In 1995, The Society for Protective Coatings (SSPC) published a paper that concluded, “It is obvious that ASTM-B117 Salt Spray (Fog) Testing and similar methods offered a rapid but unreliable means for predicting coating behavior.” For the 20+ years since that paper was published, the ASTM B-117 salt spray procedure has continued to be tested for correlation to real-world environments. Each time, the evaluation produced the same results – the correlation coefficient of ASTM B-117 to real-world exposure is around 0.11. A perfect correlation coefficient is 1, which means that the number correlates 100% of the time. A correlation coefficient of 0.11, such as the results of the ASTM B-117 test, is very close to a pure random number – meaning there is almost no correlation. In 2009, a company that manufactures environmental chambers, including one of the salt spray test chambers used in the B-117 test, published a paper compiling industry research studying the correlations of rapid environmental testing to real world exposure. This company also summarized that the B-117 correlation with severe marine environments was around 0.11, while a combined corrosion/weathering cycle test has a 0.71 correlation with severe marine environments.
The problem with the ASTM B-117 Salt Spray Test Procedure
The ASTM B-117 salt spray test uses a closed testing chamber in which a salt water (5% NaCl) solution is atomized by means of spray nozzles using pressurized air. The test subject is continuously exposed to the same corrosive atmosphere inside the chamber, then evaluated after removal from the environment. The test was originally developed in 1939, and the only updates to this procedure have been to increase the duration of the test. In an effort to demonstrate the effectiveness of the coatings being tested, some tests now run for 15,000 – 20,000 hours, which is the equivalent of 20+ months of continuous exposure to salt spray. While this high level of exposure might seem like a reasonable way to test durability, there is unfortunately no location on earth with the same weather conditions for months on end. Temperatures fluctuate, humidity changes, the sun shines, and the chemicals in the atmosphere will vary depending on wind, nearby equipment, and a host of other factors. ASTM’s notes on the significance and use of the B-117 test states, “Prediction of performance in natural environments has seldom been correlated with salt spray results when used as stand alone data.” Even without statistically valid test data, it is easy to understand why the static conditions of a B-117 test cannot possibly provide a result that can correlate with an actual marine environment. Marine environments are simply not static.
Why ASTM B-117 continues to be used, and a better alternative
B-117 is well-known, rather inexpensive, easy to perform, and therefore – entrenched in multiple industries, including HVAC-R protective coil coatings. Fortunately, the industry has responded to the obvious deficiencies in the B-117 test by developing new procedures that more closely mimic real-world environmental exposure. Cyclic corrosion tests such as ISO 12944-9 (formerly ISO 20340) have been adopted as the pre-qualification offshore performance standard for barrier coatings that may be broken down due to long-term exposure to sunlight, moisture, sea water spray, and wind chills. A study by NACE Corrosion focused on combined corrosion/weathering cycles and showed a high real-world atmospheric correlation (0.79) when compared to ASTM B-117 value of 0.11. In another study, SSPC analyzed a similar combined corrosion/weathering cycle test and determined that the correlation coefficient of that cyclical test is 0.71. The research results report stated, “It is generally recognized that combined corrosion / weathering is a substantial improvement to the traditional salt spray test (ASTM B-117) in terms of reproducing atmospheric corrosion in an accelerated fashion.”
ISO 12944-9 (formerly ISO 20340) is a much better testing procedure for HVAC-R coatings that will be used in highly corrosive marine environments. The test consists of 25 one-week cycles of varying conditions consisting of:
- 72 hours of UV exposure
- 72 hours of salt spray
- 24 hours of freezing
After 25 weeks, (4,200 hours) the test panels are evaluated for corrosion. A number of research papers have presented a correlation coefficient of at least 0.71 for the cyclic nature of ISO 20340/ISO 12944-9, in which atmospheric conditions change. With a correlation coefficient of at least 0.71, this shows that ISO 20340/12944-9 is more statistically valid when compared to the ASTM B-117 at 0.11.
Why Heresite tests against ISO 12944-9 (formerly ISO 20340)
At Heresite, we pride ourselves on the effectiveness of our coatings. Our in-house chemists work to ensure that our coatings provide the best possible protection for the environments in which they are used. While we understand that no single test can perfectly characterize a coating’s ability to perform in a complicated environment, we believe ISO 20340/12944-9 is the best test currently available to help our customers predict coating effectiveness. That’s why our P-413 coating became the first HVAC-R coil/radiator coating to meet the ISO 20340/12944-9 standard.