So that stainless steel stays rust-free

Stainless steel does not carry its protection inside, but on the surface: a wafer-thin, chromium-rich passive layer. It forms by itself when enough oxygen is present. Where oxygen is missing, for example in narrow gaps, at mechanical joints or under deposits, the surface becomes "active" and can rust. The result is pitting and crevice corrosion, often exactly at the weld seams.

Welding and fabrication produce scale and heat tint. They look harmless, but they are not passive layers, they are weak points. On top of this comes extraneous rust: contact with tools made of ordinary steel leaves ferritic particles that continue to rust on the stainless steel surface. Such contamination cannot simply be wiped away, it has to be removed chemically.

Pickling is a chemical process. A pickling acid dissolves scale, heat tint, ferritic deposits and the chromium-depleted edge zones caused by welding until the surface is metallically clean. This is decisive, because a closed, uniform passive layer can only form on a metallically clean surface.

Depending on the component, different methods are used:

• Immersion pickling: the part is fully immersed in the pickling bath, ideal for sheets, profiles and complete constructions.
• Spray pickling: for large tanks, black-and-white constructions or parts with gap problems where immersion is not practical.
• Circulation pickling: especially for pipe constructions, where the pickling solution is pumped through the inside.

Before pickling, the part is degreased and rinsed, and afterwards rinsed until free of acid, so that nothing remains.

Passivation is the targeted formation and optimisation of the passive layer of stainless steel, the self-healing rust-free layer. It does form by itself after pickling through atmospheric oxygen, but that takes days, and during this time the part is not fully protected. Chemical passivation produces the chromium oxide layer in minutes with a uniform thickness and thus immediately ensures reliable corrosion protection.

This lowers the susceptibility to pitting and crevice corrosion, removes remaining ferritic contamination and restores the corrosion resistance of attacked surfaces. Especially for highly stressed components or lower-alloyed steels, passivation is the step that determines service life.

Pickling cleans purely chemically and establishes metallic cleanliness, but it does not smooth the surface and produces no gloss. Electropolishing goes further and removes the micro-roughness electrochemically; the result is smooth, glossy and particularly easy to clean. In practice the methods are often combined: first pickle, then electropolish, finally passivate. Anyone whose main aim is to remove weld scale and corrosion starting points is in the right place with pickling and passivation.

We offer both process routes. In a consultation you will learn the difference and which is better suited to your application.