CORROSION OF
METALS In the Manufacture of Phosphoric Acid by the Electric Furnace Process
CHARLES E. HARTFORD1 AND RAYMOND L. COPSON Tennessee Valley Authority, Wilson Dam, Ala.
.
L
Serious corrosion problems are encountered in the manufacture of phosphoric acid by the electric furnace process. During the past several years, studies of these corrosion problems have been in progress in the TVA phosphate fertilizer plant at Wilson Dam. The present article describes the results of tests of metals exposed to corrosion by crude phosphoric acid and by crude phosphorus under actual plant conditions. It also describes how some of the practical corrosion problems have been met in the plant.
PHOSPHORUS STORAGE TAXKS DURING CONSTRUCTION, SHOWIXG STAINLESS STEELCOVERS AND BRASSSTEAM-HEATING COILS
I
N THE phosphate fertilizer plant of the Tennessee Valley Authority at Wilson Dam, phosphate rock is smelted in electric furnaces to produce phosphorus, which is then converted into phosphoric acid and subsequently into superphosphate containing 45-50 per cent available Pz0s. This plant has been described in detail by Curtis and co-authors (1). Serious corrosion problems, some of which were mentioned in the previous papers, have been encountered in connection with the production and handling of crude phosphoric acid. During the past several years studies of these corrosion problems have been in progress. A certain amount of information on the corrosion of metals and alloys by phosphoric acid is available in the literature and from manufacturers. I n general, it is known that lead, silicon-irons, high-chromium-iron alloys, and alloys high in nickel and chromium have been used, and that chromiumnickel-molybdenum steel has been particularly satisfactory in resisting phosphoric acid. The importance of low carbon in chromium-nickel-molybdenum steel and in chromium-iron alloys, in affecting the resistance to phosphoric acid, has been stressed by Rohrman (4). Practically all of the published quantitative data on rates of corrosion by phosphoric acid have been derived from laboratory tests, made both with c . P. and with crude acid. I
Present address, Southern Kraft Corporation, Panama City, Fla.
I n gener,al, the rate of corrosion was found to increase greatly with rise of temperature (8, 3, 4). I n c. P. acid the rate of corrosion of some materials increased as the acid concentration increased, whekeas that of others decreased ( 2 ) . In crude acid low concentrations were almost invariably found to be more corrosive than higher concentrations, possibly because the dilute acid contained more fluorides (4). The resistance of metals to crude phosphoric acid is known t o be affected greatly by the presence of impurities, particularly fluorides. The presence of sulfuric acid was found to increase the corrosion rate of iron, brasses, etc., but to reduce the corrosion of lead; arsenic inhibited the corrosion of iron and ferrous alloys; halogens destroyed the passivity of stainless irons and steels and so caused rapid corrosion (8). Aeration of the acid was found to cause an increase in corrosion rates (2, 3 ) . 1123
1124
INDUSTRIAL AND ENGINEERING CHEMISTRY
Laboratory tests inevitably suffer from certain disadvantages (6). It is practically impossible to determine in advance exactly what the conditions of service will be and to reproduce these conditions in the laboratory. Furthermore, in many instances the laboratory tests show a considerable variation in corrosion rate with time, and it is uncertain whether the initial rate or the subsequent rate will prevail in practice. Prior to construction of the TVA phosphoric acid plant, a series of laboratory tests was made in which various metals and aIloys were tested against strong phosphoric acid a t elevated temperatures. The information gained from these tests was used in selecting materials of construction for the plant, but it was recognized that the probable rate of corrosion of a particular alloy under actual plant conditions could not be predicted satisfactorily from laboratory tests.
