FAQs – Liquid Penetrant Inspection

Liquid penetrant inspection (LPI) is a simple, cost-effective and reliable method of non-destructive testing that can be used for the inspection of surface-breaking defects on a wide variety of non-porous material types, including metals and ceramics. Unlike magnetic particle inspection (MPI), LPI can be used for the inspection of non-ferrous metals.

The nature of the technique lends itself well to the inspection of large areas and parts, and the inspection of complex geometric shapes.

Magnaflux manufactures both UV fluorescent and visible penetrants.

Magnaflux fluorescent penetrants contain dyes that fluoresce when exposed to ultraviolet (UV) radiation. Our visible penetrants contain a red dye that provides high contrast when viewed against a white developer background.

Under AMS 2644, penetrant systems are classified as:

• Type 1 – fluorescent dye
• Type 2 – visible dye

Penetrants are then classified by the method used to remove the excess penetrant from the part. The five methods are as follows:

• Method A – Water washable
• Method A(W) – Water washable – water containing (≥ 20% water by volume)
• Method B – Post emulsifiable, lipophilic
• Method C – Solvent removable
• Method D – Post emulsifiable, hydrophilic

Water washable (Method A) penetrants contain surfactants, which means they can be removed from the part by rinsing with water alone.

Post emulsifiable penetrants can be removed using two different methods of emulsification, namely lipophilic and hydrophilic. A lipophilic emulsifier (Method B) acts by essentially changing the chemistry of a penetrant so that it acts like a water washable penetrant, thus allowing the excess penetrant to be removed using water. A hydrophilic emulsifier (Method D) works like a detergent by emulsifying the excess penetrant so that it can then be removed using water. 

Solvent removeable penetrants require the use of a solvent (Method C) to remove the penetrant from the part.

Penetrants are then classified based on their ability to detect an indication. The five sensitivity levels are as follows:

• Level ½ – ultra low sensitivity
• Level 1 – low sensitivity
• Level 2 – medium sensitivity
• Level 3 – high sensitivity
• Level 4 – ultra high sensitivity

In many cases, the particular penetrant inspection technique, and therefore the choice of penetrant type, will be dictated by a procedure relevant to the part being inspected. This is particularly common within the aerospace sector, where there are strict guidelines associated with NDT inspection. In other industry sectors, this will be less strict.

Often, the choice as to whether to use a UV fluorescent or visible penetrant will come down to several factors, including the following:

• The nature and size of the part(s) to be inspected.
• The number of parts to be inspected. For example, clients that are inspecting a large number of parts as part of a production operation invest in a penetrant line incorporating fluorescent dye penetrant.
• The environment where the inspection takes place, i.e. indoors or outside in the field. For example, testing welds on a pipeline in a desert would favor the use of a visible penetrant.
• The nature of the discontinuities to be detected. For example, small discontinuities would favor the best inspection conditions, which would be a darkened area using a UV fluorescent penetrant.

Penetrant sensitivity is a complex area. Although sensitivity relates to the ability to find indications, other factors play a key part too. For example, on a rough casting a high sensitivity penetrant would be difficult to remove from the background areas, so you would look to use a low sensitivity penetrant. Conversely, with a polished titanium part, you would use a high sensitivity penetrant so you can be sure that the penetrant will remain present in any flaws after the washing process.

Different industries have different requirements in relation to penetrant sensitivity. However, in general, you should look to use a low sensitivity penetrant on rough parts, a medium sensitivity penetrant on general engineering components, and a high sensitivity penetrant on more precision or polished components.

Type testing

When seeking approval for a penetrant in accordance with material specifications such as AMS 2644 and EN ISO 3452-2, the penetrant manufacturer is responsible for getting the product Type tested. This involves sending a sample of the penetrant off to an independent laboratory for verification against a comprehensive set of standard tests.

Batch testing

During the routine manufacture of a penetrant and its associated consumable products, it is a requirement under material specifications such as AMS 2644 and EN ISO 3452-2 to carry out a number of batch-specific tests, including flash point, viscosity and fluorescent brightness (Type 1 systems only). These specifications also specify the sampling rate and dictate that we should publish these results on a batch certificate of conformance. For Magnaflux penetrants, cleaners, developers and emulsifiers, these certificates of conformance are referred to as Batch Inspection Certificates.

In-use testing

When a penetrant is being used, there are a set of in-use tests that need to be carried out by the customer. ASTM E1417 has established standards for performance checks for in-use materials. You can find out more information on these tests in ASTM E1417.

The sensitivity of a penetrant is not just simply its ability to find different sized cracks/defects; it also relates to aspects such as the type of surface finish. For example, on a rough casting you would not want to use a high sensitivity penetrant as it would be too difficult to remove, and you would see a lot of penetrant in the background. Conversely, on a highly polished part you would not want to use a low sensitivity penetrant as the risk of over washing and removing penetrant from the cracks/defects would be high. The sensitivity of a penetrant is ultimately determined by the independent laboratories that assess the penetrant in accordance with material specifications such as AMS 2644 and EN-ISO 3452-2.

