Acid Corrosion of Steel Effect of Carbon Content on the Corrodibility of Steel in Sulfuric Acid GLENN H. DAMON Michigan College of Mining and Technology, Houghton, Mich.
the rate of hydrogen evolution increased with the concentration of the sulfuric acid u p to about 16 N . I n more concentrated solutions the steel became passive. The investigation here reported was made to obtain related data for all of the factors mentioned.
The corrosion rate of five different carbon steels has been determined for thirteen different concentrations of sulfuric acid ranging from 1 to 35.5 normal. In all cases the maximum corrosion rate falls be tween 11 N and 14 N , the higher carbon steels being much more corrosive than the lower carbon steels. The data show that a steel containing between 0.06 and 0.37 per cent carbon has the lowest corrosion rate for all concentrations of acid. All steels tested became passive in acid more than 17 N . The passivating film was shown to be ferrous sulfate.
Materials and Methods Because depolarization by atmospheric oxygen plays a relatively unimportant part in the acid corrosion of steel and because the corrosion products are soluble in water or dilute acids, the unaerated loss-in-weight method was used in this investigation. The specimens tested were all industrial steels; the percentage analyses are as follows: Specimen C A 0.06 B 0.19
C
D
E
T
HE experiments described in this paper were carried out to investigate the influence of the carbon content and of the acid Concentration on the rate of corrosion of steel in sulfuric acid. These factors are of both practical and theoretical interest to those working with unalloyed carbon steels. The influence of carbon on the corrosion rate of steel has been studied in several different corrosive media. Chappell (8) tested six different steels, varying from 0.1 to 0.96 per cent carbon content, in sea water. He found that the corrosion rate increased u p t o about 0.8 per cent carbon and then decreased. The same author stated that tests were made with 1 per cent sulfuric acid as the corrosion media, but he gave no data for this case. Since the copper content of his steels was not specified, i t is difficult to correlate his results with the data furnished by other authors. Hadfield and Friend (6) investigated the effect of carbon and manganese on the corrosion of steel in tap water, sea water, and 0.1-0.5 per cent solutions of sulfuric acid. They reported a maximum corrosion rate in sulfuric acid for 0.81 per cent carbon steel. The authors found an optimum carbon content for each concentration of acid. Daeves and Eisenstecken (4) tried to correlate atmospheric corrosion of steel with the corrosion rate in 30 per cent sulfuric acid. Apparently no definite relation between the two conditions could be established. Owing to the passive character of steel in the more concentrated acid solutions, the effect of acid concentration on the corrosion rate is of considerable importance. The corrosion of steel in the acid solutions investigated is due almost exclusively to hydrogen evolution. Bryan (1) found that a high concentration of oxygen tends to inhibit the evolution of hydrogen from a solution of high acidity, but this can have little effect on the acid corrosion of steel in the presence of air. Ram (8) studied the effect of acid concentration on the corrosion of two relatively low-carbon steels. He found that
0.37 0.57 0.84
hIn 0.21 0.45 0.75 0.63 0.66
P
s
0.007
0.027 0.025 0.027 0.022 0.020
0.014 0.015 0.027
0.048
cu 0.034 0.055 0.042 0.041 0.043
Ni 0,004
None None None None
Cr 0.15 0.05 None None None
