A Technical Bulletin of FEMA and the National Flood Insurance Program – Part 2 of a two-part series.
Corrosion of metal fasteners and connectors is accelerated when a certain level of surface wetness is exceeded, initiating electro- chemicalreactions among the metal, salts, and air. The conditions that accelerate the rate of corrosion near the coast have been studied at a few field stations and in research laboratories. The studies identified the major factors that contribute to accelerated corrosion rates as including proximity to the shoreline, high temperature, high humidity, elevation, exposure class and sheltering, and certain airborne pollutants. While it may be infeasible to determine the rate of corrosion at specific sites, it is helpful to understand the factors that contribute to corrosion so that appropriate design, construction, and maintenance activities can be implemented.
(Much of the FEMA and NFIP guidance is based on the work the work of Frank LaQue, a Canadian- American who conducted lifelong research into corrosion and served as president of both the American Society for Testing Materials and the National Association of Corrosion Engineers.)
Salt spray from breaking waves and onshore winds
Salt spray from breaking waves and onshore winds significantly accelerates the rate of corrosion of metal connectors and fasteners. Ocean salts, which are primarily sodium chloride but include other chlorides and compounds, accumulate on metal surfaces and accelerate the electrochemical reactions that cause rusting and other forms of corrosion. The combination of salt accumulation on the surface and the high humidity common in many coastal areas further accelerates the corrosion rate of untreated steel and other metals commonly used in connectors, fasteners, and other building materials. The longer a surface remains damp during the normal daily fluctuations in humidity, the higher the corrosion rate. Onshore winds carry both salt and moisture inland from the ocean. Therefore, corrosion rates are higher along shorelines with predominantly onshore winds than along shorelines with predominantly offshore winds.

Distance from ocean
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 five to 10 miles. Farther landward, corrosion can be similar to the rates that occur in milder, inland conditions.
Although the width of high-corrosion areas varies along the shoreline, it is appropriate to assume that oceanfront and nearshore buildings can be more severely affected than buildings farther inland. Tests in North Carolina in the 1940s—as reported in a 1975 paper by Frank LaQue—found that samples of iron corroded 10 times faster 80 feet landward of the shoreline than samples of the same material 800 feet landward of the shoreline. Similar results have been noted around the world.
Elevation above ground
LaQue determined that elevation above the ground, in addition to distance from the ocean, affected rates of corrosion in tests at Kure Beach, N.C. The tests showed that the rate of corrosion reached a peak at approximately 12 feet above the ground near the shoreline, approximately equal to the lowest floor elevation of an elevated building with parking underneath. In several rows of buildings farther inland, the corrosion rate was found to be lower, but the rate was highest at an elevation above the roofs of small buildings. The tests also indicated that the highest corrosion rate near the ocean was more than twice the corrosion rate farther inland.

Exposure to corrosion and building
Orientation
Both exposure and building orientation affect corrosion rates. Exposed areas such as building exteriors are often coated with large amounts of salt spray and can be expected to suffer high corrosion rates. LaQue found that the metals on the side of a building facing the ocean corrode much faster than those facing away from the ocean.
Perhaps less obvious is LaQue’s finding that partially sheltered exposures, such as areas under piling-supported 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.
Weather and rates of corrosion
Weather affects the rate of corrosion of metal in both coastal and inland locations. Most chemical reactions, including corrosion, are affected by temperature, humidity, wind speed, and other factors. Higher temperatures and higher humidity increase corrosion rates. Like any weather-driven condition, corrosion rates can vary considerably from year to year.
Short-term measurements of corrosion rates at specific locations can be misleading unless compared to long-term averages for nearby locations because average weather conditions for factors such as rainfall seldom occur. In any given year, measured rainfall can be much higher or lower than the average rainfall.
As a result, the corrosion rate for a given area in an individual year may be significantly higher or lower than the long-term average rate. Because corrosion rates vary, inspections and maintenance should be done at least annually, and preferably more often, to identify connectors and fasteners that need to be replaced.
Identifying areas with increased corrosion rates
Corrosion tests can help define the coastal areas where corrosion is most severe, and extra precautions should be taken to minimize corrosion of metal connectors and fasteners. Unfortunately, corrosion data are not available for most coastal communities, and building professionals must rely on local experience to estimate the areas where corrosion-resistant materials and methods are needed. Areas of increased corrosion concern can be identified by observing the state of corrosion of metal connectors and fasteners in older buildings located at various distances from the shoreline. The observations can be documented and used to delineate areas with higher corrosion rates. Alternatively, communities or builders can conduct field tests using a test kit or laboratory to determine areas where increased corrosion protection should be used.
Reprinted from Marine Construction Magazine, Issue II, 2023