Post emulsifiable penetrants are a better choice if you are concerned about removing too much penetrant from a part due to over washing. These penetrants require a separate emulsifier to break them down and make them water washable; as such, you have better control over the washing process. 

The acronym PE refers to the lipophilic method (Method B) for removing post emulsifiable penetrants.

The acronym PR refers to the hydrophilic method (Method D) for removing post emulsifiable penetrants.

From a practical point of view, it is not always possible to remove penetrant using a water wash or post emulsification process. For example, this may not be practical:

• if you are inspecting a small part;
• if you are applying the penetrant using an aerosol;
• if the location of the inspection is distant from a source of water or a facility to collect and process the rinse water.

In these instances, it is possible to remove the excess penetrant using a solvent wipe technique.

To perform this method of removal, apply the solvent (for example, SKC-S) to a cloth, and then perform one or two gentle wipes across the surface of the part to remove the excess penetrant. With each wipe, ensure that a clean area of the cloth is used. Take care not to wipe too hard or to wipe too many times as this could remove the penetrant from any discontinuities that are present.

Use the bulk variant of the product (for example, SPOTCHECK SKL-SP2 is available in aerosol and bulk variants).  The penetrant may be applied by a spray applicator or by brushing or flowing on the inspection parts.

The way penetrant inspection works is by the penetrant liquid wetting the surface of the part and finding cracks and surface-breaking defects. This process will be inhibited by the presence of surface contamination such as grease, oil, metal treatment chemicals or even residues from previous inspections.

Pre-cleaning of parts is best done using a solvent-based cleaner. The action of the solvent will remove any residues from the surface and evaporates to leave a clean and dry surface.

When working in outdoor environments, the pre-cleaning process will also need to remove all traces of water, moisture and, at  low temperatures, ice.

Magnaflux offers cleaners with different solvent make-ups to meet the requirements of our customers.

In our Product Data Sheets, we offer the following guide information on penetrant dwell times (penetration time, contact time):

‘Minimum penetration time is 2 to 5 minutes, with 10 minutes being adequate for most situations. Lower temperatures will thicken the penetrant and require longer penetration times’. Most specifications require a minimum penetrant dwell time of 10 minutes.

Please note that this information is for guidance purposes only. With any penetrant – fluorescent or visible – the penetrant dwell time will be dependent on the type of material (metals, plastics, ceramics, castings, forgings, rolled materials, welds, etc) as well as temperature.

Magnaflux recommends that prior testing is carried out when inspecting different material types, or when inspecting at different temperatures, to establish the most appropriate dwell time.

The purpose of a developer is to effectively pull the penetrant back out of a defect to allow it to be seen by an inspector. The action of pulling it back to the surface also allows the penetrant to spread along the surface around the defect, thus magnifying the strength of the indication.

With UV fluorescent penetrants, the developer particles both reflect and refract the UV light, which makes the indications brighter.

With visible penetrants, the developer, in addition to pulling the penetrant back to the surface, creates a white background to create an ideal contrast between the indication and the surrounding surface.

Under AMS 2644, there are five primary classifications of developer, namely:

• Form a – dry powder developer
• Form b – water soluble developer
• Form c – water suspendible developer
• Form d – nonaqueous solvent based developer for fluorescent penetrants
• Form e – nonaqueous solvent based developer for visible penetrants

Dry powder developers

Dry powder developers (ZP-4D) are generally considered to be easy to use and inexpensive. They can be applied using the following techniques:

• Dipping
• Powder spray applicator – for example, powder spray bulb dispenser
• Powder storm cabinet
• Electrostatic spray gun

Water suspendible developer

With a water suspendible developer like ZP-5B, the developer particles are suspended in water. It is important to keep the developer suspension stirred or agitated to prevent the particles from settling out of the suspension. After application of the developer, you will need to dry the parts to remove excess water. Water suspendible developers can be applied using the following techniques:

• Immersion dip
• Flow on

Water soluble developers

With a water soluble developer (ZP-14A), the developer particles are dissolved within a water carrier. After application, a film of developer  will form on the surface of the part during the drying stage. As with water suspendible developers, water soluble developers can be applied using the following techniques:

• Immersion dip
• Flow on

Non-aqueous developers

Non-aqueous developers are solvent based, where the developer is suspended in a solvent based carrier. Being solvent based, there is no need for heat drying as the solvent evaporates quickly from the part.

In general, these products are supplied and used as aerosols for convenience and ease of use. If used in bulk, they will need to be applied using a spray gun, with care being taken to keep the developer stirred/agitated.

