What is stainless steel? What makes it has the characteristic of higher corrosion resistance to other types of steel material? Let us deep into more details about it now.
Steel is primarily made from iron and carbon, as we know that ordinary steel is easy to be corroded is because of the iron. When iron is artificially made into a pure element to make steel, it becomes very fragile and easy to be combined with oxygen and make it to be corroded. In this case, if add-in chromium as one of the elements of steel, it will form chromium oxide as a protective surface to prevent the corrosive by air and moisture. Other alloying elements like nickel (Ni) also added to create an outstanding corrosion-resistant product. For instance, stainless steel must have at least 10.5% of chromium. Depending on the application and environment conditions, it may contain a higher level of chromium ranging from 10.5 to around 30%, and additional alloying elements such as titanium, molybdenum, aluminium, copper, phosphorous and nitrogen. With the combination of these elements, there are more than 100 different grades of stainless steel available and each has slightly different physical characteristics.
Austenitic stainless steels are usually to be selected when exposing to a highly corrosive environment. It has a combination of the high amount of chromium and nickel which provide excellent corrosion resistance and heat resistance. Austenitic steel is widely used in industrial steel, construction, facades and housewares. There are two most commonly used of Austenitic stainless steel which is grade 304 and 316. Here we will look into more details about these two types of stainless steel.
Stainless Steel Grade 304
Grade 304 is the most commonly used austenitic stainless steel around the world. It consists of high-level chromium at around 18 - 20%, nickel content between 8 - 10.5% both vary depends on the weight. It also includes other major alloying elements such as silicon, manganese, low carbon and of course the rest of the chemical composition mainly is iron. Due to its components, it can resist corrosion from oxidizing acids and also easy to be sanitized. In addition, Grade 304 provides a variety of finishes, ease of fabrication and cleaning procedure. However, 304 is most likely to be corroded by chloride solution or saline environment with salt exposure. Estimation of 25 ppm of sodium chloride can have a negative effect on its internal structure as Chloride ions can spread beneath the chromium protective layer by creating localized areas of corrosion.
Grade 304 can be used for Storage tanks, Kitchen appliances, Tubing/Piping, Pressure Vessels, Food processing equipment, Hardware (Bolts & Nuts, screws) or other Indoor architectural hardware/decoration.
Stainless Steel Grade 316
Grade 316 is the second common use of stainless steel products in the market. It has a similar composition of elements as Grade 304, with a high amount of chromium and even more nickel found in 316. The key difference between them is the addition 2 - 3% molybdenum to be incorporated in 316 in compare with a minute amount in 304. Molybdenum used to strengthen and making harder for the steel, it increases the level of corrosion resistance and particularly fights against the corrosion from chlorides, sulfuric acid, iodides, bromides and other chemical corrodents. 316 also contain manganese, carbon, silicon which made of being iron.
With these characteristics, 316 is widely used in an industrial and marine application, especially those involving chemical processing, manufacturing of medical instruments, refinery equipment and marine equipment exposed in high-saline environments. The price of 316 will be more expensive than 304. But you can save a lot in term of a long term perspective as it can be last longer with superior corrosion resistance and greater strength.
Other types of Staineless Steel
Except for the Austenitic Stainless Steels, the stainless steel products can be categorized into another 4 main categories which are-:
1. Ferritic Stainless Steel: similar to mild steel, but better in heat, crack and corrosion resistance. Most commonly used in indoor architecture products.
2. Martensitic Stainless Steel: This is less corrosion resistance than the austenitic and ferritic grade, but has a strong and hard feature. Most commonly to be used as turbine blade or knife.
3. Duplex Stainless Steel: A mixed composition of Austenite and Ferrite which is flexible and hard. Widely used in pulp, paper and petrochemical industry.
4. Precipitation Hardening Stainless Steel: Addition components of copper, niobium and aluminium into the steel make extremely stronger.
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