Orange peel, fisheyes and holidays: the defect vocabulary of coatings
Powder coaters, thermal sprayers and pipeline inspectors share a rich vocabulary for failure, and every word points straight at a cause.
The inspector holds a flashlight flat against the panel so the light rakes across the surface, then starts calling it out. "Orange peel on the doors. Two fish eyes on the rail. And the detector found a holiday on joint 40." To an outsider that sounds like a fruit stand, a fish market and a long weekend. To a coater, each word is a diagnosis with a likely cause attached, and that is why the vocabulary of coating defects is so specific. Name the defect correctly and you are halfway to fixing it.
Surface defects you can see
Orange peel is the dimpled texture of an orange's skin, and on a powder line it is the most common appearance complaint. It usually means the melted powder did not flow out before it gelled, often because the film went on too thick or too thin. Its cousin is picture framing, a darker, heavier border around the edges of a flat panel where the electric field piled extra powder.
A fish eye is a small round crater where the coating pulled away from a spot of oil or silicone, leaving a ring with a dimple in the middle. One fish eye means one fingerprint. A rash of them means something upstream is contaminated: the air line, a lubricant on the conveyor or a spray of silicone across the shop.
Then come the lumps. Seeds are hard bits of gelled powder cured into the film. Trash is lint, dust and debris that landed before the oven. On a thermal spray booth, spitting is a gun shedding blobs of molten buildup into the coating, and striping is a pattern of bands where the passes did not overlap evenly. Two-part epoxies add one more: amine blush, a waxy film that forms when the coating cures cold and damp, and that the next coat will not stick to.
Holes you cannot always see
A pinhole is a tiny hole running through the cured film, often down to bare metal. On a powder line the usual suspect is outgassing: gas escaping from a porous casting or a galvanized part during cure and blowing holes through the melted film. Coaters call the guilty metal gassy, and the cure is a pre-bake to drive the gas out before any powder goes on.
A holiday is any skip, void or pinhole that leaves metal exposed. Some are visible, most are not, so they are hunted with a holiday detector. On thin films that means a wet sponge and a low voltage. On thick pipeline coatings it means a high-voltage spring electrode walked along the pipe, which pipeliners call the jeep. When it sparks, the crew marks a jeep and patches it before the pipe goes in the ditch.
Thermal spray coatings come with holes built in. Porosity is the share of the coating that is voids between the flattened droplets, and some of it is unavoidable. Where it has to be closed, a sealer is soaked in after spraying. Under the microscope, unmelts show up as round grains among the flattened splats, particles that hit the surface without fully melting. The usual causes are powder too coarse for the settings, too little power or injection that keeps particles out of the hot core.
Where the coating will not go
Charged powder does not like inside corners. On the floor, a recess that will not take powder is simply a cage, short for the Faraday cage effect: the electric field crowds onto nearby edges, so the recess stays thin or bare. The fixes are turning down the voltage, changing the gun angle or switching to a tribo gun.
Too much charge causes trouble of its own. Back ionization is a layer so overcharged that tiny discharges inside it repel new powder and pit the surface. And when the hook holding the part is crusted with old cured powder, the charge has nowhere to go: a bad ground, with patchy coverage to match.
Liquid coatings on steel have their own thin spots. Spray runs thin over edges, welds and bolts, which is why painters brush on a stripe coat before the full coat. Inspectors check the result with a gauge, reading DFT in mils at a set pattern of spots.
Coatings that let go
The worst defects are failures of grip. Thermal sprayers say a coating is lifting when it peels up at an edge, often after tape removal or grinding. Bridging is coating that skins across the edge of a mask and tears when the mask comes off. Delamination is a coating separating from the part or from the layer below. Spalling is chunks breaking away, usually after heating and cooling or a hard knock. Mud cracking is the network of cracks like a dried riverbed that appears when an inorganic zinc primer or a waterborne coat goes on too thick or dries too fast.
Most of these trace back to surface prep. Blasting cuts an anchor profile into the steel, a peak-to-valley roughness measured in mils with replica tape. Painters call any texture that gives a coating something to grip tooth, whether it is blast profile, sanding scratches or a primer still in its recoat window. Without it, nothing grips. Invisible soluble salts left on the steel draw water through the film and raise blisters. On a buried pipe with cathodic protection, a holiday can grow into cathodic disbondment, as the alkali produced at the bare spot creeps under the surrounding coating and lifts it.
Adhesion tests have their own verdicts. In a thermal spray bond test, an adhesive failure means the coating let go of the metal. A cohesive failure means it broke within itself. A glue failure means the epoxy holding the test fixture broke first, which is good news for the coating and annoying for the lab.
Too much heat, too little
Powder has to cure, and both directions go wrong. An undercure looks perfect and performs badly, because the film never fully crosslinked. The floor check is a solvent swab, the MEK rub test: if the film softens, smears or leaves far more color on the cloth than a known good part does, the parts go back through. An overbake shows itself as yellowing in whites and light colors, lost gloss and a brittle film. Line workers just say the parts got cooked.
Five defects to learn first
- Orange peel: dimpled texture from poor flow. The most common appearance complaint on powder.
- Fish eye: a round crater around a speck of oil or silicone. Find the contamination.
- Holiday: any bare spot through the coating. Find it with a detector, not your eyes.
- Outgassing: gas from the metal blowing pinholes during cure. Pre-bake the part.
- Cathodic disbondment: coating creeping loose around a holiday on a protected pipe.