Outgassing
Outgassing turns up in 3 fields: powder coating, waterproofing, vacuum. Here is what it means to the people in each one.
Outgassing in powder coating
Gas escaping from a porous substrate during cure and blowing pinholes, bubbles or craters through the film.
Air and moisture trapped in cast aluminum, sand castings, hot-dip galvanizing, welds and some extrusions expand as the part heats and push up through the melting powder. If the film gels first the gas leaves pinholes, bubbles or volcano craters. Remedies are pre-baking, coating parts warm, using degassing-additive powders, slowing the ramp and keeping film build down.
"Every casting is pinholed, that's outgassing, they need a pre-bake before the booth."
Outgassing in waterproofing
Gas escaping into curing sealant or membrane from punctured rod or damp concrete, leaving bubbles and pinholes.
Punctured closed-cell backer rod releases gas that pushes up bubbles and blisters through a curing sealant bead. Damp or warming concrete pushes air and vapor up through fluid-applied membranes, causing pinholes. Prevention is careful rod installation, using open-cell or bi-cellular rod where needed, applying membranes when the substrate is cooling rather than heating, and filling bugholes.
"Bubbles in the urethane every couple of feet, they jabbed the rod in with a screwdriver, that's outgassing."
Outgassing in vacuum
Slow release of gas from the surfaces and bulk of materials inside a vacuum system.
Gas evolving from inside the system itself: water and other molecules desorbing from surfaces, gas diffusing out of the bulk (hydrogen from stainless steel), and vapour from plastics, adhesives and lubricants. Rates are quoted per unit area in mbar·L/(s·cm²); unbaked stainless steel gives off around 10⁻⁹ after about ten hours of pumping, falling roughly in proportion to one over time. Outgassing, not leaks, sets the base pressure of most high vacuum systems.
"Pressure isn't a leak, it's outgassing from that new cable bundle, it'll come down if we bake it."