Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

stainless steel has been supplied. The molybdenum spot test is most often utilized in this regard. The cost of such a procedure is appreciable, but becomes insignificant in comparison with the failure of a piece of equipment once the unit is in operation. Simple items such as the drain plug in a pump, a welding elbow in a hot acid line, a few incorrect tubes in the heat exchanger and many other small items can create disastrous problems if an inadvertent substitution of a lower grade of stainless steel has been made for the Types 316 or 316L analysis identified for the use. A materials identification procedure on the site to provide assurance of proper alloy installation is very easily justified economically. Kits are commercially available with complete instructions for doing such work on the site very quickly and easily. One person assigned to this work throughout the life of a project may pay for the services many times over. The standard AISI grades of martensitic and ferritic stainless steels generally do not possess sufficient corro- sion resistance for use in acetic acid service, except possibly at low concentrations and temperatures. Table XVIII shows typical corrosion data for the martensitic Type 410 stainless steel. Included for comparison are steel, cast iron and a nickel alloyed cast iron. When evaluating these materials for an application, it is important to assure that the service conditions are reproduced as closely as possible. Laboratory tests can show a considerable disparity in results because of the possibility of forming a fragile protective film on the alloy in a short time. After a high initial rate of attack, the rate will subside to a low value if the film is undisturbed by flow or other mechanical effects. Duplex structured austenitic-ferritic stainless steels and certain precipitation hardening stainless steels can show remarkable resistance to organic acids depending on the ratio of nickel to chromium and other minor alloying constituents. Table XIX illustrates the resistance of several precipitation hardening stainless steels in acetic acid at various temperatures. It is important to understand that the selection of such alloys for a specific application is more critical than when appraising an austenitic stainless steel. Prior processing of the alloy can have a significant effect on the corrosion resistance. The influence of heat treatment on the corrosion resistance of three precipitation hardening stainless steels in acetic acid is shown in Table XIX. It is obvious that the metallurgical condition of the alloy must be known when considering these alloys for acid service. Certain treatments of the alloys can greatly reduce their corrosion resistance. The data also reveal the borderline passivity of these alloys in such service, par- ticularly in the intermediate concentration of acid. The effect of heat treatment on the molybdenum-containing C. Martensitic and Ferritic Stainless Steels D. Duplex Austenitic-Ferritic and Precipitation Hardening Stainless Steels

not observed. Type 316L stainless steel, when heated for prolonged periods in certain temperature regions above 500 ºC (932 ºF), can produce sigma or chi phase in the alloy. Type 317L stainless steel with higher molybdenum con-tent is slightly more prone to formation of these phases. These phases are rich in chromium (chromium and molybdenum in the case of chi phase) and can have much the same effect as the more commonly known M C and 23 6 M C carbide precipitation in the alloy. Such a metallurgi- 6 cal phase change can occur in the fabrication of the clad vessel when it becomes necessary to stress relieve the steel backing. At the 500-650 ºC (932-1202 ºF) stress relief desired, sigma or chi phase can be produced to create severe corrosion of the clad material on the interior during process operations. Lower stress relieving temperatures are required to avoid such an undesirable metallurgical condition if these grades of stainless steel are to be used. 6. Quality Control Qualification tests are often used to assure that the initial material is of proper quality and that any heat treatment of the equipment has not produced undesirable effects. Clippings from sheet and plate, small sections of tubes and other small sections removed from pieces of equip- ment are sent to the laboratory for validation of the existing condition of the material and its ability to maintain appropriate corrosion resistance. These qualification tests have been standardized by the American Society for Testing and Materials (ASTM) and are divided into practices A through E of Recommended Practice A 262. Each of these is designed to detect specific types of phase formation in the alloy. Of these, Practice A, the electrolytic oxalic acid etch (EOAE) test, is the most sensitive. Normally, if a heat of stainless steel fails to pass the EOAE test, samples are tested in accordance with one of the other practices before rejection of the heat is allowed. However, because of the sensitivity of the EOAE test, some workers have advocated that acceptance or rejection be based upon this test alone to assure maximum corrosion resistance in the alloy. Major losses in equipment and even more expensive, extended periods of downtime may be avoided by these simple procedures. Castings to be used for pumps, valves and other critical parts of the equipment can be tested in the same manner. Solution annealing of castings is mandatory to assure the optimum corrosion resistance desired. Small amounts of ferrite provided in the matrix to assure crack-free castings of the best strength and quality are not harmful. However, carbides and other constituents which might be isolated along the dendrites of a casting should be in solution to prevent selective attack of such areas. The quality control program for assuring that the stainless steels used in acetic acid manufacture meet specification requirements is sometimes extended to qualitative chemical analysis by means of spot testing of all material received by the fabricator of the equipment and by those in the field responsible for installing piping, heat exchangers, vessels and all other equipment to be exposed to the hot acid to help assure the proper grade of

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