Stainless steel owes its durability and corrosion resistance to the addition of chromium, creating a passive protective layer. Its versatility across types like ferritic, austenitic, and duplex makes it indispensable for construction, utensils, and transportation.
The mystery of stainless steel lies in maintaining the surface free from environmental corrosion for a pretty long time. It can be few hours to several years. The catch is favourable chromium content in stainless steel.
Iron is the most abundant element on the earth but has a limited worth unless it forms an alloy with carbon which gives it the strength and this new combination is called Steel, the most useable alloy on this earth. There are several varieties of steel based upon “C” level and other elements such as Si, Mn, S and P. There are several such steels which are used in numerous construction activities such as structures, bridges, flyovers, buildings, rails, etc. All are affected by one factor which is Corrosion. Corrosion causes scales on the surface and makes it dull in appearance. It results in material degradation and finally cracking and breaking. Addition of Chromium helps to reduce scale formation or resulting in a protective layer to avoid corrosion on the surface.
However, if one wants to make such steel free from corrosion and without any attack from the environment, then this element can be added with a concentration more than 9% when it becomes stable to any scale formation. It always shines and does not react with environment. This situation of steel is termed as Stainless Steel. A stainless steel always shines even in a wet environment, without leading to any scale or rust formation. This simple addition of 9% Cr in steel is known as Ferritic Steel. There are other stainless steels which are basically divided into 5 categories:
Ferritic Stainless Steel, Austenitic Stainless Steels, Duplex Stainless Steel and the martensitic Stainless Steels and precipitation hardened Stainless
All have common properties of shining surface with varying corrosion resistance which also depends upon the temperature. The mechanism of corrosion protection is mainly due to the formation of a passive chromia layer (Cr2O3) layer which controls the attack from the pollutants. Cr in combination with Mo and or Nb, especially enhances the protectivity of the Chromia layer and makes it stronger than a pure chromia layer (especially with Mo). Another change is their microstructure. While ferritic SS has cubic microstructure, austenitic as FCC structure the duplex has a combination of both. That is why latter is considered to be super stainless which beats strong acids and saves material from a perineal problem of sensitization.
Stainless steel is therefore called a mysterious material as a slight change in the chemical composition will make it very reactive (< 9Cr) while another combination makes the surface very stable (Cr + Mo). Also, a combination of Ni+Cr+Mo can make SS very its mechanical properties drastically (9Cr – ferritic, Ni+Cr – Austenitic and Ni+Cr+Mo Duplex). Properties like hardness and sensitization resistance comes from martensitic SS and Nb presence and creating small precipitates in the steel matrix.
Another important factor in stainless steels is the cost factor. The cost factor varies from ferritic to austenitic to Duplex by a factor of 2 to 4 times. The Stainless Steel Type 304 is perhaps the cheapest in austenitic SS variety and cost goes to double to Type 316 and 317 varieties which contain Mo and hence resist chloride attack. Duplex Stainless steels are about 4-5 times costlier as they have extreme resistance to high concentration of chlorides and sulphur attack.
The utility of the Stainless steels is wide spread as seen in the enclosed figure which shows (Fig.1(a)) it is available in any kind of shape and size, angles, rods, tubes, plates, wire, Fig1(b) shows its utility as household utensils and Fig. 1(c) shows its use in building structure and construction. One widespread use of stainless today is Railway Compartments.