8.1 Selecting the Appropriate PT Method for the Job

ASNT's liquid-penetrant general exam treats Selection of the Appropriate PT Method as official topic 2. The heading is not a brand catalog and it is not a request to recite every AMS dash number from memory. It asks whether you can match a type, a removal method, and a sensitivity class to the part you are holding, the surface you can actually reach, the discontinuity the drawing cares about, and the lighting you can actually control — and whether you will refuse a combination the physics or the chemistry cannot support.

ASTM E1417/E1417M, Standard Practice for Liquid Penetrant Testing, is the usual U.S. process practice that names those families. ASTM E165 is the companion technique guide. Aerospace material qualification often sits in SAE AMS 2644. Those documents are industry and code references, not ASNT exam publications. Attribute every numeric window to the standard or to the employer's written procedure. If a procedure tightens a value or names a qualified product list, the procedure wins.

This chapter unpacks topic 2 into the decision, the process window, and the station controls the written exam actually tests. Developers, indication classification, and acceptance standards belong in Chapters 7 and 9. Here the job is to pick the right system and keep it inside a valid process.

Type I versus Type II

Industry classification in AMS 2644 / ASTM E1417 language is:

Choose Type I when the discontinuity is expected to be tight or shallow and you can deliver the UV-A intensity and the low ambient visible light that Chapter 8.2 treats as process controls. Choose Type II when you cannot darken the work, you do not have a qualified UV-A lamp, or the customer specification names visible dye. Type II is not "the same indication printed in red." In ordinary industrial use it is the lower-sensitivity family. Do not pick Type II on a fatigue-critical machined aerospace fitting just because the aerosol is cheaper or because the booth light is burned out.

If the stem gives you a darkened booth, a radiometer, and a high-sensitivity class, the answer is almost always Type I. If the stem puts you on a scaffold at noon with no hood, Type I is not a real examination no matter what the can says.

Removal methods A, B, C, and D

Method A is fast on a production line and needs no separate emulsifier station. That convenience is why it is also the method most likely to overwash a shallow crack. The same surfactant that lets water lift background will, if you linger or blast, lift penetrant back out of a wide, shallow, or slightly open discontinuity. Use Method A when the procedure qualifies it for the sensitivity you need and — for Type I — you can watch the wash under UV-A so you stop when the background is gone. Method A is a poor default on a smooth, fatigue-critical journal if the wash is aggressive and the cracks are tight and shallow.

Method C is the portable field method. No rinse booth, no dip tank, no emulsifier. One technician, wipe-on or aerosol penetrant, solvent wipes, and a developer can cover a single repair weld, a local blend, or a part that cannot come to the line. It is the wrong mental picture for a 400-piece lot of fittings. Method C's sensitivity is limited as much by removal technique as by the dye: a solvent flood will empty the discontinuity just as surely as an overzealous Method A hose.

Method B (lipophilic) and Method D (hydrophilic) keep the penetrant immiscible with water until you deliberately add an emulsifier. That is why post-emulsifiable systems are the usual high-sensitivity choice on machined aerospace and pressure parts: you can clear background without as readily emptying a tight crack. The two emulsifiers are not interchangeable and must not share a tank.

• Lipophilic emulsifier is oil-based and is used as supplied (neat). It works by diffusion into the oily penetrant film. Contact time is the primary control. Leave it on too long and you emulsify the reservoir inside the discontinuity — over-emulsification, the Method B cousin of overwash. Too short and the background will not wash.
• Hydrophilic emulsifier is detergent-like and is used as a percent solution in water. Both concentration and contact time are controls. Many procedures use a gentle pre-rinse to knock off bulk penetrant, then the emulsifier (immersion or spray), then a final rinse. Hydrophilic systems are generally the more controllable high-sensitivity shop process, which is why Method D Type I dominates qualified aerospace lines.

Do not treat "post-emulsifiable" as a single can. A stem that says the emulsifier is diluted to a procedure percent is Method D. A stem that says the emulsifier is applied as received and timed is Method B.

