Another widely used austenitic steel is 316 and 316L stainless steel, the corrosion performance of SS 304 is not enough, 316L is often consider as the first alternate. The higher Nickel content in 316 and 316L over SS 304 and the Molybdenum addition in 316 and 316L gives it an edge in performance in corrosive and high temperature environments.
As with 304 and 304L, the difference between the 316 and 316L grades is the quantity of carbon contained. The L stands for low carbon, both L grades contain a maximum 0.03% carbon, while the standard grades can comprise up to 0.07% carbon. It may not seem like a big difference, but it means that L grade versions of stainless steel alloys suited better for the larger welding projects. The lower Carbon content of L grades reduces cracking in the heat affected zones of welds and improved weld quality.
Stainless Steel 316 Chemical Composition – Chrome and Nickel
As with the 304 grade, 316 stainless steel owes much of its corrosion resistance to its chromium content. The passivated chromium oxide film which develops on the surface acts as a protective barrier against corrosion. It’s the chromium in the 304 and 316 grades that differentiate stainless steel from carbon steel.
The chemical composition of stainless steel 316 is almost identical to that of the 304 grade. There are a few distinct differences with the quantities of chromium (18 – 20% for 304, 16 – 18% for 316) and nickel (8 – 10.5% for 304, 10 – 14% for 316).
Molybdenum Content of Stainless Steel 316L
Also, 316 stainless steel contains between 2 – 3% molybdenum to give it its chloride-fighting properties. Molybdenum is a key ingredient in 316 stainless steel because it prevents small carbide particles that weaken the Chromic Oxide layer on the surface of the steel from forming at grain boundaries of the base metal.
The corrosion resistance of molybdenum-bearing 316L austenitic steel is particularly effective in chloride-ion environments. The molybdenum component prevents chloride ions pitting the and crevassing the surface of the steel. The presence of molybdenum makes 316 a good material for marine environments for the oil and gas industry. Even with the Molybdenum addition, 316 stainless steel is not entirely immune to sea corrosion. Warm seawater can corrode the surface of 316 grade marine parts over time, leaving the finish stained brown and rough. However, with careful maintenance the finish can be restored to its original clean, bright condition.
The molybdenum in 316L stainless steel also protects it against aggressive, acidic environments. 316 stainless steel offers superior protection against hydrochloric, acetic, tartaric and sulfuric acids. This makes it ideal for food processing and paper handling operations. This grade also resists corrosion from de-icing salts added to the roads.