Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

TABLE XXII

Corrosion of High Nickel Alloys in Acetic Acid

Corrosion Rate

Temperature

%

MONEL alloy 400

Nickel 200

INCONEL alloy 600

Acetic Acid

ºC

ºF

mm/y

mpy

mm/y

mpy

mm/y

mpy

.03 B

1 B

2

30

86

.05

2

2

70

158

.10

4

2 5 6

116 116

241 241

.01 .08

0.2

.03

1

.28

11

3

.30 A ,.05 B .33 A ,.08 B

12 A ,2 B 13 A ,3 B

1.19 A ,.10 B

47 A , 4 B

26-30

79-86

.10 B

4 B

10

26-30

79-86

.02

0.8

1.37 A

54 A

10

70

158

10

116

241

.33

13

1.30 A

51 A

20

70

158

.41 A ,.08 B

16A,3 B

25

26-30

79-86

3.30 A

130 A

30

26-30

79-86

.46 B

18 B

30

60

140

.74 A ,.10 B

29 A ,4 B

4.32 A ,.25 B

170 A , 10 B

50 50 50

26-30

79-86

1.68 B

66 B

80

176 241

116

.05

2

.48

19

70

116

241

.36

14

.36 A ,.05 B .23 A ,.08 B

14 A ,2 B

75

26-30

76-86

9 A ,3 B

.13 B .61 B

5 B

99.9

26-30

79-86

24 B

99.9

80

176

99.9

116

241

.15

6

.36

14

100

26-30

79-86

.10

4

100

116

241

.30

12

3.05

120

.99

39

A = Aerated B = Unaerated

Reference 46 primarily.

There are process conditions which require that essen- tially no corrosion of the material of construction occur. Critical items of equipment required to operate with close tolerances such as orifice plates or control valve trim are examples. Another possibility is that the catalyst system used in the reactor of the process will not tolerate contamination with foreign metallic ions. In these cases, the maximum in corrosion resistance is demanded of an alloy, and only the nickel-base Mo-Cr-Fe, the nickel-base molybdenum, zirconium, titanium and tantalum alloys are potential candidates as solid or clad materials of construction. Although the organic acids are less aggressive than mineral acids in detecting sensitization of this class of alloy, prolonged exposure of the sensitized alloy in hot acetic acid can produce intergranular attack. The newer wrought materials, such as HASTELLOY alloys C-276, 12 C-4 and INCONEL alloy 625 are stabilized to forestall such attack on fabricated items of equipment. Castings of this type of alloy should be purchased in the fully solution-annealed condition. A test for susceptibility to intergranular attack is defined in reference 13. These alloys usually provide the ultimate in corrosion resistance to hot organic acid streams. If the environmental

conditions are such that general attack or pitting of this type of alloys is excessive, the use of tantalum, zirconium, graphite and brick-lined construction may be explored.

I. Nickel-Copper Alloys

Alloy 400 and other nickel-copper alloys have very good resistance to pure acetic acid solutions in the absence of air or other oxidants. Tables XXII and XXV, among others, show the low rate of corrosion of MONEL* alloy 400 when the exposure is free of oxidants. As with other alloys, the maximum corrosion appears to occur in the 50-70 per cent acid range. The data agree well with the curve (Figure 3) published by Uhlig for corrosion of the alloy in acetic acid at 30 ºC (86 ºF). MONEL alloy 400 withstands the effects of oxidants added to acetic acid better than do either nickel or copper alone, as shown by Table XXIII. However, the presence of air or an oxidizing agent such as ferric or cupric ion in solution is cause for concern and may lead to excessive attack. Corrosion tests should be run to ascertain the behavior of these alloys under operating conditions if oxidants are suspected to be present.

* Trademark of the INCO family of companies

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