Dye penetrant testing

What the method can establish

Dye penetrant testing, also called liquid penetrant testing or PT, reveals discontinuities that are open to the inspected surface. Penetrant enters an opening, excess material is removed from the surface, and developer helps produce a visible indication. It is applicable to suitable nonporous materials; magnetic properties and electrical conductivity do not define eligibility. Porous surfaces are unsuitable because background retention interferes with interpretation. National Board, liquid penetrant examination

The result is an indication to evaluate, not a direct map of crack depth or proof that the entire component is sound. A negative examination concerns the accessible surface and the capability of the applied process. Select another appropriate examination when the question concerns internal discontinuities or material hidden beneath an intact coating. TWI, penetrant examination advantages and limitations

Define the inspected area, stage of manufacture, surface condition, applicable procedure and acceptance criteria before choosing consumables. For code work, completing a generic spray-can sequence is insufficient: the National Board describes a written procedure addressing the variables required by the applicable ASME examination rules. Personnel and procedure requirements must come from the governing specification and employer's qualification arrangements. National Board, procedure requirements

Select a complete penetrant system

Under the AMS 2644 classification summarized by Magnaflux, Type 1 is fluorescent and Type 2 is visible dye. Removal method is a separate choice:

Developer forms are also classified separately. A product's type does not determine its removal method or authorize every developer combination. Magnaflux's guide states that AMS 2644 sensitivity levels apply to fluorescent penetrants, not visible penetrants. Do not transfer a sensitivity number from another classification system without identifying its basis. Magnaflux, penetrant selection guide

Higher nominal sensitivity is not automatically the best choice. Excess background can obscure useful indications, and surface finish affects the useful contrast. Select the system required by the procedure and demonstrate its suitability on the actual application rather than promising a universal minimum detectable crack. Magnaflux, brightness and contrast

Preparation and controlled processing

The examination surface must allow penetrant to enter the discontinuity. Oil, grease, scale, paint, dirt and cleaning residues can interfere. Mechanical preparation also needs care: the UK HSE warns that cleaning operations can close crack openings and prevent detection. Specify how coatings and contamination will be removed without concealing the feature being sought. A polished-looking surface alone does not establish adequate preparation. HSE, inspection and NDT limitations

An oily discontinuity may remain blocked even after the surrounding surface has been wiped. ASNT describes a case where hydraulic fluid inside a crack prevented penetrant entry. Treat previous service fluids and manufacturing residues as part of the preparation problem; an apparently clean external surface is not a guarantee that the opening is available. ASNT, penetrant testing on oily surfaces

The process has distinct stages: preclean and dry; apply penetrant and allow the specified contact time; remove excess penetrant by the selected method; apply the specified developer; allow development; inspect and evaluate; then perform required postcleaning. Each stage affects the next. Dwell requirements depend on material, temperature and application, and Magnaflux recommends prior testing to establish suitable times when these change. Manufacturer guidance does not replace a contractual minimum or maximum. Magnaflux, liquid penetrant FAQs

For solvent-removable processing, distinguish precleaning from removal of excess penetrant after dwell. Magnaflux's instructions call for wiping with a cloth moistened with remover, rather than flushing the surface, which can impair sensitivity. Developer must be thin and even: excessive thickness can mask indications, while too little may develop them inadequately. These instructions describe the specified solvent-removable route, not a substitute for water-washable or post-emulsifiable processing. Magnaflux instructions, January 2018, Rev. A

Bounded visible-product example

Magnaflux's European SKL-SP2 product data sheet, revised July 2025, identifies the red penetrant with SKC-S cleaner/remover and SKD-S2 solvent-based developer among its recommended materials. Its solvent-wipe sequence applies penetrant to a clean component, wipes excess with a cleaner-moistened cloth, dries the component and applies developer before inspection. SKL-SP2 data sheet

That sheet gives minimum penetration guidance of 2–5 minutes, describes 10 minutes as adequate for many situations, and calls for at least 10 minutes development before white-light inspection. These are product guidance values, not a universal PT recipe. The same document warns against inspecting plastics with SKL-SP2 because it can stain, soften or dissolve them. Confirm the exact regional product, packaging, current instructions and governing procedure before use. SKL-SP2, instructions for use

Viewing, process checks and interpretation

Visible dye needs adequate white-light contrast; fluorescent systems need suitable UV-A illumination and controlled ambient light. Inspector vision and dark adaptation also affect observation. Use the selected procedure's measured lighting requirements and checks rather than judging a lamp by apparent brightness. Magnaflux, brightness and contrast controls

In-use materials require control as well as correctly labelled new containers. Magnaflux describes checks covering system performance, penetrant contamination, developer condition or concentration, and emulsifier condition. Their applicability and frequency depend on the system and governing requirements. Maintain the applicable checks and records; avoid treating a consumable's qualification listing as proof that an operating bath remains satisfactory. Magnaflux, in-use penetrant testing

FHWA distinguishes crack-related indications from nonrelevant indications caused by trapped dye in surface features, debris or incompletely removed coatings. Rough weld profiles and undercut can complicate interpretation. Document the location and appearance, investigate relevance using the approved process, and apply the specified acceptance criteria. Do not equate every red line with a rejectable crack or dismiss a line solely because it lies near surface roughness. FHWA, dye penetrant interpretation

A useful inspection record

Retain part and area identification, procedure revision, surface preparation, product names and batches, temperatures, processing times, viewing conditions, applicable process checks, inspector identity and indication locations. Preserve photographs with scale and orientation where useful, together with evaluation and disposition. Specify postcleaning and restoration of removed corrosion protection where required: FHWA notes that coating removal compromises that protection. FHWA, application limitations

A report should separate what was observed from the decision made under the acceptance rule. “No relevant indications observed in the defined examination area” is more informative than an unsupported claim that the part is defect-free.

Related reference

Inspection and quality documents