The liquid penetrant inspection method is based on the ability of certain types of liquids to enter into voids and crevices by capillary action and to remain there when the surface liquid is removed. Thus, liquid penetrant inspection, when done properly is a reliable and revealing method for detecting discontinuities open to the surface. Very small and tight imperfections can usually be shown. There are several variations of the method each having their advantages and disadvantages.
As-welded surfaces are suitable for penetrant inspection
methods if the surface contour does not contain sharp depressions between beads
or weld ripples. These surfaces could interfere with complete cleaning and complete
excess-penetrant removal causing excessive background.
Any one of these could result in false or non-relevant
indications. Alternatively, magnetic particle inspection may be more suitable that liquid penetrant
for ferromagnetic materials.