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4C 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

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The quality of an inspection made withliquid penetrants can be no better thanthe quality of the liquid penetrant testmaterials used for the inspection.Recognizing this basic fact requires thatliquid penetrant materials meet certainindustry standards before they arepurchased for use. It also requires thatthese materials and the liquid penetrantsystems in which they are used bemonitored and tested on a periodic andregular basis. To ensure the materialspurchased are of desired quality and thatthe proper tests are made at the properintervals during their use, a variety of specifications have been developed. A list

of some of the better knownnondestructive testing specificationsappears in the references.1-19

These specifications can be broadlyclassified as to their origin, typically theUnited States military, various UnitedStates technical societies or corporateusers. Many of the specificationsdescribing the materials have commonthreads running through them whereassome have been specifically tailored to thespecialized use of a particular industry orcustomer. For example, although virtuallyevery liquid penetrant in use has had tomeet the requirements of MIL-I-251351

(or its commercial replacement,AMS-26442), liquid penetrants used in thenuclear industry must additionally haveparticularly low contents of certain lowmelting metals, as well as sulfur andhalogens. Aircraft turbine enginemanufacturers require products with lowfluoride, sodium, chloride and sulfurcontent. Specifications detailing thetesting of materials in use have revolvedmainly around the military specificationMIL-STD-68663 (or its commercialreplacement, ASTM E 14174) but othercorporate specifications may also exist.These specifications, in turn, often refer to

tests which are called out in otherspecifications. For example,MIL-STD-68663 refers to tests which areoutlined and described in MIL-I-251351 orAMS 2644,2 and NAVSHIPS 250-1500-120

refers to tests documented by theAmerican Society for Testing andMaterials (ASTM).

When using any specification, it isimportant to find out whether the latestversion of the specification is used.Specifications are usually under constant

review and the latest changes are oftenimportant improvements over previousissues. Using the version specified bycontract is essential if the liquid penetranttest system is subject to audit, because theauditor will immediately check to see thatthe correct issues are being followed.

100 Liquid Penetrant Testing

PART 1. Importance of Maintenance of Liquid Penetrant Materials

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The following admonitions apply to thestorage of all liquid penetrant materials,that is, of liquid penetrants, of emulsifiersor removers and of developers. Materialsin storage can suffer either deteriorationor contamination if they are not properlycared for.

Deterioration

New Liquid Penetrant TestMaterials

Deterioration is largely the result of timeand storage conditions. Most liquidpenetrant materials are not greatlyaffected by time as long as they are keptin closed storage containers. Liquidpenetrants stored in open or looselycovered tanks or in improperly sealedcontainers are subject to evaporationlosses.

Hot and cold storage conditions canaffect liquid penetrants adversely. Coldstorage will cause freezing of many liquidpenetrant materials. Freezing will notusually prevent the liquid penetrant testmaterial from performing properly afterwarming to the temperature of use, but in

a few cases, freezing of liquid penetranttest materials has caused irreversibleseparation of constituents andperformance failure. This separation israre with modern liquid penetrants andMIL-I-25135E contains liquid penetrantqualification tests designed to detect thisirreversible separation.1 Hot storage up to65 °C (150 °F) for limited periods of timeusually has little effect. However, hotstorage for long periods of time (monthsor years) could cause internal reactionbetween some components anddegradation of fluorescent dyes, withsubsequent loss of performance.

Aerosol Containers

Materials packaged in aerosol spraycontainers are also not affected by normalstorage conditions. Cold storage reducesthe internal pressure so the can must bewarmed to nearly room temperature tospray properly. Conversely, high storagetemperatures raise the pressure andextremely high pressures can causebursting 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 slightleakage of propellant through the valve.This leakage usually does not cause asignificant change in performance until acouple of years have passed. Aerosolcontainers can eventually becomedepressurized after storage for three to fiveyears, although there are many instanceswhere aerosol containers spray well afteras long as 20 years.

Contamination

Deterioration is unlikely in storage butcontamination is always a possibility if care is not taken. For example, if drumscontaining liquid are stored outside, theycan have water cover the top if it rains.Then, if the openings are not sealedtightly, this water can be sucked into thedrum as the temperature changes. If containers are stored after they have beenopened and they are not properlyresealed, dust, dirt or other foreignmaterials can possibly get into thecontainers. Depending on what type of contamination enters the container, thematerial can suffer a loss in performance

or even fail completely when it is used.For developer powders, it is especiallyimportant to use proper storage. Drydeveloper powders can become damp andthen clump, rendering them less sensitivewhen used. Foreign material getting intodry developer could react with the liquidpenetrant and cause it to lose some of itsbrightness. If the contamination in thedry developer is fluorescent, it can causefalse calls.

Soluble and suspendible developerpowders contain surfactants that arehygroscopic. If their containers are nottightly sealed, they can pick up moisture.This can cause them to become difficultto use and can also make them subject tobiological decay. Microorganisms find thesurfactants very tasty. They can live in thedeveloper and eat all of the surfactant.When this happens, the developer will nolonger wet the surface of the parts and theinspection will fail.

General Rules

Many tests can be performed to ensurethat stored materials are still good but

101Care and Maintenance of Liquid Penetrant Test Materials

PART 2. Care and Maintenance of Liquid

Penetrant Testing Materials in Storage

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common sense is one of the best things touse. First, make sure that liquid penetrantproducts are stored properly. This meansindoors and at temperatures which arenot either extremely hot or extremelycold. Second, make sure that thecontainers are well sealed. Third, makesure that the containers are clearlylabeled. When removing something fromstorage, first inspect the container. Look

for evidence of incorrectly sealedcontainers. Look for evidence of waterthat has been on the top of the container.Look for evidence that someone hasopened the container, removed some of the contents and then not resealed thecontainer properly. If everything looksgood, the contents are probably good.Nevertheless, when taking material fromthe container, look at it critically. If it is aliquid, it should be clear in color, withoutevidence of any foreign material. It shouldhave no milky streaks in it if it is a waterwashable liquid penetrant or a lipophilicemulsifier. There should be no particulate

matter evident. The liquid at the bottomof the container should appear nodifferent than that that at the top.

If the container has developer in it, seethat it has no clumps that are damp. Lookfor evidence of fungus or algae — darkspots which indicate that a colony of bugsis there, eating the developer ingredients.If all appears clean, dry and relatively freeflowing, the developer is probably inexcellent condition.

Finding any of the indications listedabove should make one suspect that thematerial may not be in condition to use.In that case, tests should be made asoutlined below for in-process materials.


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Liquid Penetrants in OpenTanksProper maintenance is more difficult withliquid penetrants used in open tankswhere parts are dipped into them thanwhen sprayed from storage cans. Whenexpended as used, particularly in thesmall portable test kits, liquid penetrantsare subject to very little contamination ordegradation. This is particularly true of liquid penetrants that are packaged inaerosol spray cans. When used in diptanks or open containers, contaminatingmaterials can get into liquid penetrantsquite easily.

Some materials such as certain cleanersand solvents commonly used inmanufacturing plants can affect thewetting ability of liquid penetrants whenpresent in sufficient quantity. In addition,the greater liquid surface areas exposedwith dip tanks make evaporation and lossof light volatile constituents of liquidpenetrants more likely. The large liquidsurface can absorb or condense moisturefrom the atmosphere. This water can havea deleterious effect on liquid penetrantperformance, as can water carried intoliquid penetrant tanks on test objects.

In-Use Contamination

Effects of Water Contamination inWater Washable Liquid Penetrants

Presence of moisture in a liquid penetrantis probably the most common cause of failure in liquid penetrant testing. This isparticularly true of water washable liquidpenetrants, which contain emulsifiers sothey can be readily removed by waterwashing. Water washable liquid

penetrants have a definite water tolerancelimit beyond which they do not functionproperly. Added water reduces theirfluorescence and penetrating ability andalso adversely affects their washability.

Water contamination in liquidpenetrants may be first seen as a slightcloudiness. This increases with risingwater content and is usually accompaniedby an increase in liquid penetrantviscosity. With more added water, theliquid becomes somewhat striated and the

liquid penetrant eventually separates intotwo distinct phases. The liquid may forma gel.

