a surface treatment

A surface treatment is anything you do to just the outer skin of a part to make it harder, tougher against fatigue, more corrosion-resistant, or better-looking, without changing the bulk material underneath. Think of galvanizing a steel fence, anodizing an aluminium phone body, or peening a spring. The idea works because so many failures, corrosion, wear, and fatigue cracking, all begin at the surface, so improving only the surface can buy a big gain cheaply.

The family is broad. Shot peening blasts the surface with tiny beads, plastically stretching the skin so it ends up in compression, and that compressive residual stress squeezes fatigue cracks shut, which is why springs, gears, and aircraft parts are peened. Coatings add a protective layer: paint, electroplated or hot-dip galvanized zinc, and thin hard PVD films. Anodizing grows a thick, hard, protective oxide on aluminium electrolytically. Case hardening (carburizing or nitriding) diffuses carbon or nitrogen into the surface to make a hard skin over a tough core.

The honest point is that a surface treatment is a skin, and its benefit lasts only as long as the skin does. Shot peening's compressive layer is thin, and once a crack grows past it or the surface is machined off, the protection is gone. A coating that is scratched through can actually accelerate localized corrosion, and a hard case over a soft core can spall if the core deforms under it. So surface treatments are powerful and cheap, but they protect against the right threat only if the surface stays intact.