PENETRANT

         

Penetrants

Penetrants are carefully formulated to produce the level of sensitivity desired by the inspector. The penetrant must possess a number of important characteristics:

- spread easily over the surface of the material being inspected to provide complete and even coverage.

- be drawn into surface breaking defects by capillary action.

- remain in the defect but remove easily from the surface of the part.

- remain fluid so it can be drawn back to the surface of the part through the drying and developing steps.

- be highly visible or fluoresce brightly to produce easy to see indications.

- not be harmful to the material being tested or the inspector.

Penetrant materials are not designed to perform the same. Penetrant manufactures have developed different formulations to address a variety of inspection applications. Some applications call for the detection of the smallest defects possible while in other

Introduction to Non-Destructive Testing Technique

applications, the rejectable defect size may be larger. The penetrants that are used to detect the smallest defect will also produce the largest amount of irrelevant indications.

Standard specifications classify penetrant materials according to their physical characteristics and their performance.

 Penetrant materials come in two basic types:

Type 1 - Fluorescent Penetrants: they contain a dye or several dyes that fluoresce when exposed to ultraviolet radiation.

Type 2 - Visible Penetrants: they contain a red dye that provides high contrast against the white developer background.

Fluorescent penetrant systems are more sensitive than visible penetrant systems because the eye is drawn to the glow of the fluorescing indication.

 However, visible penetrants do not require a darkened area and an ultraviolet light in order to make an inspection.

 Penetrants are then classified by the method used to remove the excess penetrant from the part. The four methods are:

Method A - Water Washable: penetrants can be removed from the part by rinsing with water alone. These penetrants contain an emulsifying agent (detergent) that makes it possible to wash the penetrant from the part surface with water alone. Water washable penetrants are sometimes referred to as self-emulsifying systems.

Method B - Post-Emulsifiable, Lipophilic: the penetrant is oil soluble and interacts with the oil-based emulsifier to make removal possible.

Method C - Solvent Removable: they require the use of a solvent to remove the penetrant from the part. 

Method D - Post-Emulsifiable, Hydrophilic: they use an emulsifier that is a water soluble detergent which lifts the excess penetrant from the surface of the part with a water wash.

Introduction to Non-Destructive Testing Techniques.

 Penetrants are then classified based on the strength or detectability of the indication that is produced for a number of very small and tight fatigue cracks. The five sensitivity levels are:

Level ½ - Ultra Low Sensitivity

Level 1 - Low Sensitivity

Level 2 - Medium Sensitivity

Level 3 - High Sensitivity

Level 4 - Ultra-High Sensitivity

The procedure for classifying penetrants into one of the five sensitivity levels uses specimens with small surface fatigue cracks. The brightness of the indication produced is measured using a photometer.

Developers

The role of the developer is to pull the trapped penetrant material out of defects and spread it out on the surface of the part so it can be seen by an inspector. Developers used with visible penetrants create a white background so there is a greater degree of contrast between the indication and the surrounding background. On the other hand, developers used with fluorescent penetrants both reflect and refract the incident ultraviolet light, allowing more of it to interact with the penetrant, causing more efficient fluorescence.

According to standards, developers are classified based on the method that the developer is applied (as a dry powder, or dissolved or suspended in a liquid carrier). The six standard forms of developers are:

Form a - Dry Powder

Form b - Water Soluble

Form c - Water Suspendable

Form d - Nonaqueous Type 1: Fluorescent (Solvent Based)

Form e - Nonaqueous Type 2: Visible Dye (Solvent Based)

Form f - Special Applications

Dry Powder

Dry powder developers are generally considered to be the least sensitive but they are inexpensive to use and easy to apply. Dry developers are white, fluffy powders that can be applied to a thoroughly dry surface in a number of ways; by dipping parts in a container of developer, by using a puffer to dust parts with the developer, or placing parts in a dust cabinet where the developer is blown around. Since the powder only sticks to areas of indications since they are wet, powder developers are seldom used for visible inspections.

Water Soluble

As the name implies, water soluble developers consist of a group of chemicals that are dissolved in water and form a developer layer when the water is evaporated away. The best method for applying water soluble developers is by spraying it on the part. The part can be wet or dry. Dipping, pouring, or brushing the solution on to the surface is sometimes used but these methods are less desirable. Drying is achieved by placing the wet but well drained part in a recirculating, warm air dryer with the temperature 21°C. Properly developed parts will have an even, pale white coating over the entire surface.

Water Suspendable

Water suspendable developers consist of insoluble developer particles suspended in water. Water suspendable developers require frequent stirring or agitation to keep the particles from settling out of suspension. Water suspendable developers are applied to parts in the same manner as water soluble developers then the parts are dried using warm air.

Nonaqueous

Nonaqueous developers suspend the developer in a volatile solvent and are typically applied with a spray gun. Nonaqueous developers are commonly distributed in aerosol spray cans for portability. The solvent tends to pull penetrant from the indications by solvent action. Since the solvent is highly volatile, forced drying is not required.

Special Applications

Plastic or lacquer developers are special developers that are primarily used when a permanent record of the inspection is required.

Preparation of Part

One of the most critical steps in the penetrant inspection process is preparing the part for inspection. All coatings, such as paints, varnishes, plating, and heavy oxides must be removed to ensure that defects are open to the surface of the part. If the parts have been machined, sanded, or blasted prior to the penetrant inspection, it is possible that a thin layer of metal may have smeared across the surface and closed off defects. Also, some cleaning operations, such as steam cleaning, can cause metal smearing in softer materials. This layer of metal smearing must be removed before inspection.

