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348
Tests by Immersion in Sea Water
I n these tests the specimens were removed at the end of various intervals, cleaned with a stiff bristle brush, and weighed. They were then replaced for further test. Figure 1, taken from Bassett and Davis,2shows graphically in a relative way the cumulative rates of corrosion at the end of various intervals. Discussion
The results by both the salt spray tests and by the longtime immersion test showed a decided minimum loss in weight for the alloys between the limits 85:15 and 70:30. Observation of the specimens after corrosion in both tests showed that a t 75 per cent copper and below there was dezincification and above the composition 85: 15 there was pitting. T h e effects increased, in a general way, towards the extremities of the series. Depth of either dezincification or pitting is a most vital factor in determining the life of copper-zinc alloys. It will be noted in Table I11 that the actual depth of pits was from six to twelve times the depth of calculated average penetration by solution and the depth of dezincification was apparently as great or greater than the depth of pits. I n the process of corrosion by either salt spray or sea water, the copper-zinc alloys become covered with a coating of corrosion products, which appears to have a protective influence on the most resistant alloys, whereas the attack on those alloys whose tendency is to become pitted or dezincified is accelerated by these deposits. The mechanism of these phenomena is doubtless electrochemical in nature.
100
a0 COPPER
Fiaure 3-Salt
70
- PLR LCNT
60
Spray Corrosion Tests. Copper-Zinc Alloys. of Corrosion
Rates
Figure 3 shows some very interesting relations between the rate of corrosion by salt spray and the continuous duration of the test. The rates for tests of 3 and 6 months’ duration showed remarkably good agreement, whereas the rates ob-
Vol. 17, No. 4
served a t the end of 1 month were about twice as great, and might have led to erroneous conclusions in comparing the different alloys. The effects of coatings of corrosion products are clearly brought out, in slowing down the general rate of loss in weight as time advances, and in accelerating pitting and dezincification in those alloys which are subject to these effects. A minimum duration of continuous test of a t least 3 months has seemed essential in these tests in order to obtain reliable results and to allow sufficient development of pits and dezincification. Summary
1-Tests of a series of commercial copper-zinc alloys by salt spray and long-time immersion in sea water have shown that those alloys containing between 70 and 85 per cent of copper offered the greatest resistance to corrosion by salt water. Alloys richer in copper showed a tendency toward pitting and those richer in zinc a tendency toward dezincification, and these factors really determine the life of alloys in the copper-zinc series. 2-Results obtained by the accelerated salt spray tests were found to be in very satisfactory general agreement with those obtained by a long-time immersion test. It has been possible to reach conclusions in a few months by means of the accelerated tests, whereas several years were required by the immersion tests. It is gratifying that these results are also quite in accord with accumulated data representing the performance of a number of these alloys under conditions of actual service, not only in sea water, but in natural waters of corrosive nature.
The Acid Corrosion of Metals Effect of Oxygen and Velocity ’
By W. G. Whitman and R. P. Russell
MASSACHUSETTS INSTITUTE
OF
TECRNOLOCY, CAMBRIDGE,
MASS.
Part I presents an experimental survey of the effect of dissolved oxygen in the corrosion of steel, aluminium, lead, copper, nickel, tin, and several alloys by sulfuric, hydrochloric, nitric, and acetic acids. The method used was to compare the corrosion rates in two solutions, one of which was saturated with oxygen and the other with hydrogen. Part I1 deals with the effects of velocity on the corrosion of copper by sulfuric, acetic, and hydrochloric acids, and on the corrosion of steel by concentrated sulfuric acids. An apparatus in which the samples are suspended in the acid from a horizontal rotating wheel was used, and provision was made for air saturation or for total exclusion of oxygen. The results emphasize the importance of oxygen in corrosion by dilute nonoxidizing acids. They also show that dissolved oxygen may act as a passivifying agent in some cases, thereby actually reducing corrosion. Velocity increases corrosion when oxygen is a vital factor, and also markedly accelerates corrosion where protective films may beremoved.
HE importance of dissolved oxygen in the corrosion of metals by neutral waters has been emphasized by many investigators, and the prevention of corrosion by removal of oxygen is widely practiced in hot water systems and boiler installations. The role of oxygen in acid corrosion, on the contrary, has received little attention, despite such well-known facts as the resistance of copper to corrosion by nonoxidizing acids when air is excluded. Similarly, the effect
T
I S D C S T R I S L L4dYDE S G I S E E R I S G CHEMISTRY
April, 1925
349
of velocity has been studied for neutral water corrosion, but very little work has been done in acids. This paper presents certain data illustrating the effects of oxygen and of velocity in the acid corrosion of metals. In view of the nature of the preceding papers in this symposium, the discussion of theory is brief.
the results show an increase of corrosion rate with velocity, this increase being attributed to the more rapid diffusion of corroding agent in to the metal surface and of reaction products away from it.
