T.O. 33B-1-1
3-65
3.5.6
Wet Visible Particles.
CAUTION
The wet visible method SHALL NOT be used on aerospace vehicles or aerospace
vehicle parts without specific approval of the appropriate engineering authority for
the individual inspection requirements.
3.5.6.1
General.
Wet method magnetic particles are fundamentally similar to each other, once they are dispersed in the suspending
liquid. In past years, the most common form of the material concentrate was a paste. Today, however, the pastes have
been almost exclusively reformulated and produced as dry powder concentrates. These powders incorporate the needed
materials for dispersion, wetting, corrosion inhibition, etc. The powders are much easier to use, as they need merely to
be measured out and added directly to the agitated bath. The agitation system of the modern magnetic particle units
will pick up the powder and quickly disperse it in the bath.
3.5.6.2
Advantages and Limitations.
As is true of every process, the wet method has both good points as well as less favorable characteristics. The more
important good points of the wet method, which constitute the reason for its extensive use, as well as the less attractive
characteristics are tabulated as follows:
a. It is the more sensitive method for very fine surface cracks.
b. It is the more sensitive method for very shallow and fine surface cracks. It quickly and thoroughly
covers all surfaces of irregularly shaped parts, large or small, with magnetic particles.
c. It is the faster and more thorough method for testing large numbers of small parts. The magnetic
particles have excellent mobility in liquid suspension.
d. It is easy to measure and control the concentration of particles in the bath, which makes for uniformity
and accurate reproducibility of results.
e. It is easy to recover and re-use the bath.
f. It is well adapted to the short, timed shot technique of magnetization for the continuous method. It is
readily adaptable to automatic unit operation.
g. It is not usually capable of finding smaller defects lying wholly below the surface if more than a few
thousandths of an inch deep.
h. It is messy to work with, especially when used for the expendable technique, and in field testing. A
recirculating system is required to keep the particles in suspension.
i. It sometimes presents a post-inspection cleaning problem to remove magnetic particles clinging to the
surface.
3.5.6.3
Particle Selection by Visibility and Contrast.
The need to meet a variety of conditions for successful magnetic particle testing has resulted in the development of
different materials to obtain this result. The most commonly used materials, black and red, oil and water suspendible
are listed below with the special characteristics of each.
