Coastal salts (i.e. sodium, calcium and magnesium chlorides) can create a corrosive environment for any construction material. Atmospheric corrosion specialists have extensively studied the factors influencing coastal corrosion, but design professionals are often unaware of the research or how to apply it to their projects.
Designing on the Waterfront: Avoiding Corrosion Failures With Metal helps readers differentiate between typical and more severe coastal locations during a site analysis. The article compares the coastal corrosion performance of stainless steels, aluminum alloys, copper and carbon steel. Additional data on the corrosion rates of different metals in typical coastal environments and specification guidance can be found by following the reference links at the bottom of this page.
A Chinese language version of this article is also available:
Designing on the Waterfront - Avoiding corrosion failures with metal - Chinese
Most building product selection advice assumes a moderately corrosive ‘typical’ project site. Unfortunately, not all coastal sites are ‘typical.’ How does a specifier know when a more resistant stainless steel is required?
Stainless Steel for Severe Coastal Environments provides guidance on how to identify these environments and provides practical advice and project examples. It also summarizes the findings of on-going coastal corrosion research in the Middle East. While the selection advice in this article is specific to stainless steel, severe environments can cause accelerated deterioration and unexpected failure of any susceptible construction material.
Evaluating salt exposure
The salt concentration in coastal air depends on various factors, including:
- wind speeds;
- surf and sea turbulence;
- land/water temperature differences;
- wave height and direction;
- width of the surf zone;
- bottom slope;
- land projections into the sea; and
- topography differences like beaches and cliffs.
In locations where the sea is relatively calm (e.g. a sheltered harbor or river mouth), salt exposure tends to be lower. Rocky shore and high surf areas, on the other hand, have much higher levels. Weather conditions and surf action often vary and there can be significant seasonal differences in salt exposure and surface accumulation levels.
Coastal salts (e.g. mainly sodium, calcium, and magnesium chlorides) on surfaces begin to absorb moisture and form a corrosive chloride electrolyte solution at the critical humidity and temperature level combinations. This can begin to occur at 0 C (32 F) and a 45 percent humidity level. As typical or seasonal temperatures and humidity levels increase, more salts are activated.
The oft-cited ‘typical’ coastal construction sites are:
- at least 0.2 and up to 16 km (0.12 to 10 mi) from a large saltwater body with low to moderate salt, pollution and particulate levels;
- feature boldly exposed surfaces; and
- are subjected to regular heavy rain.
‘Severe’ and ‘extremely severe’ coastal zones have higher salt, corrosive pollution and/or particulate levels, and a regular source of moisture.
The distance that significant concentrations of salt travel inland is determined by weather patterns, topography, water droplet size, and the initial concentrations of salt aerosols. In some locations, coastal salt is only a factor within the first 1.6 km (1 mi) from shore; in others, significant concentrations have been found hundreds of kilometers inland.
Research shows the highest coastal corrosion rates are generally within 400 to 600 m (1300 to 2000 ft) of the shoreline, and they begin to rise exponentially within the closest 50 to 200 m (165 to 650 ft). Locations immediately on or extending over the shoreline are generally even more corrosive because of the potential exposure to high tide spray or splashing.