Tests of Samples in Plant Soon after the phosphoric acid plant was in operation, a program of testing small samples under actual plant conditions was started:
I
The Sam les were disks 2.25 inches in diameter and 0.0300.375 inch &ick, with a hole 0.375 inch in diameter drilled at the center. Duplicate samples were tested in each case. Before the metal samples were instaIled, they were washed in benzene and then in alcohol, dried, weighed, and measured accurately, and their specific gravities were determined. The test samples were mounted in spool-type specimen holders (6). These holders were constructed of chromium-nickelmolybdenum steel, with porcelain tubes for covering the rods on which the samples were mounted and short pieces of Pyrex tubing for spacing the samples. The specimen holders were fastened in place in various test locations in the acid plant. The samples were inspected at intervals of a few days, and as individual sampIes became corroded to a noticeable extent, they were removed. The test periods ranged from 7 to 133 days, depending upon the corrosion rates; the average duration of test was about 60 days. A t the conclusion of each test the samples were washed and cleaned with either copper or steel wool. The cleaned samples were dried and weighed. The average corrosion rate was calculated from the loss in weight, the original surface area, the original density, and the duration of the test. Samples were tested in both the No. 1 and the No. 2 acid plants. In the No. 1 acid plant the gas leaving the electric furnace, consisting principally of phosphorus and carbon monoxide, is cooled in a water spray condenser. The phosphorus is condensed and is pumped to a settling tank where it is se arated from the cooling water, which is recirculated. The cruie phos horus is emulsified with a considerable proportion of condenser fiquor and solid material, and is sometimes called “phosphorus sludge.’’ It is stored under hot water in concrete tanks. The molten phosphorus is umped t o a combustion chamber and burned, and the resulting $205 is hydrated t o orthophosphoric acid. Part of the
VOL. 31, NO. 9
phosphoric acid is collected in the h drator, and practically all
of the remainder is collected in an e?&rostatic precipitator. The No. 2 acid plant differs from the No. 1 in that the gas
from the electric furnace is burned immediately in a combustion chamber, the products of combustion passing directly to a hydrator and an electrostatic precipitator. An important difference with regard to corrosion follows from the difference in the two processes: Rock phosphate of the grade ordinarily used contains over 3 per cent of fluorine, part of which is voIatilized in the electric furnace. When the phosphorus is condensed from the furnace gas, as in the No. 1 acid plant, it is separated from the major part of the volatilized fluorine compounds. Consequently the remainder of the acid plant, in which the phosphorus is burned and converted to acid, handles gases which are relatively low in fluorine. On the other hand, when the step of condensing the phosphorus is omitted and the furnace gas is burned immediately after leaving the furnace, as in the No. 2 acid plant, all of the volatilized fluorine compounds pass into the acid recovery units. Typical percentage analyses of the acid produced in the two acid plants are as follows: No. 1 Plant
PlOS so2 F
58.9
0.004
0.009
c1
Trace 0.001
AS
No. 2 Plant 131.7 0.004
0.017 Trace 0,001
Phosphoric acid produced by the electric furnace process contains much less sulfur dioxide or trioxide than does acid produced by the sulfuric acid process. This probably explains the relatively excellent showing of copper alloys and the relatively poor showing of lead in the results presented below, as compared with laboratory tests in crude phosphoric acid reported by other authors (2, 4). The details of exposure in the different test locations are summarized in Table I. The average conditions of temperature and acid concentration over the period of test are shown, as well as the range over which these conditions fluctuated.
Corrosion Data The data on the observed corrosion rates are summarized in Tables I1 and 111. The samples in which iron is the predominating alloying element are classified as ferrous alloys, carbon steel and cast iron being included (Table 11). All other samples are classified as nonferrous alloys, including copper, lead, and nickel (Table 111). The essential nominal percentage composition of each alloy is given. Those sam-
TABLE I. TESTLOCATIONS AND CONDITIONS Location A. Phosphorus storage tank, No. 1 acid plant B. Overflow from phosphorus settling tank, No. 1 acid plant C . Launder of electrostatic precipitator, No. 1 acid plant D. Launder of electrostatic precipitator, No. 2 acid plant E. Storage tank for acid from both acid plants F. Gas outlet section of electrostatic precipitator, No. 1 acid plant G. Gas outlet section of electrostatic precipitator, No. 2 acid plant
Temp., O C. Av. Range
Acid Concn., % &Po4 Av. Range
65
60-70
..
...
65
60-70
10
5-15
85
80-130
78
70-85
Conditions of Exposure Submerged in crude phosphorus sludge ; reducing conditions Submerged in acid; conditions were reducing since the soln. was satd. with phosphorus Submerged in acid; oxidizing conditions
85
80-90
90
85-95
Submerged in acid; oxidizing conditions
60
50-75 85-100
78
..
75-80
95
95
85-100
..
...
Submerged in acid; oxidizing conditions Exposed to gas, principally PU’Z and 0 2 , containing a small proportion of phosphoric acid mist and traces of fluorine compounds Exposed to gas, principally Nz, COz, and 0 2 , containing small proportions of phosphoric acid mist and fluorine compounds
...
