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The subsequent formation of chromium carbide results in reduced corrosion resistance along the grain boundary, leaving the stainless steel susceptible to unanticipated corrosion in an environment where 304 would be expected to be corrosion resistant. This grain boundary corrosive attack is known as intergranular corrosion. [13]
Stainless steel, also known as inox, corrosion-resistant steel (CRES), and rustless steel, is an alloy of iron that is resistant to rusting and corrosion. It contains iron with chromium and other elements such as molybdenum , carbon , nickel and nitrogen depending on its specific use and cost.
Alloy 20 (Carpenter 20) is an austenitic stainless steel possessing excellent resistance to hot sulfuric acid and many other aggressive environments which would readily attack type 316 stainless. This alloy exhibits superior resistance to stress-corrosion cracking in boiling 20–40% sulfuric acid.
Type 316—the second most common grade (after 304); for food and surgical stainless steel uses; alloy addition of molybdenum prevents specific forms of corrosion. It is also known as marine grade stainless steel due to its increased resistance to chloride corrosion compared to type 304. 316 is often used for building nuclear reprocessing plants.
SAE 316 stainless steel is a molybdenum-alloyed steel and the second most common austenitic stainless steel (after grade 304). It is the preferred steel for use in marine environments because of its greater resistance to pitting corrosion than most other grades of steel without molybdenum. [1]
The pitting corrosion resistance of stainless steels is estimated by the pitting resistance equivalent number (PREN). PREN = %Cr + 3.3%Mo + 16%N Where the Cr, Mo, and N, terms correspond to the contents by weight % of chromium , molybdenum and nitrogen respectively in the steel.
AL-6XN (UNS designation N08367) is a type of weldable stainless steel that consist of an alloy of nickel (24%), chromium (22%) and molybdenum (6.3%) with other trace elements such as nitrogen. The high nickel and molybdenum contents of the AL-6XN alloy give it good resistance to chloride stress-corrosion cracking.
Pitting resistance equivalent number (PREN) is a predictive measurement of a stainless steel's resistance to localized pitting corrosion based on its chemical composition. In general: the higher PREN-value, the more resistant is the stainless steel to localized pitting corrosion by chloride .