Corrosion Protection for Metal Fasteners Quiz

Hint

Metal connectors and fasteners are important elements in transferring loads from natural hazards (e.g., flood, wind, seismic) through a building. Corrosion rates for metal are dramatically higher in coastal environments than in less harsh, non-coastal environments. Therefore, it is important to increase the corrosion protection for metal connectors and fasteners in coastal environments. See Section 6 for information on the causes of corrosion in coastal areas. Studies have shown that stainless steel and thick hot-dip galvanized (G185 or higher) metal connectors and fasteners improve corrosion protection. Selecting metal connectors and fasteners made of the same metal and either hot-dip galvanized or stainless steel will improve performance. See Section 8 for information on improving corrosion resistance.

Hint

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.

Hint

Galvanizing is the process of coating steel with zinc.

In hot-dip galvanizing, the steel is carefully cleaned and then dipped into a vat of molten zinc. The high temperature melts the surface of the steel and forms several steel/zinc alloys that tightly bond the zinc coating to the steel base metal; various thicknesses of zinc coatings are achievable. The protective zinc coating still corrodes, but the corrosion of the zinc protects the steel base metal. Galvanized steel can degrade up to more than 50 times slower than ungalvanized steel in the same coastal environment.

Hint

When waves break, salt water is aerosolized, and the wind tends to distribute the salt spray to inland areas. The amount of salt spray in the air is greatest near breaking waves and declines rapidly in the first 300 to 3,000 feet landward of the shoreline. Despite the inland reduction, studies have shown accelerated corrosion rates as far inland as 5 to 10 miles (IMOA, 2009). Farther landward, corrosion can be similar to the rates that occur in milder, inland conditions.

Hint

Perhaps less obvious is LaQue’s finding that partially sheltered exposures, such as areas under pilingsupported buildings or under decks and walkways, can experience even greater corrosion than open exposures. Tests showed that portions of buildings exposed to rain may undergo lower corrosion rates than sheltered areas because rain can periodically wash away salt accumulations. Sheltered or covered areas, on the other hand, do not benefit from occasional rinsing from rain and therefore accumulate more salt, resulting in higher corrosion rates.

Another effect of exposure and building orientation is related to the duration of surface wetness. Open exposures dry more readily because they are exposed to sunlight, and rapid drying slows the corrosion rate. Partially sheltered exposures stay damp longer and therefore may corrode faster.

Hint

Examples of unvented enclosed exposures are enclosed areas such as wall cavities, enclosures surrounded by breakaway walls under elevated buildings, and floor framing cavities created when finishes are installed on the underside of floor joists.

Hint

In addition to depicting which metals will function as anodes and cathodes, a galvanic chart can also depict the relative rate of corrosion that two metals may experience. Metals that are relatively close to each other on the chart will have low rates of corrosion, while those that are more widely separated will corrode more quickly. For example, when exposed to copper, bronze, which is relatively close to copper on the galvanic chart, will have a lower corrosion rate than steel, which is more widely separated from copper on the chart.

Hint

Figure 8 illustrates that increasing the thickness of galvanizing results in a longer service life. The extent to which service life is increased depends on the location and the amount of salt in the air, as well as pollutants such as sulfur dioxide.

Hint

Stainless steel is tougher than normal carbon steel, which means that stainless steel connectors are more difficult to fabricate. Also, stainless steel tends to be more expensive than carbon steel and galvanized steel. When considering upgrading from galvanized fasteners and connectors to stainless steel, designers, contractors, and building owners in coastal areas should compare the cost of stainless steel connectors to G185 galvanized connectors, not to G60 and G90 galvanized connectors, which are not recommended in coastal areas. Furthermore, the reduced maintenance/replacement/labor life-cycle costs when using stainless steel should be considered when specifying connectors. See Section 9.2.2 for further details.

Hint

Coatings and paint applied to galvanized steel connectors can improve corrosion resistance, but many coatings and paints commonly used for buildings do not adhere well to galvanized surfaces. The Truss Plate Institute (TPI) has evaluated the use of truss plates in corrosive environments. If specified by the registered design professional or building designer, ANSI/TPI 1-2014 design specifications, which are referenced in the I-Codes, recommend that one of two industrial paint systems be applied by brush to embedded plates after delivery of the completed truss to the job site or after truss installation (ANSI/TPI, 2014). Alternatively, using truss plates with a thicker hot-dip galvanized coating is also recommended. The three industrial coating options for increasing corrosion resistance are:

• Epoxy-polyamide primer (SSPC-Paint 22)
• Coal tar epoxy-polyamide black or dark red paint (SSPC-Paint 16)
• Post-plate manufacture hot-dip galvanizing (zinc-based) per ASTM A153/A153M (ASTM, 2016)