3. The properties of stainless steel

1. Stainless steel and its components

Stainless steels are steels to which chromium has been added. In accordance with the European standard EN 10088-12, a steel is classified as stainless steel if it contains at least 10.5% by mass of chromium and less than 1.2% of carbon.

• Carbon (C): The carbon content is limited to a maximum of 1.2% by mass in order to avoid the formation of carbides3 (in particular chromium carbides, which is a very stable chemical compound eager for chromium) which are harmful to the material. For example, the Cr23C6 carbide, which may appear in 18-9 austenite, has a negative effect on inter-granular corrosion (very significant depletion of chromium in the vicinity of the carbides formed causing the loss of the stainless character by capture of chromium).
  
• Nickel (Ni) : it promotes the formation of homogeneous austenitic structures. It provides the properties of ductility, malleability and resilience. To be carefully avoided in the area of friction.
  
• Manganese (Mn) is a nickel substitute. Some series of austenitic alloys have been developed to deal with nickel supply uncertainties.
  
• Molybdenum (Mo) and Copper (Cu) improve resistance in most corrosive environments, in particular acidic ones, but also in phosphoric and sulphur solutions, etc. Molybdenum increases the stability of the passive layer. Molybdenum, added to austenitic steels, further improves corrosion resistance. Thus, type 316 stainless steels contain between 2 and 3% MOLYBDENUM. They are mainly used in chemical and petrochemical industries where, for example, chloride resistance is required. However, it is important to note that these steels do not resist all types of chemical attacks (such as hydrochloric or oxalic acids, especially when they are hot and/or very concentrated).
  
• Tungsten (W) improves the high temperature resistance of austenitic stainless steels.
  
• Niobium (Nb) has a much higher melting point than Titanium and has similar properties. It is used in filler metals for electric arc welding in place of titanium which would be volatilized during transfer in the electric arc.
  
• Silicon (Si) also plays a role in oxidation resistance, in particular with respect to strongly oxidizing acids (concentrated nitric acid or hot concentrated sulfuric acid).
  
• Titanium (Ti) should be used at a content that exceeds four times the carbon content. It prevents the alteration of metallurgical structures during hot work, in particular during welding work, where it takes the place of chromium to form titanium carbide (TiC) before chromium carbide is formed, thereby maintaining the stainless nature of the steel by avoiding the depletion of chromium in the matrix near the carbonized zones.
  

2. Influence of various environments

• Industrial water: pure water has no effect but chlorides (and to a lesser extent many other salts), even in trace amounts, are particularly harmful for stainless steels; grades containing molybdenum are then the most suitable.
  
• Water vapor: normally without effect, it can however cause problems if it contains certain impurities.
  
• Natural atmospheres, except marine atmospheres : they pose even fewer problems as the steel contains more noble elements and the better the surface is polished.
  
• Marine and industrial atmospheres: chromium steels deteriorate very slowly and it is generally preferred to use molybdenum steels.
  
• Nitric acid: it attacks most industrial metals but stainless steel in general is particularly resistant to it, due to the passivation of its surface: molybdenum is only interesting if the acid contains impurities.
  
• Sulphuric acid: the resistance depends a lot on the concentration and the presence of oxidative impurities improves passivation. In general, austenitic grades containing molybdenum are the best.
  
• ‍Phosphoric acid: the resistance is generally good but impurities must be monitored, in particular hydrofluoric acid.
  
• Acid sulphites: corrosion can be catastrophic because these solutions, which are often found in paper mills, contain a lot of impurities; again, molybdenum alloys are preferable.
  
• ‍Hydrochloric acid: corrosion increases regularly as the concentration increases, so the association should be avoided.
  
• Organic acids: they are generally not as corrosive as mineral acids and those found in the food industry (acetic, oxalic, citric acids, etc.) have virtually no effect.
  
• Alkaline solutions: cold solutions have practically no action but the same is not true for concentrated and hot solutions.
  
• Saline solutions: the behavior is generally quite good, except in the presence of certain salts such as chlorides; nitrates, on the contrary, promote passivation and improve resistance. Nitric acid mixed with saturated brines can cause destruction to stainless steel (even 316L grades).
  
• Food products: there are generally no corrosion problems except with certain products that contain natural or added sulphur components, such as mustard and white wines.
  
• ‍Organic products: they generally have no effect on stainless steels, unless they are chlorinated and hot (bleach at more than 60°C and at high concentrations can destroy -black pitting- stainless steel). Glues, soaps, tars, petroleum products, etc. are no problem.
  
• Salts and other molten mineral products: alkaline products corrode all stainless steels but not nitrates, cyanides, acetates, etc. Most other salts and molten metals cause rapid damage.