Effects of Water Contamination inPostemulsifiable Liquid Penetrants

Water contamination of postemulsifiabletypes of liquid penetrant is seldom aserious problem, because these materialsare not usually compatible with water.With no emulsifier present, water in theliquid penetrant dip tank will settle to thebottom and may cause undetectedcorrosion at the bottom of the tank.Violent agitation may bring some water

into suspension in the oil, causing slightliquid penetrant turbidity, but when theagitation ceases, the liquid will clear quiterapidly as the water settles out. Becausewater will not stay suspended inpostemulsifiable liquid penetrants, it isseldom necessary to conduct a water test.However, if test parts are dipped to thebottom of the tank, the accumulatedwater can be a problem during operations.

Water is frequently introduced intoliquid penetrants and processing materialsfrom wet parts, through careless oversprayfrom a wash station or from leaking pipesor roofs. Water can also enter a liquid

penetrant from the air, if the air is veryhumid and the liquid penetranttemperature is below the dew point.Condensation of water from ambient aircan happen during a humid morningfollowing a night that was cold enough tochill the liquid penetrant in theimmersion or storage tank.

Effects of Contaminants (Otherthan Water) in Liquid Penetrants

There are other contaminants that canaffect the performance of fluorescentliquid penetrants. Overall tests to disclose

their presence are not very practical. Suchtests would involve complicatedlaboratory analysis and each test wouldhave to be directed specifically to aparticular contaminant. Fortunately, mostof these contaminant materials have to bepresent in fairly large quantities beforethey can seriously affect liquid penetrantperformance and their presence will bemade evident by a change in behavior of the process material. Foreign materialssuch as cleaning solvents, heavy oils,



Penetrants in Use

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acids or chromates in liquid penetrantswill make their presence known bychanges in liquid penetrant performancesuch as in wetting ability, dryingcharacteristics or loss of fluorescentbrightness.

Changes in wetting ability and dryingcharacteristics of liquid penetrants cangenerally be seen by operators. Changesin fluorescent brightness can be detected

during operations by placing a single dropof used liquid penetrant on a filter paperalongside a drop of new liquid penetrantand viewing the two under an ultravioletlamp. This permits a very roughcomparison but is adequate in most cases.More accurate tests can be made by use of photometers, or fluorometers describedelsewhere in this volume.

Effects of Organic Contaminationof Test Objects on LiquidPenetrantsAnother source of contamination of liquid

penetrants and processing materials isformation of organic coatings on surfacesof test objects. These organiccontaminants include grease, oil,preservative, paint and residues fromprevious processing. These contaminantscause serious problems when not removedfrom the surface of test parts because theymay fill cracks and prevent the entranceof liquid penetrant. The organiccontaminants are usually soluble in liquidpenetrant and slowly increase inconcentration in the liquid penetrant.Undesired effects of organic contaminantsin liquid penetrants include the following:

(1) diluting the dye in the liquidpenetrant; (2) absorbing ultravioletradiation before it reaches the dye in theliquid penetrant indications; (3) changingliquid penetrant viscosity; (4) unbalancingemulsifier systems; and (5) deteriorationof liquid penetrants and process materials.Most of these contaminants never getbeyond the liquid penetrant tank or drainstations and do not affect the emulsifieror developer.

Effects of Organic SolventContamination of LiquidPenetrant Test Materials

Still another type of contaminant oftenencountered is the organic solvent, suchas degreaser fluid, gasoline or kerosenecarried on test object surfaces. Thesecontaminants usually originate in aprevious cleaning operation. Smallamounts of these organic solventcontaminants are not serious but largeamounts affect liquid penetrantfluorescence and ease of washing toremove excess surface liquid penetrant.Carryover of some solvent degreasing

fluids can result in corrosion conditions.Residues from deburring, tumblepolishing or burnishing operations mayfall in this third category.

Effects of Contamination of LiquidPenetrants by Acids, Caustics andChromates

Acids and caustics have an additional

adverse effect on any type of liquidpenetrant in which they can be dissolved.These active chemical contaminants causeloss of fluorescence in fluorescent liquidpenetrant dyes. They have an adverseeffect on nonwater washable(postemulsifiable) liquid penetrants inwhich they are not soluble. Sometimes,chromate residues from etchingoperations may become trapped indiscontinuities and subsequently destroythe fluorescent response of a liquidpenetrant entrapment. This effect is duemainly to the powerful ultravioletabsorbency of the chromate ion.

In-Use Deterioration

Effects of Evaporation of LiquidPenetrant Test MaterialsOne source of deterioration of materials isevaporation. The liquid penetrants andsome of the emulsifiers often containlight oil fractions that evaporate atvarious rates, leaving an unbalancedliquid penetrant formula. Wet developersand water base penetrants contain water

or organic solvent that will evaporate if exposed for long periods. Someevaporation is unavoidable but the rate of evaporation is increased by highertemperatures and large exposed surfaceareas.

The effects of evaporation of volatileconstituents of liquid penetrantprocessing materials include increasedviscosity, higher developer concentration,changes in fluorescence, changes in watertolerance and increased dragout.

Also, evaporation will either speed orslow washing characteristics, dependingon the type of liquid penetrant material.

Therefore, evaporation of liquid penetrantmaterials cannot be ignored. Evaporationof liquid materials in tanks occurswhether the material is used or not and ata rate so gradual that it is oftenoverlooked. Periodic checks are thereforerequired to ensure that excessiveevaporation has not occurred.


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Effects of Heat on LiquidPenetrant Test Materials

Heat can be a cause of seriousdeterioration of liquid penetrantmaterials. A little heat continuouslyapplied increases the rate of liquidpenetrant evaporation. As the lighterconstituents evaporate, liquid penetrantviscosity increases. This may actually

upgrade the liquid penetrant materialbecause, when lighter fractions evaporate,dyestuff becomes more concentrated inthe less volatile oil that remains. However,an increase in liquid penetrant viscosityincreases dragout on test parts, slowspenetration into discontinuities andchanges the wash characteristics. Liquidpenetrants that include coupling agentsthat are more volatile than otherconstituents may separate or gel as thecoupling agent evaporates. Such couplingagents are used to prevent separation of liquid penetrant constituents or to controlliquid penetrant tolerance for water.

High temperatures cause all of thepreceding effects and, in addition, can killfluorescence of dyes in liquid penetrantsor leak tracers. Therefore, heating of liquid penetrant materials much aboveroom temperature, either locally orcompletely, should be avoided if at allpossible.


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In-Use Contamination

Deterioration of Emulsifiers byContamination with Water orLiquid Penetrant

Whenever the washing of surface liquidpenetrant from test parts becomesnoticeably difficult, the emulsifier(whether lipophilic or hydrophilic) shouldbe checked immediately. The mostprobable cause of deterioration of lipophilic emulsifiers is contamination of the oil base emulsifier with water. All

lipophilic emulsifiers have a definite watertolerance that may vary from five percentto practically unlimited amounts of water.When water concentrations exceed thewater tolerance limit, the lipophilicemulsifier is no longer effective as a liquidpenetrant remover. This loss ineffectiveness is generally accompanied bychanges in emulsifier appearance orphysical properties. The low toleranceemulsifiers usually become viscous or gelcompletely, whereas the high toleranceemulsifiers become cloudy or show atendency to thicken.

Another important source of

contamination of emulsifier baths isliquid penetrant carried into emulsifiersduring processing. If excessive liquidpenetrant contamination builds up, theeffectiveness of the emulsifier willdecrease noticeably.

