Accelerated Tests vs. Service Life

Q. We manufacture coated pipe products, and I’ve been asked to come up with a guaranteed lifetime period for a coated pipe. This is in regards to a  “Single Wall Steel Tube” pipe having a coating that is 25 microns thick. The coating withstood the CASS test for 60 hrs. I would like to know how many years it will with stand up in the field. C.K.

A. CASS is the ASTM B 368 Copper Accelerated Acetic Acid Salt Spray (fog) Test. The B-368 CASS test is more severe than the B 117 Salt Spray (Fog) test.

Unfortunately I am neither a magician nor a fortune-teller. I have no clue as to how long you can guarantee your coated pipe to last in real world exposure. I was my company’s paint consultant for 40 years and ran all the available accelerated tests in my laboratory. We even had a test fence on the roof on one of the buildings. I can’t tell you how many times other scientists and engineers would ask me how many years of outdoor exposure is equal to X hrs in the salt spray or Y hrs in the Weather-o-meter. My answer always was X hrs in salt spray equals X hrs in salt spray and Y hrs in the Weather-o-meter is equal to Y hrs in the Weather-o-meter. These accelerated tests were designed as research tools to make comparisons between coatings. They are not longevity indicators. They are also used at the plant level for quality control. Be very careful with your guarantees. Accelerated testing can be deceiving. I can only tell you that 60 hrs in CASS equals 60 hrs in CASS and 40 hrs in CASS equals 40 hrs in CASS. Incidentally, if the pipe is to be used underground, you should also run the so-called crock test where a voltage is impressed across the coating.