Theoretical Considerations
Scope of Investigation
According to the electrochemical theory of corrosion, the process operates through a corrosion cell. At the anode of this cell metal ions go into solution according to the reaction
The program for the investigation of the effect of dissolved oxygen comprised a series of short-time tests of the corrosion of steel, aluminium, lead, copper, nickel, monel metal, and tin in varying strengths of sulfuric, nitric, hydrochloric, and acetic acids. Two series of tests were to be run in parallel, one determining the corrosion under an atmosphere of oxygen and the other in the presence of an inert gas-either hydrogen or nitrogen. Also, in view of the possible importance of oxygen a t the slightly elevated temperatures encountered in pickling practice, it was decided to study the effect of temperature. These experiments were initiated with the object of determining the importance of dissolved oxygen in acid corrosion under widely differing conditions. It was clearly recognized that in an attempt to cover quickly the broad field outlined above, the tests could not be run for a sufficient length of time to permit accurate estimates of the life of the material under service conditions. The results, however, should be of both practical and theoretical value as a preliminary survey.
M+$=M+ (1) -4corresponding reduction must occur simultaneously a t the cathode of the cell, the mechanism of the reduction being either H+=H+@
+
+
(2)
'/d& '/zHzO = (OH)(3) Any other oxidizing agent, such as nitric acid, could replace the dissolved oxygen used in Equation 3 and thus furnish other possibilities for cathodic reduction. If the process represented by Equation 2 takes place, the hydrogen formed by the reaction may escape as gas. This occurs, for example, in the corrosion of zinc or iron in nonoxidizing acids. ,Metals cathodic to hydrogen, such as copper, do not liberate hydrogen gas, and the cathodic reduction occurs with oxygen by Equation 3, or with other oxidizing agents. It is quite possible for Reactions 2 and 3 to proceed simultaneously with zinc or iron, in which case the total corrosion will be determined by the sum of the separate actions.' The effect of velocity is usualfy to increase corrosion because of such factors as the following: (1) the relatively quiet film of liquid adjacent to the metal is thinned down and diffusion of the corroding agent-e. g., dissolved oxygento the effective cathode area is made easier; ( 2 ) protective films on the metal surface are frequently removed by turbulence in the corroding liquid; and (3) it becomes easier for the products of corrosion to diffuse away from the metal.
or
Previous Work
Clement and Walker,* in their experiments on electrolytic corrosion prevention, noticed that the corrosion of iron was about doubled by bubbling oxygen through a 0.01 N sulfuric acid solution. Seligman,3 in an investigation of the resistance of aluminium to attack by acetic acid, found that the corrosion rate and amount of pitting were increased by the presence of dissolved oxygen. Experiments by the authors and others' on the corrosion of steel by sulfuric acid showed that, a t low acid concentrations, the presence of dissolved oxygen increased the corrosion rate, and that this increase became more and more marked as the velocity was raised. McKay4 investigated the effect of air in the corrosion of copper and monel metal by acids. Since these metals are cathodic to hydrogen, the cathode reaction in nonoxidizing acids depends primarily upon the presence of dissolved oxygen. McKay found that an air-saturated, 6 per cent solution of sulfuric acid caused five times as much corrosion of monel metal as an air-free solution. I n common with the investigations mentioned above, however, the range studied was relatively narrow, and it was felt that series of exparimerits with other metals and other acids would prove of vklue, The effect of velocity on corrosion by nonoxidizing acids has been studied by several investigators.1.4.5 I n gflneral, Whitman. Russell, Welling,.and Cochrane, THISJOURNAL,, 15, 672 (1 923). Trans. A m . Electrochem. Soc., 22, 193 (1912). J . SOL.Chem. Ind., 86, 409 (1917). R. I. McKay, Trans. A m . Electrochem. Soc., 41, 201 (1922 ). Friend and Dennet, J . Chem. SOL.(London), 121, 41 (192 2 ) , Calcott and Whetzel, Proc. A m . Insl. Chem. Eng , 1924.
