Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds

TABLE XLIII

Effect of Oxygen on Corrosion of Stainless Steel in Propionic Acid

Conditions: 95% propionic acid containing 2% water, alcohol, ketone and higher acids used in laboratory tests and processed in field.

Corrosion Rate

Type 316 Stainless Steel

Type 304 Stainless Steel

Type 202 Stainless Steel

Type of Test

Additive

Temperature

Exposure

ºC

ºF

mm/y

mpy

mm/y

mpy

mm/y

mpy

None

122 to 135 122 to 135 122 to 135 122 to 135 122 to 135 110 to 137

252 to 275 252 to 275 252 to 275 252 to 275 252 to 275 230 to 279

Liquid Vapor Condensate Liquid Vapor Condensate Liquid Vapor Condensate Liquid Vapor Condensate Liquid Vapor Condensate

.15 .20 .05 Nil Nil Nil 01 01 Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil

6 8 2

.30 .28 .36

12 11 14 64 6 01

– – – – – – – – – – – – – – –

– – – – – – – – – – – – – – –

Laboratory

Air Sparged

Nil Nil Nil

1.63 .15 Nil

Laboratory

9 ppm H 0 2 2

0.3 0.2 0.1

.04 .02 Nil Nil Nil Nil Nil Nil Nil

1.5 0.6 0.1

Laboratory

1 ppm H O + 2 2 air sparged

Nil Nil Nil

Nil Nil Nil

Laboratory

200 PPM CuSO 4

<0.1 <0.1 <0.1

<0.1 <0.1 <0.1

Laboratory

Air and H O 2 2 injected in feed stream

Kettle Based on column Feed line Middle of column Top of column

<0.1 <0.1-0.1

Nil Nil-.03

<0.1 <0.1-1

<0.1 <0.1-0.1 0.1

Field Column processing the acid*

Nil Nil Nil Nii Nil

0.1 0.1 <0.1

0.1 0.1 0.3

Nil Nil 01

<0.1 <0.1

*Three separate field exposures made of 168-254 hours.

Table XLVI shows data generated by the laboratory immersion test of five alloys in C 2 through C 10 acids. The difficulty with such laboratory tests relates to the exposure of the copper alloys and the stainless steels. Organic acids are excellent retainers of air in solution. Heating of the acid at temperatures below the boiling point does not expel all the oxygen, and corrosion rates on the copper alloys will be higher than would be experienced in a closed system devoid of oxygen. On the other hand, the stainless steels retain passivity for a longer time in such media before corrosion is initiated. Longer test periods, dynamic test apparatus and a close control of the entire environ- ment are important when attempting to identify specific materials of construction for a proposed application. However, the data of this table are consistent with field experience. As indicated by the laboratory tests, Types 316 and 316L stainless steels have excellent resistance to the acids to temperatures approaching the boiling point at atmospheric pressure. For this reason, the approximate boiling point temperature of each acid is listed in the table. The more extensive listing of alloys exposed in four, six and eight-carbon acids is given in Tables XLVII and XLVIII. The essential resistance of Type 316 stainless steel in organic acids is maintained in these higher acids

whether the acid is refined or contaminated with the lower acids (crude). The higher iron or nickel-base alloys containing chromium and molybdenum exhibit the same excellent stability in the higher acids as noted in the one and two-carbon compounds. Corrosion to be anticipated in a more modern process for the preparation of the longer acids is indicated in Table XLIX. Here, the catalyzed oxidation of a straight-chain hydrocarbon to an eight-carbon acid produced no signifi- cant corrosion of the stainless steels. Although only .13 mm/y (5 mpy) corrosion of Type 304 stainless steel was obtained in this instance, the choice of Types 316 or 316L stainless steels for such a reactor would be advisable to assure adequate resistance to variations in process condi- tions that might occur. When working with the higher organic acids, it is difficult to provide test conditions and a length of ex- posure sufficient to produce intergranular attack on sen- sitized stainless steel. The higher acids will produce selective attack on a structure containing carbide pre- cipitation, however, and the use of the L-grade or sta- bilized stainless steels at temperatures above 100 ºC (212 ºF) is suggested as a safeguard, regardless of the test data obtained.

Page 40