Our assessment of atmospheric corrosivity is based on international standard ISO 9223. The classification is based on SO2 pollution, chloride deposition, and time of wetness while considering wind loads. These results, in addition to a geo-statistical approach will provide a map of atmospheric corrosion in your region of interest.
We also monitor airborne salts carried by the wind from the ocean. Airborne chloride concentrations are not monitored by weather stations and the models that we use to determine them are only accurate up to a few miles from the shore. As such, most estimates using the model in conjunction with ISO 9223-1992 will be utilized for atmospheric corrosion maps. On-site measurements will be used to obtain more accurate deposition rates for chlorides, sulfates and time of wetness per ISO 9223-1992.
Layers Used for Atmospheric Corrosion Risk Maps
- Time of wetness is a measure of how much time the material will be in contact with a conducting solution. Wet surfaces are caused by factors such as dew, rainfall, melting snow, or high humidity. These conditions are estimated by looking at the time during which the relative humidity is greater than 80% at temperatures greater than 0 °C.
- Sulfur dioxide prevalent in industrial and urban environments.
- Chlorides–A major component of most salts, which accelerate corrosion due to their hydrophilic nature. When a salt attracts water and dissociates, it produces a highly conductive electrolyte. Moreover, chlorides are a main catalyst for pitting corrosion, which is an autocatalytic, localized attack. Chlorides are known to cause hydrolysis and create acidic chlorides.
These three factors are combined to determine an overall corrosion environment classification. Based on the classification, the corrosion rates are estimated using Matergenics GIS’ existing data. For example, our estimates show that Los Angeles is typically either C3 or C4 (moderate to high corrosion rates). For carbon steel, this equates to 25 to 80 µm/year. For zinc, this equates to 0.7 to 4.2 µm/year. This agrees with data from the American Galvanizers Association (21.4 µm/year for carbon steel and 1.09 µm/year for zinc). It should be noted that this assumes that the structure in question is not too close to the ocean.
Along the coastline, a C5 (very high corrosion rate) can be expected due to higher chloride deposition rates. Concentration of sea salt aerosols, which are the main atmospheric pollutants in coastal regions, gives an indication of the probability of the atmospheric corrosion. A combination of the results with a geo-statistical approach and modeling will be used to construct the corrosion map. Specific environmental conditions, which are affecting the source and distribution of airborne salinity, will also be considered in the construction of corrosion risk maps.
The construction of atmospheric corrosion risk map consists of two phases:
- In Phase I, the relevant data will be collected, categorized, and analyzed with respect to the project objectives. The information will consist of several distinctive sets of data such as chloride deposition rates, sulfate deposition rates, time of wetness and wind data.
- In Phase II, a knowledge-based approach along with adequate and accurate equipment, and advanced techniques, will be used to collect, analyze, and verify the Phase I corrosion mapping at statistically representative selected sites. Matergenics recommends that investigators should not only consider atmospheric parameters, but also corrosion sources – such as presence of chemical plants emitting corrosive gases, electric generation plants, salt sprays sources, wind loads… – in order to determine and comprehensively assess corrosion risks.
A proprietary method is utilized in Matergenics’ corrosion risk assessment. The method includes an algorithm to assign a corrosivity index to each location on the map based on atmospheric data, wind data, and corrosive gases by chemical plants. The accuracy of this algorithm has been field tested and proven in several projects in California for major Utility companies.