Stainless steel and its components:
Stainless steels are steels with added chrome. In accordance with the European standard EN 10088-12, a stainless steel is classified as such if it contains at least 10.5 wt% chromium and less than 1.2% carbon.
Carbon (C): The carbon content is limited to a maximum of 1.2% by weight to avoid carbide formations (especilally chromium carbide which is a very stable chemical compound hungry for chrome) that are harmful to the material. For example, the Cr23C6 carbide which may appear in the austenite 18-9 has a negative effect in regard to intergranular corrosion (very important depletion of chromium around formed carbides, causing the loss of inoxidability by chromium uptake).
Nickel (Ni): Promotes the formation of austenitic type structures. It brings ductility, malleability and resilience. To be avoided in the friction field.
Manganese (Mn) is a nickel substitute. Some series of austenitic alloys have been developed to cope with nickel supply uncertainties.
Molybdenum (Mo) and copper (Cu) improve the resistance in most corrosive environments, particularly those that are acidic, but also in phosphate solutions, sulfuric, etc. Molybdenum increases the stability of the passive layer. Added to austenitic steels, it further enhances corrosion resistance. Thus, 316 type stainless steels contain between 2 and 3%. Molybdenum is mainly used in the chemical and petrochemical industries where, for example, chloride resistance is required. Nevertheless, it is important to clarify that these steels are not resistant to all types of chemical attacks (such as hydrochloric or oxalic acids, especially when hot and / or highly concentrated).
Tungsten (W) improves the resistance to austenitic stainless steels high temperatures.
Titanium (Ti) should be used at an exceeding level of four times the carbon content. Avoids metallurgical structures deterioration during hot-work, especially in welding work where it takes chromium's place to form titanium carbide (TiC) before any formation of chromium carbide thereby preserving the stainless character of the steel and avoiding the matrix chromium depletion nearby carburised areas.
Niobium (Nb) has a melting point much higher than titanium but has similar properties. It is used in filler metals for electric arc welding instead of titanium which would be volatilised during transfer in the electric arc.
Silicon (Si) also plays a role in the oxidation resistance, especially regarding the strong oxidising acid (concentrated nitric acid or hot concentrated sulfuric acid).
Influence from various backgrounds
Industrial water: Pure water has no effect but chlorides do (and to a lesser extent many other salts), even in trace amounts, are particularly harmful for stainless steels; grades containing molybdenum are more appropriate.
Water vapor: usually has no effect, however, can pose problems if it contains some impurities.
Natural atmospheres, with the exception of marine atmospheres: they pose fewer problems than steel, contains more noble elements and the surface is more polished.
Marine and industrial atmospheres: chromium steels deteriorate very slowly and it is generally preferred to use molybdenum steels.
Nitric acid: attacks most industrial metals, but stainless steel can resist it particularly well due to the passivation of its surface, molybdenum is interesting only if the acid contains impurities.
Sulphuric acid: resistance depends largely on the concentration and the presence of oxidising impurities, improves passivation. Generally austenitic grades containing molybdenum are the best.
Phosphoric acid: overall has good resistance but must keep a close watch on impurities, particularly hydrofluoric acid.
Sulfites acids: corrosion can be catastrophic because these solutions, which are often found in paper mills, contain many impurities; once again molybdenum alloys are preferable.
Hydrochloric acid: corrosion increases regularly as and when the concentration increases, the association is therefore to be avoided.
Organic acids: they are generally not as corrosive as mineral acids and those encountered in the food industry (acetic, oxalic, citric acids, etc.) have practically no effects whatsoever.
Alkaline solutions: cold solutions have virtually no action although it is not the same for hot and concentrated solutions.
Saline solutions: its behavior is usually quite good, except in the presence of certain salts such as chlorides; on the contrary nitrates promote passivation and enhance the strength. When mixed with saturated brines, nitric acid can damage stainless steel (even 316L grades).
Food: in general there are no corrosion problems except with products containing natural or added sulfur components, such as mustard and white wines.
Organic products: they usually have no effect on stainless steels, unless they are hot and chlorinated (bleach above 60 ° C and high concentrations can destroy stainless steel -black specks-) . Adhesives, soaps, tars, petroleum products, etc. cause no problem.
Salts and other molten minerals: alkalis corrode all stainless steels but not nitrates, cyanides, acetates, etc. Most of other salts and molten metals can quickly cause damage.