Sources of Liquid PenetrantContamination in Emulsifiers

The most common source of liquidpenetrant contamination in emulsifiers isthat carried into emulsifiers by dragout ontest parts processed through a dipoperation. This liquid penetrant reduces

the emulsifying ability of the emulsifier,making it less effective. If the liquidpenetrant contamination reaches a highenough level, it destroys the emulsifier. Inmany operations, the liquid penetrantcontent reaches an equilibrium becauseemulsifier is carried out on the parts andthe necessary makeup emulsifier dilutesthe carried-in liquid penetrant. If theliquid penetrant content does reachexcessive levels, the reason is often poortest procedure. Liquid penetrant should be

fairly well drained off the parts beforethey enter the emulsifier. Particularattention should be paid to reservoirs(openings in test objects) or recessed areasthat hold more than a thin surface layerof liquid penetrant. This may mean thatthe parts must be turned over during thedrain period but the extra effort will bejustified by the reduced material loss andcontamination.

Sources of Water Contaminationin Emulsifiers

Any liquid penetrant testing system thatuses emulsifiers also uses water to wash

the parts after they have been emulsified.The close proximity of water to theemulsifier makes it relatively easy tocontaminate the emulsifier with water. Inany case where it has been found thatwater contamination has taken place, thefirst thing to suspect is that water fromthe wash operation has gotten into theemulsifier tank. In virtually every case,careful attention to procedures will revealhow contamination is happening. In oneinstance, the operators were steadfast inmaintaining that it was not possible forwater to get in the tank but when theoperation was observed, the situation was

very clear. The operators were holding theparts by hand and spraying them withwater. They held the parts in such aposition that the overspray was goingdirectly into the emulsifier tank. It waslike rain.

Other sources of water are rare and notas obvious. In one case, a night janitorfound that, if he wanted to really cleanhis mop, all that he needed to do was todip it into the emulsifier tank and thenwash it with water. He repeated this manytimes each night and each time that themop went into the tank, it put somewater into it. In just a few days, the

emulsifier was ruined. In another case, anew maintenance man had been hiredand he was anxious to show that he didgood work. The emulsifier tank was emptyand he cleaned it with water, doing a verygood job. However, the tank had pipingconnected to it and he left these pipes fullof water. When the tank was refilled withemulsifier, it was almost instantlycontaminated.



Penetrant Emulsifiers and Removers in Use

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There are four types of developer incommon use. These are dry powderdeveloper, water soluble developer, watersuspendible developer and nonaqueousdeveloper. Each of these has distinctlydifferent characteristics and differentsusceptibilities to deterioration andcontamination.

Dry Powder DeveloperDry powder developer is not prone todeteriorate. It is composed of chemicallystable absorbent powders that are unlikelyto decompose or otherwise change theircomposition in a way that would affecttheir performance. However, dry powderdevelopers are prone to contamination of various types. The most obvious of theseis contamination by fluorescent liquidpenetrant. When parts have been dippedinto the dry developer, the developeradheres to the wet opening of discontinuities because there is liquidpenetrant there. The liquid penetrantthen is wicked into the developer and theliquid penetrant makes the dry powderwet and fluorescent. If particles of thiswet, fluorescent powder fall free from thepart, they mix with the rest of the dry

powder, creating spots of fluorescencethat can later adhere to another part andcause a false indication.

Dry powder developer can also becomedamp or wet, causing it to clump. Whenthis happens, it does not form a smootheven coat on the parts being inspectedand it also may not stick to the wetopenings of the discontinuities, therebymissing indications. Wetness can resultfrom parts that are not completely drybefore being put into the developer and itcan occur because of excessively highhumidity.

Water Soluble DeveloperWater soluble developer may deterioratein use as a result of infections of bacteria,algae or fungus, commonly termed bugs.In order for the soluble developer to forma smooth coat on the parts, a surfactant isincluded in the formulation. Surfactantsare a favorite food for bacteria, algae andfungi and airborne bugs can land in thesolution and infect it. Soluble developer

formulations also include biocides tocounteract this undesirable occurrence butthey do not last indefinitely. The bestpractice to avoid infections is to keep thetank covered when not in use. If aninfection occurs, the cure is to drain thetank, sterilize it and then refill it withfresh developer solution. The developermanufacturer has complete instructionsfor this procedure.

Soluble developers can also becomecontaminated in various ways. Theconcentration can go up or downdepending on evaporation of water fromthe tank or the addition of water fromwet parts being immersed. Fluorescent

liquid penetrant can get into thedeveloper bath from the parts beinginspected. Also, it is possible tocontaminate the developer solutionthrough electrolytic action. The developeris a solution of chemicals that ionize insolution, creating an electrolyte. If abasket of dissimilar metals is placed intothe solution, electrolysis can occur inwhich metal is dissolved from one of thedissimilar metals. The result can be cloudyor murky solution that reduces thedeveloping ability of the solution.

Water SuspendibleDeveloperWater suspendible developers are prone tosome of the same contaminants thatsoluble developers are, because they alsocontain surfactants and because they alsohave ionized components. This meansthat it is possible to have an infecteddeveloper bath and that it is possible tohave electrolytic effects. It is also possibleto contaminate the suspension with liquidpenetrant from the parts being inspectedand it is possible that the concentrationof the suspension will change as a result

of evaporation of water from the tank orthe addition of water from the parts.

Nonaqueous WetDeveloperNonaqueous wet developers are usuallyused in aerosol form. Properly used, thedeveloper is not prone to eitherdeterioration or contamination. If the


PART 5. Care and Maintenance of Developers

in Use

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cans are not agitated before use, thedeveloper powder will not be properlydispersed and this improper dispersionwill cause the developer coating to beeither too thick or too thin. The onlyother failure that can occur is the loss of pressure in the can, in which case thedeveloper may be good but cannot beexpelled from the can.

Nonaqueous developers used in bulk

form are prone to concentration changesbecause of solvent evaporation andshould be kept in tightly closedcontainers. If applied with a spray gun,agitation must be used to keep thedeveloper particles in suspension.


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The following information appliesgenerally to the testing of liquidpenetrant materials to determine whetherthey are still suitable for use after havingbeen used for some period of time. Mostliquid penetrant processes are runaccording to certain specifications orstandards, such as MIL-STD-68663 or itsreplacement, ASTM E 1417.4 Thesestandards or specifications contain theexact requirements for testing andreference the exact test techniques to beused. Liquid penetrant users may refer tothe following discussion for generalinformation but should carefully followthe requirements of the standard or

specification under which they areworking. If no standard or specification iscalled out, it is wise to use ASTM E 14174

to ensure that the test process is in goodcondition.

Water Tolerance of WaterWashable LiquidPenetrants and LipophilicEmulsifiersA useful property test that may beperformed on liquid penetrants is thewater tolerance test. This test is applicableonly to water washable liquid penetrantsand to lipophilic emulsifiers used withpostemulsifiable liquid penetrants becausethese are the only materials affected bysmall amounts of water contamination.The water tolerance test is not usuallyprescribed for postemulsifiable liquidpenetrants themselves as they are notwater miscible in their original form.Typically, water will sink to the bottom of the postemulsifiable liquid penetrant tankand form a separate layer.

When water is introduced into waterwashable liquid penetrants, it will mix, up

to a point, causing some characteristicchanges. First, it will usually result inincreased viscosity, thereby causing someof the same changes as evaporation. Thepeak viscosity is often reached with arelatively small amount of added water.More water then reduces viscosity, insome cases to a value lower than normal.In addition to the viscosity changes, waterreduces the dye concentration, therebydecreasing the performance of waterwashable liquid penetrants. Further,addition of water (with its high surface

tension) upsets the liquid penetrantformula’s balance between adhesion andcohesion. This modifies the characteristicsof a liquid penetrant, even though thepenetrant-and-water mix remains clear.Also, chemicals with substantial watercontents and without inhibitors willcontribute to rusting of test parts as wellas liquid penetrant processing equipment.A liquid penetrant with a high watercontent could be a source of corrosion.

Enough added water eventually causesmany liquid penetrant test materials toseparate into two layers. Generally, butnot necessarily, the first sign of suchseparation is a cloudiness of the fluid.

Sometimes, this cloudiness is localized ina small area of a tank, and stirring todilute the local high water content willclear the fluid. Therefore, care must betaken to ensure that a representativesample is used. If the whole tank iscloudy, the layers will often separate bygravity. Even if they do not, theeffectiveness of the liquid penetrant testmaterial is gone. The material must thenbe replaced.

