Wind is responsible for the salinity present in marine atmospheres. The wind picks up particles of salt from breaking waves and can carry them inland. The quantity of salt aerosol entrained by the wind is affected by many factors, such as wind strength, wave height, the width of the generation zone, and the contours of the seabed and coastline. These factors along with the persistence of the wind from a given quarter determine the corrosivity of a shoreline.
While salt deposits are measurably present in inland areas such as Taupo, the main effect of marine atmospheres reaches just a few hundred metres from the shore. Particles of salt in the air deposit on adjacent surfaces through gravity and contact; the rate at which deposits settle is affected by the roughness of the ground that the salt-laden air passes over. Obstacles such as trees slow the wind down, increasing the rate of gravitational deposit, and bringing the salt aerosol in more contact with surfaces on which they can deposit.
On the other hand, open flat land and natural “wind tunnels” can allow quite high concentrations of salt to travel several hundred metres inland.
A site’s location, relative to the sea or marine inlets, is a common method used to assess the corrosivity of a location. The distance from salt water for a given Zone varies with the location, depending on the prevailing winds and roughness of water in those areas, as well as the evenness of the terrain it passes over.
Where environmental Zones overlap, a site-specific evaluation may help define the category into which it best fits. Visual evidence of corrosion on adjacent metal surfaces may be present, ground roughness can be assessed, industrial influences can be evaluated and data about the persistence of onshore winds can be obtained from NIWA.
More local factors that affect the corrosivity of a specific location include:
- Overhanging shade increases the time of wetness of a structure and corrosion rate.
- High levels of water roughness such as caused by strong tidal flow against the wind direction, as is often experienced in areas such as Cook Strait, increases salt spray.
- Surfaces not receiving regular and effective rain washing or sufficient manual washing may experience corrosion rates two to three times that of cleaned surfaces.
There are many ways of more accurately determining the actual corrosivity of a given location. The most commonly accepted method as outlined in ISO 9223 is determined by measuring first-year corrosion rate of different metals: mild steel (MS), zinc, aluminium and copper. The COP uses the first-year corrosion rate of mild steel as the most relevant and reliable indicator of a location’s corrosivity.
The names given by different Standards for given corrosion zones vary. The Corrosion Zones in the Code of Practice are similar to those published in NZS 3604:2011 except that:
- the COP makes a distinction between Harbours, West Coast, and East Coast shorelines, and
- NZS 3604 Zone D (High) is further broken down into E (Very High) and F (Extreme Marine) because, in NZS 3604 Zone D, the first-year mild steel corrosion rate can vary from 200 g/m2 to 1000 g/m2.