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This is not simple, since the penetrant system must still be capable of detecting very small flaws. Normal penetrants
are designed to resist removal from cracks and crevices and the organic dyes are oxygen reactive.
2.8.2.3
Lox Compatible Penetrant Types.
There are three approaches used in formulating LOX compatible penetrant systems:
a. Use of materials that are soluble in water and lend themselves to complete removal during postcleaning.
b. Use of materials that are completely volatile and evaporate from the parts without leaving a residue.
c. Use of non-reactive liquids that maintain the dyes in solution and are completely wetted by the liquid at
all times.
2.8.2.3.1
Water Base Lox Compatible Penetrants.
There are dyes and developer materials that are soluble in water. Water base penetrants, if their water content is high,
are LOX insensitive. However, when the water evaporates, the residues can become LOX sensitive. Water base
penetrant systems have been approved for some LOX related applications since their residues are water soluble surface
agents similar to detergents. Approval for LOX applications is based on their ease of removal from surfaces and flaw
entrapment using plain water.
2.8.2.3.2
Volatile Penetrants.
There is a class of dyes that sublimes at room or up to temperatures in the range of 130° to 200°F (50° to 90°C). These
and other materials will fluoresce from a discontinuity and will dissipate entirely from the flaw on setting or when the
part is slightly heated. The materials have been used in formulating volatile penetrant systems. The problems to be
considered are:
a. Even though the materials evaporate from a surface or flaw, there is still the possibility of them re-
depositing at another location.
b. Determination of 100% dissipation as judged by the disappearance of an indication does not mean a
residual-free surface or crack.
2.8.2.3.3
Non-Volatile Lox Penetrants.
Another method of formulating penetrants that are not LOX-reactive is to dissolve the dye in a non-reactive, non-
volatile liquid or vehicle. The liquid serves to quench the reactivity of the dye and, since it is non-volatile, does not
produce a reactive residue. Water based penetrants described in paragraph 2.8.2.3.1 do not meet this criteria, since
they evaporate, leaving a reactive residue. There are some useful fluorinated hydrocarbon liquids, commonly called
fluorocarbon or fluorolube oils that may be employed as penetrants. Fluorolube oils are quite non-volatile and are non-
reactive with LOX. They also act to quench any LOX reactivity of dye that is dissolved in or wetted by the fluorolube
oil. Unfortunately, they are not good solvents for fluorescent dye. One particular LOX compatible penetrant utilizes a
volatile solvent, methylene chloride, to dissolve the dye and mixes this solution with fluorolube oil. This produces a
high dye content with the LOX reactivity quenched by the non-volatile fluorolube oil.
2.8.3
Low Sulfur, Low Chlorine Penetrant Systems.
NOTE
Low sulfur and low halogen penetrant material requirements are not covered by the
military specification on penetrant materials.
There is considerable concern over the effects of small quantities of sulfur and halogens present in penetrant materials.
This concern is due to the increased use of high temperature alloys such as nickel and cobalt-base alloys, austenitic
stainless steel, and titanium in aircraft and engines. These alloys are susceptible to hydrogen embrittlement,
