Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

F. Naphthenic Acids The naphthenic acids have received much attention during the past 20 years as a corrosive in process streams of the oil refineries. A considerable volume of data has been generated relating to the operation of equipment handling streams contaminated with these acids. 29, 36-40 The term “naphthenic acids” describes a group of aro- matic compounds containing one or more carboxyl groups and does not refer to a specific structure. The term em- braces acids from benzoic through those of the true naph- thenic structure, all of which can contribute to corrosion at the very high temperatures of oil refining. The corrosive potential for streams containing these acids is defined by “neutralization number” rather than acid content. Thus, all acidic materials in the stream are categorized by the term naphthenic acid. Providing materials of construction to resist naphthenic acid corrosion is not difficult, although the economics of selection are critical. When the neutralization number exceeds 0.5, the streams are considered to be corrosive to steel. The use of an austenitic stainless steel will provide resistance to corrosion during processing of the streams. However, the economics of providing materials of con- struction for such large process equipment requires that the optimum material be found. Thus, the various alloy materials between steel and the 300 series austenitic

TABLE LXIV

Corrosion of Stainless Steels in Molten Pimelic (Heptanedioic) Acid

Conditions: Metal specimens completely immersed in molten acid under quiescent conditions at 225 ºC (437 ºF). Unreported contaminant suspected to be present. Results shown are averages of duplicate tests.

Corrosion Rate

Initial 117 hr

Second 73 hr

Alloy

mm/y

mpy

mm/y

mpy

9.36 5.89 1.65

369 232 65

12.89 7.26 2.77

508 286 109

Type 304 Stainless Steel Type 347 Stainless Steel Type 316 Stainless Steel

(heptanediocic) acid are summarized in Table LXIV The greater corrosive activity of the acid in these tests is probably attributable to a process contaminant. Although the higher iron-base and nickel-base alloys were not tested, it is probable that these alloys would be satisfac- torily resistant under such conditions, particularly the nickel-base molybdenum-chromium-iron alloys. The tricarboxylic acids without other functional groups are found in nature (e.g., tricarballylic acid in beets), but are produced by industry only as a development chemical. No corrosion data are known to have been published concerning those compounds. It is possible that such a structure would generate corrosion comparable to observed for citric acid. (See Section G-4, Part III.) that

A vacuum distillation column at a major petroleum company. This photo shows the crossover piping loops in foreground and 1,500 mm (60-inch) transfer line entering column tangentially. The transfer line and the column are lined with Type 316 stainless steel to resist naphthenic acid and sulfidic corrosion.

Fig 13– Corrosion Isotherms for Various Steels and MONEL alloy 400 in White Oil/Naphthenic Acid Blends at 235 º C (455 º F) Tempera- ture

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