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Introduction to Magnetic Particle Inspection

Basic Principles
History of MPI

Magnetic Domains
Magnetic Fields
Electromag. Fields
Field From a Coil
Mag Properties
Hysteresis Loop
Field Orientation
Magnetization of Mat'ls
Magnetizing Current
Longitudinal Mag Fields
Circular Mag Fields
Measuring Mag Fields

Equipment & Materials
Portable Equipment
Stationary Equipment
Multidirectional Equipment
Field Strength Indicators
Magnetic Particles
Suspension Liquids

Testing Practices
Dry Particles
Wet Suspension
Magnetic Rubber
Continuous & Residual Mag
Field Direction & Intensity
L/D Ratio

Process Control
Particle Concentration
Suspension Contamination
Electrical System
Eye Considerations

Example Indications
Visible Dry Powder
Fluorescent Wet


Suspension Liquids

Suspension liquids used in the wet magnetic particle inspection method can be either a well refined light petroleum distillate or water containing additives. Petroleum-based liquids are the most desirable carriers because they provided good wetting of the surface of metallic parts. However, water-based carriers are used more because of low cost, low fire hazard, and the ability to form indications quicker than solvent-based carriers. Water-based carriers must contain wetting agents to disrupt surface films of oil that may exist on the part and to aid in the dispersion of magnetic particles in the carrier. The wetting agents create foaming as the solution is moved about, so anti-foaming agents must be added. Also, since water promotes corrosion in ferrous materials, corrosion inhibitors are usually added as well.

Petroleum based carriers are primarily used in systems where maintaining the proper particle concentration is a concern. The petroleum based carriers require less maintenance because they evaporate at a slower rate than the water-based carriers. Therefore, petroleum based carriers might be a better choice for a system that gets only occasional use or when regularly adjusting the carrier volume is undesirable. Modern solvent carriers are specifically designed with properties that have flash points above 200oF and keep nocuous vapors low. Petroleum carriers are required to meet certain specifications such as AMS 2641.