Salt Mist, IP66, and the Coating Spec
The first conformal coat passed lab spray tests and failed in service. The second passed in service and failed thermal cycling. The third one stuck.
A vehicle deck is not a chemistry lab. It is salt mist plus diesel residue plus brake dust plus thermal cycling, and a coating that passes one of those tests on a bench can fail the combined exposure in months. We learned this the slow way.
The three iterations
v1 — acrylic conformal
Passed salt-spray (ASTM B117) in the lab. In service, brake dust adhered to the surface and trapped moisture. Boards started showing leakage current within four months.
v2 — urethane conformal
Survived the brake-dust problem and the salt-mist problem. Failed thermal cycling at the connector joint over the first 30-day hot-route sailing — coating cracked, moisture wicked.
v3 — parylene + selective urethane
Conformal parylene under selective urethane at connector boundaries. Survived 14 months of pilot operation with zero coating-attributable failures.
Sources
- ASTM B117 — standard practice for operating salt-spray (fog) apparatus.
- IEC 60068-2-11 and IEC 60068-2-52 — environmental testing: salt mist and cyclic salt-mist tests.
- IPC-CC-830 — qualification and performance of electrical insulating conformal coatings.
- IEC 60945 — general requirements for shipborne marine equipment (environmental and durability).
- [VERIFY: 14-month coating service-life results are RoRoSafe pilot-operation data, not third-party qualified.]
Questions, answered
Why did the conformal coating spec change twice?+
Because a vehicle deck is salt mist plus diesel residue plus brake dust plus thermal cycling, and a coating passing one bench test can fail the combined exposure in months. v1 acrylic passed salt-spray but trapped brake dust and leaked within four months; v2 urethane cracked at connector joints under thermal cycling; v3 parylene with selective urethane survived 14 months with zero coating failures.
Why isn't a lab salt-spray pass enough to qualify a coating?+
Because lab tests isolate single stressors. The acrylic v1 coating passed ASTM B117 salt-spray cleanly, then failed in service when brake dust adhered and trapped moisture — an interaction the lab test never reproduced. Coating spec is set by field data, with iteration cycles measured in months.
What coating finally worked for marine deck service?+
Conformal parylene under selective urethane at the connector boundaries. It survived 14 months of pilot operation with zero coating-attributable failures, after the earlier acrylic and urethane revisions each failed a different combined-exposure mode in service.
Continue the thread
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