Visible Determination of

Water Tolerance LimitsA simple test for water tolerance is to addknown amounts of water to a knownamount of material with mixing. Whenpermanent cloudiness or gelling occurs,the water tolerance limit has beendetermined. The amount of water used toproduce cloudiness or gelling is a directindication of how much more water thematerial will accept before breaking down.

It is wise to know the water toleranceof new liquid penetrant test materials.This gives a base point that allows one todetermine if a large variation from thebase value has occurred in the test

material. If such a large variation doesoccur, not only should one be prepared toreplace the material but the probablecause of the contamination should bedetermined and, if possible, eliminated.

Regardless of the water tolerance of theliquid penetrant, it must be checkedperiodically in accordance withMIL-STD-68663 or ASTM E 14174 todetermine that the maximum allowablewater concentration has not beenexceeded.


PART 6. Quality Control Tests for Liquid

Penetrant Materials

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Quantitative WaterContent MeasurementsA water content measurement may bedesired instead of a water tolerance testand is necessary if one must know theactual water content. This is usually doneby the ASTM D 9521 technique where aknown amount of material is refluxed

with the specific solvent xylene and thecondensed vapors are caught in a watertrap. Water and xylene boil at similartemperatures and so leave the mixturetogether. The condensed liquids are notmutually soluble, so they separate in thecondensation trap. The water is heavier soit settles to the bottom of the trap formeasurement. Most of the xylene isreturned to the original mixture.

It is essential to use the specific solventknown as xylene for these water contentmeasurements. Many liquid penetrantscontain water soluble glycols or glycolethers which, though much less volatile

than water, are still distilled out to a smallextent. They collect with the water andact to increase its volume. Solvents with ahigher boiling point than xylene extractmore of these glycols or glycol ethers andso yield higher apparent water contents.Solvents that are more volatile thanxylene extract less water. Use of xyleneonly, in water content tests, permitsequivalent results to be obtained indifferent facilities. One way to avoid theerrors that glycols or glycol ethers cancause is to use a Karl Fischer analysis, achemical titration technique for precisemeasurement of moisture.

Precautions in SelectingSamples of LiquidPenetrant MaterialsWhen liquid penetrant materials are to betested for contamination or deterioration,the first consideration is to ensure that atruly representative sample is taken. Theproduct must be well mixed so that anysludge or separation layers are included inthe sample. The container used to storeliquid penetrant samples must be cleanand made of material that is not attackedby and is not permeable to the liquidpenetrant material.

Selecting StandardMaterials for Evaluation of Liquid PenetrantDeteriorationMost test procedures usable for thedetermination of liquid penetrantmaterial conditions do not give absolute

results but rather are comparisons to somestandard. Therefore, the choice and careof standard samples of liquid penetrantsand processing materials for each liquidpenetrant test system in use is veryimportant. A good standard is newmaterial saved from the original containerat the time the material is put into use.This type of standard eliminates anyeffects of batch-to-batch materialvariation and previous storage. The newmaterial standards should, of course, bestored in such a way that they will notdeteriorate. Therefore, standards shouldbe kept in tightly closed glass or metal

containers (not plastic containers) andstored in a cool, dry place. Even with suchproper storage, these liquid penetrant andprocessing material standard samplesshould be checked against brand newmaterials of the same types every two orthree years to ensure that no deteriorationhas occurred.

Procedures for ComparingStandard Materials withIn-Use Liquid Penetrant

MaterialsIt is possible to use very sophisticatedinstrument analyses to check liquidpenetrant materials during their use butsuch sophistication is rarely justified. Allthat is really necessary at the field level isto know whether the material isperforming as required and, if not, whereand how bad are its shortcomings. Thesecan be determined with a series of relatively simple, inexpensive testprocedures requiring a relatively smallinvestment in test equipment.

The most important test is the systemperformance test in which the in-usematerials are tested against standardliquid penetrant test materials of the sametype on actual cracked reference testobjects. The specimens may be parts of the type being tested and containingknown, naturally occurringdiscontinuities or they may be test panelswith synthetic discontinuities. In anycase, the test parts must be clean and drybefore the test. With some test specimens,half of the specimen can be treated within-use material and the other half withstandard material. With other specimens,


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it will be necessary to have two similartest pieces, one for in-use material andone for new.

Processing ReferenceSpecimens with Standardand In-Use Liquid

Penetrant MaterialsIn the reference test procedure, standardmaterials are applied to one part of thetest piece, the used materials are appliedto the other part (or to a similar piece)and the two systems are processedequally. If actual cracked parts are beingused, the processing should be the sameas that used in normal production tests. If synthetic discontinuity test pieces arebeing used, procedures may have to bemodified to handle the different surfaceand crack conditions of the syntheticdiscontinuity pieces. The mainrequirement is to ensure that standard

and used material systems are processedidentically.

Close comparison of test indicationswith standard and used material systems

will show whether there is a noticeabledifference between them. If there is nodifference, the used material can beassumed to be acceptable and further testsare not necessary (but may be performedif a numerical status rating is desired). If adifference is noted, a decision must bemade as to whether this difference (1) issignificant enough to reject the usedmaterials at this point or (2) justifies

further tests to determine the cause anddegree of the problem.

Laboratory Fluorometersfor Fluorescent BrightnessMeasurementsOne type of filter photometer that hasbeen used for fluorescent brightnessmeasurement is shown in Fig. 1. Thesefluorometers normally contain thephotometer sample holder and theultraviolet light source, all in one package.

They are normally designed to measuretest tubes full of fluorescent liquid. With aspecial sample holder (Fig. 2) theseinstruments can be used to measure


FIGURE 1. Laboratory fluorometer specified by MIL-I-251351 and ASTM E 11356 andcontaining built-in ultraviolet radiation source and interchangeable filters for lightmodification.

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brightness of filter paper soaked withhighly diluted liquid penetrant. ASTME 11356 contains procedural details forcomparing the brightness of fluorescentliquid penetrants. These instruments areno longer manufactured but many are stillin use.

Figure 2 shows a sample holder thatcan be built to accept soaked filter papersamples. When such a sample holder is

used, it must be positioned properly inthe instrument or no reading will beobtained. Because most laboratoryfluorometers are arranged so that theangle between the illumination sourceand the photometer is 90 degrees, the faceof the sample must be set halfwaybetween these devices to permit passageof light. Furthermore, the angle must beexactly the same for each samplemeasured if extreme measurementvariations are to be avoided. Usually stopsare installed on the instrument case andsample holder for this purpose or theholder may be manually rotated until a

maximum reading is obtained.Another instrument that can be usedwith soaked filter paper is shown in Fig. 3.The sample holder is pictured lying on itsside on top of the instrument. Formeasurement, the sample holder isinserted in the slot in the front of theinstrument.

Selection of Filters forFluorometer Measurementof Fluorescent BrightnessWhen fluorometers are used for liquidpenetrant brightness measurement, theymust be equipped with proper filters.First, of course, is the primary filter thatmust pass ultraviolet-A radiation butabsorb visible light. Generally, this filtermust be made of cobalt glass, often of a

heat resistant type because it may beplaced very near the hot light source.

The fluorometer must also be equippedwith a secondary filter that corrects thephotodetector to the desired wavelengthresponse. The first job for the secondaryfilter is to remove stray ultravioletradiation. If ultraviolet radiation is notremoved, one obtains a reflectancemeasurement rather than a fluorescence

measurement and spurious results areobtained. Many secondary filters willremove enough ultraviolet radiation togive correct measurements. An ultravioletradiation filter may be added if excessultraviolet radiation becomes a problem.

In addition to removing the ultravioletwavelengths from the fluorescentbrightness measurement, the secondaryfilter should normally cause theinstrument to respond to color as theaverage human eye does. Mostfluorometers use photodetectors with theblue sensitive response. A filtercombination must be used to correct the

response of these phototubes toapproximate the human eye response.