I-Effect of Dissolved Oxygen
Apparatus and Experimental Method
Duplicate test pieces of the different metals were suspended glass hooks in 458-cc. (16-ounce), wide-mouth bottles containing 400 cc. of acid. Gas a t the rate of about 250 cc. per minute was passed through the acid in the form of relatively fine bubbles from a specially constructed distributing device a t the bottom of the bottle. Impingement of gas bubbles on the test pieces was prevented. Whenever different acids were used in a run, contamination of acid by the gas stream from any preceding bottle was prevented by passing the gas stream through soda-lime tubes between bottles. Two chains of bottles were used in parallel, one with oxygen and the other with either hydrogen or nitrogen, the gas streams being equal to give comparable velocity effects. The test pieces 3.2 X 7.6 X 0.079 cm. (11/*X 3 X inches) were slightly roughened with KO.0 emery paper t o remove any adhering scale, and were then washed, dried, and weighed immediately before each run. The amount of corrosion was determined by a second weighing after each run. Since all the runs reported were for 5 hours only, it was impracticable to determine the degree of pitting.
011
Results
The results are given in Tables I and TI. Table I shows corrosion a t 20' C., under oxygen and under hydrogen (or nitrogen) for a series of metals and acids. In the last three columns the results are contrasted by showing, first, the ratio of the corrosion under oxygen to that under hydrogen, and second, the difference between the two corrosion rates. Table I1 gives the results of the investigation of the effect of temperature on the corrosion of steel, copper, and aluminium by various strengths of sulfuric acid. Discussion of Results
The reported results are sometimes not reproducible to within better than 20 per cent. During a preliminary 40hour run, the rate of corrosion of steel and copper was determined for each &hour period and was found to remain
I N D U S T R I A L A X D ENGINEERING CHEMISTRY
350
Table I-Corrosion METAL Mild steel
ACID HzSOi
HCI "03
HAC Mixedc
Concentration Per cent by weight
96.5 50.0 20.0 6.0 4.0 0.04 70.0 1.2
Glacial
6.0 50
Chi:omium steel:
8.3% 9.2% 13.0% 16.0% Aluminium
Mixedc HC1 HAC
96.5 50.0 6.0 70.0 30.0 3.0 4.0 Glacial
50 6
Lead
Citric HzSO,
50 93 50 20 6 20 4 70 30 Glacial
6 96.5 20.0
Copper
6.0 HCI
20.0 4.0
HAC
Glacial
50 R
Nickel
Citric HzS04 HCI HAC
50 20 2 20 4 Glacial 50 R
Monel
HISO, HCI HAC
20 2 20 4 Glacial
50 6
Tin
Citric HnSOi HCI HNOa HAC
50 6 6 3 6
-AVERAGE -Under Cm. 0.201 0.67 0.69 0.91 1.22 0.99 0.155 4.6 1.64 1.39 0.17
5.7 6.9 10.9 13.5 0.81 0.288 0.150 0.084 0.135 0.200 0.lli 0.183 76 0.015 0.008 0.013 0.003 0.114 0.043 0.038
