Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

PART I. INTRODUCTION

B. Scope This bulletin attempts to characterize the corrosion resis- tance of alloys in the wide range of exposure conditions employed today in the production and handling of the organic acids. Space does not allow the complete coverage of alloy use in all organic acid processes, or even full treatment of such a large subject as acetic acid production. However, once the basic properties of the alloys in such media are established, along with adequate warning of problems to be avoided, the judicious choice of an alloy for a similar application can usually be made. The major pitfall in such use of data is assurance that the recorded conditions of exposure are indeed the same as those existing in the proposed application. Only parts per million of certain contaminants in an organic acid process stream can have a profound effect on the corrosion rate of an alloy. Thus, it is critical to learn the details of proposed operating conditions, as well as the possibilities for inadvertent changes in stream composition. Corrosion data reported throughout this bulletin must be interpreted as providing valuable information regarding the relative corrosion resistance of the various alloys in specific environments and modes of testing. Retesting of the alloys, particularly those containing chromium, under the same apparent conditions may provide variations in corrosion rates of two to three times. However. the relative resistance of the various alloys normally remains the same. Corrosion data for alloys in all of the many organic acids are reported when they are available. Extensive data for the more common acids encountered are reported. In addition, data for representative homologues of the various types of organic acids are reported. With this information as a guide, the interested party should be able to select candidate materials for an organic acid exposure of any type. The nominal composition of alloys cited in the tables and text are shown in the table on pages 2 and 3. An attempt has been made to provide as comprehensive a listing of alloys as possible to achieve the maximum utility from these data. Some of the proprietary alloys have been improved by compositional modifications. Where data exist for the newer modification they are included; however, some data on the obsolete alloys are included. Corrosion rates on the newer, improved alloys may be assumed to be approximately equivalent. Trademarks of proprietary alloys have been used in the text and are listed on the inside back cover. All materials are assumed to be in the mill annealed condition unless notations to the contrary are shown.

A. The Organic Acids

The organic acids constitute a group of the most important reactive chemicals of industry today. Billions of pounds of acetic acid are produced in the United States every year to provide the precursor for numerous products from aspirin to the recovery of zaratite minerals. Acetic acid is best known as the astringent compound in vinegar, but the acid and its anhydride are used in the manufacture of cellulosic fibers, commercial plastics, agricultural chemicals, dyes, plas- ticizers, certain explosives, ester solvents, metal salts; pharmaceuticals such as aspirin, sulfa drugs, vitamins, and as a precursor for a host of other organic compounds used in the preparation of drugs. Other organic acids are produced in much smaller volume, but constitute important chemicals for the prepara- tion of compounds used daily in our lives. The reactive acid (carboxyl) group present in these organic molecules is responsible for their wide use as ready building blocks for many commercial compounds. Research efforts to provide these chemicals in greater quantity at less cost has paralleled their increasing impor- tance. A multitude of processes have been commercialized for the production of acetic, acrylic, adipic, lactic and the higher acids. The volume and use of corrosive by-product formic acid has continually increased. In all of these processes, nickel-containing alloys are standard materials of

construction to withstand maintain product purity.

the corrosive environment and

C. Corrosion Testing in Organic Acid Media Some of the techniques used for determining corrosion rates and changes in environment in aqueous systems are difficult to apply in organic acid media. The specific conduc- tance of the higher acid concentrations is low for elec- trochemical studies and the low dissociation constant of the common organic acids requires major dilution of the com- pounds before reliable electrochemical data can be obtained.

Type 316L stainless steel tanks and piping and cast ACI CF-8M pumps and valves are utilized in this plant handling organic acids. Courtesy Walworth Company-Aloyco Valves.

Page 4