Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

The presence of oxidizing agents in an organic acid stream completely changes the corrosive characteristics of the medium. Parts per million of oxygen, cupric or ferric salts, or peracid compounds in the stream will react stoi- chiometrically with alloys which do not produce protective oxide films. For instance, copper is essentially immune to attack by pure, uncontaminated acetic acid. Yet a small ingress of air at a circulating pump can drive the corrosion rate in a copper column to > 2.5 mm/y (hundreds of mils per year). Indeed, copper can be used as a scavenger of oxidizing species in an organic acid medium and has been so used. The addition of nickel to the copper moderates the effect of oxidants. In general, the greater the amount of nickel in the alloy, the less the effect of oxygen on the corrosion rate. This is illustrated by the data of Table XXV. The addition of nickel to copper appears to have little influence on the rate of attack in acid contaminated with heavy metal ions. The accelerating effect of these ions produces higher rates of attack which remain excessive regardless of the alloy composition. It is interesting to note the effect of dilution on the corrosive properties of the various mixtures. As would be anticipated, the corrosion rate is greatly accelerated when adding water to an air sparged solution or one containing ferric ions. However,

FIG 3—Corrosion of MONEL alloy 400 in Acetic Acid

The effect of liquid velocity on the corrosion of MONEL alloy 400 is shown in Table XXIV. No acceleration of the corrosion rate occurred up to 12.5 ft/sec velocity at a temperature of 30 ºC (86 ºF). It is believed that velocities of this magnitude would not increase the attack on MONEL alloy 400 up to temperatures of 100 ºC (212 ºF). Alloy 400 and the cast counterpart of Alloy 400, ACI M-35 alloy, have found useful service for many years in some dilute acetic acid solutions handled in the food industry at the lower temperatures. Alloy 400 is attractive because contamination of the food products with ferric or cupric ions is undesirable. Small amounts of iron can contaminate the products if ferrous alloys are used and excessive copper pickup can be experienced if the copper content of the alloy is higher than that of Alloy 400. Corrosion rates for MONEL alloy 400 in a typical dilute acetic acid solution of this type are shown in Table XXII. Mason has covered the subject of the alloy’s use in food products very well. 14 J. Copper-Nickel Alloys All of the copper alloys excepting those with high (> 15%) zinc are resistant to acetic acid in the absence of air and other oxidants. Until the advent of the stainless steels, copper was used almost exclusively for the handling of acetic acid.

TABLE XXIII

Effect of Aeration on Corrosion of Nickel, Copper and Their Alloys in Acetic Acid Conditions: Laboratory tests in 6% acetic acid at 30 ºC (86 ºF)

Corrosion Rate

Without Aeration With Aeration

Alloy

mm/y

mpy

mm/y

mpy

Nickel 200 MONEL alloy 400 C 71500 (70-30 Cupro-nickel) Copper C 10300

.08 .05 .08 .08

3 2 3 3

.28 .20 .81 .48

11 8 32 19

Reference 47

TABLE XXIV

Effect of Velocity on Corrosion of MONEL Alloy 400 in Acetic Acid

Temperature

Corrosion Rate

Test Period, hr

Velocity ft/sec

Medium

ºC

ºF

Aeration

mm/y

mpy

50% aqueous Acetic Acid

30

86

48

100 cc/min

0 1.8 3.8 8.7 12.5

.38 .41 .43 .41 .46

15 16 17 16 18

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