Corrosion protection finishes and material for metal framing connections

As with any products, selection of the right materials with the proper ratings for the specific environment will help deliver safety and durability.

Why is the proper finish or material so important?

There are several types of plating finishes. Their primary function is to prevent corrosion of the parent material when used in certain applications. So, how do you know which finish is best suited for your specific application?

Some basic questions need to be asked, such as, will the metallic channel and/or accessories be used under indoor or outdoor conditions. If outdoor, will this be in a predominately dry or wet environment, such as a desert or coastal region? If the application will be in an indoor environment, will conditions be primarily dry, high humidity, or even acidic/caustic conditions?

The answers to these questions can help put you on the right track to select the most appropriate solution. Conversely, if an unsuitable finish is selected for a certain environmental condition the installed product will not be afforded the protection that the suitable finish can provide. For instance, the wrong finish selected for an outdoor costal or beach-front setting will quickly exhibit “red” rust. The red rust will attack the bare metal at a microscopic level, eating away the material and eventually causing structural or functional failure. An important by-product of selecting the most optimal solution for the environmental and conditional situation being considered is enhanced service life.

Corrosion-resistant finishes and materials for Metal Framing channel, fittings, and accessories

The following common finishes and materials are offered in Superstrut® and Kindorf® brands. Their benefits are as follows:

Pregalvanized (PG) sometimes referred to as “hot-dipped mill galvanized” or “mill galvanized” and having a zinc coating applied by hot-dipping the steel coil at the mill prior to fabrication. This conforms with ASTM A-525/G-90 specification standards. The standard requires subjecting the product to approximately 375 hours of salt-spray testing in accordance with ASTM B117.
Application: indoor environments

SilverGalv®(EG), often referred to as zinc plated or electroplated zinc, consists of 0.5 mils of zinc bonded by an electrolysis process. After plating, a clear conversion coat is applied over the zinc to seal it and stop the formation of white rust. The finish provides a non-porous and non-crystalline surface, and is an excellent bond for the paint. SilverGalv is more resilient to rough handling compared to the PG finish. The standard requires subjecting the product to approximately 420 hours of salt-spray testing in accordance with ASTM B117.
Application: indoor environments

GoldGalv®/ Galv-Krom® is the result of a multi-step electro-galvanizing and zinc trivalent chromium process. The 0.5-mil trivalent chromium finish is applied over zinc plating, producing a chemically bonded non-porous barrier for protection from moisture and air. It is RoHS compliant and OSHA safe. The finish complies with ASTM B633 and is subjected to approximately 950 hours of salt-spray testing in accordance with ASTM B117.
Application: severe or harsh indoor environments

Green or White Urethane Powder Coat (GR or WH) is the result of urethane powder resins applied electrostatically and baked on to the steel product, creating a strong chemical bond and providing a finish that delivers excellent resistance to chipping and peeling. It has a minimum of 1.5 mil thickness.
Application: indoor environments; mild outdoor environments

Hot-Dipped Galvanized (HDG) consists of a thicker coating of zinc and offers a superior resistance to corrosion. It maintains a minimum of 2.5 mils in accordance to ASTM A-123, Thickness Grade 65. It is subjected  to approximately 600 hours of salt-spray testing in accordance with ASTM B116.
Application: harsh outdoor environments

PVC Coated (PVC) consists of a polyvinyl chloride (PVC) plastic coating fused to the metal channel or accessory with a finish thickness of 15 mils (.015”) ± 5 mils. In high temperature environments, PVC coatings will soften. Caution is required in the handling of PVC coated product. For example, care must be taken when attached fittings or accessories so as not to penetrate through the coating.  In addition, field alterations such as cutting, or drilling will compromise the sealed PVC coating necessitating the application of a coating on exposed areas coated to maintain the protective features of the PVC.  Field alteration may void the manufacturer’s warranty.
Application: outdoor environments

Stainless Steel is a material that provides protective qualities without dipping or coating and is available in high-quality austenitic Type 304 and Type 316. Stainless steel contains high chromium and nickel content and is non-magnetic.
Application: highly corrosive environments,  such as cleanroom, medical, and food grade

Aluminum (AL) is a material that provides protective qualities without dipping or coating and manufactured from Type 6063 aluminum alloy and heat treated to a T6 specification. While its load rating is approximately 1/3 of the load bearing capacity of steel framing channel, it provides an effective solution in installations demanding a light weight solution to support  relatively light loads.
Application: outdoor and extremely corrosive environments

 

As with any products, selection of the right materials with the proper ratings for the specific environment will help deliver safety and durability.

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About the author

Kevin Jolly

Kevin is the US R&D Product Engineer for Metal Framing Systems supporting Superstrut® and Kindorf® metal framing systems and Steel City® Hangers, Clamps and Fasteners. Kevin has worked for ABB Installation Products Division, and formerly Thomas & Betts, for over 16 years in various R&D product lines as a Design and Sustaining Engineer.
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