Corrosion Resistance and ACQ Compatibility

With Hurricane Irene having paid us a visit this past weekend, I’ve received a flurry of questions around corrosion resistance and ACQ compatibility with regard to fencing, decking and the like.  Sounds like a good first topic to me!

Metal fasteners WILL corrode and may lose their load carrying capacity when installed in a corrosive environment  or when exposed to corrosive materials.  Many of the day-to-day materials and climates we face here on the Eastern Seaboard can cause corrosion including the ocean salt air, fire-retardants, fumes, fertilizers, dissimilar metals and of course, preservative-treated wood.

With all of these variables, it is impossible to predict “if” or “when” corrosion will begin to reach a critical level and impact the performance of the fastener.  This uncertainty makes it crucial that users be knowledgable of the potential risks and select a product coating or metal suitable for the intended use.  Regular maintenance and periodic inspections are also important, particularly for outdoor applications.

It is very common to see some corrosion on fasteners in outdoor environments.  Even stainless steel will corrode.  The presence of some corrosion does not mean that the load capacity of the fastener has been affected or that a failure will occur.  If significant corrosion is apparent or suspected, then the wood, fasteners and connectors should be inspected by a qualified professional engineer or general contractor and may need to be replaced.

In the last decade, preservative-treated wood formulations have changed significantly. Some of the new formulations have more corrosive qualities than the traditionally used formulation of CCA-C.  The wide variety of wood-treatment chemicals available in the market today has generated much confusion and created complexity around what was once a very simple issue for builders and end users.  This article, (click here) has some great information on wood preservatives and what to be aware of and consider.

For the Paranoid – I give you Stainless Steel

There is much discussion around what is the “best” fastener and fastener material to use for either corrosive environments, preservative-treated wood or both and the answer will usually end with something like, “it depends.”  Whenever I am asked, the “it depends” portion is usually specific to what material grade of stainless steel makes the most sense.

Laboratory studies clearly demonstrate the superior corrosion resistance of stainless steel. Stainless steel is composed of iron base alloys containing a minimum of 10.5% chromium. Chromium is the key to 300 series stainless steel’s corrosion resistant qualities.  The chromium combines with oxygen to form a tough, thin, clear film of chromium oxide on the metal’s surface, insulating the alloy against caustic attack.

304 or 316?  Type 304 stainless steel provides more than adequate protection against corrosion in most commonly encountered applications.  Type 316 contains slightly more nickel than 304, plus 2-3% molybdenum, giving it better corrosion resistance than 304, especially in high chloride environments prone to cause pitting such as those exposed to immediate coastal environments and sea water.

One of the first companies to conduct any type of laboratory investigations into the corrosive behavior of wood in contact with ACQ and other preservatives was Simpson-Strong-Tie (Dublin, California).  In 2007, Simpson Strong-Tie acquired Swan Secure Fasteners and now offers a line of 304 and 316 stainless fasteners that are considered by many to be some of the best available.

If stainless steel is cost prohibitive for you or just not an option I have one last piece of advice, “Trust but Verify.”  It is important to confirm for yourself that the product you are using is the correct one.  Normally, product packaging will clearly state “ACQ compatible” or “for use with ACQ treated lumber” or words to that effect.  If you have any doubts, contact the manufacturer and double-check that what you’re using is the correct product for the application.

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