Types 316 and 316L are more resistant to atmospheric and other mild types of corrosion than the 18-8 stainless steels. In general, media that do not corrode 18-8 stainless steels will not attack these molybdenum-containing grades. One known exception is highly oxidizing acids such as nitric acid to which the molybdenum-bearing stainless steels are less resistant.
Type 316 is considerably more resistant than any of the other chromium-nickel types to solutions of sulfuric acid. At temperature as high as 120° F (49° C), Type 316 is resistant to concentrations of this acid up to 5 percent. At temperatures under 100° F (38° C), this type has excellent resistance to higher concentrations. Service tests are usually desirable as operating conditions and acid contaminants may significantly affect corrosion rate. Where condensation of sulfur-bearing gases occurs, these alloys are much more resistant than other types of stainless steels. In such applications, however, the acid concentration has marked influence on the rate of attack and should be carefully determined.
The molybdenum-bearing Type 316 stainless steel also provides resistance to a wide variety of other environments. This alloy offers excellent resistance to boiling 20% phosphoric acid. It is widely used in handling hot organic and fatty acids. This is a factor in the manufacture and handling of certain food and pharmaceutical products where the molybdenum-containing stainless steels are often required in order to minimize metallic contamination.
Generally, the Type 316 grade can be considered to perform equally well for a given environment. A notable exception is in environments sufficiently corrosive to cause intergranular corrosion of welds and heat-affected zones on susceptible alloys. In such media, Type 316L is preferred over Type 316 for the welded condition since low carbon levels enhance resistance to intergranular corrosion.
Resistance of austenitic stainless steels to pitting and/or crevice corrosion in the presence of chloride or halide ions is enhanced by higher chromium (Cr), molybdenum (Mo), and nitrogen (N) content. A relative measure of pitting resistance is given by the PREN (Pitting Resistance Equivalent, including Nitrogen) calculation, where PREN = Cr+3.3Mo+16N. The PREN of Type 316 and 316L (24.2) is better than that of Type 304 (PREN=19.0), reflecting the better pitting resistance which Type 316 (or 316L) offers due to its Mo content.
Type 304 stainless steel is considered to resist pitting and crevice corrosion in waters containing up to about 100 ppm chloride. The Mo-bearing Type 316 alloy on the other hand, will handle waters with up to about 2000 ppm chloride. Although this alloy has been used with mixed success in seawater (19,000 ppm chloride) it is not recommended for such use. The Type 316 alloy is considered to be adequate for some marine environment applications such as boat rails and hardware, and facades of buildings near the ocean which are exposed to salt spray. Type 316 stainless steel performs without evidence of corrosion in the 100-hou, 5% salt spray (ASTM-B-117) test.
Type 316 is susceptible to precipitation of chromium carbides in grain boundaries when exposed to temperatures in the 800° F to 1500° F (427° C to 816° C) range. This “sensitized” steel is subject to intergranular corrosion when exposed to aggressive environments.
For applications where heavy cross sections cannon be annealed after welding or where low temperature stress relieving treatments are desired, the low carbon Type 316L is available to avoid the hazard of intergranular corrosion. This provides resistance to intergranular attack with any thickness in the as-welded condition or with short periods of exposure in the 800-1500° F (427-826° C) temperature range. Where vessels require stress relieving treatment, short treatments falling within these limits can be employed without affecting the normal excellent corrosion resistance of the metal. Accelerated cooling from higher temperatures for the “L” grade is not needed when very heavy or bulky section have been annealed.
Type 316L posses the same desirable corrosion resistance and mechanical properties as the corresponding higher carbon Type 316, and offers an additional advantage in highly corrosive applications where intergranular corrosion is a hazard. Although the short duration heating encountered during welding or stress relieving does not produce susceptibility to intergranular corrosion, it should be noted that continuous or prolonged exposure at 800-1500° F (427-816° C) can be harmful from this standpoint with Type 316L. Also, stress relieving between 100-1500° F (593-816° C) may cause some slight Embrittlement of this type.
Stress Corrosion Cracking
Austenitic stainless steels are susceptible to stress corrosion cracking (SCC) in halide environments. Although the Type 316 alloy is somewhat more resistant to SCC than the 18 Cr-8 Ni alloys because of the molybdenum content, they still are quite susceptible. Conditions which produce SCC are: (1) presence of halide ion (generally chloride), (2) residual tensile stresses, and (3) temperatures in excess of about 120° F (49° C).
Stresses result from cold deformation or thermal cycles during welding. Annealing or stress relieving heat treatments may be effective in reducing stresses, thereby reducing sensitivity to halide SCC. Although the low carbon “L” grade offers no advantage as regards to SCC resistance, it is a better choice for service in the stress relieved condition in environments which might cause intergranular corrosion.