Decision table — match the job, not the storeroom

Field weld versus production line is an access and logistics decision. A production shaft lot belongs on a Type I line with a rinse booth, a dryer, and a UV inspection station. A single pipe-fillet repair on a scaffold belongs in a Method C kit after local coating removal. Swapping those pictures is a common written-exam trap.

Rough versus machined is a sensitivity decision. A coarse water-washable visible dye on a rough weld can still find a wide-open crater crack. It will skip a tight fatigue crack on a ground radius. An ultrahigh-sensitivity Type I Method D on a rough casting will leave so much background that you cannot tell a relevant bleed-out from the surface texture. Match the class to the finish and to the discontinuity the drawing actually cares about.

Coatings and closed surfaces

Paint, powder coat, anodize, conversion coating, heavy oxide, carbon, and oil are barriers. The exam default is: remove them. Anodize and paint are not "porous enough to skip stripping." Penetrant cannot cross an intact organic film or a dense anodic layer and still be a valid capillary test of the metal underneath. If a procedure qualifies examination through a specific thin conversion film, that waiver is written — it is not a field improvisation. After coating removal, restore a surface that is clean, dry, and not smeared (Section 8.2).

Local stripping for a field weld must expose bare metal over the zone of interest plus the overlap the procedure names. Residue from the stripper is itself a contaminant. Rinse and dry before penetrant.

Titanium, nickel alloys, and chemistry limits

Penetrant, emulsifier, remover, and developer are chemicals left on or in the metal. Many aerospace and high-temperature procedures impose sulfur and halogen (chlorine / fluorine) limits when the part is titanium, a nickel-base alloy, or sometimes austenitic stainless steel, because residual sulfur or halides can contribute to corrosion or embrittlement in later heat or service. Treat those limits as typical procedure chemistry controls, not as an unpublished ASNT number and not as a single ppm figure you should invent for the exam. The Level II reads the qualified-product list and the material certification: if the procedure says "low sulfur / halogen only," an unmarked hardware-store dye is not a substitute.

Compatibility also includes the substrate family. Liquid penetrant testing needs a nonporous surface that the liquid can wet. It is the usual surface method on aluminum, carbon steel, stainless, titanium, nickel alloys, and many copper alloys. It is the wrong method for unsealed porous iron, concrete, or a polymer the solvent will attack. When the chemistry of the part or of a previous coating is unknown, stop — Section 8.3 makes the SDS a hard gate.

Sensitivity class is part of the selection

Type I penetrants are graded by sensitivity class (commonly ½ through 4 in AMS 2644 language). Higher is not automatically better. Class 4 on a rough weld is a background problem. Class 1 on a fatigue-critical turbine disk is an under-test. The procedure names the class. The Level II refuses a substitution that drops class unless the written practice allows it. Method and class travel together: a high class does not rescue an overwashed Method A, and Method D does not rescue a Type II examination performed in a parking lot.

Realistic exam scenarios

A production line of machined nickel-alloy fittings with a darkened booth and a rinse station is a Type I job, usually Method D at a high sensitivity class, using materials that meet the procedure's sulfur and halogen limits.

A scaffold weld on painted carbon steel, noon sun, no water, is Type II Method C after local coating removal, viewed under white light that meets the procedure's intensity. Spraying fluorescent Method A and "inspecting" in sunlight is not a Type I examination.

A shallow grinding crack on a smooth journal is a poor Method A candidate if the wash is aggressive. Post-emulsifiable Type I (Method D) is the usual high-sensitivity answer because the penetrant stays water-immiscible until you choose to emulsify the background.

A lipophilic station that "soaks until the part looks clean" has lost the contact-time control. A hydrophilic station that never checks concentration has lost the percent-range control. Both are selection-plus-process failures, not housekeeping notes.

An anodized aluminum housing offered "as coated" is not ready. Strip or otherwise remove the anodic layer if the procedure requires metal-base examination; do not treat the anodize as a built-in developer.

Selection is a decision table, not a favorite can. Name the access, the finish, the coating state, the lighting you can control, and the chemistry the alloy will tolerate — then pick the type and method that survive that table.