Porosity
Porosity turns up in 2 fields: thermal spray, welding. Here is what it means to the people in each one.
Porosity in thermal spray
The fraction of a coating's volume made of voids between and within splats, usually reported in percent.
Voids from incomplete filling between splats, gas trapped during solidification, shrinkage and unmelted particles. It is measured by image analysis of a polished cross section, typically giving under 1 percent for HVOF carbides, a few percent for good plasma metals, 10 to 20 percent for thermal barriers and 20 to 35 percent for porous ceramic abradables. Connected porosity lets corrosive fluids reach the substrate, which is why coatings are sealed.
"Porosity on the mount is 4 percent, spec max is 2, so that's a coupon fail on the whole lot."
Porosity in welding
Gas bubbles trapped in weld metal as it freezes, showing as round holes on the surface or on film.
Cavities left by gas that could not escape before the weld solidified. Common causes are lost shielding from wind, leaks or a spatter-clogged nozzle, damp electrodes or flux, and oil, paint, rust or galvanizing on the joint, plus an overly long arc. Codes classify scattered, cluster and linear porosity and limit the size and amount, with elongated piping porosity treated more strictly.
"Surface porosity all along that bead, check your gas, I bet the fan's blowing it away."