Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

TABLE XLI

Corrosion of Stainless Steels During the Preparation of  -Methacrylic Acid

Corrosion Rate

Type 304 Stainless Steel

Type 316 Stainless Steel

CARPENTER alloy 20

HASTELLOY alloy C

HASTELLOY alloy B

Silicon Bronze

Aluminum 3003

Exposure

DURIRON

mm/y mpy mm/y mpy mm/y mpy

mm/y mpy

mm/y mpy

mm/y mpy

mm/y mpy

mm/y mpy

.03

1

<.03

<1

<.03

< 1

–

–

–

–

–

–

–

–

Oxidation of the acid by

.03

1

air blowing of the aldehyde at 40 ºC (104 ºF) Distillation of the crude oxidation product at ca. 55 ºC (131 ºF) (liquid exposure) As above (vapor exposure)

<.03

<1

<.03

<1

<.03

<1

<.03

< 1

.03

1

.05

2

Nil

Nil

.51

20

<.03

<1

<.03

<1

<.03

<1

<.03

<1

.38

15

1.88

74

<.03

<1

.23

9

TABLE XLII

pylene-acrolein process. The oxidation product of the ß - methacrylic aldehyde would contain some oxidized de- composition compounds such as formic and acetic acids. However, the reasonable low temperatures at which the product must be handled, combined with the low con- centration of such contaminants, does not produce an aggressive medium for the stainless steels. The data of Table XLI show results obtained in oxidation and primary distillation steps of the process. Further information relating to the acrylic acids is contained in Table XLII. In processing the more stable dimethyl acrylic acid at the higher temperatures, it is apparent that a Type 316 stainless steel is required. The temperature of the operation exceeds the point where the Type 304 stainless steel is adequately resistant. C. C 3 Through C 8 Acids The first of the remaining higher acids, the 3-carbon propionic acid, is produced in considerable quantity. The acid and its unsaturated counterpart, acrylic acid, are very similar to acetic acid in reactivity with metals. The corrosion rate of the common materials of construction is essentially the same in propionic and acrylic acid as in acetic acid at the same temperature. Certainly, all factors described as influencing the corrosion of alloys in acetic acid are applicable to corrosion mechanisms in the 3- carbon acids. Corrosion rates of various alloys in boiling propionic acid solutions are shown in Figure 12. 5 Elder points out the anomalous results that can result from the short test period used for these tests and the effect of dissolved oxygen on the results. The beneficial effect of added oxygen on austenitic stainless steels is not restricted to laboratory tests but was also attained in the field as shown in Table XLIII. It is interesting to note that a maximum rate of attack on the stainless steels appears to occur at approximately the same concentrations (60-80 per cent) as found for acetic acid in boiling solutions. For welded construction, the low carbon stainless steel grades should be employed unless it has been definitely established that

Corrosion of Alloys in Dimethyl Acrylic (Seneceoic) Acid

Field test obtained by exposure of alloys in the overhead stream of a refining column at 145ºC (293ºF) for three days.

Corrosion Rate

Alloy

mm/y

mpy

1.83 .18 .18 <.03 .05 .46 1.45

72 7 7 < 1 2

Type 304 Stainless Steel Type 316 Stainless Steel (annealed) Type 316 Stainless Steel (sensitized)

HASTELLOY alloy C HASTELLOY alloy B MONEL alloy 400 Copper

18 57

The major commercial approach to acrylic acid produc- tion today is the direct oxidation of propylene to acrolein with subsequent oxidation to acrylic acid or a one-step oxidation with only the acid recovered. One advantage of the process is the milder corrosive conditions existing throughout the unit. Steel and the austenitic stainless steels may be used for all equipment except where chloride stress-corrosion cracking of the stainless steels requires the use of Alloy 600, Alloy 400, or other crack- resistant alloys. Table XL provides data regarding the corrosion of a number of alloys in significant portions of a propylene oxidation process. Although the austenitic stainless steels are resistant to the primary corrosive agents throughout the process, the use of INCONEL alloy 600 and other high alloys have been used in the process for the reason cited. Similar data were obtained by the exposure of alloys in a plant preparing ß-methyl acrylic (crotonic, 2-buteneoic) acid. Types 304L and 316L stainless steels and CARPEN- TER alloy 20Cb-3 were unattacked in process handling of the acid up to 90 ºC (194 ºF) in a process similar to that described for the production of acrylic acid by the pro-

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