Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

normal carboxylic acid terminus to the molecule, and, in addition, the incorporation of a halogen, an amino, a hydroxy addition, or other active ion added to the mole- cule which brings unique characteristics to the product. 1. Glycolic Acid The simplest of the organic acids in this category is glycolic (hydroxyacetic) acid. As an acid in aqueous solution, the material does not appear to be excessively corrosive at the lower temperatures. For instance, Type 304 stainless steel will show only .003 mm/y (0.1 mpy) corrosion rate or less in a 6% solution of glycolic acid at ambient temperature. At 50 ºC (122 ºF) during tests of eight days, both Types 304 and 316 stainless steels showed no attack in a 50% aqueous solution. Thus, the acid could not be described as exceedingly corrosive at conditions normally encountered. However, it has been found to be corrosive when heated to higher temperatures when contained in process streams as a contaminant. Again, the stainless steels resist attack at the high temperatures, but areas where steel would normally be acceptable become impractical with contamination of the streams by glycolic acid. Type 304 stainless steel is then required. 2. Lactic Acid Lactic acid (hydroxypropionic acid) is familiar to most persons as the corrosive agent in milk. To maintain the purity of the milk, tanks and tank trucks of Types 302 and 304 stainless steels have been constructed for many years for the handling of this precious commodity. As indicated by Tables LXV and LXVI, Type 304 stainless steel and its cast counterpart CF-8 is most satisfactory for the handling of aqueous lactic acid solu- tions at the lower and intermediate temperatures. At some point between 2 and 10%, aqueous solutions of lactic acid begin to attack Type 304 and CF-8 stainless steels exces- sively. One can then use Type 316 stainless steel and its cast counterpart CF-8M which shows good resistance throughout the range of concentrations and temperatures explored (Table LXV11). Thorough testing of Type 316

stainless steels are explored as potential candidates. The ease of fabrication of the Type 304 stainless steel, com- bined with its more than satisfactory corrosion resistance, makes the material a prime candidate for such service. No corrosion of Type 304 stainless steel in the most basic of the aromatic acids (benzoic) is apparent. Tests using a two per cent aqueous solution at 100 ºC (212 ºF) or of 10 per cent in an anhydrous octanol solution at 130 ºC (266 ºF) produced no attack on the alloy. In many instances, the corrosion attributable to organic acids in such systems is compounded by the presence of sulfur compounds, the lower aliphatic acids, chlorides and other contaminants. Thus, in making a choice of materials for such service, the possible effect of chloride ion, sulfur ions, or other contaminants that may accumulate at times in the equipment must be considered. Stress-corrosion cracking of the austenitic stainless steels can be experi- enced under certain circumstances and must be evaluated thoroughly before the choice of such an alloy is made. Gutzeit has pointed out that the corrosion in such systems is directly related to the neutralization number. Curves showing the corrosion rate for various alloys as related to the neutralization number are provided in his paper. 40 One of those is reproduced here as Figure 13. Corrosion occurs in the liquid phase with only mild corrosion experienced in the vapor areas. Thus, hot condensate is always a potential corrosive in such a system. The use of Alloys 400 or 600 and 800 in such systems has merit. If stress-corrosion cracking of the stainless steels are experienced, the use of these alloys should be considered.

G. Organic Acids with Other Functional Groups

There are a large number of organic acids of complex structure which have found extensive use in industry and home. The corrosion characteristics of this group of compounds varies widely, as do the organic structures. Organic acids with other functional groups describe the

TABLE LXV

Corrosion of Alloys in Various Concentrations of Aqueous Lactic Acid

Conditions:

% Lactic Acid Temperature, ºC Temperature, ºF Test Period, days Other

0.5 100 212

1 65 149

2 100 212

5 26 79 21 –

45 26 79 14

10-50 54 129 15

1 –

1 –

1 –

Aerated; agitated

Field Test in vacuum evap.

Corrosion Rate

0.5%

1%

2%

5%

45%

10-50%

mm/y

mpy

mm/y

mpy

mm/y

mpy

mm/y

mpy

mm/y

mpy

mm/y

mpy

Alloy

Nil

Nil

.03

1 –

– –

– –

–

–

–

– 8

Type 304 Stainless Steel

.03

1 –

–

–

–

Nil

0.1

.20

INCONEL alloy 600

–

C71000

–

–

–

–

.02

0.9

–

–

–

–

(Cupro- nickel 80- 20)

–

–

C71500

–

–

–

–

–

–

–

–

1.4

57

(Cupro-nickel 70-30)

–

–

References 19, 49

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