PT04

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C
H A P T E R
William E. Mooz, Met-L-Chek, Santa Monica, California James S. Borucki, Gould Bass NDT, Pomona, California Donald J. Hagemaier, Boeing Company, Long Beach,California
J. Thomas Schmidt, J.T. Schmidt Associates,Incorporated, Crystal Lake, Illinois
Amos G. Sherwin, Sherwin Incorporated, South Gate,California
Care and Maintenance of
Liquid Penetrant Test
Materials
The quality of an inspection made with liquid penetrants can be no better than the quality of the liquid penetrant test materials used for the inspection.
Recognizing this basic fact requires that liquid penetrant materials meet certain industry standards before they are purchased for use. It also requires that these materials and the liquid penetrant systems in which they are used be
monitored and tested on a periodic and regular basis. To ensure the materials purchased are of desired quality and that the proper tests are made at the proper intervals during their use, a variety of specifications have been developed. A list of some of the better known
nondestructive testing specifications appears in the references.1-19
These specifications can be broadly classified as to their origin, typically the United States military, various United States technical societies or corporate users. Many of the specifications
describing the materials have common threads running through them whereas some have been specifically tailored to the specialized use of a particular industry or customer. For example, although virtually every liquid penetrant in use has had to meet the requirements of MIL-I-251351(or its commercial replacement,
AMS-26442), liquid penetrants used in the nuclear industry must additionally have particularly low contents of certain low melting metals, as well as sulfur and halogens. Aircraft turbine engine
manufacturers require products with low fluoride, sodium, chloride and sulfur content. Specifications detailing the
testing of materials in use have revolved mainly around the military specification MIL-STD-68663(or its commercial
replacement, ASTM E 14174) but other corporate specifications may also exist.These specifications, in turn, often refer to tests which are called out in other specifications. For example,
MIL-STD-68663refers to tests which are outlined and described in MIL-I-251351or AMS 2644,2and NAVSHIPS 250-1500-120refers to tests documented by the American Society for Testing and Materials (ASTM).
When using any specification, it is important to find out whether the latest version of the specification is used.
Specifications are usually under constant
review and the latest changes are often important improvements over previous issues. Using the version specified by
contract is essential if the liquid penetrant test system is subject to audit, because the auditor will immediately check to see that the correct issues are being followed.
100Liquid Penetrant Testing
P ART 1.Importance of Maintenance of
Liquid Penetrant Materials
The following admonitions apply to the storage of all liquid penetrant materials, that is, of liquid penetrants, of emulsifiers or removers and of developers. Materials in storage can suffer either deterioration or contamination if they are not properly cared for.
Deterioration
New Liquid Penetrant Test Materials
Deterioration is largely the result of time and storage conditions. Most liquid penetrant materials are not greatly affected by time as long as they are kept in closed storage containers. Liquid penetrants stored in open or loosely covered tanks or in improperly sealed containers are subject to evaporation losses.
Hot and cold storage conditions can affect liquid penetrants adversely. Cold storage will cause freezing of many liquid penetrant materials. Freezing will not usually prevent the liquid penetrant test material from performing properly after warming to the temperature of use, but in a few cases, freezing of liquid penetrant test materials has caused irreversible separation of constituents and performance failure. This separation is rare with modern liquid penetrants and MIL-I-25135E contains liquid penetrant qualification tests designed to detect this irreversible separation.1Hot storage up to 65°C (150°F) for limited periods of time usually has little effect. However, hot storage for long periods of time (months or years) could cause internal reaction between some components and degradation of fluorescent dyes, with subsequent loss of performance.
Aerosol Containers
Materials packaged in aerosol spray containers are also not affected by normal storage conditions. Cold storage reduces the internal pressure so the can must be warmed to nearly room temperature to spray properly. Conversely, high storage temperatures raise the pressure and extremely high pressures can cause bursting of the can. Therefore, aerosol can
temperatures should never exceed 55°C
(130°F).
Aerosols do not have infinite shelf life,
because there is always some slight
leakage of propellant through the valve.
This leakage usually does not cause a
significant change in performance until a
couple of years have passed. Aerosol
containers can eventually become
depressurized after storage for three to five
years, although there are many instances
where aerosol containers spray well after
as long as 20years.
Contamination
Deterioration is unlikely in storage but
contamination is always a possibility if
care is not taken. For example, if drums
containing liquid are stored outside, they
can have water cover the top if it rains.
