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FIELD TECHNIQUE: Combating Corrosion

   Author: Joe Stager

IT IS DIFFICULT TO BELIEVE that it has been more than four years since chromated-copper-arsenate-, or CCA-treated lumber was replaced by alkaline-copper-quarternary-, or ACQ-, treated lumber. Since that time, there have been many corrosion tests conducted, numerous articles written and countless fasteners installed into ACQ-treated lumber. Yet with all this information available, questions persist about ACQ, corrosion of metals and fastener selection.

The Glenview, Ill.-based Metal Construction Association recently approved a position paper addressing the most frequently asked questions surrounding metal-roofing fasteners and ACQ-treated lumber. The following questions are a sampling of what "Metal Roof and Wall Panel Components in Contact with Preservative-Treated Lumber" covers:

Why was CCA-treated lumber removed from the market?
The preservatives used in CCA-treated lumber are chrome and arsenic, which are carcinogens. Removing them is a benefit to the environment.

Why was ACQ treatment selected in place of CCA?
According to the U.S. Environmental Protection Agency, Washington, D.C., ACQ is a water-based wood preservative. It was chosen because of its ability to guard against decay and insects while being environmentally friendly.

What problems can ACQ-treated lumber cause to metal roofing fasteners?
The copper in ACQ has a tendency to accelerate the corrosion of zinc-plated fasteners. The zinc plating corrodes in the form of white rust (zinc oxide) until the bare carbon steel is exposed and corrodes in the form of red rust (iron oxide).

One of the most common causes of rusting steel fasteners is galvanic corrosion, which takes place when all the following occurs:

> Dissimilar metals are present.
> An electrical contact path between the metals is present.
> An electrolyte is present.

All three conditions exist when metal-roofing fasteners are installed into ACQ lumber.

What can be done to fasteners to show or eliminate corrosion?
There are many ways to minimize fastener corrosion. One way is by using materials with similar electro-potential. (See the Galvanic Series Chart on page 81). Using similar metals will reduce the amount of corrosion that occurs to the least noble material.

There also are two basic types of finishes applied to fasteners that enhance corrosion resistance. These finishes are referred to as a barrier coating, such as Kynar paint, or sacrificial plating, including zinc. Barrier coatings and sacrificial plating have advantages and disadvantages. The thicker the coating or plating, the more corrosion resistance they provide. However, at some point, coatings and plating will fail and the fastener will rust.

What coatings are best to use on carbon-steel fasteners?
A popular coating system often used on carbon-steel fasteners is an organic barrier style applied over a metallic sacrificial plating. There are many types and brands of barrier coatings available and most will provide six times more corrosion resistance than standard zinc-plated fasteners. Barrier coatings widely are specific for use in construction, industrial and automotive applications. They protect the base metal from corrosion as long as the coating is not chipped, peeled or cracked. Because of this condition, a sacrificial base coat often is used under the barrier-style topcoat.

Can standard zinc-plated fasteners be used in ACQ-treated lumber?
Standard zinc-plated fasteners will corrode quickly and should not be used in ACQ-treated lumber.

It is important to remember that carbon-steel fasteners used in ACQ-treated lumber must provide corrosion resistance equal to hot-dipped galvanized per ASTM A153.

What fasteners are approved for use in ACQ-treated lumber?
The Southern Pine Council, Kenner, La., has stated certain carbon-and stainless-steel fasteners are acceptable for use in ACQ-treated lumber. These fasteners also are recognized by MCA and the Rosemont, Ill.-based National Roofing Contractors Association.

>Carbon steel
It is important to remember that carbon-steel fasteners used in ACQ-treated lumber must provide corrosion resistance equal to hot-dipped galvanized per ASTM A153. Carbon-steel fasteners coated with proprietary anti-corrosion coatings also may be considered.

>Stainless Steel
Although stainless-steel fasteners are three to four times more expensive than carbon-steel-coated fasteners, stainless steel provides greater resistance to corrosion in ACQ-treated lumber. Fasteners made from 302, 304, 305 or 316 stainless steel (18-8) are good options. (Although 410 series stainless steel is not listed as an approved material by the Southern Pine Council, it has been tested and is supplied by some fastener companies.) However, under certain conditions, the use of stainless-steel fasteners may accelerate galvanic corrosion of the steel panel. Discuss the application with your panel supplier to determine the best material for your condition.

Joe Stager is in product development for Triangle Fastener Corp., Cleveland. He contributed to the writing of the "Fastener Selection Guideline" published by the Glenview, ILL.-based Metal Construction Association.

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