Anti-Corrosive Treatments

May 1, 2018

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A bit of history and science explains today’s corrosion issues

 

There’s a bit of history and a bit of science required to understand why fasteners (even treated fasteners) and steel hangers can rust so quickly around pressure-treated wood…and why it’s potentially worse now than it was just 15 years ago. 

 


Steel that does not have an adequate level of protection can corrode when water or moisture in this seaside location enabled contact between the copper in treated lumber and the steel in the hanger.

 

 

First, the history: In residential wood products markets until 2004, the dominate pressure-treated wood choice was lumber treated with chromated copper arsenate (CCA).  

After years of pressure from consumer health advocates (concerned about the toxins arsenate and chromium, but not the copper), CCA lumber was banned from most uses. 

Second, the science: Most pressure-treated wood contains copper, because it is a natural anti-microbial. Copper is toxic to most wood-eating fungi, but non-toxic to humans.

When copper is embedded in wood fiber, it can protect the wood from rot, but won’t affect people who have contact with the wood.

 

This hanger has the proper level of protection. Corrosion in an oceanfront location is virtually unnoticeable. 

 

The Challenge of Using Copper

The challenge of using copper as a wood preservative is two-fold.  
First, we know copper is an effective wood preservative. So, wood treaters are always looking for better ways to embed the copper, which sometimes means A) dissolving it in an aqueous liquid to be forced into the wood in pressurized vats, a.k.a. pressure treating, or B) more recently, using clever non-toxic treatments like finely grinding the copper (at a microscopic scale) that it can be carried into the wood by just plain water. These embedding methods have improved over time, and will continue to improve. 

Second, when water runs across wood that contains copper (e.g. rain running across a deck), the water becomes an electrolyte that carries copper ions.

When that water comes in contact with untreated or under-treated steel, the copper ions will corrode steel by galvanic action. In other words, the steel rusts. 

(Water that contains copper is not the only potential source of corrosion.  Other elements can cause corrosion as well, especially salt water, or caustic chemicals like fertilizer and pool chemicals.)

When CCA pressure-treated was used before 2004, the chromate and the arsenate actually inhibited release of copper ions. But today, the two main replacements for CCA – alkaline copper quaternary (ACQ) and copper azole (CA) – do not offer those protections.  

So, the responsibility to defeat the galvanic reaction to copper in pressure-treated wood falls entirely with the anti-corrosion treatments on the fasteners and hangers that come in contact with the pressure-treated wood. Let’s review how that is accomplished.

Corrosion of fasteners and steel connectors (joist hangers, tie downs, etc.) can result from any contact with water (especially salt water) caustic chemicals (like fertilizer and pool chemicals), and preservatives found in pressure treated wood. So, virtually every application is subject to corrosion risks. But with a range of anti-corrosion treatment options, corrosion can be reduced or entirely stopped.

 

What to Look for in Anti-Corrosion Treatments 

As mentioned above, at the top of the list of risks to non-stainless steel fasteners and connectors is the copper that’s a common ingredient in treated wood.  The steel starts to corrode when water in any form enables an “ionic exchange” between the copper and the steel.

To slow or stop this corrosion, manufacturers will A) coat the steel or B) offer stainless steel products.  Although there is a new hybrid coating on the market (explained below), the most common anti-corrosive coating is zinc.  It’s long been known that zinc is an effective “sacrificial” coating; it freely gives up its ions to protect the steel substrate.  For fasteners and connectors, the most common types of zinc treatments are zinc plating, and hot-dipped galvanized zinc coating.

Zinc-coated protection has many levels, determined by the thickness of the zinc coating.

 

Zinc plating. Zinc plating is typically not a long-term anti-corrosion treatment.  Most plating is merely done to keep the steel from rusting during transportation and storage; most plated steel is not protected from corrosion in installed applications. 

Zinc coating. Using a manual hot-dipping process, galvanized zinc coatings can be applied before or after fabrication of the fastener or connector.  In the case of connectors, it is either applied to steel coils at the steel mill before it is stamped and formed into hangers, or for fasteners and connectors, it is applied after fabrication in a manual hot-dipping process.

 

3 Classes of Anti-Corrosion 

To understand the full range of anti-corrosion treatments available for fasteners and steel connectors, let’s take a look at three classes of product: 1) zinc-coated, 2) stainless steel, and 3) zinc-polymer hybrids.  

For hot-dipped, zinc-coated products, the thickness of the zinc determines the protection, and the product’s rating depends on its compliance with ASTM standards.  