SEPrEMBER. 1939
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE11. CORROSION RATESOF SMALLSAMPLES OF FERROUS ALLOYSEXPOSED UNDER PLANTCONDITIONS Sample No. 1. 2. 3. 4. 5. 6.
Alloy
....... c;:*e*.
'
Trade Name Carbon steel Cast iron Croloy 9
Cr-Fe Cr-Fe Cr-Fe (cast) Cr-Ni-Fe Cr-Ni-Fe Cr-Ni-Fe
Duraloy A Allegheny 55 Amsco F-3 KA-2S Silcrome 25-20 KA-2SMo
10. 11.
Cr-Nj-Fe Cr-Ni-Fe
KA-2ST Croloy 25-20
12.
Cr-Ni-Fe
Enduro NC-3
13.
16.
Ni-Fe (cast) Ni-Resist (copper-free) Ni-Cr-Fe Zorite Ni.Cr-Fe Worthite (cast) Ni-Cu-Fe Ni-Resist
17. 18.
Si-%t!ast Si-Fe (cast]
7. 8. 9.
14. 15.
a
Durichlor Duriron
(Each figure represents the average of two samples) -Inch Nominal ChemicalCompn., % A C 0 2 6 SO05 0.005 C 3.36 S 6 0 7 0.005 0.005 Babcock and Wilcox Tube C r 8-lb. C'0.15.a M o 1.25-1.75, no. M~ - -- n.iino D;r;loy Co. C r 27-30. C 0.25, M n 0.60 0.001 Allegheny Steel Co. Cr26-30,C0.25,nMn~.0,0Ni0.6a0.000 Am. Manganese Steel Co. Cr27-29 C 0 3 N i 3 S i 1 5 0 0.000 Crucible Steel Co. Cr 17-19: Ni,?-'9,Cd.OS-b.20 0.000 Ludlam Steel Co. Cr 24-26 NI 19-20 C 0 25 0.000 Crucible Steel Co. Cr, l6-2d Ni7-11,'C0.07,aMo 0.000 2-4 Crucible Steel Co. Crl7-19 Ni7-9 C 0 . 0 7 a T i 0 3 5 0.000 Babcock and Wilcox Tube Cr25, Ni20, COh5, Mn'l.0, Si 0.000 co. 0.75 Republic Steel Corp. Cr24-28,Ni19-21,C0.25,"Si 0.000 2.0," Mn 1.50 Licensees of Interna- Ni 15-20, C 2.2, Cr 2.5," M n 1- 0.000 1.5, SI0.6-2 tional Nickel Co. Michiana Products Corp. Ni 35 Cr 15 C 0 50" 0.001 Worthington Pump & Ni 24: C r 20: C 0107," M o 5 0.000 Machinery Corp. Licensees of Interna- Ni 12-15, Cu5-7, C 2.75-3.1, C r 0.000 tional Nickel Co. 1.5-4, Si 1.25-2, M n 1-1.5 Duriron Co. Si 14.5, Mo 3 0.001 Duriron Co. Si 14.5, C 0.8,M n 0.35 0.001 Manufacturer
............... ...............
-
- --
of Corrosion per Year a t Location:B C D E F Q 0.025 8.3 3.4 0.025 0.050 0.20 5.9 1.2 0.25 0.025 0.10 0.001 1.10 0.45 0.15 0.15
...
...
0.000 0.000 0.000 0.000 0.000 0.000
0.55 0.75 0.65 0.65 2.0 0.15
0.000 4.0 0.000 4 . 5
0.005 0.10 0.20 0.005 0.30 0.005
0.001 0.001 0.005 0.001 0.050 0.005
0.001 0.001 0.001 0.001 0.025 0.001
0.001 0.001 0 003 0.001 0.050 0.001
0.60 0.20
0.15 0.010
0.001 0.001
0.005 0.010 0.025
0.000
4.5
0.35
0.025
0.001
0.025
2.0
0.35
0.10
0.10
0.050
0.001 1.1 0.000 0.025
0.050 0.025 0.001 0.001
0.050 0.001
0.050
0.010
0.025
0.025
0.10
0.10
0.001 0.001
0.001 0.001
0.05 0.025
0.025 0.025
0.45
0,001 0.025 0.000 0.001
0.001
Maximum.
