Corrosion of Metal Connections in Preservative-Treated Wood Structures - 4/16/2002 - Real Estate Products Services

Corrosion of Metal Connections in Preservative-Treated Wood Structures

September 2003

Introduction

Recent testing of alternatives to CCA (chromated copper arsenate) indicates that some of these new wood preservatives can be more corrosive to metal connectors and fasteners than CCA. As CCA-treated lumber will be phased out of the residential market for most applications by the end of 2003, the corrosion resistance of connections needs reevaluation to ensure the structural safety and durability of the buildings constructed with preservative-treated lumber. The objective of this technical note is to discuss issues associated with corrosion of connections in treated wood and to summarize available options for improved corrosion protection.

Connections are an integral part of a wood structure. Wood members are joined together using metal connectors (e.g., joist hangers) and/or fasteners (e.g., nails, bolts) designed and installed to resist loads that can be experienced by the structure over its lifetime. These loads are caused by the weight of the building materials, human activity (e.g., weight of people standing on a deck), or by the forces of nature (e.g., weight of snow on the roof or shaking due to earthquake). Corrosion causes the steel connections to lose their strength with time such that the structure may be incapable of supporting the applied loads. Therefore, it is critical to protect wood connections against corrosion in an effective and lasting manner to prevent structural failures. The most common applications for preservative-treated lumber in residential construction are sill plates, floor framing in exposed crawl spaces, decks, wood foundations (note: wood foundations are exempt from the agreed phase out), playgrounds, and fences.

Background

Corrosion is a process in which metal deteriorates through a chemical reaction with the environment. Some preservatives used for treating wood can promote the corrosion process by providing more favorable conditions for conversion of metal into the products of corrosion. Corrosion can be minimized by using one of the following methods:

1. Corrosion-resistant alloys
   Alloys such as stainless steel or silicon bronze are very effective against most types of corrosion. The main barrier for these products in the residential market is their high cost. In addition, stainless steel has increased hardness characteristics making the manufacture of these products more complicated.
2. Impermeable covering
   Covering steel with an impermeable coating (i.e., painting) can inhibit corrosion by providing a mechanical barrier between steel and corrosive agents such as oxygen, water, and other substances. However, this method is vulnerable to imperfections in the coating such as scratches where corrosion can initiate and spread to adjacent surfaces. This method is not recommended for long-term outdoor exposure of structural connections.
3. Galvanic coating
   Galvanic coatings such as zinc are the most common method for protecting connections against corrosion in residential construction. A galvanic coating serves a double function. In addition to working as a mechanical barrier similar to paint, zinc provides a chemical defensive mechanism. It works as a sacrificial material that takes steel's place in the corrosion reaction. Another type of coating often used with roofing products is referred to as Aluzink 185, an alloy of aluminum and zinc. Aluzink 185 can be several times more effective that zinc, but it is slightly more expensive.

The in-place corrosion rates of connectors in wood depend on many factors including:

• Salt water or air containing salt water
• Pollutants
• Wood preservatives
• Moisture content of wood
• Presence of liquid water (e.g., rain, dew, capillary water)
• Presence of other metals
• Artificial and natural fertilizers
• Temperature
• Relative humidity
• Cleaning products and other chemicals
• Retention levels of chemicals used for wood treatment
• Fire retardants
• Ground contact
• Wood species
• Chemical makeup of each preservative or fire retardant
• Atmosphere: rural, industrial, marine

Testing for corrosiveness

Accelerated aging tests can be used for estimating the relative degree of corrosiveness of wood preservatives. Steel coupons in contact with wood specimens are placed in an environmental chamber with elevated humidity and temperature. Other factors such as salt water can be included to promote corrosion. After testing, coupons are cleaned and weighted to calculate the corrosion rates. Visual observations are also conducted. This test method allows for relative comparison of corrosion rates of steel tested in contact with treated and untreated (control) wood. Based on the existing experience with preservatives such as CCA that have been successfully used for over 30 years, the long-term performance of other products can be inferred. However, test results cannot be directly correlated to product service life.