Test for WaterContamination inLipophilic EmulsifierThe amount of water contamination in alipophilic emulsifier can be easilydetermined by use of ASTM D 95.21 Thetest procedure and apparatus are followedexactly as described in the ASTM test


FIGURE 2. Special reflectance measuring sample holder for use with fluorescence photometer.

FIGURE 3. Another type of fluorometer for measuringpenetrant brightness.

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specification but anhydrous (waterfree)xylene must be used as the solvent(instead of the general solvent specified).It is most important that only thewaterfree xylene solvent material be used.If this material cannot be obtained, asatisfactory solvent can be prepared bydistillation of the commercial grade of xylene by placing a suitable amount of commercial xylene in a still and boiling

off ten percent by volume. Most of thewater in the commercial xylene will bedriven off in this first ten percentevaporation and the xylene residue will besufficiently dry to permit a reliable test forthe water content of emulsifiers. Althoughlipophilic emulsifiers are petroleum based,the water contamination test results areaffected by the type of solvent used in theASTM D 95 test.21 (See above for detaileddiscussion of water contentmeasurements.)

Comparison Tests forDeterminingContamination of Emulsifiers by FluorescentLiquid PenetrantFluorescent liquid penetrantcontamination in emulsifiers can often beseen by illuminating the used emulsifierwith near ultraviolet (ultraviolet-A)radiation. The actual test for liquidpenetrant content in an emulsifier can bequite simple or fairly complex, dependingon the amount of information desired.Essentially, the test involves preparation

of a standard series of emulsifier samplescontaminated with various knownamounts of liquid penetrant. These arethen compared with the test sample of used emulsifier to determine theapproximate degree of its contaminationwith liquid penetrant. The comparisoncan be made visually or with instruments,depending on the degree of sophisticationdesired.

The basic procedure for evaluatingfluorescent liquid penetrantcontamination in lipophilic emulsifiers isto make a series of mixtures of the sametypes of liquid penetrant and emulsifier

used in production. The mixtures shouldcontain amounts such as 0, 5, 10, 15, 20and 30 percent liquid penetrant. Smallerincrements are only confusing. Thesolutions should be thoroughly mixedand placed in clear, nonfluorescing glasscontainers. A similar container filled witha similar amount of test sample is then,under ultraviolet radiation, visuallycompared side by side with the standardseries. The estimation thus obtained willbe approximate but close enough to

confirm or eliminate liquid penetrantcontent of emulsifier as a cause for aperformance deficiency.

Washability Test for EmulsifierContamination Using Grit BlastedTest Panel

A practical test for emulsifiercontamination is a washability test on a

grit blasted stainless steel panel. A drop of in-use emulsifier is placed on the paneland is allowed to drain off to one side.Then the panel is washed in a very gentleflow of tap water. Failure of the emulsifierstreak to wash clean and without anyedge residues indicates the presence of excessive liquid penetrant contamination.Emulsifiers usually exhibit a sharpwashability break at some criticalconcentration of liquid penetrantcontamination, at which point the streakof emulsifier will not dissolve properly inwater. In conducting this washability test,the panel should be examined under

ultraviolet radiation for the presence of unwashed residues.

Visual Monitoring of HydrophilicEmulsifiers Bath for LiquidPenetrant Contamination

Visual monitoring of the hydrophilicremover immersion bath, although notprecise, will give clues as to the generalcondition and freedom from liquidpenetrant contamination. For example, asolution of 20 percent hydrophilicemulsifier in water has a clear, pinkish redcolor in white light and exhibits a pinkish

fluorescence under (near ultraviolet)ultraviolet-A radiation. A trace of fluorescent liquid penetrantcontamination in the hydrophilicemulsifier bath will darken the emulsifiersolution color and the fluorescence willshift to a blue-white color. Much higherlevels of liquid penetrant contaminationcause the hydrophilic emulsifier solutionto become cloudy. With still higher levelsof liquid penetrant contamination, freeliquid penetrant will float on the surfaceof the hydrophilic emulsifier bathsolution. A hydrophilic emulsifier baththat is cloudy or has free liquid penetranton its surface is generally considered to beover contaminated. Such a bath can beused but the free-floating liquid penetrantshould be skimmed from the bath surfaceto prevent liquid penetrantcontamination of test parts as they areremoved from the hydrophilic emulsifierbath.

Noticeable changes in the hydrophilicemulsifier performance, especiallyincreased fluorescent background on thewashed test parts, will accompany thevisual changes in the bath described


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above. As the emulsifier bath becomesmore contaminated with liquid penetrant,it will become less efficient in removingexcess surface liquid penetrant and willrequire increased emulsifier contact time.The addition of hydrophilic emulsifierconcentrate to a contaminated emulsifierbath is not recommended, because thisleads to loss of control over the removerconcentration. Properly diluted emulsifier

concentrate (corresponding to the originalremover bath concentration) can beadded to a tank that is subject to a heavydragout loss due to heavy use.

Concentration Control of Hydrophilic Emulsifier orRemoverThe concentration of hydrophilicemulsifier in water is important to bothits performance effectiveness and theeconomies of its use. The concentration of

hydrophilic emulsifier can be estimatedby comparing the color of standarddilutions to those of the freshly mixedbath. This procedure is used for spraysolutions rather than for the more highlyconcentrated dip solutions. This colorcomparison technique is not usable forcontaminated baths of hydrophilicemulsifier.

A useful technique for determining theconcentration of specific hydrophilicemulsifiers in water baths is themeasurement of the refractive index of the solution. Figure 4 shows arefractometer being used to determine therefractive index of a hydrophilicemulsifier solution. Charts relating theconcentration to the refractive index are

available from the emulsifiermanufacturer. Figure 5 shows an exampleof the full range of refractive index valuesfor concentrations varying from 0 to100 percent of a hydrophilic emulsifier inwater. This refractive index test isapplicable only to fresh, uncontaminatedemulsifier solutions. Therefore, the test isespecially suited to solutions applied byspray. Contamination by liquid

penetrants can cause the refractive indexof hydrophilic emulsifier baths to shiftfrom the values for uncontaminatedbaths.

Quick Tests forPerformance Loss Due toRemover Contamination

Test Procedure

A quick emulsifier bath performance test

uses reference panels for comparison teststo determine the loss in performance dueto contamination of hydrophilicemulsifier baths. (See elsewhere in thisvolume for descriptions of test panels andcracked blocks used for evaluating liquidpenetrant test system performance.) Thebackground and indication brightness anddefinition obtained when liquid penetrantcoated test panels are exposed tohydrophilic emulsifier for 15 s and 30 stime periods are compared. Two gritblasted steel test panels and two


FIGURE 4. Refractometer test provides animmediate visual determination of hydrophilic emulsifier concentration.

FIGURE 5. Variations of refractive index of specific hydrophilic emulsifier as function of emulsifier concentration in water.

100

90

80

70

60

50

40

30

20

10

0

C o n c e n t r a t i o n o f e m u l s i f i e r i n w a t e r ( p e r c e n t )

1.33 1.35 1.37 1.39 1.41 1.43 1.45 1.47

Refractive index

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chromium plated panels with fine cracksare covered with liquid penetrant andallowed to drain in air for 5 min. The firstset, including one grit blasted and onecracked chromium plated panel, isprocessed as follows: 15 s prerinse, 15 sdrain, 15 s agitated remover dip, 15 sdrain and 15 s wash.

The second set of similar panels isprocessed like the first set, except that the

agitated remover dip is of 30 s duration.In this test, fresh hydrophilic remover at20 percent dilution in water leaves nobackground residues on either grit blastedtest panel. The first cracked chromiumsensitivity panel has good crackindications after the 15 s dip. The secondcracked chromium sensitivity panel hasno indications or only faint indicationsafter its 30 s dip in agitated remover. Freshhydrophilic remover at 20 percentconcentration leaves no background coloror fluorescence on either grit blastedbackground test panel. All panels are driedand developer applied before inspection.

These results are those for fresh, cleanhydrophilic remover bath.

