Long term corrosion tests carried out in the UK have shown that weathering steels continue to corrode at a finite rate. It is therefore necessary to use thicker steel and to monitor the corrosion. A novel ultrasonic instrument to measure residual steel thickness has been assessed. This measures steel thickness through a rust layer, requires no couplant to transmit the ultrasound into the steel and gives only the steel thickness not the thickness of the steel plus a layer of rust. This instrument provides a suitable method of measuring the residual steel thickness on weathering steel structures where corrosion has been generally uniform. However, the instrument can give no information on localised roughness or pitting of the underlying steel surface and it would seem worthwhile to include test specimens in any monitoring scheme so that the surface condition of the steel can be assessed. (A)

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