The different types of protections outlined below may apply more to connectors than to fasteners, but, when reading product labels, the description of the anti-corrosion properties apply to both.

G90 Rated Zinc Coatings.  No matter the manufacturer, you will see a G90 galvanized rating.  G90 is not a trade name or brand.  The “90” in the label means that there’s 0.9 ounces of zinc applied per square foot of the steel surface.  But this 0.9 ounces is a cumulative figure that counts the zinc applied to both sides of the steel.  So, each surface of G90 has 0.45 ounces/sq./ft. of zinc. 

If you don’t want to pay for stainless steel, but you want protection that exceeds G185, there is a new product: Gold Coat by USP® Structural Connectors. It takes a G90 zinc-coated steel product and adds an additional protective organic chemical coating. The image to the left is a galvanized connector after just 480 hours of sea spray. The image to the right is a Gold Coat hanger, still intact, even after 1,500 hours of similar exposure.

G185 Rated Zinc Coatings.  The next step up in zinc protection is G185.  These products have 1.85 ounces of zinc per square foot, with .925 ounces/sq./ft. on each surface.  G185 is also known as “triple zinc,” or TZ.  (To confuse matters, is used to be that G60 was the baseline treatment, not G90 as it is today.  So G185 was “triple” the G60, and the “triple zinc” term never left the industry.)  When you see “TZ” in a product name, that likely refers to the “triple zinc.”) 

Hot-Dip Galvanized (HDG).  Strictly speaking, G90 and G185, are all “hot-dipped” galvanized.  But the term hot-dipped galvanized has come to mean steel fasteners and connectors products that are dipped after fabrication. (For steel hangers that are not just stamped or folded into shape, but welded, note this:  If you welded zinc-coated steel, the surrounding zinc would melt off, so many heavy gauge steel connectors are fabricated from untreated steel and then hot-dipped. Hot dipped coatings applied this way are typically coated with a thickness on par with G-185 coatings.) 

Stainless Steel.  If you are building in highly corrosive environments, like the sea side, near a pool, or in an industrial area where there’s chemicals in the air, you may want to step up to stainless steel for fasteners and connectors, rather than zinc-coated steel.  Stainless steel is a “noble metal,” like silver or platinum and, surprisingly, to a lesser extent, copper.  As a noble metal, the steel molecules do not freely “give” ions in reactions with dissimilar metals; so, stainless steel won’t corrode. 

 

Hybrid coatings, like Gold Coat, add an extra barrier over the zinc. The protection is near that of stainless steel.

 

Organic polymer – zinc hybrids.  If you don’t want to pay for stainless steel, but you want protection that exceeds G185, there is a new class of product, branded as Gold Coat by USP Structural Connectors.  It takes a G90 zinc-coated steel product and adds an additional protective organic chemical coating.  This provides a product that offers performance that’s between what you find in a G-185 product and a stainless steel product, but at a much lower price than stainless.  The protective top-coat layer that is applied to make a Gold Coat product is 10 to 12 microns thick, rich in aluminum, and resistant to both acids and bases.  (It was developed as a proprietary system from Magni, a global company that has long specialized in protective coatings for metal.)

 

ProTips

Fasteners.  For fasteners, always match the fastener metal or anti-corrosive treatment with the metal or anti-corrosive treatment of the connector.  Stainless connectors call for stainless fasteners, period.  You never want to have dissimilar metals in contact, if that can be avoided.

 

Match the fastener to the hanger: If you are using stainless steel hangers, use stainless steel fasteners. If you are using hot-dipped galvanized hangers, use hot-dipped galvanized fasteners. The “hybrid” Gold Coat hangers (above) can be installed using Gold Coat screws (shown).

 

Impact Drivers.  If you are using an impact driver, note that the repeating banging of the screw head as it turns can nick the anti-corrosion treatment, introducing an access point to the steel.  This is typically not an alarming occurrence, as this is the head surface and not the bearing surface.

Furthermore, the ASTM standard for corrosion resistance of fasteners does not require damaging or nicking of the fasteners when testing the anti-corrosive treatment. So, generally, minor nicks and chips in the head of the fastener should be of little concern when trying to protect your connection from corrosion.

 

Click here to see our complete line of anti-corrosive treatments.

 

—Todd Grevious, PE, is Engineering Manager at USP Structural Connectors. This article and video are from MiTek, one of ProTradeCraft's advertisers. View all of MiTek's articles and videos.

 


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