5.1 Known Concerns Related to Nominally Galvanized Metal Connectors
In this Course, the term “nominally galvanized metal connectors” refers to connectors that have the minimum galvanization provided by the manufacturer and that do not have corrosion resistance beyond the minimum.
Nominally galvanized metal connectors should be upgraded when enhanced corrosion resistance or greater strength in the connection is needed or desired.
5.1.1 Chemicals in Preservative-Treated Wood
Salt spray in coastal environments and the chemicals in the preservatives that are used to treat wood can both contribute to the corrosion of metal fasteners. One of the chemicals in preservatives, chromated copper arsenate (CCA), was removed in 2004 from formulations for preservative-treated wood for most building applications. Several formulations to replace CCA were developed, including some that are more chemically reactive and therefore more corrosive to metal connectors and fasteners. As a result, manufacturers of several of the new formulations specified that all fasteners used with preservative-treated framing be stainless steel or hot-dip galvanized metal with a defined minimum amount of galvanizing.
Which chemical is used in the treatment and the amount of chemical in the wood after treatment, referred to as retention, can both influence the corrosion rate. Wood treated for use in more severe environments, such as in direct contact with the ground, has higher chemical retention than wood treated for use in less severe environments. Higher retention can increase the corrosion rate. Connector manufacturers may have recommendations on selecting connectors and fasteners that will be in contact with treated wood.
Lumber manufacturers attach identification tags to treated wood products to indicate the type of preservative that was used (see Figure 4). The tags allow designers and builders to select connectors and fasteners that are compatible with the amount and type of preservative in the wood and its intended environment.
5.1.2 Galvanic Corrosion
Galvanic corrosion is the corrosion that results when two metals with different electrical potential are in contact with each other and are in the presence of an electrolyte such as saltwater. Galvanic corrosion is an electrochemical process in which one of the metals corrodes and the other one is protected.
Metals can be protected from galvanic corrosion by applying a sacrificial metal to a metal surface (see Section 8). The metals involved in galvanic corrosion can be (1) the metallic compounds in wood preservatives and the metal connectors the compounds are in contact with or (2) metal connectors and adjoining fasteners if they are made of dissimilar metals such as stainless steel and aluminum. It is important that fasteners and connectors be made of similar metals to prevent galvanic corrosion. Galvanic corrosion can occur anywhere but can occur more quickly in coastal environments.
