intergranular corrosion, and stress corrosion. Small amounts of sulfur and halogens, principally chloride, remaining on
the alloys during service will increase their susceptibility to attack. Sulfur and halogens are not essential compounds in
penetrant materials, nor are they deliberately added. They are usually introduced as contaminants in the raw materials.
There is considerable difference of opinion as to the allowable limits of these contaminants. Nuclear and boiler codes
specify from 0.5% to 1% by weight as the maximums. Many of the QPL materials will meet at least the upper limit.
The position is similar to that for LOX compatible materials. Namely, there is no requirement for special penetrants, if
the part to be inspected is disassembled and can be sent to the cleaning shop for the removal of all inspection residues.
The aircraft or engine manufacturers recommendations should be followed for on-aircraft and assemblies.
High Temperature Penetrant Materials.
Standard penetrant materials are limited to temperatures of 120°F (49°C) (see paragraph 126.96.36.199). There are special
penetrant systems formulated for use above 120°F (49°C). These special high-temperature penetrants contain visible
and fluorescent dyes that resist heat degradation. The vehicles and solvents are carefully chosen to remain liquid and
resist evaporation at the operating temperature. The nonaqueous wet developer must be modified since standard
developer will peel or curl on hot surfaces. The upper temperature limits are in the range of 350°F (177°C) to 400°F
(204°C). Typical applications for high temperature penetrant systems are the inspection of live steam valves and lines
and intermediate weld beads prior to laying down a covering bead.
Dye Precipitation Penetrant Systems.
Dye precipitation penetrant systems are not covered by the military specifications on
Dye precipitation penetrant systems are commonly referred to as high resolution penetrants. The penetrant contains a
high concentration of either visible or fluorescent dye dissolved in a highly penetrating, volatile solvent. The penetrant
is usually applied by brushing on the surface to be inspected. The penetrant will enter any discontinuities and, during
the dwell period, the solvent evaporates, precipitating the dye as a solid that fills the discontinuity. After removal of the
excess surface penetrant, and when using a two-step development process, a very thin layer of solvent developer is
sprayed on the surface. The developer re-dissolves the solid penetrant dye entrapped in the flaw, expands its volume,
and extracts it from the flaw. It is possible to build the indication to any desired size and resolution by applying
additional thin coats of solvent developer. When the indication reaches the desired size, it is fixed by applying a layer
of plastic developer. The plastic developer allows the developer coating with the embedded indication to be removed or
stripped from the part. There is also a one-step developer that provides the same results. Dye precipitation penetrant
systems are extremely sensitive.
Reversed Fluorescence Method.
The reversed fluorescence method is similar to a photographic-negative of the standard fluorescent penetrant
inspection. A standard visible-dye penetrant is applied to the surface to be inspected and after the dwell, the excess is
removed in the normal manner. A special developer, containing a low intensity fluorescing dye and a relatively small
amount of developer powder, is applied by spraying under a black light. The entire surface will fluoresce, except for
the flaw, which appears as a dark line where the penetrant has quenched the fluorescent dye.
A thixotropic material is one that changes form or structure as a function of time or shear stress. Thixotropic
penetrants are applied as a solid or gel and then change to a liquid after application. They are used when it is difficult
to apply the penetrant as a liquid. One example is a high temperature penetrant in the form of a crayon or stick used to
inspect welds before they have cooled.
Dilution Expansion Developers.
Dilution-expansion developers differ from the conventional powder type developers in that they do not utilize the
absorption-adsorption action of powder particles. In fact, powder particles are not required and may even interfere with
the action of dilution-expansion developers. The action of dilution-expansion developer is to dissolve the exuded and