Note: A convincing argument
can be made that the second-best coating over the best surface
preparation will perform better than the best coating over the
second-best surface preparation.
There are many ways to clean
a part, each with advantages and disadvantages. Some techniques
go beyond simple cleaning and create surface "structures"
that enable a coating to adhere better to the part.
In many instances, it is desirable
to use a combination of cleaning methods to achieve optimum
adhesion. The more common cleaning methods are listed below.
Vapor degreasing
used to be the most widely used cleaning technique, but fell
into disfavor because of regulatory restrictions on the use
of cleaning solvents.
Where permitted, degreasing
remains an excellent technique for removing a wide variety of
foreign material, from fingerprints to machine oils. It is an
economical technique for cleaning small batches of parts. Avoid
using it on parts that may be attacked by the solvent such as
plastics, composite parts or metal parts with organic inserts.
Grit blasting
(or tumble blasting) with aluminum oxide or other abrasive particles
is another common cleaning technique, preferred for parts whose
surface contaminants — rust, scale, corrosion, old coatings
— must be attacked physically to be removed. It is not,
however, the most effective technique for removing oily or fluid
contaminants.
When parts are particularly
oily, alkaline cleaning or prebaking them before blasting will
improve the effectiveness of the blast and reduce contamination
of the blast medium.
Grit blasting does more than
clean: it roughens the surface and enhances mechanical adhesion
by increasing the surface area to which the coating can cling.
Whitford recommends a grit medium
from 250 to 125 microns/60 to 120 mesh for blasting the surface
of most metal parts. (Note: The particle sizes quoted above
run from larger to smaller in both cases). Steel grit is generally
avoided because minute particles are sometimes left behind and
become starting points for oxidation.
Shot blasting
is similar to grit blasting, but employs metal or other "shot"
as the blast medium. For parts which will be used in fatigue/fretting
applications, this process can be beneficial because it imparts
residual compressive stresses on the surface of the parts, thus
lengthening their lives under cyclic loads.
Tumble blasting
is another variation in which parts — usually small parts
— are placed into a cylinder in which an abrasive medium
is blown against the surfaces. The effect varies with the medium
employed, but is much the same as grit blasting. Also, this
technique is less effective than fluid cleaners for removing
machine oils and other similar contaminants.
Alkaline washing
involves cleaning parts with neutral, moderate or high pH cleaners.
This is preferred for high volumes of parts and is generally
as effective as vapor degreasing. Parts which should not be
washed by this method are those which may be adversely affected
by the chemistry involved (such as aluminum and magnesium).
Acid or alkaline etching
is an excellent technique both for cleaning and roughening the
surface of aluminum parts. Because considerable equipment is
required, it is usually reserved for high-volume production
parts.
Pickling is
common for removing rust/scale from ferrous parts after cleaning.
It should not be used on parts that will be highly loaded, since
it can cause hydrogen embrittlement.
Phosphating
is a secondary surface preparation for steel which is generally
used after vapor degreasing, alkaline washing or grit blasting.
Whitford normally recommends a modified zinc phosphate with
a fine crystalline structure such as Aerocote (made by Aerocote
Corporation, Houston, TX).
A thin layer (7.5 - 15 grm/m²)
of zinc phosphate is deposited on the surface of the component
to promote better adhesion and dramatically increase corrosion
resistance and chemical protection. Another good alternative:
Xylan P-501 Primer, a coating that outperforms conventional
phosphating.
Iron phosphate tends to be less
expensive than zinc, although zinc phosphate has superior corrosion
resistance and better protection from corrosive creep. Whitford
prefers zinc phosphate. Note: Manganese phosphate has even better
corrosion resistance, but requires a thicker film to cover the
coarser crystalline structure.
Conversion coating/anodizing:
This serves a similar function for aluminum parts, but produces
a harder surface than phosphating. It creates a porous, corrosion-resistant
surface that is excellent for coating, provided it has not been
sealed. Specially formulated phosphates are available for use
on aluminum.
Other factors
Combinations of pretreatments are required
for the best overall coating adhesion and corrosion protection.
For common substrates, industry practice has been to use the
following pretreatments or combinations:
