FAQs – Magnetic Particle Inspection

Magnetic particle inspection (MPI) is a simple, cost-effective and reliable method of non-destructive testing that can be used  to find surface-breaking and sub-surface discontinuities in ferromagnetic materials such as iron (non-alloyed and low alloyed steels, cast steel, cast iron), cobalt and nickel. 

The MPI technique can NOT be used on non-ferromagnetic materials such as high alloyed steels (austenitic structure), aluminum, copper, plastics, composites, etc.

To test a part using the MPI technique, the part first needs to be magnetized. If there is a flaw or discontinuity within the part, the magnetic field will spread out. This spreading out is referred to as magnetic flux leakage. Using a product containing iron particles, you can detect these areas of magnetic flux leakage.

A very simple way to establish if a part can be tested using magnetic particle inspection is by using a permanent magnet to see if it is ferromagnetic. If it is, the magnet will ‘stick’ to the part and you can go ahead and test it using MPI.

Magnaflux manufactures both UV fluorescent and visible magnetic particle inspection (MPI) products.

Our fluorescent MPI products contain pigments that fluoresce when exposed to ultraviolet (UV) radiation. Our dry method visible magnetic inspection products consist of colored particles which provide contrast against a colored background. Alternatively, our black particle products can be viewed against a white background by first applying our white contrast paint.

Our fluorescent magnetic particle inspection products comprise:

• Fluorescent powders used to make up oil or water baths.
• Ready-to-use oil-based fluorescent products.
• Water-based fluorescent concentrates.

Our visible magnetic particle inspection products comprise:

• Dry powders.
• Ready-to-use oil-based black products.

Dry magnetic powder inspection is particularly suited to inspections on rough surfaces such as un-ground welds and rough castings. In addition to surface-breaking flaws, dry powders can also be used to detect sub-surface discontinuities.

With dry method testing, no carrier fluid is involved, so dry powders can be used for testing hot surfaces. However, the absence of a carrier fluid means that they will lose mobility once the particles are on the surface.

Magnaflux manufactures three dry magnetic powders – 1 Gray, 2 Yellow and 8A Red. The different colors of powder are designed to produce effective contrast on different colored backgrounds. It is also possible to use these powders on a white contrast background, as long as the contrast paint is applied lightly.

When using dry powders, it is important to ensure that the surface is dried thoroughly after cleaning. Dry powders are treated as disposable and should not be re-used on account of the risk of contamination from dirt and moisture.

Wet method inspection involves applying magnetic particles to a part while the particles are suspended in a liquid carrier. The liquid carrier can be either oil or water-based.

The advantages of this inspection method are:

• the magnetic particles are easy to apply evenly over the part to be inspected;
• their mobility within the carrier allows more time for the particles to find small magnetic flux leakage fields, making it possible to detect smaller discontinuities on smooth surfaces;
• Wet method MPI products have a high sensitivity when compared with dry powders;
• Wet method MPI products are better suited to the inspection of more complex shapes where, again, dry powders would not be able to flow properly.

When choosing between oil or water-based carriers, you need to consider:

• the surface wetting capability of the carrier;
• the flammability and safety aspects associated with the oil-based carrier; and
• the potential corrosion effect of the water-based products.

The surface of the part to be inspected should be free of any form of contamination that might affect the test, for example grease, water, dirt and other agents that may have been applied to the surface during the manufacturing or pre-treatment process. This can be done using a solvent-based cleaner such as SKC-S. The presence of a coating on the surface – such as white contrast paint – will not affect the test as long as the coating thickness is less than 50 µm. A coating thicker than 50 µm can potentially reduce the sensitivity of the test.

The mobility of the particles are greatly influenced by the presence of foreign matter such as dirt, rust, grease, scale, oils and water (in oil-based products). In addition, certain corrosion protection products can cause false indications at their boundaries.

The part will need to be thoroughly demagnetized prior to magnetizing as part of the MPI process. If the part retains some residual magnetism (for example, magnetism introduced during welding), the sensitivity of the test may be reduced or false indications produced.

All Magnaflux water-based concentrates contain low levels of corrosion inhibitors. The level should be sufficient to give protection to parts during magnetic particle inspection testing.

Corrosion of parts can occur both before and after inspection so you need to ensure the parts are kept clean and dry. Following inspection using water-based products,  it is especially important to ensure that any excess water is removed as quickly as possible to prevent any risk of corrosion.

If you require longer-lasting corrosion protection, treat your cleaned components with a temporary protective film coating.

There are many reasons why a bath may become contaminated. A typical example is the oil or protective coating on a component coming off in the bath. Over time, this degreasing effect can potentially lead to the buildup of a ‘sludge’. This is less likely to occur when using an oil-based product, as it is naturally able to solvate such contamination.

To overcome this problem, we recommend pre-cleaning your components using our solvent-based cleaner SKC-S prior to carrying out inspection.

There are several reasons for the depletion of particles during magnetic particle inspection, including:

• Drag out of particles during the inspection process. In addition to particles being aligned around a defect, it is not uncommon to get a level of particles on the background of a component, on account of its surface roughness and geometric shape.
• Over magnetization – this can occur where a component has different thicknesses or diameters.
• Particles becoming ‘stuck’ in parts of the processing equipment, such as pipework, filters, pumps etc. With this, it is not uncommon when you first fill a bath to find that you need additional particles to achieve the right settlement volume level.
• The presence of contamination on a component. Contamination in the form of oils and greases, for example, could mean that particles adhere to a component as it is removed from a bath.

The carrier fluid we produce – Carrier II Oil – is not volatile under normal operating conditions. However, with water-based products it is possible to see a low level of evaporation but, in this case, it is the water that is lost and not the additives such as wetting agents, defoamers and corrosion inhibitors. For this reason, if there is a need to top up a bath because of water loss, only water should be added, as addition of the additives could potentially cause an imbalance of the bath.

Use the bulk variant of the product (for example 14AM is available in aerosol and bulk variants) and apply using a spray applicator.

The presence of a coating on the surface – such as white contrast paint – will not affect the test as long as the coating thickness is less than 50 µm.  A coating thicker than 50 µm can potentially reduce the sensitivity of the test. Take care to apply your contrast paint carefully and evenly.

If you need to remove white contrast paint from the inspected part, use a wire brush or a common solvent such as acetone.

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 (50.8 mm) 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.

Magnaflux magnetic particles – both oil-based and water-based – have a recommended maximum usage temperature of 120°F (48°C). This is due to the stability of the particles at elevated temperatures.

Coupled with this, care must be taken when using a carrier oil because of the flash point (the lowest temperature at which a liquid can form an ignitable mixture in air). Under the AMS specifications, the flash point for carrier oil must exceed 200°F (93°C). When conducting MPI, it is best to ensure that the usage temperature conforms with the above recommendations.

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.