Atmospheric salinity distinctly increases atmospheric corrosion rates. Apart from enhancing surface electrolyte formation by hygroscopic action, direct participation of chloride ions in the electrochemical corrosion reactions is likely. In ferrous alloys, iron chloride complexes tend to be unstable (soluble), resulting in further stimulation of corrosive attack. Metals such as zinc and copper, whose chloride salts tend to be less soluble than those of iron, generally display lower chloride induced corrosion rates. The initiation and propagation of localized corrosion damage under the influence of chloride ions is most important. Pitting and crevice corrosion in passivating alloys such as stainless steel, aluminum alloys or titanium are examples of such damage.
The units used for the chloride categories (airborne salinity) in the ISO 9223 are as chloride deposition (CD) rate in mg m-2 day-1:
CD <= 3
4 < CD <= 60
61 < CD <= 300
301 < CD <= 1,500
For the purposes of the ISO 9223 Standard the airborne chlorides should be measured using the wet candle method and should be an annual average measure as the amount of airborne chlorides vary strongly with the weather. Surface chlorides can also be measured on real surfaces using an extractive solution and following a simple procedure.
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