T.O. 33B-1-1
3.3.6
Suspensions.
CAUTION
The use of water suspensions SHALL be carefully controlled to prevent
corrosion and provide wetting of ferromagnetic aerospace components. This
requires weekly monitoring of corrosion inhibitor and wetting agent
concentrations.
3.3.6.1
Water Suspensions.
Wetting agents and corrosion inhibitors SHALL be used with water suspensions. Usually the magnetic
particle concentrates provided include the correct amount of wetting agent and corrosion inhibitor for initial
use. However, these materials are available separately so the concentrations can be maintained or adjusted
to suit the particular conditions. If no corrosion can be tolerated, a higher concentration of corrosion inhibitor
is used. Acidity should be checked weekly. The pH of the water bath shall be 6 to 10. If the parts being
inspected have a residual solvent film, more wetting agent is required so the parts’ surfaces will be
completely wetted. Breaking of the bath into rivulets as it is applied over a part is an indication of a need for
additional wetting agent or part cleaning. A water break test shall be conducted daily using a clean specimen
or part having the smoothest surface finish to be inspected. The specimen should be f looded with the bath
and examined once f looding is stopped. If a smooth continuous film of bath forms over the entire surface,
sufficient wetting agent is present. If the film breaks, wetting agent is inadequate if the part is not clean.
Before adding wetting agent make certain the specimen or part is clean. Reference shall be made to the
manufacturer’s recommendations for the correct quantity of wetting agent to be added.
3.3.6.2
Petroleum Distillate Suspensions.
No additives other than the magnetic particles themselves are used with petroleum distillate suspensions.
Petroleum distillate recommendations are included in other publications or specifications.
3.3.7
Methods of Particle Application.
3.3.7.1
Dry Particles.
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 part being inspected; this is usually accomplished by using a
combination of bulb squeezing and tossing of the powder toward the area being inspected.
3.3.7.2
Wet Particles.
Many techniques are used to apply liquid suspension magnetic particles. They range from simple hand
pouring of the suspension onto a part to large industrial systems in which the suspension is applied
automatically, either by dumping or spraying. The most common technique for application is through the use
of a hand-held nozzle and recirculating pump on stationary units. Occasionally small, hand-held, lever-
operated sprayers are used. Aerosol-type containers similar to those used for spray painting are also
available.
3.3.8
Techniques for Current/Particle Application.
Two processing techniques are used in magnetic particle inspection. The approach to use in a given case
depends upon the magnetic retentivity of the part being inspected and the desired sensitivity of the
inspection to be made. Highly retentive parts may be inspected using what is called the residual technique.
The other technique, continuous, must be used on parts having low retentivity. For a given magnetizing
current or applied magnetizing field, the continuous approach offers the greatest sensitivity for revealing
discontinuities.
3.3.8.1
Residual Technique.
In the residual inspection technique, magnetic particles, either as a dry powder or in a liquid suspension, are
applied after the parts have been magnetized. This technique is used only when parts are magnetized with
DC and when parts have sufficient retentivity to form and retain adequate magnetic particle indications at
discontinuities. This technique can be used with both longitudinal and circular magnetization with either
direct contact or central conductor application. Usually it is limited to the search for discontinuities open to
the surface such as cracks. Residual inspection permits the magnetizing of parts at one time and the
application of magnetic particle media at some subsequent convenient time. When a central conductor is
used, inspection of holes or bores is facilitated since inspection takes place after removal of the central
conductor.
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