Finding Correlations to Salt Spray and UV Protection Hours
Bob Hohn with Assured Testing Services says it is often difficult to equate test duration to actual years of field service.
Q. We manufacture and paint equipment that is used outdoors, and are frequently asked by customers to provide “real-world” correlation to the salt spray and UV protection hours we provide for our products. However, our paint suppliers and applicators claim they cannot provide valid correlation. Is there a real-world duration correlation to these types of exposures?”
A. This is a legitimate and frequently asked question. Unfortunately, your suppliers are correct in asserting that there is not a universally accurate correspondence between accelerated exposure tests and results in the field. There are many complications that make this comparison difficult.
Tests such as ASTM B117 and ISO 9227 salt spray, ASTM G154 UV weathering exposure, and SAE J2334 cyclic corrosion are meant to accelerate the degradation of paints and platings, and the subsequent corrosion of the substrate. They are used so that we can choose, with reasonable certainty, the most effective coating system (including pretreatment) for a given application without having to wait for years by actually exposing samples to real-world atmospheric conditions. By exposing sample panels or actual parts with different coating systems to exactly the same controlled corrosive conditions in a laboratory, we can comparatively judge which coating system will likely provide the best protection. In addition to testing several coatings/coating processes, it is helpful to include samples from a competitor or one that has an extensive history in the field for benchmarking. Comparative testing can be a valuable tool for product engineers when choosing the optimal coating system.
However, it is very difficult to equate test duration to actual years of field service, if you desire to provide a warranty or guarantee. One main reason for this is due to the extreme variability of real-world atmospheric exposure conditions. Here is a partial list of considerations:
- Geographical latitude — there are more hours and intensity of sunlight closer to the equator.
- Effect of salt due to proximity to ocean breezes or exposure to deicing road salts (for example, Arizona vs. coastal South Carolina).
- Weather patterns, such as annual rainfall, freeze/thaw cycles, wind-driven abrasion, sand and more.
- Exposure to airborne corrosive chemicals — acid rain, industrial pollution.
- Microlocation — shade vs. open exposure.
In addition, there are considerations related to the actual parts or assemblies that can affect how lab tested samples compare to real-world conditions:
- Geometry of the part (recesses, blind holes and more).
- Substrate or substrates, including dissimilar metals or galvanic reaction (heat exchangers).
- Mechanical damage to the coating during transit or installation (installation of hardware for photovoltaic cell mounting).
Finally, most exposure test standards only address one main component of degradation, such as salt (ASTM B117, ISO 9227), UV from sunlight (ASTM G154) or humidity (ASTM D1735) when actual exposure is a combination of degradations. Cyclic corrosion tests such as SAE J2334 attempt to address this by cycling electrolyte (salt) exposure with drydown and humidity exposure. This more closely mimics real-world conditions than does ASTM B117 salt spray, but still does not feature all degradation factors. ASTM D5894 takes this concept one step further by adding alternating (weekly) UV exposure with cyclic salt spray and probably provides as realistic results as economically possible. However, there are degrading environmental conditions such as freeze/thaw that even ASTM D5894 does not include; ISO 12944-9 does combine UV, salt spray and freezing, and is a good alternative for product that will be exposed to low temperature.
Automotive original equipment manufacturers (OEMs) and their coating suppliers have parallel tested coatings on actual in-service vehicles for extended periods with standardized accelerated laboratory tests. This is done to correlate estimated years in actual service to hours in the chamber, and is typically location specific (South Florida, Nova Scotia, and more). They have generally agreed that cyclic corrosion testing yields closer parallels to real-world conditions than salt spray, and have “rules of thumb” for the cycle/years of field service correlations. It is interesting to note that with some standards, such as GMW14872 and GMW15272, these approximate cycle/years of field service correlations are tied to the location of the component on the vehicle. For example, underbody components (possibly exposed to deicing salts) will require longer cyclic testing duration than underhood components to achieve the same estimated years of field service due to the expected conditions that these components will receive.
In addition to the above complications, choosing the correct exposure test and definition of what constitutes a failure is an important part of the equation. For example, epoxy paints provide excellent chemical and corrosion resistance, but are notorious for chalking and fading when exposed to sunlight. If aesthetics are not important for a product that is likely to be used in a marine environment, then UV or Xenon exposure with gloss and Delta E color evaluations would not make sense. In this case, traditional ASTM B117 salt spray or a cyclic test with salt exposure followed by post-exposure rust evaluation would be more logical. And, even though it helps to properly choose the correct testing standard and subsequent evaluation, it still does not solve the problem of converting accelerated testing duration into real-world years of service. It can, however, ensure that comparative testing will provide the most relevant data.
Hopefully, this will help you explain to your customers why it is so difficult to accurately correlate accelerated exposure testing to real-world conditions. It can be a slippery slope to guarantee such coating characteristics (such as corrosion resistance, color retention or gloss retention) for a given period of time based on accelerated atmospheric exposure testing. However, you can assure your clientele that you have performed comparative testing and have chosen the optimal coating system for your product and its application.
Bob Hohn is sales manager at Assured Testing Services. Visit assuredtestingservices.com.
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