Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

conditions for the acid (m.p. 16 ºC or 61 ºF). For this service, the coil is usually constructed of Type 316L stainless steel. Heat transfer on the surface of Type 304 stainless steel can produce excessive corrosion at normal steam temperatures. The vessel proper can be constructed of Types 304, 304L, 321, 347, 316, or 316L stainless steels. If a truly meticulous grade of acetic acid is required, such as USP grade, it will be necessary to use the molybdenum- containing grade of stainless steel if temperatures are to exceed 50-60 ºC (122-140 ºF). In this range, a minute amount of metallic contamination of acetic acid can occur in contact with the Type 304 analysis or other grades not containing molybdenum. The use of a molybdenum- containing stainless steel moves this point of initial contamination to some 70-80 ºC (158-176 ºF) before any detectable metallic ion is picked up by the acids. 5 When using the stainless steels for tankage equipment in a meticulous service, it is advisable to clean (pickle, passivate) the interior of the vessel to remove all traces of iron contamination that might have been embedded in the stainless steel at the mill. Sulfamic acid, nitric acid, oxalic acid, or other acids as suggested by ASTM A 380 can re- move the embedded iron and at the same time provide a uniform clean surface for the stainless steel. Since the more aggressive acids can cause intergranular corrosion of sensitized stainless steels, it appears prudent to utilize a low carbon or stabilized grade if fabrication by welding is anticipated. Shipping containers constructed of stainless steel provide the greatest durability combined with the best preservation of the refined acetic acid of all materials available today. Tankers have stainless steel-lined com- partments for shipment of the organic acids and other corrosive products. If entire compartments are not justified on tankers for conveying the acetic acid, deck tanks can be added which are durable and free of harmful corrosion in the severe exposures of marine transportation. Complete barges have been constructed using stainless steel for meticulous care of the product during shipment. Tank cars constructed of stainless steel have been used for 40 years on the rails for acid shipments as well as to provide the versatility required for the shipments of other aggressive commodities. In the smaller containers, the austenitic stainless steels remain unparalleled as the material of construction for drums, cans and other items used for the shipment of acetic acid. With the higher area-to-volume ratios existing in the small containers, it is imperative that no corrosion occur on the container walls to contaminate the acid. For this reason, the stainless steels constitute standard con- struction identified as ES and ESM (DOT designation for Types 304 and 316 stainless steels) drums and cans for acetic acid shipments. These not only provide good protection of the acid during long periods of shipment and storage, but are durable and reusable for many years because of the good strength and external corrosion resistance of the stainless steel container. Stainless steel containers are readily cleaned on the inside to provide a spotless, uncontaminated surface for reuse.

The forward tanks in this double-skinned barge are clad with Type 316 stainless steel, capable of transporting organic acids as well as other liquid cargo.

3. Vinegar Production and Storage All vinegars contain acetic acid in addition to variable amounts of nonvolatile organic acids such as malic and citric acids and smaller amounts of succinic and lactic acids. The term “grain strength” is used to express the acetic acid concentration, which is ten times the acetic acid content. Protection of the vinegar from metal contamination, particularly iron and copper, has led to the use of the austenitic stainless steels for both production and storage. Table XXXI shows that both Types 304 and 316 stainless steels are unaffected in 40-320 grain vinegar at the -17 to 35 ºC (2 to 95 ºF) process temperatures involved. Because of the low temperatures, intergranular corrosion of sensitized stainless steel is not a problem. Although 120 grain (12 per cent acetic acid) is a common strength, the particular plant at which this test was run produces up to 300 grain vinegar in Type 304 stainless steel equipment and piping without corrosion problems or product contamination. Acetic anhydride has long been made as a co-product in the “dual” oxidation of acetaldehyde to acetic acid. With the many streams in this process containing both acetic acid and the anhydride, it is important to understand the effect of small amounts of anhydride residual in hot acid streams. However, the anhydride itself is only mildly corrosive. In the absence of acetic acid, distillation in Type 304 stainless steel equipment is acceptable. In the newer process where acetic acid or acetone is cracked to ketene, which is then reacted with acetic acid to form the anhydride, it is reported that Type 316 stainless steel is fully satisfactory for the construction of distillation columns and other process equipment. However, Type 316L stainless steel is often selected so that the equipment is more versatile and can be used for other organic acid services where more stringent conditions might exist. In the process utilizing acetic acid as the starting material, the cracking tubes are of prime interest. 23 Once P. Acetic Anhydride

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