An ideal series of samples would have all chemical components, with the exception of carbon, in the same proportion. The specimens tested are as near this ideal as is possible with industrial samples. The original samples were hot-rolled to approximately 18 gage, and photomicrographs indicated some but not a great deal of strain in this condition. A complete series of tests was made on specimens in this mildly strained condition, and additional tests were made on specimens which had been fully annealed at 750' C. and then furnace-cooled. The test specimens were 21/a X ll/s inches (5.7 X 2.9 cm.). They were pickled in 10 per cent sulfuric acid solution for 3 to 5 minutes or until the black mill scale was completely removed. The specimens were washed for several minutes in a stream of t a p water, thoroughly rinsed with distilled water, dried with absorbent paper toweling, dipped in alcohol and redried, and then placed in a desiccator. No visible oxide film was produced when the specimens were dried immediately after removal from the washing medium. The samples were supported a t an angle of approximately 45" by glass triangles in a la-cm.-diameter crystallizing dish. I n all cases 500 ml. of acid were added, and the dish was covered loosely with a watch glass. All tests were made at room temperature, which was approximately 25 * 2' C. The duration of each test was 24 hours, with the exception of those tests in which the corrosion rate was so high that the steel specimen would have been completely consumed before the end of the 24-hour period. The amount of sulfuric acid consumed during the test was calculated approximately, and a sufficient amount was added to keep the average concentration within *O.l N of the concentration being studied in any given experiment. After the test period was complete, the specimens were removed from the acid and scrubbed with a brush. After thorough washing, they were dried and weighed as at the beginning. 67
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Vol. 33, No. 1
INDUSTRIAL A N D ENGINEERING CHEMISTRY
NORMALITY
OF ACID
BETWEEN CORROSION RATEAND CONCENFIGURE 1. RELATION TRATION OF SULFURIC ACIDFOR DIFFERENT CARBONSTEELS
subsequent sudden drop to a negligible amount is evidently due to the well-known phenomenon of passivity. The conclusion of Speller (9) and of Evans ( 5 ) that the passivating film is ferrous sulfate was verified when the specimens were placed in acid more than 17 N ; the visible evolution of hydrogen was a t first rapid but after a few minutes stopped almost completely. When the samples were removed from the acid, they were washed quickly with cold water and then placed in a beaker of warm water. After a few minutes the white coating on the surface of the steel disappeared, and the solution gave excellent tests for both the ferrous and the sulfate ions. After the removal of the a m , the specimens corroded normally in more dilute acids. It might be expected that the maximum corrosion rate would correspond with the maximum conductivity of the solution. Since the maximum conductivity of sulfuric acid comes a t approximately 8 N ; it is evident that conductivity is not the controlling factor. The data indicate that the point of maximum corrosion rate comes at a lower acid concentration for the 0.84 per cent carbon steel and that the point of passivity is almost the same for the range of steels investigated. The fact that the 0.19 per cent carbon steel had a lower corrosion rate than the other steels tested is of considerable interest. The diierence is particularly marked in the region of high corrosion rates. Figure 2 shows the relation between corrosion rate and carbon content for annealed specimens in 13 N sulfuric acid. The point a t approximately 0.00 per cent carbon content is taken from unpublished data on electrolytic iron obtained in this laboratory. The reason for these unusual experimental results is not entirely clear. The 0.19 per cent carbon steel corroded more uniformly than did any of the other steels tested. Since the heat treatment was identical for all specimens, the difference must be due to factors
Discussion of Results Table I shows the effect of carbon content and acid concentration on the corrosion of hot-rolled and annealed carbon steels. The data are shown graphically in Figure 1. The data for the hot-rolled specimens are so similar to the data for the annealed specimens that a separate discussion is unnecessary. When two or more curves coincide on the graph, no attempt has been made to designate each individual point. The exact numerical value of the point can be determined by reference to Table I. Thus the points shown for 20, 25, 30, and 35.5 N refer to all five curves. Each point on the curve is an average of three to six independent runs. With but a few exceptions, the variation of “0.0 02 0.4 0.6 0.6 1.0 C A R B O N CONTENT - Z individual runs did not exceed =!