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 faults in ferromagnetic materials such as iron (non-alloyed and low alloyed steels, cast steel, cast iron), cobalt and nickel. MPI can also detect faults hidden beneath coatings.

The MPI technique canNOT be used on non-ferromagnetic materials such as high alloyed steels (austenitic structure), aluminium, copper, plastics, composites, etc.

To test a part using the MPI technique, the part first needs to be magnetised. 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 coloured particles which provide contrast against a coloured background. Alternatively, our black particle inks 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-based inks.
• Ready-to-use oil-based fluorescent inks.
• Water-based fluorescent ink concentrates.

Our visible magnetic particle inspection products comprise:

• Dry powders.
• Ready-to-use oil-based black inks.
• Water-based black ink concentrate.

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 faults.

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 EMEA manufactures two dry magnetic powders – 1 Grey and 8A Red. The different colours of powder are designed to produce effective contrast on different coloured 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 whilst they 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 to for the particles to find small magnetic flux leakage fields, making it possible to detect smaller discontinuities on smooth surfaces;
• liquid inks have a high sensitivity when compared with dry powders;
• liquid inks 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 ink;
• the flammability and safety aspects associated with the oil-based carrier; and
• the potential corrosion effect of the water-based products.

Within the Magnaflux EMEA range of MPI inks, we use common MPI particles. To summarise:

The key point here is that, in a number of instances, we have oil-based and water-based variants that use the same particle – for example, 14HF (oil-based) and WB-12 (water-based) both use 14A powder.

The main difference comes down to personal preference, as summarised below:

The difference is really about convenience and consistency. As described in our Product Data Sheets, it is possible to make up both an oil-based and water-based ink by dispersing magnetic particles in our Carrier II Oil (for an oil-based ink) or in water mixed with our WA-1 water conditioner (for water-based ink).

However, by using our ready-to-use oil-based inks and water-based ink concentrates, you will:

• save time, and
• have the assurance that the initial settlement volume is within the specification limits.

In addition, all of our water-based products:

• are based on deionised water, so are not subject to variations in local water hardness.
• contain the correct level of water conditioning chemicals for optimum performance.

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

The part will need to be thoroughly demagnetised prior to carrying out the inspection. 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 ink concentrates contain low levels of corrosion inhibitors. The level should be sufficient to give protection to parts during magnetic particle inspection testing.

Water-based inks should always be made up within stainless steel tanks

Corrosion of parts can also occur both before and after inspection so you need ensure the parts are kept clean and dry. Ffollowing inspection using water-based inks,  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.

Yes. We recognise that surface wetting of parts to be inspected is important so all Magnaflux water-based ink concentrates contain wetting agents. We also recognise that water-based inks are prone to foaming, especially when they are in re-circulating baths. To overcome this, our inks also contain antifoam agents.

For situations where additional wetting and foam control are needed, we offers two water bath additives – WA-1 water conditioner and WA-2 antifoam.

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 due to the surfactants present within the ink. Over time, this degreasing effect can potentially lead to the build up of a ‘sludge’ of ink/oil mixture. This is less likely to occur when using an oil-based ink, as it is naturally be more 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 a number of 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 magnetisation – 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 – MG/MX Carrier II Oil – is not very volatile. However, with water-based inks 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 as a result of water loss, only water should be added, as addition of the additives could potentially cause an imbalance of the bath.

There are two options here:

1. To use the bulk variant of the product (for example 14HF is available in aerosol and bulk variants) and then apply the ink using a pump spray bottle.
2. To use the water-based equivalent.

With our aerosol products (14HF, 7HF, 690.1), the concentration of the particles within the ink will remain constant.

With our bulk products (14HF, 622.1, WB-12, MF-655 WB), the concentration of the particles will become depleted when particles are used up during the inspection process. The same is true with inks that are made up using the magnetic powders (14A, MG 410, MG 601).

As such, you will need to check the bath strength at least once a day. The most widely-used way of checking an ink’s settlement volume is by using a graduated ASTM pear-shaped centrifuge tube.

When the settlement volume approaches the lower limit, check the bath:

• If it appears contaminated, or has been in use for a long time, replace the contents.
• If a bath of 14HF is still clean and uncontaminated, you can add 14A powder.

For the water-based inks and 622.1 oil-based ink, if it is still clean and uncontaminated, there are two options, namely:

• To add more of the concentrate to the bath, or
• Use the following table to see what powder you need to add:

Details on the recommended settlement volume are given in our Product Data Sheets.

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 these barrels, you will need a Tri-sure spanner. These often come with a tab seal cap remover.

The best way to get the product out of these barrels is to use a threaded barrel pump that screws into the main opening. On the top of a Magnaflux barrel, you will typically see two openings – the main opening is 2 inches (50.8mm) in diameter (2″ BSP) 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.

Barrel pumps come in a variety of power source types. Most are hand operated but it is also possible to source battery and electric pumps. They also come in a number of different styles – lever pumps, lift pumps, rotary pumps and syphon pumps. Some have telescopic tubes on which enables them to used with different barrel sizes. We would recommend using one with Viton seals. Viton is a brand of synthetic rubber and fluoropolymer elastomer which provides good chemical resistance to a wide range of liquids, including Magnaflux LPI and MPI products. It is not uncommon to be able to replace these seals.

When conducting magnetic particle inspection, it is possible to generate high temperatures at the contact heads. Magnaflux EMEA inks – both oil-based and water-based – have a recommended maximum usage temperature in the range 48°C to 60°C, depending on the particle being used. This is due to the stability of the particles at elevated temperatures.

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

Although, in the UK, aerosols are not a category of Hazardous Waste under the Lists of Wastes Regulations 2005, materials containing flammable solvents and/or flammable aerosol propellants are classed as Hazardous Waste. As a result, empty Magnaflux EMEA aerosols fall under the Hazardous Waste Regulations because of their residual contents.

Furthermore, depending on the product type, some aerosols may contain irritants or residual amounts of liquids or solids that have other hazardous characteristics that make them harmful, or may be dangerous to the environment.

Magnaflux EMEA therefore recommends the use of a licensed waste contractor for the disposal of Magnaflux aerosols. Please seek the advice of an approved waste disposal contractor for further information.

Further information

The British Aerosol Manufacturer’s Association provide a guide on recycling empty aerosols which is available at Recycling Empty Aerosols. This provides guidance on puncturing empty aerosols, so that the hazardous contents can be removed and the metal and plastic components of the aerosol can be recovered and recycled. Note that this should be carried out under the requirements of DSEAR¹, and that a chemical risk assessment (COSHH assessment²) of the contents should be conducted. In addition, operatives must be fully trained in the puncturing procedure and must wear appropriate Personal Protective Equipment (PPE).

Disclaimer

The information given in this FAQ is given in good faith, but does not imply acceptance of any liability or responsibility for the consequences of its use or misuse in any particular circumstances.

¹ Dangerous Substances and Explosive Atmospheres Regulations 2002

² A risk assessment of the operation should be carried out in accordance with the Control of Substances Hazardous to Health (COSHH) Regulations 2002 to identify the potential hazards arising from the chemicals likely to be present.

In 2013, the manufacture of ITW Scrubs products for the European market transferred to Kleinmann in Germany. Kleinmann were incorporated into the ITW Group in 2006 and, in 2013, the company merged with ITW Systemcare.

To find your nearest distributor, please visit the Scrubs website.