Page 254 - Handbook of Adhesives and Sealants

Surfaces and Surface Preparation 223

contaminants. Ultrasonic cleaning is ideal for lightly soiled parts with
intricate shapes, surfaces, and cavities that are not easily cleaned by
spray or immersion techniques. Commercial ultrasonic cleaning units
are available from a number of manufacturers.
Electrolytic cleaning is a modification of alkaline cleaning in which
an electric current is imposed on the part to produce vigorous gassing
on the surface to produce release of solids and contamination. With
anodic cleaning the gas bubbles are oxygen, and with cathodic clean-
ing they are hydrogen. The effect of this cleaning process on metal
surfaces and its influence on long term strength on metal-adhesive
bonding is superior to normal industrial cleaning processes. 22

6.4.2 Passive mechanical treatment
Mechanical methods for surface preparation include abrasive blasting,
wire brushing, and abrasion with sandpaper, emery cloth, or metal
wool. These methods are most effective for removing heavy, loose par-
ticles such as dirt, scale, tarnish, and oxide layers. Cleaning is gen-
erally required both before and after mechanical surface preparation.
The parts should be degreased before abrasive treatment to prevent
contaminants from being rubbed into the surface. Solid particles left
on the surfaces after abrading can be removed by blasts of clean, dry
air and solvent wiping.
Dry abrasion consists of lightly and uniformly sanding the surface
of the substrate material with medium (180–325 grit) abrasive paper.
Composite abrasive materials, such as ‘‘Scotch-Brite ’’ (3M Company),
have proved popular for mechanical surface preparation. 23 These ab-
rasive materials are available in pad and sheet form, and they can
conform easily to the shape of a surface. When combined with water
flushing, they can provide clean almost oxide free surfaces. Hand
sanding, wire brushing, and other methods that are highly related to
the operator’s skill and patience must be carefully controlled. These
methods provide inconsistency and should be used only when no other
method is possible.
Abrasive blasting is generally the preferred method for removing
contamination from most metal surfaces. It is particularly appropriate
for removal of rust, oxide layers, old coatings, and other heavy con-
tamination. Blasting is a fast, efficient, and easily controlled process.
It requires containment of the blast media and the resulting dust. The
abrasive medium must be regularly renewed and/or cleaned to main-
tain efficiency and avoid contamination. Blasting is impractical for
thin or delicate parts because of warping and possible physical damage
to the part. Blasting is also a line-of-sight process, and certain part
geometries may be inappropriate.