When removal of the penetrant from a defect due to over-washing of the part is a concern, a post-emulsifiable penetrant system can be used. Post-emulsifiable penetrants require a separate emulsifier to break the penetrant down and make it water-washable. Most penetrant inspection specifications classify penetrant systems into four methods of excess penetrant removal. These are listed below:
- Method A: Water-Washable
- Method B: Post-Emulsifiable, Lipophilic
- Method C: Solvent Removable
- Method D: Post-Emulsifiable, Hydrophilic
Method C relies on a solvent cleaner to remove the penetrant from the part being inspected. Method A has emulsifiers built into the penetrant liquid that makes it possible to remove the excess penetrant with a simple water wash. Method B and D penetrants require an additional processing step where a separate emulsification agent is applied to make the excess penetrant more removable with a water wash. Lipophilic emulsification systems are oil-based materials that are supplied in ready-to-use form. Hydrophilic systems are water-based and supplied as a concentrate that must be diluted with water prior to use .
Lipophilic emulsifiers (Method B) were introduced in the late 1950's and work with both a chemical and mechanical action. After the emulsifier has coated the surface of the object, mechanical action starts to remove some of the excess penetrant as the mixture drains from the part. During the emulsification time, the emulsifier diffuses into the remaining penetrant and the resulting mixture is easily removed with a water spray.
Hydrophilic emulsifiers (Method D) also remove the excess penetrant with mechanical and chemical action but the action is different because no diffusion takes place. Hydrophilic emulsifiers are basically detergents that contain solvents and surfactants. The hydrophilic emulsifier breaks up the penetrant into small quantities and prevents these pieces from recombining or reattaching to the surface of the part. The mechanical action of the rinse water removes the displaced penetrant from the part and causes fresh remover to contact and lift newly exposed penetrant from the surface.
The hydrophilic post-emulsifiable method (Method D) was introduced in the mid 1970's. Since it is more sensitive than the lipophilic post emulsifiable method it has made the later method virtually obsolete. The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time. While emulsification time should be controlled as closely as possible, a variation of one minute or more in the contact time will have little effect on flaw detectability when a hydrophilic emulsifier is used. However, a variation of as little as 15 to 30 seconds can have a significant effect when a lipophilic system is used.
-- Boisvert, B.W., Hardy, G., Dorgan, J.F., and Selner, R.H., The Fluorescent Penetrant Hydrophilic Remover Process, Materials Evaluation, February 1983, pp. 134-137.
-- Sherwin, A. G., Overremoval Propensities of the Prewash Hydrophilic Emulsifier Fluorescent Penetrant Process, Materials Evaluation, March 1993, pp. 294-299.