confirm Arch’s position that there is a slight increase in corrosion potential as you move from untreated and borate (DOT) treated to CCA treated to copper-amine treated wood.

Is there a standard test protocol?
AWPA E12, similar to the Military Standard Specification 19140E, is the current standardized test available today. However, the AWPA, International Staple, Nail, and Tool Association (ISANTA), the American Galvanizers Association (AGA), and the International Lead Zinc Research Organization (ILZRO) are all actively working to develop an improved consensus standard that will give better real life predictability to these tests and result in better use recommendations.

How do I know what I am buying?
For identification purposes, Wolmanized® Natural Select™ wood is labeled with an end tag or stamp, which indicates the specific type of treatment. Arch views the latest change in the ACQ formula as a positive step for the industry to address the concerns about corrosion in next-generation treated wood. If buying ACQ treated wood, be sure that it is labeled, and, for confidence in its performance properties, confirm that the label says the wood contains the carbo-quat formulation.

Arch recommendations.
Arch does not depend solely on lab results such as the AWPA tests. We couple this research with more than a decade of real life service of copper azole treated wood throughout the world, which clearly demonstrates that Wolmanized® Natural Select™ wood can be expected to perform as intended. As with CCA, nearly all Wolmanized® Natural Select™ wood is used outdoors in everyday environments which have always required hot-dipped galvanized fasteners, regardless of treatment. Our current best management practice for Wolmanized® Natural Select™ wood follows. As test protocols change, so may these recommendations.

Hot-dipped galvanized fasteners (meeting ASTM A 153) and connectors (ASTM A 653 Class G185 sheet), or better, are recommended for protection against the effects of moisture often present where treated wood is used.

For Permanent Wood Foundations, use 304 or 316 stainless steel. Stainless steel may also be advisable in corrosive environments, such as areas with saltwater spray.

Aluminum should not be used in direct contact with Wolmanized® Natural Select™ wood, unless an adequate protective physical barrier separates the aluminum from the wood or the manufacturer ensures the performance of the aluminum product in contact with Wolmanized® Natural Select™ wood.

Indoors, while galvanized fasteners are preferable, the use of non-galvanized nails or screws of sizes and types approved by the Model Code is acceptable when attaching joists, studs, or other framing to Natural Select™ sill plate, provided the wood will remain dry in service, protected from weather and water. Under similar conditions, the use of standard galvanized strapping, anchor plates, or mild steel
anchor bolts ½? diameter and larger is also acceptable for fastening Natural Select™ wood to foundation.

Arch recommends that you always use fasteners and connectors that meet these standards and are labeled as appropriate for use with the new copper based alternative wood treatments.

TECH NOTES SERIES 1.0
Revised August 30, 2004