Magnetic Particle Inspection (MPI) is a popular non-destructive testing (NDT) method. MPI helps to detect surface and subsurface faults and discontinuities in ferromagnetic metals and their alloys such as nickel, iron, and cobalt.
Steel, automobile, petrochemicals, power, and aerospace industries often use MPI to determine a component's fitness-for-use. A variety of castings, welding, and forgings can be inspected using MPI. In addition, underwater inspection of offshore structures and pipelines too can be done with the help of MPI.
In the construction industry, MPI can detect flaws of structural welds on storage tanks, bridges, and other safety-critical structures.
- Principle of MPI
- Magnetic Particles
- Steps to Perform a Dry Magnetic Particle Inspection
- Steps to Perform a Wet Magnetic Particle Inspection
Principle of MPI
MPI combines two non-destructive testing methods, namely visual testing and magnetic flux leakage.
A bar magnet has a magnetic field around and in it. The magnetic line of force in a magnet exits through the north pole and enters the magnet through the south pole. When there is a slight crack or fault in the bar (not completely broken in two pieces), north and south pole are formed on the edges of the crack. The magnetic line of force conventionally exits the magnet through the north pole and re-enters through the south pole.
The cracks create a small air gap causing the magnetic field to spread out as the air can’t support the magnetic field per volume as compared to the magnet. This phenomenon of the magnetic field spreading out appears like a leak out of the material and is referred to as a flux leakage field.
Magnetic particles are a key ingredient for performing magnetic particle testing or MPI. The magnetic particle should be finely crushed iron or iron oxide. They are pigmented in order to throw light on the defects present in a specimen.
The crushed particles should have high magnetic permeability so that the particles are easily attracted to the small magnetic leakage fields of the flaws. In addition to that, the particles should have low retentivity so that they do not stick to the surface or even each other.
Magnetic particles that are used in MPI can be either a dry mix or a wet solution.
1. Dry Magnetic Particles
The diameter of dry magnetic particles ranges from 50 µm to 150 µm making it a mixture of both finer and coarse particles. The finer particles adhere to the flawed specimen easily while the coarser bridges the discontinuities reducing the dusty nature.
In terms of shape, a blend of both rounded and elongated particles should be selected. While the elongated particles easily align themselves in the direction of the force, the rounded particles flow freely. They are available in various pigments such as yellow, red, black, etc. to depict a high contrast between the particles and the specimen.
2. Wet Magnetic Particles
The wet magnetic particles are known to be more sensitive than the dry particles. Their smaller size (10µm and lesser) provides extra mobility as well as adherence. They are available in fluorescent shades in an oil or water suspension.
It is easy to apply the wet magnetic solution to a larger area. They offer protection against corrosion. Oil-based wet magnetic particles protect hydrogen embrittlement.
Steps to Perform a Dry Magnetic Particle Inspection
1. Surface preparation
The surface of the test specimen must be clean and free from moisture, oil, or grease.
A layer of paint or rust often reduces the sensitivity of the test. But specifications allow up to 0.025 mm of the ferromagnetic coating or 0.076 mm of non-conductive coating on the specimen’s surface.
In some cases, a thin layer of coating has to be applied to the specimen. This enhances the contact between itself and the particles.
2. Application of Magnetizing force
Establish a magnetic flux using a coil, permanent magnets, an electromagnetic yoke, etc.
3. Application of Dry Magnetic Particles
Sprinkle a thin layer of the crushed particles on the specimen.
4. Removal of Excess Particles
While the magnetizing force is being applied, blow off the excess powder from the specimen’s surface gently. Diligently blow the air. The force of air shouldn’t blow off all the particles adhered to the magnetic flux leakage field.
5. Terminate the Magnetizing force
The magnetizing force (electromagnet or electromagnetic field) generating the magnetic flux should be terminated. The permanent magnets can be left in place if being used.
Particles will cluster in the damaged/faulty area.
Dry magnetic inspection works better on rough surfaces. It also detects shallow subsurface cracks.
Steps to Perform a Wet Magnetic Particle Inspection
1. Surface preparation
The preparation step is the same as that of dry magnetic particle inspection.
The surface has to free from impurities and other freely moving particles. Specifications allow a thin coating of non-conductive or ferromagnetic paint.
2. Application of Wet Magnetic Particles
Gently spray/pour the suspended magnetic particles over the specimen.
3. Application of Magnetizing force
Apply the magnetizing field immediately after pouring the suspended magnetic particles. Apply the current in two to three short bursts (1/2 second) in a horizontal inspection unit. This improves the mobility of the particles.
Particles will cluster in the damaged/faulty area. In the case of surface discontinuities, there will be a sharp indication. In subsurface flaws, the indication will be less defined. Infrared lights too can be used to get a better vision of the faults on the surface of the specimen.
This method detects minute discontinuities on smooth surfaces in a better manner. On rough surfaces, these are less effective than the dry magnetic particle inspection.
The principle of the method is that the magnetisation force is used to produce magnetic lines of force, or magnetic flux, in the test specimen
A variety of castings, welding and forgings can be inspected. Underwater inspection of offshore structures and pipelines can be done.
The standard used for MPI is ISO 9934-1:2015