Liquid Penetrant Testing vs Magnetic Particle Inspection: what’s the difference?

When carrying out non-destructive testing, it is important to make sure that you use the most appropriate testing method to identify any flaws or defects. Two of the options that you can use when looking for surface defects are liquid penetrant testing vs magnetic particle testing. It is a good idea to find out more about them before you choose which would be best for the materials or structure that you want to test. 

In this article we unpack Liquid Penetrant Testing and Magnetic Particle Inspection, explaining each test, before looking at their similarities and differences 

What is Liquid Penetrant Testing? 

Liquid Penetrant Testing (sometimes also known as Dye Penetrant Inspection) is used to detect surface-breaking defects. Put simply, a dye is used to highlight surface cracks and then it is measured under different light sources. Normally, it is used on non-ferromagnetic materials, (these are materials that are non-magnetic and contain no iron).

What is the process? 

  1. First, the surface needs to be cleaned and then the penetrant liquid can be applied. The penetrant will penetrate into any flaws, for example, cracks or pores, by capillary action. 
  2. Any excess penetrant should be removed. 
  3. A developer (usually a dry powder or spray) is then applied to pull the liquid out of the flaws, creating surface indications that can then be evaluated. 
  4. The test surface can then be inspected. The indications are much broader than the actual flaw and are therefore more easily visible. 
  5. Finally, the surface must be cleaned again. 
dye penetrant inspection 2
The penetrant liquid being applied

Liquid Penetrant testing can be used for storage tanks, small metal components, welds, shafts, heat exchangers and boilers. It is a portable method and can be carried out on-site. 

What are the advantages and disadvantages? 

Liquid Penetrant Inspection offers many advantages, such as speed, large-area coverage and a low-cost process. It can work on any non-porous clean material, metallic or non-metallic. Cracks as narrow as 150 nanometres can be detected. This makes it a very cost-effective and flexible testing method, allowing for a quick turnaround on projects. 

There are limitations to Liquid Penetrant testing as it can only detect surface-breaking defects and is unsuitable for dirty or very rough surfaces. It is also temperature dependent, so it is unsuitable for low temperatures. It is not suitable for highly porous materials. 

What is Magnetic Particle Testing? 

Magnetic Particle Testing is used to detect surface defects, particularly cracks, in ferromagnetic or magnetic materials. A magnetic field is generated inside the component and a magnetic particle powder or ink is applied. The magnetic flux leaks through any defects and attracts the particles, highlighting any defects. 

What is the process? 

  1. First, the surface needs to be cleaned and then a white contrast paint can be applied.
  2. The component is magnetised, either locally or overall.
  3. A magnetic particle powder or ink is applied to the surface. 
  4. If there is a surface-breaking flaw the magnetic field will have become distorted, causing local magnetic flux leakage around the flaw. The fine iron particles (either dry or in a liquid) will build up in the areas of flux leakage. This will be clear to see, even for narrow cracks. It is indicated as a line of iron powder particles on the surface which can be clearly seen against the white contrast paint background. 
  5. It is common to apply magnetisation more than once in different directions to fully inspect the material. 
  6. If required the component can be demagnetized and post-cleaning can be applied.
magnetic particle inspection 1
The surface being magnetised.

The method can be used on all metals that can be strongly magnetised and are often used during the manufacturing and testing of welds. 

Magnetic Particle Testing can be used for storage tanks, pipework, shafts, castings, structural steel, heat exchangers and boilers. 

What are the advantages and disadvantages? 

Magnetic Particle testing is a fast, cost-effective method. It is a portable, simple method that gives instant results. It is a sensitive test that can detect small and narrow defects up to 2 mm below the surface. 

Magnetic Particle testing can be used on relatively rough and dirty surfaces, but the flaw sensitivity may be impaired. 

The method can require a power source and uses flammable liquids so might not be suitable for certain areas on-site. It is limited in use on magnetic materials and cannot be used on non-ferromagnetic materials. 

Liquid Penetrant Testing vs Magnetic Particle Testing 

Liquid Penetrant testing and Magnetic Particle testing are both methods that supplement visual inspection, revealing defects such as fine cracks or micro-porosity that would be invisible or difficult to detect with the naked eye. 

Neither technique can give a permanent record of the inspection, but photographs of the area are useful – reference markers can be helpful for accurate recording of the size and site of indications, particularly if repairs are needed. 

As the name suggests, Magnetic Particle testing requires the material to be magnetised (and therefore made of metal), whereas Liquid Penetrant testing can be used on a wider variety of materials, such as metals, plastics and ceramics. 

Surface preparation is less critical in Magnetic Particle testing than in Liquid Penetrant Testing, but surfaces should be generally smooth for both.

LPT vs MPI: cost-effective, time-efficient 

There are pros and cons to both methods and generally, you should make your choice by considering the various factors involved, especially the type of material that you want to test, as Magnetic Particle Testing cannot be used on non-magnetic materials. Both tests, though, allow for cost-effective and time-efficient testing, meaning that reports can be turned around quickly. 

Responsive testing that delivers assurance that your equipment is safe.

Contact us for a quality testing service with a fast report turnaround. We can respond to any challenge.


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