Washing is best accomplished with a fan shaped, coarse spray with a temperature range of 60°F (15.5°C) to 100°F
(37.8°C), and line pressure water not to exceed 40 psig. The wash time will depend upon the surface roughness of the
part. Water washable penetrant can easily be over-washed and wash time must be closely controlled. Washing of
fluorescent penetrant SHALL be done under a black light in a semi-darkened area and the washing stopped when a low
background level is reached. If small defects must be detected in parts with rough surfaces, some residual background
may be necessary.
Removal of Postemulsifiable Penetrant, Methods B and D.
Postemulsifiable penetrants are oil based vehicles containing highly visible colored or fluorescent dyes. They are
formulated to optimize their penetration and visibility capabilities. They differ from water washable penetrant in that
they resist removal by water washing since they do not contain an emulsifier. A separate process step of emulsification
is required for removal. This is done by one of two methods: lipophilic process or hydrophilic process.
Derivation of the Names Lipophilic and Hydrophilic.
The words, "lipophilic" and "hydrophilic", like many other chemical and medical terms, have their basis in Greek word
elements: "lipo" comes from the Greek word for oil or fat, whereas "hydro" is from the Greek word for water. "Philio"
means a fondness or affinity for, borrowed from the Greek word "philos" for loving. Thus, lipophilic is an oil or fat
based material, and hydrophilic is a water based solution. In this chapter, the word "emulsifier" will be used when
referring to lipophilic material and the word "remover" will be used when discussing hydrophilic material. This is a
practice generally used by industry.
Lipophilic versus Hydrophilic Processes.
Details of the two processes are discussed below. Some basic differences are summarized as follows:
a. Lipophilic emulsifier is supplied in a ready to use liquid, whereas hydrophilic remover is supplied as a
liquid concentrate that has to be diluted with water.
b. The hydrophilic process requires an additional pre-rinse step following the penetrant dwell period.
c. The methods of applying the emulsifier and remover differ. Parts are dipped into lipophilic emulsifier
and then immediately removed to drain. Parts either are immersed into hydrophilic remover for the
entire removal time or are subjected to a spray of remover for the specified time.
d. The modes of action by which the emulsifier and remover remove the excess penetrant differ. This is
also why the application methods differ as explained in the previous paragraph.
Lipophilic Emulsifier Process, Method B.
Mode of Action.
Diffusion into the oil-base penetrant is the primary action. Diffusion is the intermingling of molecules or other
particles as a result of their random thermal motion. If two miscible (capable of being mixed) liquids or gases are
placed in a container, they will eventually mix into a uniform solution. For example, if a sugar solution (a heavy
solution) is placed in the bottom of a glass, and plain water (lighter medium) is placed on top, the sugar will migrate
across the boundary. After a period of time, the entire quantity of liquid will reach a nearly uniform concentration.
This is what happens when emulsifier is applied to a layer of penetrant on a part (see Figure 2-18).