Magnetic Particles and Methods of Application
The particles used in magnetic particle testing are made of ferromagnetic materials, usually combinations of iron and iron oxides, having a high permeability and low retentivity. Particles having high permeability are easily magnetized by and attracted to the low level leakage fields at discontinuities. Low retentivity is required to prevent the particles from being permanently
magnetized. Strongly retentive particles tend to cling together and to any magnetic surface, resulting in reduced particle mobility and increased background accumulation.
Particle sizes are very small, ranging from about 0.0005 to 0.015 cm (0.0002 to 0.006 in.) in commonly used formulations. Each magnetic particle formulation always contains a range of sizes and shapes to produce optimum results for the intended use. The smallest particles are more easily attracted to and held by the low level leakage fields at very fine discontinuities; larger particles can
more easily bridge across coarse discontinuities where the leakage
fields are usually stronger. Elongated particles are included, particularly in the case of dry powders, because these rod shaped particles easily align themselves with leakage fields not sharply defined, such as those that occur over subsurface discontinuities.
Globular shapes are included to aid the mobility and uniform dispersion of particles on a surface. Magnetic particles may be applied as a dry powder, or wet media, using either water or a high flash point petroleum distillate as a
liquid media. Dry powders are available in various colors so the user can select the color that contrasts best with the color of the surfaces on which they are used. Colors for use with ordinary visible light are red, gray, black or yellow. Red and black colored particles are available for use in wet baths with ordinary light, and yellow-green fluorescent particles for use with ultraviolet radiation. Fluorescent particles are widely used in wet baths because the bright fluorescent indications produced at discontinuities are readily seen against the dark backgrounds that exist in ultraviolet radiation testing areas.
Wetting agents and rust inhibitors must be used with water wet baths. Sometimes a defoaming agent is needed. Usually the magnetic particle concentrates include the correct amounts of wetting agent and rust inhibitor for initial use. However, these materials are available separately so that the concentration can be maintained or adjusted to suit the particular conditions. If no rusting can be tolerated, a higher concentration of rust inhibitor is used. If
the test objects have an oily film, more wetting agent is required so that the test object’s surfaces will be completely wetted and covered with the bath. Breaking of the bath into rivulets as it is applied over a test object is an indication of the need for test object cleaning.
Reference should be made to the manufacturer’s recommendations
for the correct quantity of wetting agent. No additives other than the
magnetic particles themselves are necessary with petroleum distillate
baths.
The use of water wet baths should be carefully controlled to prevent corrosion and provide wettability of ferromagnetic components. This requires regular chemical analysis of corrosive inhibitor and wetting agent concentration. Wet magnetic particles are available in aerosol, mix and premix forms for field tests. Magnetic particles in dry form may be applied by hand, using rubber squeeze bulbs or plastic squeeze bottles equipped with perforated caps similar to an ordinary salt shaker but with smaller holes. The objective is to lay down a light cloud of powder on the test object. This is usually accomplished by using a combination of bulb squeezing and tossing of the powder toward the area being tested.
Dry powder is also applied using handheld guns and compressed air. One such device has the gun integrated with the powder container and operates from an ordinary compressed air line. Using a trigger, the technician controls the discharge of a powder cloud of low velocity air for removing excess powder to better reveal indications.
A more elaborate gun powder blower has a motor driven compressor, with a powder container and air powder mixer. A multichannel rubber hose connects to the gun. A work light is contained in the gun tip to illuminate the testing area. A trigger on the gun controls the discharge of the powder and air mixture and the blow off air. More elaborate production systems have been built
using this same principle of operation. In these cases, the discharge nozzles are mechanically controlled, as is the movement of test objects through the machine. Spent powder is automatically retrieved and reused.