Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

TABLE LIV

Corrosion of Alloys in Animal Fatty Acids

Conditions a Temp. ºC (ºF) Time, days Exposure

1 100 (212) 130 Liquid

2 250 (482) 147 Liquid

3 250 (482) 210 Vapor

4 250 (482) 84 Vapor

Corrosion Rate

Alloy

mm/y

mpy

mm/y

mpy

mm/y

mpy

mm/y

mpy

.18

7

– – –

– – –

– – Consumed Consumed .79 31 .20 8 Nil Nil .05 2 .08 3 .10 4 .13 5

Mild Steel Cast Iron Ni-Resist Type 2

Consumed Consumed >3.46 >140 .33 13 .05* 2* .01* 0.2* .01 0.3 .13 5 .15 6 – –

1.63

64

.23

9

Nil Nil Nil

Nil Nil Nil

.36*

14*

Type 304 Stainless Steel Type 316 Stainless Steel INCONEL alloy 600 Nickel 200 MONEL alloy 400 Copper

Nil .01 .41 .58

0.1 0.3

.08 .05

3 2

16 23

–

–

–

–

*Pitted a Conditions: 1–Field test in storage tank for mixed acids from fish oils. 2–Field test in outlet of preheater to distillation column processing animal tatty acids. 3– Field test in overhead vapor of column distilling acids from fish oils. 4–Field test on feed tray of distillation column handling crude fatty acids from tallow.

References 28,49

fraction at various temperatures. It is obvious that at some place above 300 ºC (572 ºF) the use of the 300 series stainless steels is questionable in such a mixture. At this point, the use of the more highly alloyed materials should be investigated. The vegetable oils, characterized by stearic and palmitic acids among others, appear to have somewhat less aggres- sive characteristics than the tall oil acids. Table LIII shows data for the exposure of alloys in a diverse group of field exposures. It will be noted that INCONEL alloy 600 and Type 316 stainless steel are resistant to all of the processing conditions. Indeed, INCONEL alloy 600 vessels have been used for over 30 years with good success in the processing of vegetable oil acids. Other aspects of the handling of these vegetable oil fatty acids would be the same as described for the tall oil acids. Those acids derived from animal fats appear to be somewhat more aggressive. Table LIV shows data ob- tained while processing acids derived from fish oils and beef tallow. Again, the INCONEL alloy 600 and Type 316 stainless steel appear to be the most attractive materials for construction of such equipment. E. Di and Tricarboxylic Acids Although the di and tricarboxylic acids are produced in less quantity than the monobasic acids, the products constitute a most important industrial commodity. Many of the acids and corresponding anhydrides are used in the synthesis of drugs, food products, plasticizers and resins. Citric, oxalic and certain other of the acids are used extensively as metal cleaning agents. However, the most important of the products are maleic and phthalic anhydrides used to produce alkyd and polyester resins, the para-phthalic acid used in the preparation of polyester fibers and adipic acid required for nylon synthesis.

The lower acids of this series are more aggressive in aqueous solution than are the monobasic acids at the same temperature and concentration. Dissociation of these acids in water is greater than for acetic or formic acid. In addition, the multiple acid grouping has the capacity to solubilize cations by chelation. Thus, protective, insoluble corrosion products are normally not found on the surface of a metal attacked by this group of acids. This allows continuous attack on the clean metal surface. Since the rate of attack is not as severe as when using mineral acids, oxalic, citric and certain other of the dibasic acids are used to clean metal surfaces. The elemental dibasic acid is oxalic (ethanedioic) acid; sublimes at 150 ºC (302 ºF). As with other first homologues of a series, oxalic acid is extremely aggressive in its attack on most metals. Rates of corrosion are significantly higher on alloys than with acetic acid at the same concentration and temperature (Table LV). However, the relative corrosion resistance of the alloys remains essentially the same. Higher alloying is required to provide an alloy with useful resistance to attack. For instance, Type 304 stainless steel is attacked excessively in most concentrations of the acid at temperatures above ambient, and Type 316 stainless steel, although significantly more resistant, has severe limitations of use. Table LVI shows the corrosion to be expected by exposure of a variety of alloys to oxalic acid. More corrosion data are available for the 10 per cent concentra- tion of the acid at the boiling temperature than far other combinations, because: (a) 10 percent represents satura- tion in cold, 25 ºC (77 ºF), water, (b) the mixture is an aggressive cleaning solution for metals and (c) the mix- ture is often used as a corrosion test medium for the evaluation of alloys. It is obvious that higher amounts of nickel in an austenitic base are beneficial in combatting attack by

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