Penetrant Application and Dwell Time

The penetrant material can be applied in a number of different ways, including spraying, brushing, or immersing the parts in a penetrant bath. Once the part is covered in penetrant it must be allowed to dwell so the penetrant has time to enter any defect that is present.

There are basically two dwell mode options:

- Immersion-dwell: keeping the part immersed in the penetrant

during the dwell period.

- Drain-dwell: letting the part drain during the dwell period

(this method gives better sensitivity).

Penetrant Dwell Time

Penetrant dwell time is the total time that the penetrant is in contact with the part surface. The dwell time is important because it allows the penetrant the time necessary to seep or be drawn into a defect. Dwell times are usually recommended by the penetrant producers or required by the specification being followed. The time required to fill a flaw depends on a number of variables which include:

 The surface tension of the penetrant.

 The contact angle of the penetrant.

 The dynamic shear viscosity of the penetrant.

 The atmospheric pressure at the flaw opening.

 The capillary pressure at the flaw opening.

 The pressure of the gas trapped in the flaw by the penetrant.

 The radius of the flaw or the distance between the flaw walls.

 The density or specific gravity of the penetrant.

 Microstructural properties of the penetrant.

The ideal dwell time is often determined by experimentation and is often very specific to a particular application. For example, the table shows the dwell time requirements for steel parts according to some of the commonly used specifications.

Penetrant Removal Process

The penetrant removal procedure must effectively remove the penetrant from the surface of the part without removing an appreciable amount of entrapped penetrant from the discontinuity. If the removal process extracts penetrant from the flaw, the flaw indication will be reduced by a proportional amount. If the penetrant is not effectively removed from the part surface, the contrast between the indication and the background will be reduced.

Removal Method

As mentioned previously, penetrant systems are classified into four types according to the method used for excess penetrant removal.

- Method A: Water-Washable

- Method B: Post-Emulsifiable, Lipophilic

- Method C: Solvent Removable

- Method D: Post-Emulsifiable, Hydrophilic

Method C, Solvent Removable, is used primarily for inspecting small localized areas. This method requires hand wiping the surface with a cloth moistened with the solvent remover, and is, therefore, too labor intensive for most production situations.

Method A, Water-Washable, is the most economical to apply of the different methods and it is easy to use. Water-washable or self-emulsifiable penetrants contain an emulsifier as an integral part of the formulation. The excess penetrant may be removed from the object surface with a simple water rinse.

When removal of the penetrant from the defect due to over-washing of the part is a concern, a post-emulsifiable penetrant system can be used.

 The post-emulsifiable methods are generally only used when very high sensitivity is needed. Post-emulsifiable penetrants require a separate emulsifier to breakdown the penetrant and make it water washable.

 The part is usually immersed in the emulsifier but hydrophilic emulsifiers may also be sprayed on the object. Brushing the emulsifier on to the part is not recommended either because the bristles of the brush may force emulsifier into discontinuities, causing the entrapped penetrant to be removed. 

The emulsifier is allowed sufficient time to react with the penetrant on the surface of the part but not given time to make its way into defects to react with the trapped penetrant. Controlling the reaction time is of essential importance when using a post-emulsifiable system.

 If the emulsification time is too short, an excessive amount of penetrant will be left on the surface, leading to high background levels. If the emulsification time is too long, the emulsifier will react with the penetrant entrapped in discontinuities, making it possible to deplete the amount needed to form an indication.

The hydrophilic post-emulsifiable method (Method D) is more sensitive than the lipophilic post-emulsifiable method (Method B). The major advantage of hydrophilic emulsifiers is that they are less sensitive to variation in the contact and removal time.

When using an emulsifiable penetrant is used, the penetrant inspection process includes the following steps (extra steps are underlined): 

1. pre-clean part,

 2. apply penetrant and allow to dwell, 

3. pre-rinse to remove first layer of penetrant,

 4. apply hydrophilic emulsifier and allow contact for specified time,

 5. rinse to remove excess penetrant,

 6. dry part,

 7. apply developer and allow part to develop, and

 8. inspect.

Rinse Method and Time for Water-Washable Penetrants

The method used to rinse the excess penetrant from the object surface and the time of the rinse should be controlled so as to prevent over-washing.

 It is generally

recommended that a coarse spray rinse or an air-agitated, immersion wash tank be used. When a spray is being used, it should be directed at a 45° angle to the part surface so as to not force water directly into any discontinuities that may be present. The spray or immersion time should be kept to a minimum through frequent inspections of the remaining background level.

Hand Wiping of Solvent Removable Penetrants

When a solvent removable penetrant is used, care must also be taken to carefully remove the penetrant from the part surface while removing as little as possible from the flaw. The first step in this cleaning procedure is to dry wipe the surface of the part in one direction using a white, lint-free, cotton rag. One dry pass in one direction is all that should be used to remove as much penetrant as possible.

 Next, the surface should be wiped with one pass in one direction with a rag moistened with cleaner. One dry pass followed by one damp pass is all that is recommended. Additional wiping may sometimes be necessary; but keep in mind that with every additional wipe, some of the entrapped penetrant will be removed and inspection sensitivity will be reduced.

Use and Selection of a Developer

The use of developer is almost always recommended. The output from a fluorescent penetrant is improved significantly when a suitable powder developer is used. Also, the use of developer can have a dramatic effect on the probability of detection of an inspection.

Nonaqueous developers are generally recognized as the most sensitive when properly applied. However, if the thickness of the coating becomes too great, defects can be masked.