CoatingsPro Magazine

SEP 2018

CoatingsPro offers an in-depth look at coatings based on case studies, successful business operation, new products, industry news, and the safe and profitable use of coatings and equipment.

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70 SEPTEMBER 2018 COATINGSPROMAG.COM Step 6: Examine the actual coatings involved W hile the lining samples were being analyzed in the laboratory, I physically examined the as-applied condition of the linings inside a sample of the hopper cars. is is what I found: • The overall condition of the linings (visual and dry film thick- ness) was in spec. • The adhesion pull-off readings were consistently above 1,000 psi (6.9 MPa). Per ASTM D4541: Standard Test Method for Pull-Off Strength of Coatings Using Portable Adhesion Testers, I did not scribe around the dollies prior to testing. • By lightly tapping on the topcoat with a painter's 5-in-1 tool, the topcoat delaminated (in spots) cleanly from the prime coat. • Consistent with the lab findings, I found no evidence of contamina- tion on the backside of the failed topcoat chips. Step 7: Survey the literature My colleagues and I performed a study of available literature to determine whether this delamina- tion failure morphology had been reported. Our conclusion was that none of the failures reported in the literature by other shops, contrac- tors, or curing oven manufacturers applied to our situation. Step 8: Hypothesize the failure mechanism I was initially baffled by this failure. Nothing in the computer records, NACE inspector records, or laboratory analyses gave me any infor- mation except what was not wrong. To clear my head, I physically walked down the entire surface preparation and lining process. During the walk, I noted that the heat-curing ovens used direct-fired propane heaters, which is unusual. is means that heated air and exhaust gasses are introduced directly into the space to be heated. Construction propane heaters are normally indirect-fired heaters, where exhaust gasses are not intro- duced into the space to be heated. I recalled that burning 1 pound (0.5 kg) of propane produces 4 pounds (1.8 kg) of water vapor. I then referred to my psychrometric chart and deter- mined that, even though the steel components inside the hoppers were maintained at 130 °F (54 °C), the steel was actually below the dew point (the temperature at which moisture condenses out of the air), meaning that moisture had condensed on the bottom sloped plates of each hopper car and had resulted in spotty delam- ination of the topcoat from the prime coat. I had violated one of my rules and assumed that 130 °F (54 °C) steel temperature logically could not be below the dew point. Shame on me! I also recognized that the high (1,000+ psi, 6.9 MPa) pull-off adhesion tests I had performed resulted from the high cohesive strength of the topcoat material and not from the adhesion of the topcoat to the prime coat. Had I scribed around the dolly before pull-off testing, I might have detected the adhesion problems sooner. Step 9: Observe reconstruction and testing As with the old saying, "e proof is in the pudding," the facility subse- quently changed their curing oven heaters to indirect-fired heaters. e change from direct-fired curing oven heaters to indirect-fired heaters dramatically reduced the water vapor content of the air in the ovens and thus eliminated the surface moisture problems. e problem has not, to my knowledge, occurred again. Key Takeaways Regardless of whether you're working in the field or in a facility, there are a few key lessons learned from this near-miss of a failure investigation. e first, of course, is that even a seasoned profes- sional can overlook particulars. e key is to continue to persist until you find the actual root cause(s) of the problem. Additionally, the plant should have researched the effect that the use of direct-fired space heaters versus indirect-fired space heaters would have on the application conditions inside the curing ovens. Remember that this type of problem could have occurred in either shop or field work. CP Jon R. Cavallo, PE, FASTM, is a registered professional engineer in three states and holds a Bachelor of Science Degree from Northeastern Engineering in Boston, Massachusetts. Active on many national technical societies, including SSPC, NACE, and ASTM, Cavallo received the ASTM Award of Merit in 2010 and is an ASTM Fellow. He has worked in the coatings and corrosion mitigation field for 40 years. For more information, contact: Jon R . Cavallo, PE, FASTM, (603) 431-1919, Railcar Lining Delamination During the examination of the actual coatings involved, the topcoat delaminated (in spots) from the prime coat when tapped with a painter's 5-in-1 tool.

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