This method is frequently used for the detection of surface breaking flaws in nonferromagnetic
materials.
The subject to be examined is first of all chemically cleaned, usually by vapour
phase, to remove all traces of foreign material, grease, dirt, etc. from the surface
generally, and also from within the cracks.
Next the penetrant (which is a very fine thin oil usually dyed bright red or ultra-violet
fluorescent) is applied and allowed to remain in contact with the surface for
approximately fifteen minutes. Capillary action draws the penetrant into the crack
during this period.
The surplus penetrant on the surface is then removed completely and thin coating of
powdered chalk is applied.
After a further period (development time) the chalk draws the dye out of the crack,
rather like blotting paper, to form a visual, magnified in width, indication in good
contrast to the background.
The process is purely a mechanical/chemical one and the various substances used
may be applied in a large variety of ways, from aerosol spray cans at the most simple
end to dipping in large tanks on an automatic basis at the other end. The latter
system requires sophisticated tanks, spraying and drying equipment but the principle
remains the same.