0.026 2.82 0.411 0.437 22.0 16.1 2.64 0.104 0.343 0,376 5.46 3.53 0.005 0.183 0.058 0.069 0,576 0.552 1.14 1.12 1.33 0,696 0.216 0.216 0.236 1.07 0.48 0.008 0.74 0.053 0.137 2.20 5.69 0.325 1.18
u n d e r Oxygen a n d u n d e r Hydrogen PENETRATION PER YEAR01---Under Hz-Inches Cm. Inches
Vol. 17, No. 4
7
0
0.079 0.262 0.270 0.359 0.48 0,388 0.061 1.82 0.647 0.545 0.067
0.208 0.49 0.19 0.079 0.079 0.014 0.163 4.0 0.033 0.016 0.038
0.082 0.192 0.075 0.031 0.031 0.0055 0.064 1.57 0.013 0.0063 0.015
01:HP 1.0 1.4 3.6 11.6 15.5 70.8 1.0 1.2 50 87.2 4.5
2.25 2.70 4.29 5.30 0.319 0.112 0.059 0.033 0.053 0.079 0.046 0.072 30 0.006 0.003 0,005
4.7 6.4 11.2 13.7 0.83 0.338 0.157 0.024 0.155 0.236 0.068 0.193 25 0.015
1.85 2.50 4.40 5.40 0.326 0.133 0.062 0.0094 0.061 0.093 0.027 0.076 10.00 0.006
1.2 1.1 1.0 1.0 1 .o 0.8 1.0 3.5 0.9 0.9 1.7 1.0 3.0 1.0
1.0 0.5 -0.7 -0.2 -0.02 -0.053 -0.007 0.060 -0.020 -0.036 0.049 -0.010
2.5 1.0 1.2 1.4 2.5 1.0 10.0 9.5 1.8 1.1 10.9 3.8 7.3 23.4 43.8 179.0 82.0 1.0 24.0 18.0 5.7 57.0 108.0
0 608
0.001
0.045 0.017 0.015 0.010 1.11 0.162 0.172 8.65 6.34 1.04 0.041 0.135 0.148 2.15 1.39 0.002 0,072 0.023 0.027 0,227 0.217 0.451 0.440 0.522 0.274 0.085 0.085 0.093 0.422 0.189 0.003 0.290 0.021 0.054 0.865 2.24 0.128 0.465
0.0
0:005 0.003 0.094 0.030 0.015 0.025 0.282 0.043 0.246 20.8 1.47 0.69 0.0142 0.0147 0.0087 0.034 0.043 0.005 0.008 0.0033 0.0119 0.0102 0.005 0.241 0.015 0.010 0.025 0.005 0.006 0.003 0.043 0.008 0,005 0.036 0.005 0.053 0.018 0.030 0.320 0,008
0,002 0.001 0.037 0.012 0.006 0,010 0.111 0.017 0.097 8.17 0.58 0.27 0.0056 0.0058
0.0034 0.012 0.017 0.002 0.003 0.0013 0.0047 0.004 0.002 0.095 0.006 0.004 0.010 0.002 0.002 0.001 0.017 0.003 0.002 0.014 0.002 0.021 0.007 0.012 0.126 0.003
...
4.8
73.0 130 27.4 42.5 42 :5 93.0 24.8 63.0 1.5 20.7 10.5 2.6 124. 187 1.0 155
Cm.
-0.007 0.18 0.50 0.83 1.14 0.98
-0.008
0.6 1.61 1.37 0.13
51.0 0.0
:
0~. .0
0.020 0.013 0.023 0.0 2.54 0.368 0.191 1.2 14.6 1.95 0.090 0.328 0.367 5.43 3.49 0.000 0.175 0.055 0.057 0.566 0.547 0.90 1.10 1.32 0.671 0.211 0.211 0.233 1.03 0.47 0.003 0.70 0.048 0.084
2.18 5.66 0,005 1.17
-
; HtbInches
-0.003 0.070 0.195 0.328 0.45 0.382 -0.003 0.25 0.634 0,539 0,052 0.40 0.20 -0.11 -0.10 -0.006 -0.021 -0,003 0.024 -0.008 -0,014 0,019 -0.004 t20.0 0.0 0:003 0.0 0.008 0.005 0.009 0.0 1.00 0.145 0.075 0.48 5.76 0.77 0.035 0.129 0.145 2.14 1.37 0.000 0.069 0.022 0.022 0.223 0.215 0.356 0.436 0.518 0.264 0.083
0.083 0.092 0.405 0.186 0,001 0,276 0.019 0.033 0.858 2.23 0.002 0.462
corrosion under oxygen This ratio signifies corrosion under hydrogen or nitrogen' b This difference signifies corrosion under oxygen minus corrosion under hydrogen. c Mixed acid: 8.17 per cent HzSO4, 83.3 per cent "03, 8.13 per cent HzO, and 0.44 per cent Nz oxides.