Interpretation of Results

As the remover bath becomescontaminated, similar test panels givensimilar processing show differences inperformance. Background is present afterthe 15 s dip and faint background afterthe 30 s dip, on the grit blasted testpanels. The cracked chromium plated testpanels show poor performance after 15 sin the remover bath. Resolution on thefine crack panel may improve after the30 s dip, reinforcing the interpretation of

a loss in remover effectiveness withcontamination. A sample of fresh liquidpenetrant should be used periodically inthese tests to determine whethercontaminated liquid penetrant could beresponsible for the changes observed inperformance.

With a specific liquid penetrant andremover, this comparison test indicatedthat no noticeable change in performanceoccurred until the remover bath reachedabout four percent contamination. Atfour percent contamination with liquidpenetrant, heavy background appears onthe grit blasted test panel after a 15 s dip

and faint background remains even after a30 s dip. With about six percentcontamination, the difference inbackground on the grit blasted test panelsexposed to agitated remover for 15 and 30s is negligible. the comparison betweenthe two dip periods is the guide toremover condition. The test is only meantto be a quick, easy reference and is notquantitative. With different test objectsand processing requirements, the usermay find it beneficial to use an ideal test

part for performance test or to alter thetest conditions to correspond to actualliquid penetrant processing conditions inuse in the specific facility.

Visual Comparison Test forDry Developer

PerformanceDevelopers may suffer degradation, whichcan affect performance and thus shouldnot be ignored. The best test of drydeveloper is a visual comparison with newdeveloper material under both white lightand ultraviolet-A radiation. Limits may beset if desired but the results of theperformance test will normally determinewhether the material is acceptable or not.Water in dry developers usually comesfrom incompletely dried test parts,careless overspray during washing orleaking roofs or pipes. Added water causeslumps in the dry developer tank or on the

test parts, which may mask indications. If the contaminant is water, the developermaterial may dry eventually, particularlyif heated. Liquid penetrant contaminationin dry developers causes bright color orfluorescent spots that form falseindications. Liquid penetrant usuallyenters the developer through carelesshandling of washing and removal of excess surface liquid penetrant. If thedeveloper is contaminated, the drypowder developer must be replaced.

Control and Maintenanceof Aqueous Wet DeveloperBathsThe proper consistency of aqueous wetdeveloper baths must be maintained byreplacing water lost through evaporationor powder lost through dragout. Thetechnique for measuring the consistencyof the batch will depend on the type andbrand of material being used. Therecommendation of the manufacturershould be followed in all cases.

Visual Examination of Applied

DeveloperOne test involves pouring a streak of wellmixed developer bath onto a glossy blackplate supplied as part of a comparator kit.The appearance of this streak is thencompared with the developer streaks on astandard plate. The solution or suspensioncan then be adjusted by adding morewater or more powder until the test streakclosely matches the proper standardstreak. Any cracking of the developercoating during the drying operation in


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the normal test procedure indicates (1) aserious loss of water or (2) an excessivelyhigh concentration of developer powder.Both of these conditions can cause thedeveloper to obscure indications of finediscontinuities.

Hydrometer Test forConcentration of Aqueous Wet

Developer BathsAnother technique of checking the bathconcentration is by means of ahydrometer with a suitable range. Beforechecking the concentration with thehydrometer, the developer bath should beat least 24 h old, soluble powders shouldbe completely dissolved and suspensionsshould be thoroughly stirred. Thehydrometer can be floated directly in thebath or in a small vessel containing asample of the bath. Manufacturers of thedeveloper will supply information as tothe correct hydrometer range for thedeveloper and will furnish tables relating

the developer bath concentration andspecific gravity. A high specific gravityreading can be lowered by adding water tothe mixture. If the specific gravity is toolow, add powder and stir thoroughly.

Table 1 compares typical hydrometerreadings to developer bath particleconcentration. Generally, the powderconcentration in water suspension rangesfrom 40 to 240 g (1.4 to 8.5 ozm) of drypowder concentrate per 1 L (0.25 gal) of water. This corresponds to 152 to 907 g(0.33 to 2.0 lbm) of powder per 3.8 L(1 gal) of water (4 to 24 percent byweight).

Evaluating Developer Wetting andEvenness of Coating

The coating test is simply to pour orotherwise apply some of the material andsome standard mixture to a smoothhard-to-wet metal surface and inspect forsigns of pulling apart or balling up of thewet developer coating material. A

convenient surface for this test is a new,clean, tin-plated lid from a gallon can. If pulling apart or balling up of the coatingoccurs, the surface should be dried andthen checked for even, completedeveloper coverage.

Evaluation of FluorescentContaminants in Developer

CoatingsAnother test that should be applied to wetdevelopers is a check for fluorescence.This is easily done by examining the twodeveloper coatings prepared for theevenness test under ultraviolet-Aradiation. Increased fluorescence of thetest material will be obvious. Whether thefluorescence has reached a rejection levelmust be determined by the performancetest.

Wet developer becomes fluorescentmainly from liquid penetrant carried intoit on test parts or that otherwise entersthe developer. There is no way to reclaim

liquid penetrant contaminated developer,so replacement is the only answer to sucha problem. Developer should notnormally become fluorescent; therefore,the cause of the problem should bedetermined and procedures should bealtered to prevent recurrence.

Care and Maintenance of Nonaqueous WetDevelopersMost of the developers used with portable

liquid penetrant test kits are of thenonaqueous wet type. The developers areprocured in a ready-to-use form andcontain the necessary developer powdersuspended in a volatile liquid medium.With flammable solvent types, operatorsmust avoid smoking or other source of ignition while spraying developer frompressurized spray cans. Solvent developersin bulk should be kept in covered orclosed containers to limit evaporation orspreading of flammable vapors. If solventdevelopers lose appreciable liquid throughevaporation, the quality and effectivenessof the developer will decreasesignificantly. The solids must be insuspension, before use, to obtain properdeveloper performance. Solventdevelopers in pressure spray cans or otherportable containers must be thoroughlyagitated before use to ensure the propersuspension of developer in solvent whenapplied to test parts. Developers inpressurized spray cans generally do notrequire maintenance precautions, becausethey are primarily intended for use onlyonce. When developer is sealed in a spraycan, there is no way for it to become


TABLE 1. Aqueous developer concentrations of dissolvedor suspended particles as function of specific gravityhydrometer readings of the bath.

Typical Specific Gravity

Developer Concentration Hydrometer Readings______________________g·L–1 (lba·gal–1) at 22 °C (72 °F)

120 (1.0) 1.052

96 (0.8) 1.042

72 (0.6) 1.032

48 (0.4) 1.021

24 (0.2) 1.011

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contaminated with liquid penetrant orother foreign materials.

If it is necessary to check the contentof solids in nonaqueous solventdevelopers, this can best be done byweighing a well agitated sample of developer, filtering out the solids andthen weighing the dried solids content.Older techniques of allowing the solids tosettle out of a developer suspension have

fallen into disrepute.


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Maintenance Checks forExpendable LiquidPenetrant ProcessMaterialsExpendable materials, once placed inservice in a liquid penetrantnondestructive test system, are notparticularly susceptible to change orvariation. Most of the changes that dotake place will be apparent to experiencedoperators by variation in the performanceof the liquid penetrant or of theprocessing materials. In most installations,

alertness on the part of the operator maybe all that is required to maintain allmaterials in satisfactory condition.However, it is advisable to carry out aroutine check on the test materials toensure that reliable and consistentperformance is being attained with thetest system.

Selection of specific tests will dependon the make and type of liquid penetrantprocess materials being used. Testsrecommended by the manufacturer orsupplier of the expendable liquidpenetrant process materials may be usedas a guide. There are some generally

applicable tests, however, that can beapplied advantageously, as described hereand elsewhere in this volume. Note thatthese tests described are comparison testsonly; that is, a current result is comparedto a previous result or results obtainedwith in-use materials are compared toresults obtained with retained unusedmaterials.

Checking Test SystemPerformance with CombinationTest PanelThe quality of indications on processed

test parts depends on proper functioningof all phases of the entire liquid penetranttest system, including the following:effectiveness of precleaning of test objects;condition of the liquid penetrant,emulsifier and developer (all of which caneasily become contaminated or spent );liquid penetrant and emulsifier dwelltimes; temperature, pressure andcleanliness of the wash water; drying timeand temperature; lighting in theinspection area.

