Steel is basically iron alloyed to carbon with certain additional elements to give the required properties to the finished melt. Listed below is a summary of the effects of alloying various elements in steel.
Niobium (Colombium) (Nb)
 
Niobium is added to steel in order to stabilize carbon, and as such, performs in the same way as described for titanium. Niobium also has the effect of strengthening steels and alloys for high temperature service.
     
Nitrogen (N)
 
Nitrogen has the effect of increasing the austenitic stability of stainless steels and is, as in the case of nickel, an austenitic forming element. Yield strength is greatly improved when nitrogen is added to stainless steels.
     
Silicon (Si)
 
Silicon is used as a deoxidizing (killing) agent in the melting of steel, and as a result, most steels contain a small percentage of silicon. Silicon contributes to hardening of the ferritic phase in steels and for this reason silicon killed steels are somewhat harder and stiffer than aluminum killed steels.
     
Cobalt (Cu)
 
Cobalt becomes highly radioactive when exposed to the intense radiation of nuclear reactors, and as a result, any stainless steel that is in nuclear service will have a cobalt restriction, usually approximately 0.2% maximum. This problem is emphasized because there is normally a residual cobalt content in the nickel used in producing these steels.
     
Copper (Cu)
 
Copper is normally present in stainless steel as a residual element. However, it is added to a few alloys to produce precipitation hardening properties or to enhance corrosion resistance particularly in sea water environments.
     
Sulphur (S)
 
When added in small amounts sulphur improves machinability but does cause hot shortness. Hot shortness is reduced by the addition of manganese sulphide. Manganese sulphide has a higher melting point than iron sulphide, which would form if manganese were not present. The weak spots at the grain boudaries are greatly reduced during hot working.
     
Selenium (Se)
  Selenium is added to improve machinability.
     
Carbon (C)
 
The basic metal, iron, is alloyed with carbon to make steel and has the effect of increasing the hardness and strength of iron. Pure iron cannot be hardened or strengthened by heat treatment but the addition of carbon enables a wide range of hardness and strength.
     
Manganese (Mn)
 
Manganese is added to steel to improve hot working properties and increase strength, toughness and hardenability. Manganese, like nickel, is an austenite forming element and has been used as a substitute for nickel in the AISI 200 Series of Austenitic Stainless Steels e.g. AISI 202 as a substitute for AISI 304.
     
Chromium (Cr)
 
Chromium is added to steel to increase resistance to oxidation. This resistance increases as more chromium is added. 'Stainless Steels' have approximately 11% chromium and a very marked degree of general corrosion resistance when compared to steels with a lower percentage of chromium. When added to low alloy steels, chromium can increase the response to heat treatment thus improving hardenability and strength.
     
Nickel (Ni)
 
Nickel is added in large amounts, over about 8%, to high chromium stainless steels to form the most important class of corrosion and heat resisting steels. These are the austenitic stainless steels, typified by 18-8, where the tendency of nickel to form Austenite is responsible for great toughness and high strength at both high and low temperatures. Nickel also improves resistance to oxidation and corrosion. It increases toughness at low temperatures when added in smaller amounts to alloy steels.
     
Molybdenum (Mo)
 
Molybdenum when added to chromium-nickel austenitic steels, improves resistance to pitting corrosion by chlorides and sulphur chemicals. When added to low alloy steels, Molybdenum improves high temperature strength and hot hardness. When added to chromium steels, it greatly diminishes the tendency of steels to embrittle in service or in heat treatment.
     
Titanium (Ti)
 
The main use of titanium as an alloying element in steel is for carbide stabilization. It combines with carbon to form titanium carbides, which are quite stable and hard to dissolve in steel. This tends to minimize the occurrence of inter-granular corrosion as with AISI 321, when adding approximately .25/.60 % titanium, the carbon combines with titanium in preference to chromium, preventing a tie-up of corrosion resisting chromium as inter-granular carbides and the accompanying loss of corrosion resistance at the grain boundaries.
     
Phosporous (P)
 
Phosphorus is usually added with sulphur, to improve machinability. In low alloy steels, phosphorus, in small amounts, aids strength and corrosion resistance. Experimental work indicates that phosphorus present in austenitic stainless steels increases strength. Phosphorus additions are known to increase the tendency to cracking during welding.
Corrosion can be defined as the attack of a metallic material by its environment. Stainless steels all possess a high resistance to corrosion. This resistance is conferred by the naturally occuring chromium-rich oxide film which is always present on the surface of stainless steel. Although less than 130 angstrom thick (1 anstrom unit =108cm) this invisible film is extremely protective as it is inert and adheres tightly to the metal.