Alternatives to CCA

A variety of chemicals have been developed for wood preservation purposes. Wood preservatives are classified as oilborne and waterborne. Wood treated with waterborne preservatives became the material of choice for residential applications due to its cleanliness and paintability. Some economically viable waterborne alternatives to CCA include:

• ACQ - Ammoniacal Copper Quat
• CBA, CA - Copper Azole
• Borate compounds

ACQ, CBA, and CA are among the preservatives that have been shown to produce accelerated corrosion of steel compared to CCA. It should be noted that corrosion rates depend on many factors and although the accelerated corrosion rates have been evidenced by testing, the actual degree of increase for each combination of factors is still largely unknown. For example, one factor is the treating solution carrier used as the carrier that delivers the active chemicals inside the wood structure. Carriers for ACQ, CBA, and CA can be ammonia-based or amine-based or hybrid. The preservatives containing ammonia in the carrier were observed to be more corrosive than amine-based.

On the other hand, borate compounds do not promote corrosion and performed as well as control specimens (i.e., wood without treatment) during testing. However, borates leach out of wood if subjected to liquid water and are only allowed in protected conditions such as the inside of a wall cavity. Borates are not permitted for decks, use in contact with the ground, and other outdoor applications.

Hot-dipped galvanized (zinc coated) steel

The use of hot-dipped galvanized steel for wood connectors and fasteners is the most common method for preventing corrosion. The coating is typically applied through a continuous or batch dipping process when the steel is submerged into molten zinc. The continuous process is used with steel coils of lighter gage that are afterwards manufactured into connectors. The batch process is used with heavier gage products that cannot be manufactured from coil steel. Mechanical galvanizing is another method for applying zinc coating to steel.

Based on the thickness of galvanic coating, hot-dip steel products are designated as follows: G60, G90, G185, etc. The number indicates the fraction of ounces of zinc per one square foot of steel treated on both faces. For example, G90 means that the weight of zinc coating on both faces of one square foot of steel is 0.90 ounces. G90 produces a protective layer of about 0.0008 inches (0.8 mil) in thickness on each face. Thicker coatings are expected to extend the service life of connections. Recall that the galvanic layer is a sacrificial coating that extends service life through bypassing the corrosion process and is effective until the zinc is depleted.

G60 is a minimum industry norm for wood connections used with CCA-treated lumber. G90 is also used by the industry for enhanced protection. In light of the new data on the corrosiveness of alternative wood preservatives, the industry is considering requiring G185 galvanic coating or stainless steel products as a new minimum standard. However, it is recommended to contact the hardware manufacturer for specific guidelines on the use of each individual steel product with treated wood. These new requirements are not explicitly included in the model building codes. A consistent policy for use of steel connections with treated wood is currently under development.

Summary

The advent of new wood preservatives generates new challenges for homebuilders. These new chemicals can be more corrosive and can decrease service life of connections compared to CCA. As a solution, hot-dip galvanized connectors and fasteners with increased coating thickness (G185) can be used. While more expensive, stainless steel and other corrosion resistant alloys can significantly improve the corrosion resistance of connections. The development of uniform guidelines on the use of steel with treated wood is underway.

FAQ

Do scratches and other defects that occur during manufacturing, handling, and installation of hardware affect the performance of galvanic coatings?
Because zinc provides a chemical protection mechanism against corrosion, small imperfections in the zinc layer do not have an adverse affect on the performance. Adjacent zinc material is capable of offsetting the effect of such defects.

Can I combine stainless steel and hot-dip galvanized steel?
No. Stainless steel connectors or fasteners cannot be used in combination with galvanized connectors or fasteners. These two metals are sufficiently dissimilar that zinc will be protecting the entire stainless steel component and, therefore, will corrode at a more rapid rate.

How can I find adequate connectors for a new deck?
Make sure that all connectors and fasteners are approved for use with preservative-treated lumber. This approval should be clearly indicated either on the connector or it's packaging. If the designation is unclear on this subject, contact the hardware manufacturer to find out which products are suitable for this application.

What type of nails should I use with sill plates?
If the sill plate is treated with ACQ, CBA, or CA, you may be required to use galvanized nails. Consult the nail manufacturer to obtain specifications for this application. As an alternative, approved borate preservatives can be used for sill plate applications. In this case, bright basic nails used throughout the structure can be also installed into the sill plate.