Screws and Fasteners Corrosion Risks

General Corrosion Risks

 

SST connectors, anchors and fasteners feature a wide range of materials and coatings designed to meet specific performance criteria. It is important to select a material and/or coating that is suitable for the intended application and environment based upon factors such as corrosion resistance and mechanical properties of the material. For more information on selecting connectors, anchors and fasteners based upon corrosion resistance, please see General Corrosion Risks, Guidelines for Selecting the Proper Materials and Coatings, Materials and Coatings Available – Connectors, and Materials and Coatings Available – Fasteners for more information on selecting products based on corrosion resistance.

Understanding the Corrosion Issue

Many environments and materials can cause corrosion including ocean salt air, fire-retardants, fumes, fertilizers, preservative-treated wood, de-icing salts, dissimilar metals and more. Metal connectors, fasteners, and anchors could corrode and lose load-carrying capacity when installed in corrosive environments or when installed in contact with corrosive materials.

The many variables present in a building environment make it impossible to accurately predict if, or when, corrosion will begin or reach a critical level. This relative uncertainty makes it crucial that specifiers and users are knowledgeable of the potential risks and select a product suitable for the intended use. It is also prudent that regular maintenance and periodic inspections are performed especially for outdoor applications.

It is common to see some corrosion in outdoor applications. Even stainless steel can corrode. The presence of some corrosion does not mean that load capacity has been affected or that failure is imminent. If significant corrosion is apparent or suspected, then the wood, fasteners, anchors, and connectors should be inspected by a qualified engineer or qualified inspector. Replacement of affected components may be appropriate.

Some wood-preservative chemicals and fire retardant chemicals and retentions pose increased corrosion potential and are more corrosive to steel connectors and fasteners than others. Testing by Simpson Strong‑Tie has shown that ACQ-Type D is more corrosive than Copper Azole Type C, Micronized Copper Azole, and CCA-C. At the same time, others have shown that the inorganic boron treatment chemicals, specifically SBX-DOT, is less corrosive than CCA-C.

Due to the many different chemical treatment formulations, chemical retention levels, moisture conditions and regional formulation variants, selection of fasteners has become a complex task. We have attempted to provide basic knowledge on the subject here, but it is important to fully educate yourself by reviewing our technical bulletins on the topic (www.strongtie.com/info) and also by reviewing information, literature and evaluation reports published by others.

Low Level of Corrosion Resistance

Clear Zinc

Electroplated clear zinc is applied in accordance with ASTM F1941. In the ASTM B117 salt spray test, clear zinc provides 12 to 24 hours of corrosion protection before the first appearance of red rust depending on coating thickness.

Black Phosphate

Black phosphate provides a minimum level of corrosion resistance and is intended for dry, low-corrosion applications.

Yellow Zinc

Electroplated zinc applied in accordance with ASTM F1941. In the ASTM B117 salt spray test, yellow zinc provides at least 24 hours of corrosion protection before the first appearance of red rust.

Coated Zinc

This coating system consists of an electroplated zinc base layer with an E-Coat top coat. It provides corrosion resistance that is adequate for low corrosion environments. In ASTM B117 salt spray testing at 500 hours of exposure, fasteners with this coating have an average red rust of less than 5%.

Medium Level of Corrosion Resistance

Class D Hot-Dip Galvanized, ASTM A153

The Class D hot-dip galvanization is a coating that meets the requirements of ASTM A153, Class D, which is a minimum average of 1.0 oz/ft.2 [305 g/m2] of zinc applied by a hot-dip process. Hot-dip galvanized fasteners are compliant with the 2012 and 2015 IBC and IRC.

High Level of Corrosion Resistance

Types 304 and 305 Stainless Steel

Types 304 and 305 stainless steels are nickel-chromium austenitic grades of stainless steel. Types 304 and 305 stainless steels are not hardened by heat treatment and are inherently nonmagnetic.

They provide very good corrosion resistance and are suitable for use in many corrosive environments. Fasteners made from Types 304 and 305 stainless steels are compliant with the 2012 and 2015 IBC and IRC.

Severe Level of Corrosion Resistance

Type 316 Stainless Steel

Type 316 stainless steel is a nickel-chromium austenitic grade of stainless steel with 2-3% Molybdenum. Type 316 stainless steel is not hardened by heat treatment and is inherently nonmagnetic. It provides a level of corrosion protection suitable for severe environments, especially environments with chlorides. Type 316 stainless-steel fasteners are compliant with the 2012 and 2015 IBC and IRC.