the newsletter of the Chemical Coaters Association International.
Reprinted with the kind permission of
By Dave Wright [deceased]
- Mequon, Wisconsin
With sadness we note Dave's passing on Oct. 11, 2013. His longtime friend Anne Goyer offers a tribute to him in The Finishing Touch, Vol. 23 No. 4.
Excerpts from 'The Finishing Touch',
the newsletter of the Chemical Coaters Association International.
Reprinted with the kind permission of
by Dave Wright
Senior Technical Represenative
Texo Corporation
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.
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