=5per cent from the average. A brief induction period was noted, but the runs were of FIGURE 2. RELATION BETWEEN CORsufficient length so that no appreciable error could be caused ROSION RATEAND CARBON CONTENT by this factor. Relatively large amounts of free were left On the TABLE I. INFLUENCE OF ACID CONCENTRATION ON CORROSION RATEOF DIFFERENT STEELB test specimen and in the con‘Onon’ Average Corrosion Rate, Grams/Sq. Dm./Day tainer after the corrosion of Of H2s041 0.06% C 0.19% C 0.37% C 0.57% C 0.84% C The the higher I%:& Annealed Hot-rolled Annealed Hot-rolled Annealed Hot-rolled Annealed Hot-rolled Annealed Hot-rolled hydrogen evolution was so 1.1 0.60 0.97 1.4 3.4 3.3 4.0 3.46 7.7 31.2 6 5.5 1.8 3.4 3.7 11.4 8.2 10.7 10.4 44.0 48.9 rapid that only small amounts 8.5 7.9 5.1 6.3 14.1 13.6 12.9 16.0 73.2 78.0 8 IO 14.7 13.5 5.9 8.6 22.6 23.6 24.5 28.4 94.0 96.3 of carbon actual157 clung to ... .. . 1 0.... 0 1 ... ... ... 4 i : b ... 112.3 thecorrodingspecimen. Varia: 3 . 21.7 18.1 0.2 31.6 35.9 42.0 101.2 iii:i tions of the length of tests 13 28.1 31.7 11.3 17.8 39.5 52.8 59.6 61.2 85.0 122.5 proved that ,.he corrosion rate 14 28.1 27.8 10.0 14.1 37.2 53.4 49.2 58.8 69.3 102.2 15
was not altered by the presence of this carbon. The rapid rise of the corrosion rate to B maximum and its
i25g %,5
13.9 2.1 0.26 0.13 0.17
0.0s
16.9 2.2 0.34 0.15 0.29 0.09
10.0 2.5 0.28 0.16 0.13 0.08
11.7 3.1 0.32 0.14 0.30 0.09
9.5 1.3 0.28 0.16 0.18 0.09
49.2 1.7 0.32 0.14 0.31 0.09
9.1 1.2
0.28
0.17 0.23 0.10
39.7 1.7 0.29 0.12 0.31 0.09
6.9 1.1 0.27 0.18 0.23
0.09
51.0 1.7 0.31 0.12 0.32 0.09
January, 1941
INDUSTRIAL AND E N G INEERING CHEMISTRY
other than mechanical treatment. The difference in the copper content may partially account for the results shbavn. Walker (IO), Burgess and Aston (8), and Marzahn and Pusch (7) all reported a marked decrease in the sulfuric acid oorrosion rate of copper-bearing steels. However, the copper content of the steels studied in this investigation was much lower than in previously reported investigations. The effect of traces of copper on the atmospheric corrosion of steels is well established, but it is not yet clear as to whether small traces of copper have a similar effect in acid corrosion.
Acknowledgment The author is indebted to R. F. Besner for part of the data on hot-rolled specimens, and to E. Nagle for chemical analyses.
69
Literature Cited (1) Bryan, J. M., Trans. Faraday SOC., 31, 1714 (1935). (2) Burgess, C. F., and Aston, J., J. IND. ENC.CHEM.,5, 458 (1913). (3) Chappell, C.,J . I r o n Steel Inst., 85, 270 (1912). (4) Daeves, K.,and Eisenstecken. F., Stahl u. Eisen, 56, 417 (1936). (5) Evans, U. R., “Metallic Corrosion, Passivity and Protection”, p. 18,London, Edward Arnold & Co., 1937. (6) Hadfield, R., and Friend, J. N., J . I r o n Stet2 Inst., 93,48 (1916). (7) Maraahn, W.,and Pusch, A., Korrosion ZL. Metallschzctz, 7, No. 2, 34 (1931). (8) Ram,‘%, i.Soc, Chenz. Ind., 54, 107T (1935). (9) Speller, F. N., “Corrosion, Causes and Prevention”, 2nd ed., p. 504, New York, MoGraw-Hill Book Co., 1935. (10) Walker, W. H., Proc. Am. SOC.Testing Materials, 11, 615 (1911). PREBENTBD before the Divieion of Industrial and Engineering Chemistry a t the 100th Meeting of the American Chemical Society, Detroit, Mioh.
Activitv Coefficients of Gases J
Calculation from the Beattie-Bridgeman Equation of State SAMUEL H. MARON AND DAVID TURNBULL Case School of Applied Science, Cleveland, Ohio
The Beattie-Bridgeman equation explicit in volume is employed to derive an equation for the activity coefficients of gases as functions of pressure and temperature. The equation is applied to the calculation of the activity coefficients of nitrogen, hydrogen, and ammonia. The calculated results are compared with the activity coefficients for these gases obtained graphically. The agreement is shown to be satisfactory over a wide range of pressure and temperature. It is further shown how the derived equation may be utilized to obtain I(, values for gaseous equilibria as analytic functions of pressure and temperature. Applied, for example, to the ammonia equilibrium, the equation for I
