Salt spray testing is a laboratory simulation of a corrosive saline environment. It is used as an accelerated means of testing the ability of surface coatings to withstand atmospheric corrosion. Also known as salt fog testing, it is one of the oldest testing methods used in the HVAC-R coatings industry, and for this reason, is often referred to when selecting HVAC-R coatings.
Hersite Protective Coatings uses salt spray testing in two ways as we develop and evaluate our HVAC-R and radiator coatings. Salt spray testing is used as a stand-alone corrosion resistance test and also as a component of ISO 12944-9 (previously known as ISO 20340). For both tests, the salt spray process is handled the same way, conducted per ASTM B-117.
- First, small panels (typically 3×5 inches or 4×6 inches) are prepared with the coating to be tested. The edges and the backs of the panels are protected, typically with tape. Then the front of the panel is “scribed” with a sharp tool to produce a scratch through the coating to the metal substrate.
- After the panel is prepared, it is placed inside the test cabinet. The cabinet contains racks to hold the panels in position as specified by ASTM B-117 – tilted back 15 to 30 degrees from the vertical.
- The cabinet is then closed, and a 5% salt solution is delivered at a constant rate and temperature into the cabinet through an atomizer. Salt fog is sprayed upwards so that droplets will descend and settle on the panels.
- After this point, there will be periodic checks of the test panels, as well as monitoring of the salt spray cabinet to ensure that it functions properly throughout the duration of the test. During these brief evaluations of the panels, it is determined whether the test is progressing as expected or if the coating on the test panels is failing prematurely.
Although each laboratory has a slightly different set up, the basics of the test are as described above. Heresite is fortunate to have a cabinet with a digital readout displaying the total hours of test cabinet operation, as well as alarms that will notify us about a variety of important things, such as if the salt solution level becomes low. These features are important because some salt spray tests run for extremely long times. For example, Heresite P-413 was tested against ASTM B-117 for 6,000+ hours. That’s 250 straight days (or a little over 8 months) of continuous salt spray.
Although both ASTM B-117 and ISO 12944-9 are used to measure corrosion protection, there are some significant differences between the two tests. Unlike B-117, which is a static salt spray only test, ISO 12944-9 is a cyclic test, where the panels are tested for 72 hours of UV exposure, followed by 72 hours of salt spray, then 24 hours of freezing (for a total of 168 hours / one week. That cycle (UV / salt spray / freezing) is repeated for 25 weeks, for a total of 4,200 hours. Although neither test is an actual outdoor environment, ISO 12944-9 is much more like the extreme weather cycles that HVAC-R units are often exposed to in the real world. For this reason, SSPC (The Society for Protective Coatings) has concluded that “combined corrosion / weathering is a substantial improvement to the traditional salt spray test (ASTM B117) in terms of reproducing atmospheric corrosion in an accelerated fashion.” You can learn more about the differences between these two tests and why we test against ISO 12944-9 by reading Studies show ASTM-B117 salt spray test does not correlate to real-world environments.
Once the required amount of exposure is achieved for either ASTM B-117 or ISO 12944-9, the lab will evaluate the panel for degree of blistering, degree of rusting, and scribe creepage from the scribe. The results are recorded and are shared on our Technical Data Sheets. The test hours reported on our Technical Data Sheets are based on these results. You can find links to each Heresite Technical Data Sheet on the web page for that particular coating. Still have questions? You can contact us or give us a call at 920-684-6646. We want you to select the right coating for your application and we are here to help.