Then, if the openings are not sealed
tightly, this water can be sucked into the
drum as the temperature changes. If
containers are stored after they have been
opened and they are not properly
resealed, dust, dirt or other foreign
materials can possibly get into the
containers. Depending on what type of
contamination enters the container, the
material can suffer a loss in performance
or even fail completely when it is used.
For developer powders, it is especially
important to use proper storage. Dry
developer powders can become damp and
then clump,rendering them less sensitive
when used. Foreign material getting into
dry developer could react with the liquid
penetrant and cause it to lose some of its
brightness. If the contamination in the
dry developer is fluorescent, it can cause
false calls.
Soluble and suspendible developer
powders contain surfactants that are
hygroscopic. If their containers are not
tightly sealed, they can pick up moisture.
This can cause them to become difficult
to use and can also make them subject to
biological decay. Microorganisms find the
surfactants very tasty. They can live in the
developer and eat all of the surfactant.
When this happens, the developer will no
longer wet the surface of the parts and the
inspection will fail.
General Rules
Many tests can be performed to ensure
that stored materials are still good but
101 Care and Maintenance of Liquid Penetrant Test Materials
P ART2.Care and Maintenance of Liquid
Penetrant Testing Materials in Storage
common sense is one of the best things to
use. First, make sure that liquid penetrant
products are stored properly. This means
indoors and at temperatures which are
not either extremely hot or extremely
cold. Second, make sure that the
containers are well sealed. Third, make
sure that the containers are clearly
labeled. When removing something from
storage, first inspect the container. Look
for evidence of incorrectly sealed
containers. Look for evidence of water
that has been on the top of the container.
Look for evidence that someone has
opened the container, removed some of
the contents and then not resealed the
container properly. If everything looks
good, the contents are probably good.
Nevertheless, when taking material from
the container, look at it critically. If it is a
liquid, it should be clear in color, without
evidence of any foreign material. It should
have no milky streaks in it if it is a water
washable liquid penetrant or a lipophilic
emulsifier. There should be no particulate
matter evident. The liquid at the bottom
of the container should appear no
different than that that at the top.
If the container has developer in it, see
that it has no clumps that are damp. Look
for evidence of fungus or algae — dark
spots which indicate that a colony of bugs
is there, eating the developer ingredients.
If all appears clean, dry and relatively free
flowing, the developer is probably in
excellent condition.
Finding any of the indications listed
above should make one suspect that the
material may not be in condition to use.
In that case, tests should be made as
outlined below for in-process materials. 102Liquid Penetrant Testing
Liquid Penetrants in Open Tanks
Proper maintenance is more difficult with liquid penetrants used in open tanks where parts are dipped into them than when sprayed from storage cans. When expended as used, particularly in the small portable test kits, liquid penetrants are subject to very little contamination or degradation. This is particularly true of liquid penetrants that are packaged in aerosol spray cans. When used in dip tanks or open containers, contaminating materials can get into liquid penetrants quite easily.
Some materials such as certain cleaners and solvents commonly used in manufacturing plants can affect the wetting ability of liquid penetrants when present in sufficient quantity. In addition, the greater liquid surface areas exposed with dip tanks make evaporation and loss of light volatile constituents of liquid penetrants more likely. The large liquid surface can absorb or condense moisture from the atmosphere. This water can have a deleterious effect on liquid penetrant performance, as can water carried into liquid penetrant tanks on test objects.
In-Use Contamination
Effects of Water Contamination in Water Washable Liquid Penetrants Presence of moisture in a liquid penetrant is probably the most common cause of failure in liquid penetrant testing. This is particularly true of water washable liquid penetrants, which contain emulsifiers so they can be readily removed by water washing. Water washable liquid penetrants have a definite water tolerance limit beyond which they do not function properly. Added water reduces their fluorescence and penetrating ability and also adversely affects their washability.
Water contamination in liquid penetrants may be first seen as a slight cloudiness. This increases with rising water content and is usually accompanied by an increase in liquid penetrant viscosity. With more added water, the liquid becomes somewhat striated and the
liquid penetrant eventually separates into
two distinct phases. The liquid may form
a gel.
Effects of Water Contamination in
Postemulsifiable Liquid Penetrants
Water contamination of postemulsifiable
types of liquid penetrant is seldom a
serious problem, because these materials
are not usually compatible with water.