Figure 6 is a drawing of a combinationtest panel devised to verify thefunctioning of the liquid penetrant testsystem. Proper performance of the testsystem can be ensured by processing thisperformance test panel through all cyclesof the system, from precleaning toreadout of indications, at the beginning of each work shift. One half of thecombination test panel consists of a metalstrip that has a rough, grit blasted,stainless steel surface. The other half of the combination test panel has a smoothchrome plated surface with five dimplecrack patterns imposed on it by ahardness tester. The five dimple crack

patterns range in size from very fine tocoarse. The combination panel can beused for judging both rough surfacewashability and smooth surfacewashability, as well as for judging relativesensitivity of the liquid penetrant testprocess conducted with a given testsystem at specified intervals. To remainuseful, the combination test panel must


PART 7. Quality Control Tests for Test Systems

and Procedures

FIGURE 6. Penetrant system monitor panelwith rough, grit blasted section (at right) for washability check and five crack centers inchrome plated section (at left) for sensitivity

evaluation.

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be thoroughly cleaned immediately aftereach use to remove all residues of liquidpenetrant processing materials and water.(See elsewhere in this volume fordescriptions of additional artificialreference panels of various types).

Checking Test SystemPerformance Using Test Parts with

Known DiscontinuitiesTest parts with known typicaldiscontinuities and known washabilitylevels can be retained as referencespecimens and processed at the beginningof each shift or when inadequate systemperformance is suspected. These test partsserve a function similar to that of thecombination test panel. The experiencedinspector will quickly spot a radicalchange in the liquid penetrant testingsystem performance by use of either thecombination panel or retained anomaloustest parts. A problem that must berecognized is that, even after

discontinuities in test panels or test partsare cleaned, they may retain residues of liquid penetrants or processing materialsfrom previous liquid penetrant tests orinspections of other types. The gradualbuildup of retained processing materialswill then reduce the apparent size of theliquid penetrant indications and the testpanels/parts must be replaced.

Frequency of Comparison Tests of Liquid Penetrant Materials

In all probability, liquid penetrantmaterials’ deterioration due to

evaporation, contamination and othercauses will be gradual. The rate will varywith such conditions as the work load,type of test parts, climatic conditions orprior processing. The need for acomparison test between materials in useand retained standards will not be thesame for all liquid penetrant installations.The frequency of tests may vary from asoften as twice daily to weekly or evenmonthly.

Detecting Sudden PerformanceShifts in Test System OperationIt should be remembered that, in addition

to gradual deterioration, there is alwaysthe danger of sudden deterioration of materials or sudden change in thefunctioning of a processing stage. Thesudden change can be guarded against byprocessing known anomalous parts orspecial test panels with which thetechnician is familiar at the beginning of each shift or at other selected intervals.An experienced inspector will detect achange in familiar parts or test panels bynoting the following: whether the known

discontinuities appear; the contrast of thediscontinuity indications; the degree of background color or fluorescence left ontest object surfaces after removal of excesssurface liquid penetrant and applicationof developer. The stainless steelcombination grit blasted and chromeplated test panel (Fig. 6) has been used forseveral years to monitor for suddenchanges. Another suitable test panel is an

unglazed ceramic plate or disk having asurface porosity condition.

Processing special test panels such asthese can alert the inspector to a suddenshift in performance caused by thefollowing: sudden increase in oventemperature; failure of developer dustchamber; wash water temperatureincrease; solvent contamination of emulsifier; and other changes fromnormal conditions

Though these monitoring panels donot measure the gradual shift inperformance and do not replace thecomparison test, they protect against the

sudden failure and they can be processedquickly and easily.

Tests for Effectiveness of Liquid Penetrant RemovalA removability test, or washability test,often follows the fluorescence test (seeelsewhere in this volume for stepsequence). These tests measure theremovability performance of theindividual process materials. Removalperformance is very important becausesurplus liquid penetrant must be removed

from the surface of the part or the entirepart will have a high backgroundfluorescence. This will greatly reduce thecontrast between the discontinuityindication and the rest of the surface area.On the other hand, if the liquid penetrantcan be removed too easily, it will bewashed out of the discontinuities. Thiswill cause greatly reduced discontinuitydetection sensitivity capability,particularly for the shallowerdiscontinuities.

The test for effectiveness of removal of surface liquid penetrant may beperformed in a variety of ways but, with

present procedures, it must always bedone as a comparison to standardmaterials; this is because there is so muchvariation of the test parameters evenwhen rigorous controls are applied. Anaccurate comparison can be made only if standard and test materials are handledside by side.


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Selection and Renewal of Removability Test PanelsRemovability is probably best tested on afairly rough surface such as the stainlesssteel test panels sand blasted with 165 µmmesh aperture (100 lines per 1 in. mesh)grit as specified by MIL-I-251351 orAMS 2644.2 Rough surfaces on test objectsare harder to wash than smooth surfaces,

so they allow a better evaluation of washcapability. A problem with wash testpanels is that, once used, a very slightresidue remains on the surface and affectsfuture tests. An effective procedure forcleaning removability test panels for reuseincludes the following processing steps:(1) use a soft brush under running waterto remove the developer, (2) dry, (3) dipin alcohol, (4) dry, (5) immerse in asolvent and ultrasonically clean and(6) dry. This entire cycle can be repeated if necessary to remove all traces of priorprocessing materials. If this proceduredoes not completely clean the panels,they can be sandblasted again andprobably should be after five or ten uses.

Procedures for PerformingRemovability Tests

When performing removability tests,standard and test materials should beapplied to separate panels that were gritblasted simultaneously or adjoining areason a single test piece, being careful not toallow the liquid penetrants to mix. Thetest should actually be run twice: oncewith a minimum removal effort (shortemulsification or wash) that will not quitecompletely remove the standard materials

and once with a fairly strong removaleffort that will completely remove liquidpenetrant from the standards.

Interpretation of Liquid PenetrantRemovability Tests

If the test materials are more completelyremoved than the standards, the testmaterials are being removed too easilyand performance will suffer. This loss of performance usually shows up on theperformance test, so the purpose of removal test is to point out any poorremovability performance. This may be

obvious from the results being obtainedin the actual test operation where partsbeing tested may be showing sufficientlyhigh residual background that complaintsarise. If so, the main use of this test is tolocate which material or materials arecausing the poor removability.

Purpose of CrackedSample TestsA practical test of a liquid penetrant testsystem uses a test object or part of thetype actually being inspected thatcontains the smallest allowablediscontinuity. If this discontinuity isdetected, the liquid penetrant test system

is doing the minimum required job. Howwell this smallest discontinuity is detectedcan be determined by the brightness of the indication as measured with aphotometer (Fig. 7), if such an instrumentis available and the information isrequired. However, because each user’stest parts and minimum allowablediscontinuity size will be different, thereis no way that individual ratings will becomparable when using different parts.

Synthetic Crack Specimens

for ComparisonEvaluationsIn practice, there is a great need forreproducible, synthetically produceddiscontinuities that can be used to rateand compare liquid penetrant test systemsand materials. Many attempts to producesuch devices have been made over theyears. Unfortunately, none of theseattempts has produced the ideal syntheticdiscontinuity. The big problem isreproducibility. To rate liquid penetrantmaterials properly in different locations,


FIGURE 7. Narrow angle photometer with digital readout andoptical finder.

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the test parts must be identical and notwo parts of any current syntheticdiscontinuity sample are identical.Without identical test discontinuities,there is no way to cause the rating of asystem done at one location to equal thatdone at another. Even at one location anyrating of a test system, process or materialis a comparative rating only.

Over the course of many years, two

types of synthetic discontinuity testpanels have found wide use despite theirinadequacies; the cracked chrome platedbrass panel and the quench crackedaluminum block. These are brieflydescribed below and discussed in detailelsewhere in this volume. The normalprocedure for checking a complete systemby means of cracked specimens is to treatone half of each specimen with all newmaterials and the other half with allin-use materials. After processing iscomplete, the specimen is examinedclosely under the appropriate light,according to the types of materials used.

