Connect, no.148, May/June 2007, p.8

TWI/NPL collaboration on coating adhesion assessment

A painted panel showing surface damage

Thermographic image of the panel which detects areas of blistered and delaminated paint

Surface performance is critical to almost every engineering material, and the ability to apply a durable coating (functional and/or decorative) is a key requirement. The integrity of such coatings is a major performance requirement as the cost of failure may be significant. Quality assurance of coated structures at the time of manufacture is critical to ensure that sub-standard components, with a reduced lifetime, are not put into service.

TWI, in collaboration with the National Physical Laboratory (NPL), has just completed a major project to investigate techniques for the non-destructive evaluation (NDE) of coatings. Funded by the DTI, and with active involvement from industrial contributors, the 30 month project has concentrated on a range of functional engineering coatings produced by thermal spraying, plating, physical and chemical vapour deposition as well as paint systems for corrosion protection. These have been evaluated using over 10 different NDE methods including thermography, C-scan ultrasonics and laser shearography.

The wide range of coatings investigated has given diverse results, leading to the principal conclusion that there is not one technique that can be universally applied to all possible coating types, a major factor being the thickness. Flash thermography has proved particularly useful in detecting areas of delamination on vapour deposited chromium nitride (5µm) on steel, whilst terahertz pulsed imaging has been able to detect suspect areas on painted marine structures.

The key output of the project will be a Good Practice Guide which will assist end-users with selection of the most appropriate NDE techniques for their particular surface coating. This guide will be made freely available via the Adhesives Toolkit at www.adhesivestoolkit.com

For more information on inspection of engineering coatings, contact john.rudlin@twi.co.uk