With no emulsifier present, water in the
liquid penetrant dip tank will settle to the
bottom and may cause undetected
corrosion at the bottom of the tank.
Violent agitation may bring some water
into suspension in the oil, causing slight
liquid penetrant turbidity, but when the
agitation ceases, the liquid will clear quite
rapidly as the water settles out. Because
water will not stay suspended in
postemulsifiable liquid penetrants, it is
seldom necessary to conduct a water test.
However, if test parts are dipped to the
bottom of the tank, the accumulated
water can be a problem during operations.
Water is frequently introduced into
liquid penetrants and processing materials
from wet parts, through careless overspray
from a wash station or from leaking pipes
or roofs. Water can also enter a liquid
penetrant from the air, if the air is very
humid and the liquid penetrant
temperature is below the dew point.
Condensation of water from ambient air
can happen during a humid morning
following a night that was cold enough to
chill the liquid penetrant in the
immersion or storage tank.
Effects of Contaminants (Other
than Water) in Liquid Penetrants
There are other contaminants that can
affect the performance of fluorescent
liquid penetrants. Overall tests to disclose
their presence are not very practical. Such
tests would involve complicated
laboratory analysis and each test would
have to be directed specifically to a
particular contaminant. Fortunately, most
of these contaminant materials have to be
present in fairly large quantities before
they can seriously affect liquid penetrant
performance and their presence will be
made evident by a change in behavior of
the process material. Foreign materials
such as cleaning solvents, heavy oils,
103 Care and Maintenance of Liquid Penetrant Test Materials
P ART3. Care and Maintenance of Liquid
Penetrants in Use
acids or chromates in liquid penetrants will make their presence known by changes in liquid penetrant performance such as in wetting ability, drying characteristics or loss of fluorescent brightness.
Changes in wetting ability and drying characteristics of liquid penetrants can generally be seen by operators. Changes
in fluorescent brightness can be detected during operations by placing a single drop of used liquid penetrant on a filter paper alongside a drop of new liquid penetrant and viewing the two under an ultraviolet lamp. This permits a very rough comparison but is adequate in most cases. More accurate tests can be made by use of photometers, or fluorometers described elsewhere in this volume.
Effects of Organic Contamination of Test Objects on Liquid Penetrants
Another source of contamination of liquid penetrants and processing materials is formation of organic coatings on surfaces of test objects. These organic contaminants include grease, oil, preservative, paint and residues from previous processing. These contaminants cause serious problems when not removed from the surface of test parts because they may fill cracks and prevent the entrance of liquid penetrant. The organic contaminants are usually soluble in liquid penetrant and slowly increase in concentration in the liquid penetrant. Undesired effects of organic contaminants in liquid penetrants include the following: (1)diluting the dye in the liquid penetrant; (2)absorbing ultraviolet radiation before it reaches the dye in the liquid penetrant indications; (3)changing liquid penetrant viscosity; (4)unbalancing emulsifier systems; and (5)deterioration of liquid penetrants and process materials. Most of these contaminants never get beyond the liquid penetrant tank or drain stations and do not affect the emulsifier or developer.
Effects of Organic Solvent Contamination of Liquid Penetrant Test Materials
Still another type of contaminant often encountered is the organic solvent, such as degreaser fluid, gasoline or kerosene carried on test object surfaces. These contaminants usually originate in a previous cleaning operation. Small amounts of these organic solvent contaminants are not serious but large amounts affect liquid penetrant fluorescence and ease of washing to remove excess surface liquid penetrant. Carryover of some solvent degreasing fluids can result in corrosion conditions. Residues from deburring, tumble polishing or burnishing operations may fall in this third category.
Effects of Contamination of Liquid Penetrants by Acids, Caustics and Chromates
Acids and caustics have an additional adverse effect on any type of liquid penetrant in which they can be dissolved. These active chemical contaminants cause loss of fluorescence in fluorescent liquid penetrant dyes. They have an adverse effect on nonwater washable (postemulsifiable) liquid penetrants in which they are not soluble. Sometimes, chromate residues from etching operations may become trapped in discontinuities and subsequently destroy the fluorescent response of a liquid penetrant entrapment. This effect is due mainly to the powerful ultraviolet absorbency of the chromate ion.
In-Use Deterioration
Effects of Evaporation of Liquid Penetrant Test Materials
One source of deterioration of materials is evaporation. The liquid penetrants and some of the emulsifiers often contain light oil fractions that evaporate at various rates, leaving an unbalanced liquid penetrant formula. Wet developers and water base penetrants contain water or organic solvent that will evaporate if exposed for long periods. Some evaporation is unavoidable but the rate of evaporation is increased by higher temperatures and large exposed surface areas.
