Liquid penetrant classification is divided into several categories which are: the type that categorizes based on the type of contrast dye used in the penetrant process, the method that categorizes based on the method used to remove the penetrant, the level that categorizes based on the sensitivity level of the process, the form that categorizes based on the type of developer and the class that categorizes based on the class of solvent remover.
Classification of Penetrant Materials and Processes
Type
- Type I: Fluorescent liquid penetrant.
- Type II: Visible liquid penetrant.
- Type III: Dual Mode (Both visible and fluorescent liquid penetrant).
Method
- Method A: Water-Washable.
- Method B: Post-emulsifiable, Lipophilic.
- Method C: Solvent Removable.
- Method D: Post-emulsifiable, Hydrophilic.
Sensitivity (Level)
- Level ½: Very Low.
- Level 1: Low.
- Level 2: Medium.
- Level 3: High.
- Level 4: Ultra High.
Developer (Form)
- Form a: Dry-Powder.
- Form b: Water-Soluble.
- Form c: Water-Suspendable.
- Form d: Nonaqueous (Wet; for Type I).
- Form e: Nonaqueous (Wet; for Type II).
- Form f: Specific Application.
Solvent Remover (Class)
- Class 1: Halogenated.
- Class 2: Non-halogenated.
- Class 3: Specific Application.
Penetrant Types
Type I – Fluorescent Penetrant
Type I penetrants consist of liquid penetrants that demonstrate the property of fluorescence when exposing to UV radiation. The fluorescence property of a chemical means certain chemicals’ capability to emit visible light when exposed to ultraviolet radiation (Blacklight).
Type I penetrants have high sensitivity, which means excellent ability to detect small surface discontinuities. Type I’s high sensitivity is due to the highly visible indications the fluorescent penetrant will emit when exposed to blacklight even from a tiny quantity of penetrant at a tiny discontinuity.
Type II – Visible Penetrant
Type II penetrants consist of liquid penetrants that contain visible red dye dissolved in the penetrant oil. An application of a white developer layer following the penetrant removal provides a high contrast background to enhance visibility.
Type III – Dual-Mode Penetrant
Type III penetrants consist of liquid penetrants containing dyes to provide high contrast indications under normal light and have fluorescence property that emits lights and exposes indications when exposed to black light.
Methods of Penetrant Removal
Method A – Water Washable Penetrant
Liquid penetrants are typically oil-based products made of petroleum oil, which means the penetrant cannot be removed with water as the oil is not insoluble in water. Method ‘A’ penetrants use emulsifier (emulsifying agent) mixed with the penetrant oil on which it makes the penetrant removable with water (water washable).
Water-washable penetrants are the type of penetrants formulated with an emulsifier mixed with the penetrant oil to permit direct removal with water spray, immersion, or manually wiping.
Method B – Post-emulsifiable Lipophilic Penetrant
Method ‘B’ penetrants do not contain emulsifying agents, and thus, penetrants cannot be removed entirely with water. The penetrants are formulated to provide enhanced penetrating and visibility characteristics. The removal of the penetrant occurs by applying an emulsifier in a separate step post penetrant dwell time.
Lipophilic emulsifier application occurs to transfer the excess penetrant on the surface of the tested piece to an emulsifiable mixture that can be washed with water.
Method C – Solvent Removable Penetrant
Method ‘C’ penetrants are usually either post emulsifiable penetrants that are more common or water washable penetrants. However, the penetrants can also be removed with solvents used by wiping the surface to remove excess penetrants. Usually, the solvent is supplied and used in an aerosol spray can.
Method D – Post-emulsifiable, Hydrophilic Penetrant
Method’ D’ penetrants are similar to Method ‘B’ Lipophilic Penetrants. The difference is the hydrophilic emulsifier method requires a water-based remover solution; hence, removing the excess surface penetrant occurs using a detergent action rather than an emulsification action.
