Nickel Insitute - Nickel Alloys in Organic Acids & Related Compounds
Table XXXVI demonstrates the excellent corrosion resistance of HASTELLOY alloy C-276 in formic acid solutions. Throughout the entire range of temperatures and concentrations of formic acid, the nickel-base molyb- denum-chromium-iron alloy exhibits good stability. However, in formic acid exposures, more than in acetic acid exposures, the HASTELLOY alloys B and B-2 materials must be given consideration as materials of construction. Other nickel and cobalt-base alloys can be useful for specific field applications when the metallurgi- cal properties of these alloys are required. Applications involving heating are more demanding than isothermal exposures for an alloy. (See comments in the section on Acetic Acid—Effect of Temperature.) Calandria or vaporizer tubes require construction with a corrosion-resistant alloy. Table XXXVII provides data for six alloys tested under heat transfer conditions. The rate of attack on the austenitic stainless steel alloys increases sharply with a higher metal temperature under heat transfer conditions. CARPENTER alloy 20Cb-3 and HASTELLOY alloy B show rates of attack sufficiently low to warrant their selection under most of these conditions. Unfortunately, data for HASTELLOY alloy C-276 and INCONEL alloy 625 are not included, but they would be expected to be as good or better than that shown for the “B” alloy. Many commercial applications require the use of the
alloys at temperatures above those obtained at one at- mosphere of pressure. Consequently, the effect of increasing the temperature on the corrosion rate of the common alloys must be determined. Table XXXVIII presents a composite of the data contained in the report of Miller and Wachter on corrosion by acids at high temperatures. 24 Of greatest interest is the information for the Type 316 stainless steel. The rates are higher for this alloy than would be expected for a test of longer duration. The important inference to be made is that the rate of attack approximately doubles for each 15°C (27°F) increase in temperature. (It should be recognized that this is a very rough approximation that does not always hold true.) Corrosion tests in many other media show a similar relationship rather than one conforming to the ideal Arrhenius equation. Figure 5 and isocorrosion charts (Figures 6 through 11), originally published by the NACE, 25 indicate the corrosion behavior of several alloys in formic acid. Isocorrosion charts are intended only as guides; there are conditions where higher or lower rates can prevail. In fact, Figure 5 shows much lower rates for Type 316 stainless steel in boiling formic acid than is shown in the isocorrosion chart, Figure 6. It is believed that Figure 5 is more representative of pure formic acid and that the higher rates shown by the isocorrosion chart must reflect the presence of unidentified impurities.
TABLE XXXVI
Corrosion of HASTELLOYS and Related Alloys in Formic Acid Solutions
Laboratory data obtained without aeration or deaeration using five 24-hour test periods. (Courtesy of the Cabot Corporation, Stellite Division.)
Corrosion Rate
HASTELLOY alloy B
HASTELLOY alloy C-276
HAYNES alloy No. 25
MULTIMET alloy
%
Temperature
Formic Acid
ºC
ºF
mm/y
mpy
mm/y
mpy
mm/y
mpy
mm/y
mpy
10
26 66
78.8
.03 .23 .08 .05 .25 .10 .30 .08 .08 .28 .05 .05 .25 .03 08
1 9 3
<.03 <.03
<1 <1
Nil Nil .20
Nil Nil
Nil Nil .10
Nil Nil
150
Boiling
.13
5
8
4
20
26 66
78.8
2
< .03 <.03
<1 <1
<.03 <.03
<1 <1 10
<.03 <.03
<1 <1
150
10
Boiling
4
.18
7
.25
.15
6
30
26 66
78.8
3
<.03 <.03
<1 <1
– – –
– – –
– – –
– – –
150
1 2
Boiling
3
.20
8
40
26 66
78.8
3
<.03 <.03
<1 <1
<.03
<1 Nil 15 <1 Nil 20 <1 <1
<.03
<1 Nil
150
11
Nil .38
Nil .20
Boiling
2
.13
5
8
60
26 66
78.8
2
< .03 <.03
<1 <1
<.03
<.03 <.03
<1 <1
150
10
Nil .51
Boiling
1
.18
7
.15
6
90
26 66
78.8
<.03
<1
<.03 <.03
<1 <1
<.03 <.03
Nil Nil .08
Nil Nil
150
.03
1
Boiling
<.03
<1
.05
2
.15
6
3
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