Published on 02-Mar-2024

Liquid Penetrant Testing Vs Magnetic Particle Inspection: What’s The Difference?

Liquid Penetrant Testing Vs Magnetic Particle Inspection: What’s The Difference?

Table Of Content

  • What is Liquid Penetrant Testing? 
  • Dye Penetrant Testing Explained
  • What are the Advantages and Disadvantages of Liquid Penetrant Inspection? 
  • What is Magnetic Particle Testing? 
  • MPI- Magnetic Particle Inspection Procedure
  • What are the Advantages and Disadvantages of Magnetic Particle Inspection?
  • Liquid Penetrant Testing vs Magnetic Particle Testing 
  • LPT vs MPI: Cost-effective, Time-efficient 

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 dye penetrant test 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 Inspection (sometimes also known as Dye Penetrant Inspection) is used to detect surface-breaking defects.

The penetrant testing definition states that 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).

Dye Penetrant Testing Explained

Penetrant Inspection involves cleaning the surface, followed by the application of the penetrant liquid. The penetrant will penetrate any flaws, for example, cracks or pores, by capillary action. 

1. Any excess penetrant should be removed. 

2. 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. 

3. The test surface can then be inspected. The indications are much broader than the actual flaw and are therefore more easily visible. 

4. Finally, the surface must be cleaned again. 

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. 

Also Read, Procedure for Liquid Penetrant Examination

What are the Advantages and Disadvantages of Liquid Penetrant Inspection? 

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, also called Magnetic Penetrant Testing is used to detect surface defects, particularly cracks, in ferromagnetic or magnetic materials. 

Magnetic Particle Inspection (MPI NDT Test) is a widely employed Non-destructive Testing (NDT) method used to assess the integrity of welds and various metallic components.

This technique is particularly valuable in industries such as welding and manufacturing. 

During an MPI test for welding, the material or weldment under examination is magnetized, typically by inducing a magnetic field.

This magnetic field allows iron or steel particles (dry powder), often suspended in a liquid solution, to be applied to the surface. 

The magnetic particles are drawn to areas with Magnetic Flux Leakage, such as cracks, weld discontinuities, or other defects, revealing these flaws.

The inspection can be conducted at different magnetic particle inspection temperatures, with variations in the test procedure to accommodate specific environmental conditions and material properties.

By effectively detecting surface and near-surface defects, MPI plays a pivotal role in ensuring the reliability and safety of welded structures and critical components across various industries.

Also Read, Basic Principles of Magnetic Particle Testing

MPI- Magnetic Particle Inspection Procedure

1. First, the surface needs to be cleaned and then a white contrast paint can be applied.

2. The NDT MPI Testing component is magnetized, 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 seen against the contrasting white paint background. 

5. It is common to apply magnetization more than once in different directions to fully inspect the material. 

6. If required, the component can be de-magnetized, and post-cleaning can be applied.

The method can be used on all metals that can be strongly magnetized 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 of Magnetic Particle Inspection?

Magnetic Particle Testing is a fast, and 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 belong to Non-destructive Testing 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 liquid, nor MPI testing techniques 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 magnetized (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. 


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