Levels of Penetrant Sensitivity
The penetrant test sensitivity is the testing process’s ability to measure the smallest discontinuity and display it as a clear indication.
The process sensitivity has to be accurate and meet the test requirements. If the process sensitivity is high, the result will display irrelevant indications and creates misleading results during the evaluation process. If the process sensitivity is low or not sensitive enough, the process may not reveal discontinuities beyond the application’s acceptance criteria, causing defects to remain without detection.
The sensitivity level of the penetrant testing materials is directly connected to the material’s cost; the higher the sensitivity, the higher the material’s cost; hence, the selection must be appropriately made based on the application to avoid unnecessary expenses.
Fluorescent penetrant is defined by the sensitivity level category as follow:
- Sensitivity Level 1/2 – Ultra-Low sensitivity.
- Sensitivity Level 1 – Low sensitivity.
- Sensitivity Level 2 – Medium sensitivity.
- Sensitivity Level 3 – High sensitivity.
- Sensitivity Level 4 – Ultra-high sensitivity.
Forms of Developer Application
Form a – Dry-powder Developer
A dry developer is the type of developer which is in powder form; the powder is fluffy and forms a thin film on the workpiece whenever it is applied.
The dry developer is suitable for fluorescent penetrants that give good development properties. Whenever dry powder is used with fluorescent penetrant and under blacklight, the developer provides good contrast between the workpiece and the penetrant on the discontinuities.
The dry developer is not suitable for visible penetrant as the developer does not provide a contrasting background.
Form b – Water-soluble Developer
A water-soluble developer is an aqueous developer that is dissolved in water. The water-soluble developer is suitable for fluorescent penetrants but not recommended for use with visible penetrants.
The water-soluble developer is not recommended for use with the water-washable removal method. The developer’s water may remove water-washable penetrant from discontinuities, erasing defects and affecting the process reliability.
Water-soluble developer comes in the form of dry concentrate; the dry concentrate is then mixed with water to a proper ratio. The ratio of developer concentrate to water is regularly checked as well as the percentage of penetrant contamination.
Form c – Water Suspendable Developer
Water suspendable developer is an aqueous developer that does not dissolve in water as a water-soluble developer but remains suspended in the water. The mixture must be thoroughly agitated before application. Water suspendable developer is suitable for visible penetrants.
Water-soluble developer comes in the form of dry concentrate; the dry concentrate is then mixed with water to a proper ratio. The ratio of developer concentrate to water is regularly checked as well as the percentage of penetrant contamination.
Form d – Nonaqueous, Type I, Fluorescent Systems (solvent-based), and Form e – Nonaqueous, Type II, Visible Dye Systems (solvent-based) Developer
Nonaqueous developers are the type of developers that suitable to use with all penetrant types and methods. Nonaqueous developers are the most sensitive developer method available.
Nonaqueous developers are supplied in aerosol spray cans or bulk containers; whenever the developer is in aerosol spray cans, no further process control is required. However, the developer must be thoroughly agitated and mix appropriately before spraying whenever the developer is in bulk containers.
Form f – Special Applications Developer
Special application developers are developers that have been produced for a specific application; the developers are usually made to be suitable to use with certain penetrant materials. Before the developer’s use, a qualification process takes place to verify and approve the system’s reliability.
Class of Solvent Remover
Class 1 – Halogenated
Halogenated solvent removers are solvents that contain a halogen such as fluorine, chlorine, bromine, or iodine.
Class 2 – Non-halogenated
Non-halogenated solvent removers are solvents that do not contain a halogen.
Class 3 – Specific Application
Special application solvent removers are solvents that have been produced for a specific application; the solvent removers are usually made to be suitable to use with a particular process or penetrant materials.
Reference:
- NAVAIR 01-1A-16 – TECHNICAL MANUAL – NONDESTRUCTIVE INSPECTION METHODS, BASIC THEORY.
- SAE AMS 2644 – Inspection Material, Penetrant.
