Corrosion In Steel

Corrosion in steel Corrosion is caused by air and moisture, embedded sheet plies do not corrode – water may be present but air is not. Corrosion spreads in steel due to electrical activity. A corrosion protection system should consist of good surface preparation and application of a suitable coating. Steel can also corrode if in contact with other metals this is bimetallic corrosion. Steel preparation • Millscale to be removed (forms during fabrication process) this can be done by blast cleaning – standard clause says Blast clean steelwork to SA2½ (or 2nd quality). Below are the environmental categories for steel

Corrosion protection of steel structures by protective paint systems is detailed in BS EN ISO 12944-5: 2007

Protective treatment Assessment of the level of protection steel requires depends on: • Aggressiveness of the environment • Required life of structure • Maintenance schedule • Method of fabrication and erection • Aesthetics.

Types of protection: • Paint • Metallic coatings o Hot dip galvanizing (should not be used for plate thicknesses less than 5mm) and is dependant on bath size. Steel is blast cleaned and then acid pickled, then dipped into a zinc bath. o Thermal spray: blast cleaned steel, is sprayed with molten aluminium or zinc. • Weathering steel: steel that forms a protective rust coating in air which reaches a critical level after 2-5 years which prevents further corrosion. • Stainless steel: corrosion resistant steel Steel work in a cavity Steel work sat on a foundation should be cased to at least ground level, it not 150mm above ground. The cavity should be filled with concrete to ground level. The steel work in a cavity wall should be painted with bitumen for its full height in the cavity. For tables indicating types of paint system refer to attached pages. For a C2 environment a zinc rich epoxy (2 pack) is ok.