One of the most important advantages of stainless steel is its high resistance to corrosive environments. The principle alloying element imparting corrosion resistance is Chromium. Chromium forms a thin and self-repairing layer around the metal surface, thus protecting it from outside environment. Other alloying elements which further strengthen its resistance to corrosion include Nickel, Molybdenum.
Having high resistance does not make it immune to corrosion. Even stainless steel suffers from corrosion but the rate of corrosion is much slower, implying stainless steel offers better up-time.
Following are a few types of corrosion affecting Stainless Steel
This form of corrosion occurs uniformly across the surface. It is the least dangerous form of corrosion and can be easily predicted. Its unit of measurement is mm/year. The corrosion rate of 304 series is about 0.03mm/year which is a miniscule rate in comparison to other types of steel.
Carbon has high affinity for Chromium. At higher temperature Carbon combines with Chromium to form Chromium Carbide (Cr23C6). This formation generally takes place at the boundaries thus depleting the chromium layer and making the metal prone to corrosion.
Inter-granular corrosion can be minimised by using grades containing alloying elements like Titanium, Niobium. These alloying elements have higher affinity to form carbides thus preventing depletion of chromium. Other option is to use low carbon grades for example 316L.
This is a form of corrosion usually occurs in Chloride containing environment. Small pits are formed on the surface of the metal where chloride ions accumulate and there is depletion of oxygen. The outer surface acts as cathode and galvanic corrosion takes place. Such corrosion occurs in both acidic and basic solutions and is more severe in acidic solutions.
Pitting corrosion can be prevented by using stainless steel of higher alloying content. Pitting Resistance Equivalent Number (PREN) is a number used to indicate resistance of metal to this form of corrosion. PREN higher than 32 is better suitable for such applications.
Crevice Corrosion takes places in locations where the metal is shielded, example under bolt heads, washers and pipe fittings. It is similar to Pitting corrosion and is further accelerated in chloride containing environments. Similar to pitting corrosion, areas deplete of oxygen are formed. This gives rise to galvanic effects caused due to formation of cathode at the surface and anode in the crevice regions.
Crevice corrosion can be addressed by proper design of the products by minimising of shielded areas and using stainless steel containing higher alloying elements.
Stress Corrosion Cracking (SCC)
Three forces combine to cause this type of corrosion. Tensile stress, High temperature and solutions containing greater concentrations of chlorides, hydroxides contribute towards corrosion of metal. Corrosion initiates with a small crevice and then accelerates rapidly due to high stress concentrations at base of the crevice finally leading to rupture.
SCC limits the use of the most popular austenitic 304 and 316 grades, since these grades are most prone to SCC failure. The ferritic grades offer high resistance to this form of corrosion. Grades containing higher Chromium, Nickel, and Molybdenum are more resistant to SCC. Duplex grades also offer good resistance to this type of corrosion, their resistance to SCC is inferior to ferritic but superior to austenitic grades.
Design and material selection both go hand in hand. A good design can minimise failure of material due to corrosion. Pairing design with proper selection of stainless steel grade can help further reduce the impact of corrosion. This combination not only insures safety of the product but also enhances its life and thus reduces the overall life cycle cost of the product.
About Ambica Steels Limited
Ambica Steels Limited has over 5 decades of experience is Stainless Steel manufacturing. The Company offers 150+ grades of stainless steel in Round, Square, Hexagonal and Flat profiles. Ambica exports these products currently to 6 continents.
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Aniruddha Bauskar is responsible for New Business Development and Sales at Ambica Steels Limited. He likes reading and writing articles on various industries including Steel, Automobile and FMCG. You can find him on LinkedIn.