5. New Techniques and Phenomena
The failure to detect small through-the-thickness flaws in non-metallic coatings on metallic substrates can lead to corrosion rates several orders of magnitude greater than those of the bare metal. Therefore, the detection of such defects has high priority even though presently used inspection procedures are both expensive and difficult to perform. It has recently been shown that it is possible to make a penetrant that can detect those through-thickness flaws to the exclusion of all others. These penetrants are made by adding chelating agents to a carrier fluid such as an alcohol. The chelating agent becomes fluorescent when it contacts a metal substrate. Initial work utilized the chelating agent 8-hydroxy quinoline, and is sensitive to most metals. Current work is concentrating on chelating agents that are specific to certain metals. Thus, cracks in chromium coatings or steel can be detected if they penetrate on the steel base. Work on organic and biological agents show promise.
Crane, R. L. and Allinikov, S., "Defect Specific Penetrants" (1978). Proceedings of the ARPA/AFML Review of Progress in Quantitative NDE, September 1976–June 1977. 30.