Alloy 304 (UNS S30400, 1.4301) is perhaps the most widely used (austenitic) stainless steel, based upon 18% chromium and 8% nickel additions, with smaller amounts of manganese added to a base of iron. It can contain a small amount of carbon which can limit corrosion resistance after welding, so a lower carbon grade 304L is widely used. Due to it’s lower alloy content it is a cost-competitive grade for general applications, used in an array of domestic and industrial applications including utensils, tubing and sanitary ware.
In comparison, Alloy 316 (UNS S31600, 1.4401) contains 16% chromium, 10% nickel and 2% molybdenum. The addition of molybdenum greatly improves the pitting corrosion resistance of Alloy 316, albeit at an additional cost. As with Alloy 304, a lower carbon variant is more widely specified as 316L (UNS S31603, 1.4404). Improved corrosion performance extends the range of applications where it can be used, including more aggressive industrial and coastal applications.
The most common form of corrosion in stainless steels is pitting corrosion. The Pitting Resistance Equivalent Number (PREN) gives an indication of resistance to pitting corrosion based upon the alloys composition, where PREN = %Cr + 3.3 %Mo + 16 % N. Therefore the PREN for Alloy 316 is around 25, which is 4-5 points higher than Alloy 304.