Stainless steel is a science unto itself

In 1913 Harry Brearley, a scientist and metallurgist in Sheffield, England, was tasked with solving the problem of corrosion in British gun barrels. In one test he alloyed iron with chromium. And so began the development of stainless steel.

Today we know that when iron is alloyed with at least 12 % chromium, its resistance to rust increases through the reaction of chromium with oxygen in the air and the formation of an extremely thin protective film on the surface of the steel. This is known as passivation and protects the steel against what is commonly referred to as corrosion. If the film is damaged, a new one is formed through contact with oxygen.

Stainless steels also contain other alloying elements that protect it against corrosion and provide other desirable characteristics. For example, nickel improves plasticity and weldability, while molybdenum improves the material’s resistance against corrosion from chlorides and other types of acids.

The most common stainless steel alloys:

• EN 1.4301 with approx 18 % chromium and 8 % nickel. This stainless steel is suitable for environments with minor corrosive impact e.g. indoors.
  AISI 304 is its American designation.
• EN 1.4404 with approx 18 % chromium, 11 % nickel and 2,1 % molybdenum. This stainless steel is usually termed acid proof and is often used in coastal environments and polluted industrial and urban environments. Swimming pools and facilities with chloride compounds combined with high heat and humidity also requires this type of stainless steel.
  AISI 316L is its American designation.

Pickling

When stainless steel is exposed to high temperatures such as at welding wanders (diffuses) chromium away from the steel surface. The chromium content in the surface of the material drops below 12% on the heat-affected part and the steel loose its ability to "self-heal" its passive layer.

Upon pickling, acids dissolve the weld oxides as well as the chromium-depleted zone and the steel's original corrosion-resistant properties are restored. Pickling is, from a corrosion point of view, superior to other treatment methods such as grinding and blasting.

In addition to removing "chromium-depleted" material, pickling usually gives a better surface finish to the material than other methods, which in itself facilitates the construction of the steel passive layer.

We offer pickling in our plant where we dip products with sizes up to 3x2x2 m.