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TABLE111. CORROSION RATESOF SMALLSAMPLES OF NONFERROUS ALLOYS EXPOSED UNDER PLANT CONDITIONS (Each figure represents the average of t w o s a m d e s ) Sample -Inch No. Alloy Trade Name Manufacturer Nominal Chemical Compn., % A 19. c u Plain.copper ................. .................. 0.001 20. c u Deoxidized copper Chase Brass & Copper Co. C u 99.96, P 0.035 0.010 21. c u Beryllium copper Am. Brass Co. 0.001 Cu 97.75, Be 2.25 22. c u Olympic D Chase Brass & Copper Co. Cu 98.25, Si 1.5, Mn 0.25 0.001 Olympic A 23. C u Chase Brass & Copper Co. C u 96, Si 3 M n 1 0.001 24. Cu-AI Aluminum bronze Am. Brass Co. Cu 95, A1 k 25. Cu-AI (cast) Resistac Am. Manganese Bronze Co. C u 88 A1 10 Fe 2 o:ooi 26. Cu-AI-Ni Nickel-aluminum Chase Brass & Copper Co. Cu 92: A1 4, 'Ni 4 ... (cast) bronze 27. Cu-Pb-Sn Anti-Acid metal H. Kramer & Co. C u 75, Pb 15, Sn 10 0.001 (cast) 28. Cu-Ni Super-Nickel Am. Brass Co. Cu 70, N/ 30 0.001 29. Cu-Ni Advanoe Driver Harris Co. Cu 55 NI 45 0.001 30 Cu-Ni cast) Everbrite Am. Manganese Bronze Co. C u 6O'Ni 30 Fe 3 Cr 3, Si 3 0.000 31: Cu-Ni-hn C uDro- Nick el Chase Brass & Copper Co. Cu 74 Ni 2b, Zn'5 0.001 32. Cu-Sn Bronze cu 90: Sn 10 0.001 33. Cu-Sn Phosphor bronze Am. Brass Co. Cu 94.5, Sn 5.4, P 0.01 34. Cu-Sn Phosphor bronze C Am. Brass Co. C u 92, Sn 8, P trace 0:001 35. Cu-Zn Red brass ................. Cu 85, Zn 15 0.001 36. Cu-Zn Admiralty Chase Brass & Copper Co. Cu 70, Zn 29, Sn 1 0.001 37. Cu-Zn Alpha brass Chase Brass & Copper Co. C u 6 6 Zn33.5 P b 0 5 0,001 38. Cu-Zn Tobin bronze Am. Brass Co. C u 6 d 1 , Zn 39:1, Snb.7 0.001 39. Cu-Zn Manganese bronze Am. Manganese Bronze Co. Cu 59, Zn 39, Fe 1.2, Sn 0.7, Mn ....... 0.06 40. Cu-Zn-A1 Su er Strength Am. Brass Co. C u 6 3 , Z n 2 7 , A l 6 , F e 3 , M n l 0.001 (cast) EroLe 41. Pb Chemical lead National Lead Co. P b 99 9 Cu 0 1 0.001 42. P b Tellurium lead National Lead Co. P b 99:Q: T e 0.'1 0.001 43. Ni Nickel International Nickel Co. Ni 99+ 0.001 44. Ni-Cr Nichrome V Driver Harris Co. Ni 80 Cr 20 0.000 Illium G 45. Ni-Cr-Cu Burgess-Parr Co. Ni_5C&:2_4, C u 8 , M o 4 , Mn (cast) Z W Z He 46. Ni-Cr-Mo Pioneer metal Pioneer Alloy Products Co. Ni i 5 , Cr125, Mo 5 , F e 0.000 (cast) 47. Ni-Cu Monel International Nickel Co. Ni65-70, Cu26-3O,SiO.25 0.001 (max.) 48. Ni-Cu (cast) Monel S International Nickel Co. Ni64,Cu29,Si3.75,Fe2.5, 0.000 M n 0.05, C 0.1 49. Ni-Mo (cast) Hastelloy A Haynes Stellits Co. Ni58 Mo20 M n 2 Fe 0.000 50. Ni-Mo-Cr Hastelloy C Haynes Stellite Co. NiT$E(Mo17,'Cr 1 4 , b e 6 , 0.000 (cast) 51. Ni-Si (cast) Hastelloy D Haynes Stellite Co. Ni"i
