5. Corrosion allowance

A steel structure that will not be protected against corrosion by painting or zinc coating can be made to run its planned service life by adding a corrosion allowance to its material thicknesses, determined according to the service conditions.

Corrosion in air:

Humidity, temperature, rain, wind, impurities and metal wet times have an effect on the corrosion rate. Corrosion occurs when the relative humidity of the air is 70 to 80 %. Corrosion reaction is possible generally when the temperature is above 0 °C and the relative humidity is over 80 % (the surface is wet). Air impurities that dissolve in condensed water or rain water may accelerate corrosion. Settling of dust and dirt on the metal surface accelerates atmospheric corrosion.

Information about steel corrosion rates in different atmospheres is given in table 4. It should be noted that localised corrosion can occur, which can greatly exceed the corrosion rates given in the table.

Table 4. The corrosion rate of steel in different atmospheres (uniform corrosion).

Corrosion rates of steel in water and soil are given in different information sources.

Weathering steels

Weathering steels are high strength, low alloy, weldable structural steels that possess good weather resistance in many atmospheric conditions without the need for protective coatings. They contain up to 2.5% alloying elements, e.g. chromium, copper and nickel. On exposure to air, a protective rust patina forms that adheres to the surface of the steel. This layer causes the rate of corrosion to slow so that after 2-5 years, corrosion almost ceases. Requirement for the formation of the protective corrosion product layer is regular wetting and curing of the surface. Long wet periods may prevent the formation of the protective layer.

Wet environments, immersed or buried conditions are unsuitable for weathering steels.

Consideration for use of weathering steels:

• the actual loss varies with the environment. For long-life corrosion allowance must be considered
• crevices and water/dirt traps should be avoided
• rust stains may run to adjacent surfaces and cause staining
• fasteners should be made of weathering steel
• specific low alloy welding rods should be used
• for an even weathering result, surface blasting may be necessary
• weathering steels are unsuitable for use in marine and aggressive industrial environments.

References

• ESDEP WG 4A, Protection: Corrosion, Lecture 4A.1: General Corrosion
• Erkki Huhdankoski & working group. Rautaruukki steels under critical conditions 2000

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