The oxide film has the unique property of self-repair which is unattainable in applied films. This means that if the film is removed or damaged or a new metal surface is created by cutting then in the atmosphere or other source of oxygen the protection will be instantaneously re-established.

The more highly alloyed grades of stainless steel possess the best corrosion resistance and are able to withstand more aggressive environments. Selection of the correct grade of stainless steel is the key to the avoidance of corrosion problems.

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Corrosion Resistance
 
Corrosion takes many different forms. Its initiation and subsequent rate of progress is affected in varying degrees by numerous material and environmental factors. A comprehensive assessment of the exact corrosion resistance of a material is therefore difficult. However, corrosion tables covering a vast range of stainless steels and environments are available.
     
Design Criteria
 
To achieve optimum corrosion performance care must be taken at the design stage. In particular design should employ smooth contours and radiused corners whilst avoiding sharp edges and crevices. Design should also promote material flow and mixing to avoid localized concentrations and/or stagnant conditions. Other considerations are ease of cleaning and maintenance as well as avoidance of dissimilar metal contact.
     
     
     
     
Acid Corrosion
 
Occurs due to aggressive attack by acids which may be accelerated by the presence of other chemicals. A large number of acid environments are resisted by stainless steels whose resistance to oxidising solutions is particularly good provided the correct grade is used.
     
Atmospheric Corrosion
 
Occurs due to the attack from oxygen, water and the pollutants therein such as chlorides, sulphur compounds and solids. The problem is particularly prevalent in coastal and industrial areas which necessitates the use of type 316 for outdoor applications in such environments.
     
Bacterial Corrosion
 
Occurs due to the presence and activity of certain types of bacteria and tends to be localized, for example in crevices. It is overcome by good design, continuous flow and regular cleaning.
     
Crevice Corrosion
 
Is a form of galvanic corrosion which occurs in crevices such as joints, cavities, holes, corners, and gaps between components. Good design should eliminate such crevices.
     
Fretting Corrosion
  Which could also be called corrosion-abrasion is caused by continuous removal of corrosion product due to surfaces rubbing together which leads to progressive wasting of material. Stainless steels do not suffer from this form of attack.
     
Galvanic Corrosion
 
Covers situations where attack is caused by a potential difference. This potential difference can be set up in a number of ways including contact between dissimilar metals in an aqueous or conducting solution, differential aeration (variation in oxygen concentration) and local variations in concentration of the solution. Correct material selection and good design eliminate this activity.
     
General Corrosion
 
Is the uniform overall attack of a component across its whole surface. It is avoided by correct grade selection.
     
Inter-Granular Corrosion
 
Is the uniform overall attack of a component across its whole surface. It is avoided by correct grade selection.
     
Oxidation
 
Oxidation is the combination of a metal with oxygen to form the metal oxide which occurs in dry conditions. When this process is ongoing the whole of the metal may be converted. Stainless steels are oxidation resistant, even at elevated temperatures. Special grades, such as type 310, are operated at temperatures of up to 1100 degrees centigrade. The surface of all stainless steels is oxidized to form a chromium rich oxide film which is inert, self-repairing and forms a proctective barrier keeping oxygen away from the metal surface.
     
Pitting Corrosion
 
Is the highly localized attack seen as small spots across a surface occuring mainly at sites of metallurgical heterogeneity. It is particulary prevalent in chloride environments especially if oxygen is in plentiful supply. Higher chromium, nickel and molybdenum contents improve pitting resistance with type 316 being used extensively in such situations.
     
Stress Corrosion
 
Cracking can occur in austenitic stainless steels when they are operated under tensional stress in chloride environments at temperatures in excess of about 60 degrees centigrade. The stress could arise through in-service loading, pressurization of pipework and vessels or as residual stress from cold working. Ferritic stainless steels are immune to this form attack.
     
Waterline Corrosion
  Is a form of Galvanic corrosion taking place at the liquid surface.
     
Weld Decay Corrosion
  Is a form of inter-granular corrosion occuring in the heat-affected zone of the parent metal parallel to the weld. Susceptibility to this attack is assessed using one of these standard tests:
    1.)Test as given in BS 1449/BS 1501 using boiling copper sulphate/sulphuric acid.
    2.)Test as given in ASTM A262-75 Practice C using boiling nitric acid.
     Low carbon grades perform better in the more severe (latter) test.