Scale
Scale turns up in 2 fields: heat treating, boilers. Here is what it means to the people in each one.
Scale in heat treating
The oxide crust that forms on steel heated in air or in an oxidizing atmosphere.
Layer of iron oxides (wustite, magnetite and hematite) that grows on steel surfaces above about 570 °C in air and thickens fast as temperature rises. Scale eats material, insulates parts against the quench and causes soft spots, and has to be removed by shot blasting or pickling. Protective atmospheres, vacuum, salt, stainless foil wraps and anti-scale coatings prevent it.
"Box furnace parts come out caked in scale, wrap the tool steel in foil next time."
Scale in boilers
A hard mineral crust that forms on boiler heat transfer surfaces from calcium, magnesium and silica in the water.
Hardness salts become less soluble as water heats, so they precipitate right on the hottest metal, forming calcium carbonate, sulfate or silicate layers. Scale insulates the metal from the water, so fuel use and stack temperature rise and the steel underneath runs hotter until it bulges or cracks. Softening makeup, controlling cycles and internal treatment chemicals prevent it; removing it takes acid cleaning or mechanical work.
"Opened the handholes and the tubes are scaled up, eggshell thick on the bottom row, call the chemical company in."