The effects of evaporation of volatile constituents of liquid penetrant processing materials include increased viscosity, higher developer concentration, changes in fluorescence, changes in water tolerance and increased dragout.
Also, evaporation will either speed or slow washing characteristics, depending on the type of liquid penetrant material. Therefore, evaporation of liquid penetrant materials cannot be ignored. Evaporation of liquid materials in tanks occurs whether the material is used or not and at a rate so gradual that it is often overlooked. Periodic checks are therefore required to ensure that excessive evaporation has not occurred.
104Liquid Penetrant Testing
Effects of Heat on Liquid
Penetrant Test Materials
Heat can be a cause of serious
deterioration of liquid penetrant
materials. A little heat continuously
applied increases the rate of liquid
penetrant evaporation. As the lighter
constituents evaporate, liquid penetrant
viscosity increases. This may actually
upgrade the liquid penetrant material
because, when lighter fractions evaporate,
dyestuff becomes more concentrated in
the less volatile oil that remains. However,
an increase in liquid penetrant viscosity
increases dragout on test parts, slows
penetration into discontinuities and
changes the wash characteristics. Liquid
penetrants that include coupling agents
that are more volatile than other
constituents may separate or gel as the
coupling agent evaporates. Such coupling
agents are used to prevent separation of
liquid penetrant constituents or to control
liquid penetrant tolerance for water.
High temperatures cause all of the
preceding effects and, in addition, can kill
fluorescence of dyes in liquid penetrants
or leak tracers. Therefore, heating of
liquid penetrant materials much above
room temperature, either locally or
completely, should be avoided if at all
possible.
Care and Maintenance of Liquid Penetrant Test Materials
105
In-Use Contamination
Deterioration of Emulsifiers by Contamination with Water or Liquid Penetrant
Whenever the washing of surface liquid penetrant from test parts becomes noticeably difficult, the emulsifier
(whether lipophilic or hydrophilic) should be checked immediately. The most probable cause of deterioration of
lipophilic emulsifiers is contamination of the oil base emulsifier with water. All
lipophilic emulsifiers have a definite water tolerance that may vary from five percent to practically unlimited amounts of water.When water concentrations exceed the water tolerance limit, the lipophilic
emulsifier is no longer effective as a liquid penetrant remover. This loss in
effectiveness is generally accompanied by changes in emulsifier appearance or physical properties. The low tolerance emulsifiers usually become viscous or gel completely, whereas the high tolerance emulsifiers become cloudy or show a tendency to thicken.
Another important source of
contamination of emulsifier baths is liquid penetrant carried into emulsifiers during processing. If excessive liquid penetrant contamination builds up, the effectiveness of the emulsifier will decrease noticeably.
Sources of Liquid Penetrant Contamination in Emulsifiers
The most common source of liquid
penetrant contamination in emulsifiers is that carried into emulsifiers by dragout on test parts processed through a dip
operation. This liquid penetrant reduces the emulsifying ability of the emulsifier,making it less effective. If the liquid
penetrant contamination reaches a high enough level, it destroys the emulsifier. In many operations, the liquid penetrant content reaches an equilibrium because emulsifier is carried out on the parts and the necessary makeup emulsifier dilutes the carried-in liquid penetrant. If the liquid penetrant content does reach
excessive levels, the reason is often poor test procedure. Liquid penetrant should be
fairly well drained off the parts before they enter the emulsifier. Particular attention should be paid to reservoirs
(openings in test objects) or recessed areas that hold more than a thin surface layer of liquid penetrant. This may mean that the parts must be turned over during the drain period but the extra effort will be justified by the reduced material loss and contamination.
Sources of Water Contamination in Emulsifiers
Any liquid penetrant testing system that uses emulsifiers also uses water to wash the parts after they have been emulsified.The close proximity of water to the emulsifier makes it relatively easy to
contaminate the emulsifier with water. In any case where it has been found that water contamination has taken place, the first thing to suspect is that water from the wash operation has gotten into the emulsifier tank. In virtually every case,careful attention to procedures will reveal how contamination is happening. In one instance, the operators were steadfast in maintaining that it was not possible for water to get in the tank but when the operation was observed, the situation was very clear. The operators were holding the parts by hand and spraying them with water. They held the parts in such a position that the overspray was going directly into the emulsifier tank. It was like rain.
