The basic groups of Environment (related to ISO 9223) are given in the table opposite, and can be used as a guide to the risk and rate of corrosion for differing environments.
Where the relative humidity is below 60%, the corrosion rate of iron and steel is negligible. A relative humidity higher than 60%, or exposure to wet/immersed conditions results in more serious corrosion. Temperature also influences the corrosion rate, with temperature fluctuations having a greater effect than the average temperature value. Normal atmospheric corrosion occurs within the temperature range of -55°C-+66°C. Contaminents such as chlorides and sulphates on the surface also accelerate attack and although there has been a decrease in atmospheric pollution over the last 30 years, the underlying tendency for corrosion in different countries or areas differs. It is therefore of vital importance to select the most accurate atmospheric environmental category in order to ensure the correct specification.
It is a misconception that interior environments are not subject to corrosion. Frequent condensation, a harsh chemical environment or exposure to the atmosphere due to delayed construction all contribute to corrosion.
Corrosion In Water
The type of water has a major influence on corrosion and therefore on the selection of protective coatings. Salt water is more aggressive than freshwater, but other factors such as temperature, pressure, flow rate, fluctuation zones, agitation and oxygen availability are all important considerations. Please contact us for guidance.
Corrosion In Soil
Corrosion in soil is dependant upon both the mineral and moisture content of the soil. Clay and clay marl soils are corrosive to a certain extent, bog and peat soils contain corrosive acids, the least corrosive soils being lime and sandy soils. Corrosion can also form at the soil/air and soil/ground water interfaces. These areas should be given special consideration and specialist advice should be sought for full guidance on all conditions involved, please contact us for further information.
In Contact With Wood
Direct contact with very acidic woods such oak, sweet chestnut, beech and teak is a corrosion risk due to the wood acids or wood treatment chemicals eg the corrosion of steel cables wound on to wooden drums.
Corrosion By Abrasion
Natural mechanical exposure can occur in waters, and by particles such as sand carried by the wind, leading to increased corrosive attack. Areas which are walked or driven on are subject to severe corrosion by impact and abrasion. The zinc /iron alloy such as that produced by Sherardizing has a very high abrasion resistance and helps to limit such effects.