entrap penetrant and some over-emulsification can be tolerated. When shallow flaws must be detected in parts with
rough surfaces, some under-emulsification or residual background may be necessary.
There are at least two general types of emulsifiers. The two general types are similar in composition with the principal
difference being viscosity. However, viscosity and diffusion rates are related and viscous materials diffuse slowly, while
thinner, less viscous emulsifiers diffuse more rapidly.
Penetrant Dwell Time.
Long penetrant dwell times permit more penetrant to drain from the part, resulting in a thinner surface layer. Since
diffusion rate for a given emulsifier is constant, the emulsifier dwell time required is proportional to the thickness of
the penetrant layer, i.e., thicker layers require more emulsification dwell time, and thinner layers require less time.
As parts are processed, the emulsifier becomes contaminated with penetrant from both the initial immersion and the
drain cycle. While penetrant and emulsifier are soluble in all combinations, the gradual increase of penetrant in the
emulsifier slows the emulsification action. With combined build-up, the mixture will eventually stop functioning as an
emulsifier. The slowing action due to penetrant contamination is very gradual, and at concentrations of less than 25%,
penetrant in emulsifier is not noticeable for practical purposes.
Water Rinsing the Emulsified Layer.
When diffusion of the emulsifier has reached the desired end point, further diffusion is stopped by spraying with water.
The same water spray serves to remove the emulsified penetrant surface layer. Details of the water spray wash are
given in section 188.8.131.52. One variation in technique is the initial requirement to rapidly and lightly water spray the
entire surface at the end of the emulsification dwell period, without regard for removal of the emulsified layer. This
stops the diffusion process and eliminates excessive emulsifier dwell on one surface, while removing penetrant from an
adjacent surface. The emulsified surface layer can be removed after the entire surface has been wetted and the diffusion
process has been stopped. Postemulsifiable penetrant entrapped in flaws and not diffused with emulsifier is relatively
resistant to water spray and rinse time is not critical. However, excessive spray pressure or hot water can remove
entrapped penetrant and must be avoided.
Insufficient or Excessive Emulsification.
The part SHALL be completely reprocessed if, during or after the rinse step, it is suspected that a too short (insufficient
emulsification) or too long (excessive emulsification) dwell time has occurred. Correction of dwell time cannot be
made by immersing in penetrant or emulsifier. The part must be cleaned to remove all residual penetrant and
reprocessed through the entire process.
Hydrophilic Remover Process, Method D.
The objective in both the lipophilic and the hydrophilic methods is the removal of excess surface penetrant without
removing any of the penetrants entrapped in discontinuities. However, the hydrophilic method is completely different
from the lipophilic method. The differences are in the materials, mechanism or mode of action, and the procedures
used. The removal of excess surface penetrant using hydrophilic removers can be accomplished using immersion or
spray techniques. A combination of both immersion and spray is most often used in hand lines and is recommended.
Mechanism or Mode of Action.
Hydrophilic removers are basically detergent/dispersant concentrates consisting of water soluble chemicals, usually
non-ionic surface active agents called surfactants. They are supplied as concentrated liquids and are mixed with water
either before or during the removal process. The surface active agent in the remover displaces a small quantity of
penetrant from the surface and disperses or dissolves it, preventing it from recombining with the remaining penetrant
layer. Unlike lipophilic emulsifier, hydrophilic remover is immiscible with penetrant and diffusion does not occur. All
of the removal action takes place at the exposed surface, and penetrant just below the surface is not involved until it
becomes exposed. Gentle agitation of the liquid helps remove the displaced penetrant and allows fresh remover to