comcom
by Dave Wright
Senior Technical Represenative
Texo Corporation
Send your Questions to davewrit@execpc.com
Q: My salt spray is not what it should be. We have increased our
iron phosphate coating weight from approximately 20 milligrams per
square foot to over 50 and have tried a number of "non-chrome" seals.
We are getting 144 hours. We want 500 hours. What avenue should we go
down to improve our salt spray? G.L. , Minneapolis, MN.
A: This is not an easy question, and will take more time to answer
(if I can) than most. First of all, you must take a look at your
needs versus wants. What you didn't mention is your systems field
performance. Are you getting a lot of field failures? A salt spray of
144 may not be bad for your purpose / product. Building more into the
product than it needs may be a waste of effort and money.
You need to understand some of the strengths and weaknesses of salt
spray to really determine your needs. Salt spray as a predictor of a
pretreatment or finish systems performance seems here to stay. This
is unfortunate, since a given corrosion prevention system (substrate
/ pretreatment / finish) may perform perfectly in it's intended
environment, yet perform poorly in salt spray.
A classic example of this unexpected behavior is presented by the
case of galvanized steel. Hot dipped and electrogalvanized steels are
used heavily in the automobile and appliance industries, and with
good reason. This system provides the best corrosion performance
available short of stainless steel. However, in salt spray, painted
galvanized steel performs very poorly. This is because the
accelerated corrosion environment found in a salt spray cabinet is
ideally suited to reacting away the zinc in the galvanized coating,
yielding premature failure. The salt spray corrosion mechanism bears
little resemblance to real-world observed corrosion behavior of
painted galvanized steels, now the most dominant material used in
automotive body skins.
The salt spray test has been the traditional laboratory method for
evaluating the corrosion resistance of metals protected by organic
coatings. In recent years, however, there has been increasing
acknowledgment, particularly among automotive companies, that the
salt spray test alone does not accurately predict the performance of
corrosion-resistance systems in actual service conditions. In a salt
spray cabinet, samples are exposed continuously to a five percent
solution of sodium chloride. But in service, painted metal products
are exposed to a wide range of humidity, temperature, and ionic
contaminants. This wide range of possible exposure conditions causes
corrosion mechanisms to perform well in service, which do not perform
in the salt spray test.
This lack of known correlation between salt spray corrosion
mechanisms, and corrosion known to occur in service, is the principal
weakness of the salt spray test. Put more simply, excellent
performance in salt spray does not necessarily guarantee excellent
performance in service.
One of the principal strengths of salt spray testing is the speed
with which useful corrosion data can be obtained. Indeed, the most
time required to complete a test is about six weeks, and the least
time can be as little as a few hours. Compare this with alternative
tests like field exposure (years to get useful data) and various
cyclic corrosion tests (months), and it can be concluded that the
efficiency of salt spray in inducing and accelerating electrolytic
corrosion is the major reason for its continuing popularity.
One of the myths often propagated about salt spray testing is that it
is a useful technique for comparing performance of candidate
pretreatments, paints, and substrates. The truth is that the data
provided by such comparison testing is useful only for predicting
corrosion performance of candidate systems in a salt spray cabinet!
The data is generally useless for ranking corrosion performance of
candidate systems in field service. The reason, as stated above, is
that excellent performance in salt spray does not necessarily
guarantee excellent performance in service. The chemical corrosion
mechanisms are vastly different. The use of salt spray testing alone
to select components of a corrosion-resistant system often leads to
misleading data and incorrect selection.
If you really do need to improve your corrosion resistance, I will
lay out the factors in order of importance that influence salt spray
performance.
#1. By far the most influential factor in salt spray performance is
the composition and thickness of the organic coating. This is the
first barrier to corrosion. The extent to which this layer resists
permeation of moisture, will make it more salt spray resistant. A
TGIC polyester powder coating is (generally) going to perform better
than a polyester solvent spray paint. An epoxy electrocoat is going
to vastly outperform an acrylic electrocoat. Cathodic electrocoat is
going to outperform anodic electrocoat. It all has to do with the
ability of these films to resist permeation of water.
#2. The next most influential factor is the presence or absence of a
primer layer. Obviously, a chromate bearing epoxy primer is an
example of a preferred material. These are often omitted by
manufacturers in an effort to trim costs. Simply adding a primer
layer can often double the corrosion resistance in salt spray of a
given system. Field service is often improved even more.
#3. After the consideration of the paint film(s), the next strongest
influence in salt spray performance is the presence or absence of a
pretreatment conversion coating. In general, zinc will perform better
than iron. With iron phosphate, there has been very little
correlation observed between the coating weight of an phosphate and
the systems performance. Let me say this again. In general, efforts
to improve salt spray resistance by simply increasing iron phosphate
coating weight will not be successful. The reason is that the
pretreatment is rapidly consumed during the salt spray corrosion
mechanism, regardless of it's coating weight. Remember, we are
talking about salt spray performance only. Field corrosion experience
tells us that heavier coating weights generally do provide somewhat
better field corrosion performance.
#4. Next in importance is the final rinse or "seal". Mixtures of
hexavalent and trivalent chromium in acid solutions are preferred.
"Non-chrome seals" are available that can improve performance over
straight water final rinses. But low solids water (D.I. or R.O.) can
also be a benefit without the cost or control issues of other
chemistries. Rinsing off as many of the "salts" that will cause paint
film failures when the film is permeated by moisture can do quite a
bit in itself.
Not surprisingly, the automotive companies have been the largest
driving force in developing alternatives to salt spray testing for
predicting field exposure performance. Procedures like the GM Scab
Test, Volvo Test, and Nissan Cyclic Corrosion Test have been
developed to more closely simulate field corrosion mechanisms in an
accelerated environment. All such tests use a cyclic change to the
sample's environment: Cold to hot, wet to dry, presence and absence
of various ionic components (not just chloride) in an effort to
duplicate the corrosion products on field-exposed samples. Their
efforts have met with varying degrees of success, and these tests are
now standards in the automotive industry. The tests suffer from a
time constraint, however. It was discovered that the more closely a
given accelerated test resembled the corrosion performance observed
in the field, the longer the test took to yield useful data. About
the best level of acceleration (one year in the test correlates to
five years of field exposure) which has been achieved is in the GM
Scab test, which generally yields a factor of five acceleration (one
year of test correlates to five years of field exposure).
The appliance industry and many of our other industries have been
slow to adopt alternatives to salt spray testing. Humidity testing is
the most often seen alternative, but it is generally not quick enough
to yield results. Several new cyclic corrosion tests are being tried,
with varying results (i.e. Prohesion).
The bottom line is to really evaluate your need. If you are seeing
field failures or warranty returns (or any other method of tracking
your performance) you may have a need to increase your systems
performance. But don't take let salt spray alone drive you to making
potentially unnecessary, costly changes.
*A special thanks to David B. Chalk Ph.D. for his input on this
subject.