The first iron alloys and corrosion resistant steels were sunk in antiquity: the iron pillar of Delhi, erected by order of Kumaragupta I in fifth century still exists today in perfect condition. However, a distinction must be made in vocabulary: these alloys were resistant to its high phosphorus content, not chrome (stainless steel fabricator). So it was not stainles-steels in sense that currently gives the term. In these alloys, and under favorable weather conditions, forms a surface layer of iron oxide passivation and phosphate that protects the rest of metal much better than a layer of rust.
Other elements may be added, in particular nickel, which improves the mechanical properties in general and particularly ductility, and other elements such as molybdenum or titanium, which enhance the stability of alloy at temperatures other than room as well as elements with high melting points such as vanadium and tungsten in general accompanied by an increase in chromium content, for resistance to high temperatures ignited (refractory steels).
The corrosion of metals are electrochemical in nature: the metal returns to its thermodynamically stable state, the oxidized state. In presence of an oxidizing environment (water, air), the metal reacts with the environment, this reaction taking place with exchange of electrons. Iron, major constituent of steels, is easily oxidized; the corrosion product, rust, crumbles or dissolved in water, creating a deterioration of part. When hot, the diffusion of oxidants atoms in metal thickness can further complicate the problem.
This layer, compact, adherent and protective therefore, is called "passive layer": it forms a barrier between the steels. Normally, it is invisible because very fine. Thus, contrary to its name, the metal is not: it oxidizes quickly, but forms a protective oxide, unlike rust. Relative to a standard hydrogen electrode, the potential of stainlesss-steels is between molybdenum and mercury, not far from the silver and platinum. The addition of various alloying elements can be adapted to specific environment in which the steels is to be used, and change its mechanical properties.
Nickel is an austenite forming element, it provides an austenitic structure and therefore have sheets that are taking shape easily. High carbon content makes dipping the steels and to obtain a martensitic steels, very hard. But carbon overnight at weldability, and furthermore, it can trap the chromium and hinder the formation of passive layer. Other alloying elements, mainly metals relatively "noble" as molybdenum, titanium, copper further improve chemical resistance, especially in non-oxidizing environments.
There are actually numerous grades of stainles-steels and the choice is difficult because they do not all have the same behavior in a given environment. Are often referred to by the weight percentages of nickel and chromium. Thus, a 18/10 stainles-steel, such as those used in cutlery for cutlery and for cooking in general, contains 18% by weight of chromium and 10% by weight of nickel.
In 1924, William Herbert Hatfield (en), who succeeded Harry Brearley at the head of Brown-Firth laboratories, worked steels "18/8" (18 wt% chromium and 8% nickel) is probably the representing the most used stainles-steel iron-nickel-chromium. In 1925 is developed the process Ugine-Perrin in factories of Savoy Society of Electrochemistry, electrometallurgy and steels mill Ugine, future Ugitech, a method to obtain a stainles-steel both pure, reliable and cheap, by stirring steels with previously molten slags, for a complete treatment of steels.
The metal is exposed (grinding, machining, deformation of workpiece cracking the passive layer, friction, erosion, cavitation), but the oil or grease prevents air arriving to oxidize; then the surface is "active". Non-stainles-steel particles pollute the surface (pollution iron): these particles rust, forming halos, but can also initiate corrosion of stainless-steel in some cases.
Other elements may be added, in particular nickel, which improves the mechanical properties in general and particularly ductility, and other elements such as molybdenum or titanium, which enhance the stability of alloy at temperatures other than room as well as elements with high melting points such as vanadium and tungsten in general accompanied by an increase in chromium content, for resistance to high temperatures ignited (refractory steels).
The corrosion of metals are electrochemical in nature: the metal returns to its thermodynamically stable state, the oxidized state. In presence of an oxidizing environment (water, air), the metal reacts with the environment, this reaction taking place with exchange of electrons. Iron, major constituent of steels, is easily oxidized; the corrosion product, rust, crumbles or dissolved in water, creating a deterioration of part. When hot, the diffusion of oxidants atoms in metal thickness can further complicate the problem.
This layer, compact, adherent and protective therefore, is called "passive layer": it forms a barrier between the steels. Normally, it is invisible because very fine. Thus, contrary to its name, the metal is not: it oxidizes quickly, but forms a protective oxide, unlike rust. Relative to a standard hydrogen electrode, the potential of stainlesss-steels is between molybdenum and mercury, not far from the silver and platinum. The addition of various alloying elements can be adapted to specific environment in which the steels is to be used, and change its mechanical properties.
Nickel is an austenite forming element, it provides an austenitic structure and therefore have sheets that are taking shape easily. High carbon content makes dipping the steels and to obtain a martensitic steels, very hard. But carbon overnight at weldability, and furthermore, it can trap the chromium and hinder the formation of passive layer. Other alloying elements, mainly metals relatively "noble" as molybdenum, titanium, copper further improve chemical resistance, especially in non-oxidizing environments.
There are actually numerous grades of stainles-steels and the choice is difficult because they do not all have the same behavior in a given environment. Are often referred to by the weight percentages of nickel and chromium. Thus, a 18/10 stainles-steel, such as those used in cutlery for cutlery and for cooking in general, contains 18% by weight of chromium and 10% by weight of nickel.
In 1924, William Herbert Hatfield (en), who succeeded Harry Brearley at the head of Brown-Firth laboratories, worked steels "18/8" (18 wt% chromium and 8% nickel) is probably the representing the most used stainles-steel iron-nickel-chromium. In 1925 is developed the process Ugine-Perrin in factories of Savoy Society of Electrochemistry, electrometallurgy and steels mill Ugine, future Ugitech, a method to obtain a stainles-steel both pure, reliable and cheap, by stirring steels with previously molten slags, for a complete treatment of steels.
The metal is exposed (grinding, machining, deformation of workpiece cracking the passive layer, friction, erosion, cavitation), but the oil or grease prevents air arriving to oxidize; then the surface is "active". Non-stainles-steel particles pollute the surface (pollution iron): these particles rust, forming halos, but can also initiate corrosion of stainless-steel in some cases.
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