Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

TABLE LIX

taining the molten acid or in scrubber waters rich with the water-soluble acid. As an example, the majority of equip- ment used in the butane oxidation process for maleic anhydride was originally of Type 304L stainless steel construction. However, unforeseen accumulations of mal- eic acid in portions of the equipment dictated a shift to the use of the more resistant Type 316L stainless steel. The benzene process to produce the anhydride is even more corrosive, and Type 316L stainless steel is used exten- sively throughout the process chain. Corrosion to be expected from exposure of alloys in various aqueous concentrations of maleic acid at the boiling point is summarized in Table LVIL These data relate to the corrosion found in process scrubber systems where water is used as the scrubbing medium. Note the loss of Type 304 stainless steel as a usable material of construction at concentrations of 10 per cent or more acid. Higher iron-base stainless steel alloys, such as Type 316L and above, show acceptable resistance in all aqueous concentrations. The determination of corrosion rates for the stainless steels in both aqueous and molten maleic acid composi- tions is difficult. The maleic acid in the absence of more aggressive anions is slow in penetrating the oxide film on the stainless steels to initiate corrosion. Consequently, multiple tests of sufficient duration must be conducted to provide meaningful “rate” data for the corrosion process. Also, during the test period, a conversion of a portion of the maleic acid to insoluble fumaric acid will occur, which must be taken into account if the data are to be precise. Pure maleic acid in the molten form is not encountered normally in industry, but does exist in certain of the anhydride process streams. See Table LVIII for field corrosion data obtained in streams containing the molten acid as a contaminant.

Corrosion of Alloys in Phthalic Acid and Phthalic Anhydride

Conditions: Laboratory test of duplicate specimens at 150 ºC (302 ºF) for 13 days without aeration ordeaeration.

Corrosion Rate

1:1 Phthalic Anhydride: Phthalic Acid Mixture

Phthalic

Phthalic Acid

Anhydride

Alloy

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

<.03

<1

.03

1

Mild Steel

<.03

<1

Type 304 Stainless Steel Type 316 Stainless Steel

<.03

<1

<.03

<1

<.03

<1

<.03

<1

<.03

<1

<.03

<1

Phthalic acid, decomp. ca. 200 ºC (392 ºF), is found in many of the same process streams containing the maleic acid. However, the contribution of phthalic acid to corro- sion of the equipment is minimal. Table LIX shows corrosion data for steel and Types 304 and 316 stainless steels exposed to hot phthalic acid, phthalic anhydride and a mixture of the two. These chemicals are not aggressive. However, the austenitic stainless steels are often used to process these chemicals to prevent contamination of the product and to provide a surface readily. that can be cleaned

TABLE LX

Corrosion of Alloys in Terephthalic Acid Media Laboratory Test 6% Terephthalic Acid in Water Laboratory Test 6% Terephthalic Acid 84.6% Acetic Acid 9.4% Water

Field Test TPA Leach Feed Slurry (TPA + Acetic Acid)

Field Test Leach Crystallizer Liquid (14.1 % TPA, 82.7% Acetic Acid, 2.7 % water)

Temperature, º C Temperature, º F Test Period, days

232 450 24

232 450 24

260 500 14

177 351 523

Corrosion Rate

Alloy

mm/y

mpy

mm/y

mpy

mm/y

mpy

mm/y

mpy

.06 .02

2.4 0.8

– –

– –

–

–

Type 304 Stainless Steel Type 316 Stainless Steel Type 216 Stainless Steel Type 317 Stainless Steel CARPENTER alloy 20Cb-3

.01 .01

0.4 0.3

.03

1.0

– – – –

– – – –

.19

7.3

–

–

– – – –

– – – –

– – –

– – –

.01 .02 .35 .03

0.2 0.7

13.8

INCOLOY alloy 800

Nil Nil

0.1 0.1

.04

1.5 0.1

1.1

HASTELLOY alloy C- 276

Nil Nil

0.1

Titanium

<0.1

Nil

Nil

<0.1

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