Corrosion of Fasteners and Connectors
“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
Are connector manufacturers’ test results
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 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
TECH NOTES SERIES 1.0
Revised August 30, 2004