Other sources of water are rare and not as obvious. In one case, a night janitor found that, if he wanted to really clean his mop, all that he needed to do was to dip it into the emulsifier tank and then wash it with water. He repeated this many times each night and each time that the mop went into the tank, it put some water into it. In just a few days, the
emulsifier was ruined. In another case, a new maintenance man had been hired and he was anxious to show that he did good work. The emulsifier tank was empty and he cleaned it with water, doing a very good job. However, the tank had piping connected to it and he left these pipes full of water. When the tank was refilled with emulsifier, it was almost instantly contaminated.
106Liquid Penetrant Testing
P ART 4. Care and Maintenance of Liquid
Penetrant Emulsifiers and Removers in Use
There are four types of developer in common use. These are dry powder
developer, water soluble developer, water suspendible developer and nonaqueous developer. Each of these has distinctly different characteristics and different susceptibilities to deterioration and contamination.
Dry Powder Developer
Dry powder developer is not prone to deteriorate. It is composed of chemically stable absorbent powders that are unlikely to decompose or otherwise change their composition in a way that would affect their performance. However, dry powder developers are prone to contamination of various types. The most obvious of these is contamination by fluorescent liquid penetrant. When parts have been dipped into the dry developer, the developer adheres to the wet opening of
discontinuities because there is liquid penetrant there. The liquid penetrant
then is wicked into the developer and the liquid penetrant makes the dry powder wet and fluorescent. If particles of this wet, fluorescent powder fall free from the part, they mix with the rest of the dry powder, creating spots of fluorescence that can later adhere to another part and cause a false indication.
Dry powder developer can also become damp or wet, causing it to clump. When this happens, it does not form a smooth even coat on the parts being inspected and it also may not stick to the wet
openings of the discontinuities, thereby missing indications. Wetness can result from parts that are not completely dry before being put into the developer and it can occur because of excessively high humidity.
Water Soluble Developer
Water soluble developer may deteriorate in use as a result of infections of bacteria,algae or fungus, commonly termed bugs.In order for the soluble developer to form a smooth coat on the parts, a surfactant is included in the formulation. Surfactants are a favorite food for bacteria, algae and fungi and airborne bugs can land in the solution and infect it. Soluble developer
formulations also include biocides to
counteract this undesirable occurrence but they do not last indefinitely. The best practice to avoid infections is to keep the tank covered when not in use. If an
infection occurs, the cure is to drain the tank, sterilize it and then refill it with fresh developer solution. The developer manufacturer has complete instructions for this procedure.
Soluble developers can also become contaminated in various ways. The concentration can go up or down
depending on evaporation of water from the tank or the addition of water from wet parts being immersed. Fluorescent liquid penetrant can get into the developer bath from the parts being inspected. Also, it is possible to
contaminate the developer solution
through electrolytic action. The developer is a solution of chemicals that ionize in solution, creating an electrolyte. If a basket of dissimilar metals is placed into the solution, electrolysis can occur in which metal is dissolved from one of the dissimilar metals. The result can be cloudy or murky solution that reduces the developing ability of the solution.
Water Suspendible Developer
Water suspendible developers are prone to some of the same contaminants that soluble developers are, because they also contain surfactants and because they also have ionized components. This means that it is possible to have an infected developer bath and that it is possible to have electrolytic effects. It is also possible to contaminate the suspension with liquid penetrant from the parts being inspected and it is possible that the concentration of the suspension will change as a result of evaporation of water from the tank or the addition of water from the parts.
Nonaqueous Wet Developer
Nonaqueous wet developers are usually used in aerosol form. Properly used, the developer is not prone to either
deterioration or contamination. If the
107
Care and Maintenance of Liquid Penetrant Test Materials P ART 5.Care and Maintenance of Developers in
Use
cans are not agitated before use, the
developer powder will not be properly
dispersed and this improper dispersion
will cause the developer coating to be
either too thick or too thin. The only
other failure that can occur is the loss of
pressure in the can, in which case the
developer may be good but cannot be
expelled from the can.
Nonaqueous developers used in bulk
form are prone to concentration changes
because of solvent evaporation and
should be kept in tightly closed
containers. If applied with a spray gun,
agitation must be used to keep the
developer particles in suspension.
108Liquid Penetrant Testing。