Our solvent-based (non-aqueous) developers contain inorganic white pigments dispersed in a solvent mixture. When sprayed onto a component, the solvent evaporates to leave the fine layer of white developer particles. By their very nature, it is hard to keep these inorganic pigments in suspension; without agitation – for example, the shaking of an aerosol can – they will fall to the bottom of the container. This is why we recommend spraying, either by aerosol or conventional spray gun. 

While Magnaflux penetrants can be used on ceramic, plastic and/or composite materials, the dyes within them can cause staining. It is also possible that the penetrant will soften or even dissolve the base material of some plastics.

A key point here is that the penetrants are only suitable for non-porous materials. We do not recommend their use on porous ceramic or composite materials. We strongly recommend that you carry out a pre-test with the penetrant when using such materials, in order to check the compatibility of the penetrant.

When operating below 50°F (10°C) the following risks are presented:

• The presence of moisture in the form of condensation, frost, or ice. At the very least, all moisture, frost and ice must be removed prior to carrying out a test, and the part must be dry.
• Differences between the air temperature and the temperature of the component to be inspected. For example, if an inspection is being carried out on an external vessel, the air temperature within that vessel is likely to be higher than the temperature of the vessel wall.
• Differences in the structure of the component to be inspected. For example, as a rule, metallic structures will contract as the temperature becomes lower, and different metals contract at different rates. This contraction effect could mean that any surface-breaking defects at lower temperatures are potentially finer and may be harder to detect.
• The penetrants are likely to have lower mobility into a defect. As a result, the penetration dwell times may need to be lengthened.

For all these reasons, we strongly recommend that any inspection carried out at temperatures below 50°F (10°C) is subject to an application test using known test materials/reference materials, that will validate the test results under these conditions.

The penetrant can be applied using the following techniques:

• Immersion dip
• Brush (painting)
• Flow on
• Conventional spray – air gun or pump spray bottle
• Electrostatic spray

No, we discontinued our penetrant pens as they were prone to leaking during transportation.

To open Magnaflux products in sealed drums, use nonsparking adjustable pliers to take the drum seals off and use a nonsparking drum wrench to loosen the ¾-inch and 2-inch drum bungs.

The best way to get the product out of drums is to use a threaded drum pump that screws into the main opening. On the top of a Magnaflux drum, you will typically see two openings – the main opening is 2 inches (50.8mm) in diameter and the smaller breather hole is 0.75 inch (19.1 mm) in diameter.  Before pumping the liquid out, you will need to unscrew the seal on the breather hole.

Drum pumps come in a variety of power source types. Many are hand operated, but it is also possible to source air driven pumps. Some have telescopic tubes which enables them to be used with different drum sizes. We would recommend using a drum pump with Teflon seals. Teflon provides good chemical resistance to a wide range of liquids, including Magnaflux LPI and MPI products.

If a component is found to have cracks and surface breaking defects, or is relatively porous, penetrant residues are likely to be present following the inspection process. Penetrant residues can be removed using one of the following techniques:

Water wash: It may be possible to remove penetrant residues from component(s) using further water washing – applicable to water washable penetrants and post emulsifiable penetrants (the latter would also require the use of an emulsifier).

Solvent wipe using a remover: It may be possible to remove penetrant residues from component(s) using a solvent wipe technique – applicable to all penetrants. In this case, the remover – for example, SKC-S – is applied to a cloth which is then used to wipe the component surface.

Solvent soak: This process involves the use of solvents in liquid form to clean the component(s) and remove the penetrant residues. Using this method it is possible to soak the component(s) in a cleaner – for example, SKC-S. Alternatively the component(s) can be soaked in an alcohol (for example, isopropyl alcohol) or a ketone (for example, acetone). Use of an ultrasonic bath will further enhance this process. In all cases, this process would require a risk assessment.

To check for removal of any penetrant residues:

• A visual inspection should be used for red visible penetrants.
• Inspection using a UV(A) light source should be used for fluorescent penetrants.

Over time, penetrant residue can build up within test panels, which will affect their performance. Also, in a hard water area, it is possible to get salt and limescale deposits within the panels. Our recommendations on cleaning test panels are:

• Pre-soak the panels in hot water 122-158ºF (50-70ºC) for a few minutes. This will soften any residual penetrant.
• Use a polar solvent such as acetone, methyl ethyl ketone (MEK) or isopropanol in combination with an ultrasonic bath. This way you get a combination of the solvent action and a vibrational effect to release any residues. Prior to doing this, we recommend that you carry out a workplace risk assessment to minimize or eliminate any risk to an operator.
• If you are in a hard water area, you could also use a weak solution of a limescale remover. Avoid using strong acids or prolonged contact with a limescale remover as this will cause damage to the chrome plating on the test panels. Ensure that all traces of limescale remover are removed by rinsing with deionized water.
• Ensure that the panels are fully dried before reusing.

Since the U.S. and Canada switched to the GHS regulation, SDS’s no longer have an expiration date. Instead SDS are updated within 3 months of any new information that we receive or discover. Therefore, the SDS found on our website are the most up to date.