If a careful comparison reveals nooutstanding difference between the twohalves of the specimen, it can be assumedthat all materials are in usable condition.If the half that was processed with in-usematerials bears indications that aredefinitely inferior, it indicates that at leastone of the in-use materials is not up tostandard. The actual material(s) at faultcan be pinpointed by carrying out furthertests, substituting new materials, one at atime, for the in-use ones. This should becontinued until the block shows equal

indications on the two halves, whereuponthe last used material replaced by the newmaterial may be considered at fault.

Once the faulty material is found, thenecessary corrective measure may beobvious. If not, further tests may benecessary but need be carried out on theinferior material only. If no material isfound faulty, one or more processingparameters or procedural steps may be at

fault. An audit of the test process orhaving a different operator process thecracked specimens may help reveal theproblem.

In most cases comparable crackpatterns appear immediately adjacent toeach other on the two halves of aspecimen. The patterns should beobserved not only for brightness but alsofor crack definition as well. Photographicrecords of crack patterns can aid observersto make valid conclusions. Somecontaminants, particularly in liquidpenetrants, can cause bleeding of indications to such a degree that,

although the indications may be larger,they are less well defined and may evenobscure other fine indications. In such acase, indications are usually less bright.

Cracked Nickel-Chromium PlatedSpecimens for Evaluating thePerformance of Test Systems

One synthetic discontinuity test panel isthe cracked nickel-chrome plated panel.In this case the two halves are two


FIGURE 8. Two examples of matched halves of chrome plated, cracked panels for evaluatingliquid penetrant system performance: (a) coarse cracks, relatively deep; (b) fine cracks,relatively shallow.

(b)(a)

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separate panels made by cutting theoriginally prepared panel completely inhalf. Two examples of this type are shownin Fig. 8.

The cracks extend entirely through thebrittle nickel-chrome plating to the brassbelow, so the depth of the cracks iscontrolled by the plating thickness.Variation of the coating thickness thusallows the production of different ranges

of crack size that can be used to test liquidpenetrant test systems with differentsensitivity levels. The nickel-chromeplated crack specimens, especially the finegrade, contain some of the smallestdiscontinuities normally detectable by theliquid penetrant method. Thus, they canbe used as go/no-go standards for therating the relative performance of a liquidpenetrant test system over a period of time.

A further advantage of these panels isthat they may be easily and completelycleaned for reuse, by washing andbrushing off the developer coating and

then by soaking the panel in solventovernight.

Quench Cracked AluminumSpecimens for Evaluating thePerformance of Test Systems

A second type of synthetic discontinuitysample in common use, especially forvisible dye liquid penetrant test systems,is the quench cracked aluminum blockspecified in AMS 26442 and elsewhere.1,22

This sample is produced by nonuniformlyheating a piece of rolled 2024-T3aluminum alloy to nearly its melting

point, then by quenching the piecequickly in cold water. This produces anetwork of cracks of various sizes, as seenin Fig. 9. Naturally, there is considerablevariation in crack patterns from piece topiece and no two pieces are exactly alike.Therefore, these samples also must beused strictly for comparison of two liquidpenetrant test systems or materials on thesame piece at the same time.

These samples are suitable only forsystems of low sensitivity. To clean themfor reuse, it is necessary to wash andbrush off the developer, soak the panels insolvent at least overnight and then reheatthem to about 30 °C (50 °F) less than theoriginal temperature. This procedure canbe repeated only three or four times for apanel before it must be replaced.

Need for ImprovedSynthetic Crack SpecimensVarious other liquid penetrant testreference samples have been tried over theyears but none has survived in general use

except the cracked aluminum blocks andthe cracked nickel-chromium platedpanels. All others were too difficult to

make, not reproducible enough or notsufficiently indicative of performance.There is still need for a better syntheticdiscontinuity sample than those presentlyavailable.


FIGURE 9. Quench cracked aluminum test panel specified byMIL-I-25135,1 AMS 26442 and ASME Boiler and Pressure Vessel

Code .22

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1. MIL-I-25135, Inspection Materials, Penetrants. Washington, DC: United

States Department of Defense (1989).(Cancelled January 1998; replaced bySAE AMS 2644.)

2. SAE AMS 2644, Inspection Material, Penetrant. Warrendale, PA: SAEInternational (1996).

3. MIL-STD-6866, Inspection, Liquid Penetrant. Washington, DC:Department of Defense (1985).(Canceled 22 November 1996;replaced by ASTM E 1417.)

4. ASTM E 1417, Standard Practice for Liquid Penetrant Examination. WestConshohocken, PA: American Society

for Testing and Materials (1995).5. ASTM E 165, Standard Test Method for

Liquid Penetrant Examination. WestConshohocken, PA: American Societyfor Testing and Materials (1995).

6. ASTM E 1135, Standard Test Method for Comparing the Brightness of Fluorescent Penetrants. West Conshohocken, PA:American Society for Testing andMaterials (1992).

7. ASTM E 1208, Standard Test Method for Fluorescent Liquid Penetrant ExaminationUsing the Lipophilic Post-Emulsification Process. West Conshohocken, PA:American Society for Testing and

Materials (1994).8. ASTM E 1209, Standard Test Method for

Fluorescent Liquid Penetrant ExaminationUsing the Water-Washable Process. WestConshohocken, PA: American Societyfor Testing and Materials (1994).

9. ASTM E 1210, Standard Test Method for Fluorescent Liquid Penetrant ExaminationUsing the Hydrophilic Post-Emulsification Process. West Conshohocken, PA:American Society for Testing andMaterials (1994).

10. ASTM E 1219, Standard Test Method for Fluorescent Liquid Penetrant ExaminationUsing the Solvent-Removable Process.

West Conshohocken, PA: AmericanSociety for Testing and Materials(1994).

11. ASTM E 1220, Standard Test Method for Visible Liquid Penetrant ExaminationUsing the Solvent-Removable Process.West Conshohocken, PA: AmericanSociety for Testing and Materials(1992).

12. ASTM E 1418, Standard Test Method for Visible Penetrant Examination Using theWater-Washable Process. West

Conshohocken, PA: American Societyfor Testing and Materials (1992).

13. KSC-SPEC-Z-0013, Penetrant, Magnetic Particle and Ultrasonic Inspection, Requirements for, Specification for.Kennedy Space Center, FL: NationalAeronautics and Space Administration(1969).

14. MIL-STD-271, Nondestructive Testing Methods, Requirements for.Washington, DC: Department of Defense; United States GovernmentPrinting Office (June 1986). (CancelledMay 1998; superseded by NAVSEATechnical PublicationT9074-AS-GIB-010/271.)

15. MSFC-STD-366(1), Penetrant Inspection Method. Huntsville, AL: Marshall SpaceFlight Center (1976). (Cancelled.)

16. SAE AMS 2647B, Fluorescent Penetrant Inspection Aircraft and EngineComponent Maintenance. Warrendale,PA: SAE [Society of AutomotiveEngineers] International (1995).

17. SAE AMS 3155C, Oil, Fluorescent Penetrant Solvent-Soluble. Warrendale,PA: SAE [Society of AutomotiveEngineers] International (1994).

18. SAE AMS 3156C, Oil, Fluorescent Penetrant Water Washable. Warrendale,PA: SAE [Society of Automotive

Engineers] International (1983).19. SAE AMS 3161A, Oil, Odorless Heavy

Solvent. Warrendale, PA: SAE [Societyof Automotive Engineers]International (1993).

20. NAVSHIPS 250-1500-1, Welding Standard. Washington, DC: UnitedStates Department of Defense (1995).

21. ASTM D 95, Standard Test Method for Water in Petroleum Products and Bituminous Materials by Distillation.West Conshohocken, PA: AmericanSociety for Testing and Materials(1990).

22. ASME Boiler and Pressure Vessel Code:

Section V, Nondestructive Examination.New York, NY: American Society of Mechanical Engineers (1995).

References

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