Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

Processes employing halide catalysts in the reaction system to produce acetic acid must be assessed thoroughly to determine where the less costly stainless steels can be used in the process train. Type 316 stainless steel usually cannot be used in the reaction area or in the first separation steps. More highly alloyed materials are required. Once the halide ion is removed, the overhead acid stream from the distillation train can be processed safely in stainless steel. (See section on Process and Plant Corrosion Data.) Stress-corrosion cracking of the 300 series stainless steels may occur readily in aqueous acidic media containing chlorides. Presumably the cracking will not occur in a completely anhydrous medium, but such a water-free sys- tem is obtained rarely and some water must be assumed to be present. Where the chloride-containing acid solution can concentrate on the surface of stainless steel under stress, cracking of the metal can occur. Such areas as gasket joints, crevices and liquid-vapor interfaces in the equipment are examples of zones where such cracking (and pitting) often occurs in chloride-containing acetic acid. Cracking may also occur beneath deposits or at the base of pits on the surface of the stainless steels. Where the metal surface is washed continually with fresh liquid, there is little likelihood of stress-corrosion cracking. If the process temperature is less than 80-90 ºC (176-194 ºF) the cracking process may be sufficiently slow to allow a respectable service life for the equipment before failure occurs. At temperatures below 50-60 ºC (122-140 ºF), stress-corrosion cracking usually does not occur. Stress- corrosion cracking may be avoided by the use of higher nickel alloys or duplex stainless steels. With the exception of formic acid, (see Section on Formic Acid), other contaminants found in the usual acetic acid process stream only serve to dilute the acid and reduce the rate of attack. Aldehydes, ketones, esters and higher acids are in this category. 4. Effect of Temperature It has been shown that Types 316 and 316L stainless steels are satisfactorily resistant to attack by all concentrations of acetic acid to the boiling point and that Type 304 stainless steel is acceptable for use in all concentrations of acid less than approximately 90 per cent to the boiling point. As the temperature is increased beyond these points, the rate of attack on the stainless steels in the liquid acid increases, but certainly not as rapidly as the Arrhenius equation would indicate. Laboratory and field data presented in Tables V and XI through XIII show that for both wrought and cast alloys the stainless steels remain useful at temperatures well above the atmospheric boiling point. Various techniques of testing can produce significantly different results and ingenuity is required to establish stable conditions for the desired test environment. Figure 1 condenses considerable data generated by Ohio State University personnel when exploring the corrosion resistance of the cast alloys in acetic acid up to 200 ºC (392 ºF). 9 The cast CF-8 alloy corrodes at in- creasingly greater rates as the temperature is increased

until excessive rates of attack are obtained. However, CF- 8M resists the effect of increased temperature quite well and has potential for use at the 200 ºC (392 ºF) temperature. Field applications utilizing CF-8M pumps in acid near this temperature confirm the utility of the alloy for handling hot acid when oxidizing conditions exist. Table XII shows other data obtained in the upper temperature region of Figure 1. Note the lower corrosion rate for a Type 316 stainless steel at 190 ºC (374 ºF), although the test period is longer. Sufficient peroxide appears to be effective in reducing corrosion, even at these high temperatures. The presence of ferric ion was detri- mental at these temperatures as opposed to the beneficial effect noted at lower temperatures. Vapors of the acid at higher temperatures are not aggressive in the absence of condensation (Tables VIII and XIV). However, condensation or drippage of liquid on a hot metal surface can produce excessive attack. In addition, pitting of the austenitic stainless steels in acetic acid exposures at the higher temperatures is possible. It is obvious that careful assessment of the stability of the 300 series stainless steels in an acetic acid environment must be made before discounting their use at even the higher temperatures.

TABLE XIII

Corrosion of Nickel-Containing Alloys in Buffered Acetic Acid at High Temperature

Test Conditions: Specimens exposed in a high pressure autoclave at temperature of 200 ºC (392 ºF) for 8 days to the following solution without aeration or agitation: 15% acetic acid plus 19% ammonium acetate aqueous solution at 250 psi.

Corrosion Rate

Alloy

mm/y

mpy

HASTELLOY alloy C-276 INCONEL alloy, 625 INCOLOY alloy 825 HASTELLOY alloy G Nickel 200 IN-862 Cast Alloy Type 315 Stainless Steel (sensitized)

02 02 02 03 04 05 13*

0.6 0.7 0.8 1.0 1.5 1.8 5.2*

Type 316 Stainless Steel (annealed)

04*

1.5*

*Incipient pitting

TABLE XIV Corrosion of Stainless Steels in Vapors Over 52 Per Cent Aqueous Acetic Acid at High Temperature

System Pressure

Corrosion Rate

Temperature

Alloy

ºC

ºF

psig

mm/y

mpy

142

288

35

.03

1

Type 304 Stainless Steel

153

308

55

.10

4

Type 304 Stainless Steel

Type 316 Stainless Steel

142

288

35

<.03

< 1

Type 316 Stainless Steel

153

306

55

.05

2

Each datum is average of eight tests conducted in closed pressure vessel.

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