Stainless Steel

Stainless steel plays an important part in many domains: daily life, mechanical industry, food-processing industry, chemistry, medicine, surgery and more. Like other steels, it is an alloy of chrome and other elements such as nickel, and sometimes molybdenum and vanadium to improve its resistance to corrosion.

Composition of Stainless Steels:
To be classified as “stainless”, the steel must contain at least 10,5% of chrome and at least 1,2% of carbon.

(Chemical analysis in weight %) Most stainless steels meet European (especially EN 10088) and American (AISI) standards. Other standards exist but are less known internationally.

Stainless Steel

Industrial Waters
Pure water has no effect, but chlorides (and other salts), even in small traces, are particularly harmful to stainless steels; steels containing molybdenum are more convenient.

Water Steam
Normally without effect, it can still cause problems if it contains certain types of impurities.

Natural Atmospheres, except for Marine Atmospheres
The higher the content of noble elements or surface polish, the higher is its resistance to natural elements

Marine and industrial atmospheres
Chrome steels alter very slowly but generally molybdenum steels constitute a better choice..

Nitric Acid
It attacks most industrial metals but stainless steels generally resist very well to nitric acid after the passivation of it’s surface: molybdenum is interesting only when the acid contains impurities

Sulphuric Acid
Its resistance depends on its concentration and the presence of oxidant impurities enhancing passivation. Generally, austenitic variations containing molybdenum are the best.

Phosphoric Acid
Resistance is generally good, but not with certain impurities, particularly hydrofluoric acid.

Acid Sulphites
Corrosion may be catastrophic since these solutions, commonly found in paper mills, contain a high degree of impurities. Once again, molybdenum alloys are preferable.

Hydrochloric Acid
Corrosion regularly increases with its concentration. Hydrochloric acid should not be combined with Stainless steel.

Organic Acids
They generally are less corrosive than mineral acids and acids found in food industry ; acetic, citric, and oxalic acids have no effect on the steel.

Alkaline Solutions
Cold solutions have no effect but concentrated and warm solutions do.

Saline Solutions
The behaviour is generally good, except when there are salts such as chlorines; nitrates facilitate passivation and improve resistance.

Food Products
Generally, there aren’t any corrosion problems, except with certain products that contain natural or added sulphurous components such as mustard and white wine.

Organic Products
They generally have no effect on stainless steels, except if they are chlorinated: glues, soaps, tars, oil products, etc. cause no problem.

Salts and other Melted Mineral Products
Alkaline products corrode all stainless steels, but nitrates, cyanides and acetates won’t attack stainless steels. Most salts and melted metals cause rapid damage.

Stainless Steel

In numerous cases, cleaning with water and soap is adequate. There are some appropriate cleansers but they cannot be compared to Nitric Acid. This acid eliminates all deposits and leaves a well passivated surface.

See Cleaning and Maintenance section for learn morel.

Stainless Steel

Other than it’s resistance to corrosion, stainless steels have a great virtue: mechanical resistance.

Chrome is the main alloy element of stainless steels. Contrarily to what people generally believe, chrome is a very chemically reactive metal that oxidizes easily, but it’s oxide forms a transparent and protective layer. Alloyed to iron and nickel, it causes the formation of an oxided surface component capable of reducing or stopping corrosion.

Chrome and nickel oxide this way :

4 Cr + 3 O2 -> 2 Cr2O3
2 Ni + O2 -> 2 NiO

Stainless is a misleading and inappropriate term

There is a variety of stainless steels, and the type may be difficult to choose. Each stainless steel reacts differently to its environment. The type is often represented by the mass percentage of nickel and chrome. An inox 18\10, used in cutlery and cooking in general, contains 18% of its mass in chrome and 10% in nickel. This designation is insufficient since it does not refer to its metallurgical structure.

The chrome content is always at least 12%. Other alloying elements, for most “noble” metals such as nickel, molybdenum, copper or brass, improve its chemical resistance mostly in non oxidant environments.

The characteristics of the resistance of these alloys were discovered in 1913 when it was noticed that some polished samples for laboratory exams wouldn’t oxidize. In fact, we could say that:

  • Stainless Steels can be corroded in cold temperatures only in the presence of humidity. Stainless steels resist to chlorine, very corrosive gasses, as long as it is perfectly dry.
  • In the presence of aqueous solutions, electrochemical corrosion takes over direct chemical corrosion ; The resistance of the material depends, as said earlier, on the electrochemical potentials of the surface and its distribution.
  • Like aluminium which is very likely to oxidize and covers itself with a protective oxide; stainless steels actively behave when just manufactured, stripped, or polished, and passively when exterior attacks allows it to form its protective “skin”
  • A good use of stainless steels requires a very homogeneous metal to avoid local corrosion and a transition from the active to the passive state everywhere on the exposed surface.

Compared to a referenced hydrogen electrode, stainless steel’s potential is set between molybdenum and mercury, close to silver and platinum.

Ferrous deposits on stainless steel surfaces are very dangerous in humid environments since rust is used as a catalyst and the surface ends up being punctured.