essentially constant. In spite of this, however, since results obtained in the initial stages of corrosion are usually erratic, it is probable that a number of the corrosion rates reported are considerably in excess of those which would be obtained in long, continued runs. Others may be somewhat lower than the results of prolonged tests. On the whole, however, particularly as regards the relative magnitude of the oxygen effect, the results seem to offer a good basis of comparison. MILDSTEEL-In sulfuric acid the effect of oxygen is practically negligible with 96.5 per cent acid. With increasing dilution oxygen causes a great increase in corrosiveness, and with 6 per cent acid the rate is over eleven times as fast as in an oxygen-free solution. Considered in another way, the most dilute acid is the most corrosive of the four solutions when saturated with oxygen and is the least corrosive when oxygen is absent. I n dilute hydrochloric acids oxygen is mainly responsible for the rapid corrosion, accounting for well over 90 per cent of the attack a t strengths below 4 per cent. Glacial and 6 per cent acetic acids show the same characteristics,
Seventy per cent nitric acid does not attack steel rapidly, and shows practically no oxygen effect. Dilute nitric is very corrosive, but the presence of oxygen, although it does increase the corrosion, is only a minor factor. Thus the last column shows a definite effect due to oxygen, but this is quite unimportant as compared with the 2orrosion due to the oxidizing acid itself. CHROMIUMSTEEL-The results with chromium steels shnw the susceptibility of these steels to sulfuric acid. Chl*ome steels are excellent resistors of strongly oxidizing acidfi but seem to corrode under nonoxidizing conditions. The pyesence of oxygen has little effect, and may even slightly reduce', the corrosion of the samples with 13 and 16 per cent chromiJtm. Such a reduction of attack caused by the presence of oxygen indicates that passivity and the formation of protet'tive films due to oxidizing conditions can be brought about to a certain extent by such a weak agent as dissolved oxygen. Similar indications of passivifying action due to dissolved (3xygen can be observed from the data on the action of strong : d f u r i c and nitric acids on mild steel.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1925
Table 11-Effect Acid
MIZIAL Mild steel
concentration Temperature Per cent c. by weight -96.5 20 50 20.0 6.0
35
50
Copper
20 35 50
Aluminium
20
35 50
96.5 50.0 20.0 6.0
96.5 50.0 20.0 6.0
96.5 20.0 6.0
96.5 20.0 6 0 96.5
20.0
6.0 96.5 50.0 6.0 96.5 50.0 6.0 96.5 50.0 6.0
of Temperature on Corrosion by Sulfuric Acid
-----AVERAGE ---Under Cm. 0.158 0.520
0.685
0.854 0.270 2.06 1.28 0.70 0.467 7.9 1.97 1.13 0.104 0.338 0.376 0.125 0.525 0.965 0.214 0.246 1.10 0,284 0.150 0.086 1.18 0.580 0.147 4.45 2.26 0.467
351
02-
PENETRATION PER YEAR---Under HtCm. Inches 0.165 0.065 0.460 0.181 0.190 0.075 0.076 0.030 0.315 0.124 1.89 0.743 0.221 0.56 0.132 0.052
Inches 0.062 0.205 0.270 0,336 0.106 0.809 0,504 0,276 0.184 3.1 0.773 0,443 0.041 0.135 0.148 0.049
0.207 0.380 0.084 0.097 0.434 0.112 0,059 0.033 0,464 0,228 0.058 1.75 0.888 0.184
ALuMINIuM-Dissolved oxygen does not markedly increase the corrosion of aluminium by sulfuric and nitric acids, and may possibly reduce the attack by these acids when con-. centrated. Since aluminium is normally protected by a surface film of oxide which must be removed by the corroding acid, it is not surprising that oxygen does not increase the rate of solution. There may well be a close parallel in this case with the suggestion offered above that oxygen actually gives further protection under conditions where passivity is induced. The run with 4 per cent hydrochloric acid shows the extremely rapid corrosion of aluminium when the protective surface coating is removed. Furthermore, the oxygen effect is very large under these conditions. LEAD-Lead corrodes only slightly in sulfuric acid because of film protection, and the presence of oxygen does not cause a marked increase. In hydrochloric acid, however, no such film is formed, and about 90 per cent of the total corrosion in 20 and 4 per cent acids is due to oxygen. Thirty per cent nitric shows very severe action, and here again, as with all the experiments on nitric, the effect of oxygen is unimportant as compared with the oxidizing action of the acid itself. Glacial acetic shows the oxygen effect in the absence of protective films even more strikingly than does the 20 per cent hydrochloric acid. COPPER-Copper is practically immune to attack by nonoxidizing acids saturated with hydrogen. Oxygen accelerates corrosion quite markedly except with the strong sulfuric and glacial acetic acids. I n particular, the attack by hydrochloric acid is increased 180-fold in 20 per cent solution and 75-fold in 4 per cent. KOruns were made with nitric acid because of the rapid rate a t which the metal would be dissolved under such oxidizing conditions. NIcmL-The results with nickel correspond somewhat with those on copper, the corrosion under hydrogen being very low except for 20 per cent hydrochloric acid. Oxygen accelerates the corrosion of nickel in all the cases studied here, and although the action with hydrochloric acid is less drastic than it is on copper, acetic acid corrodes the nickel more rapidly. I n particular, glacial acetic acid with oxygen is quite corrosive to nickel but nearly inert towards copper. MomL-Monel follows the general behavior of its constituents, copper and nickel, but is superior to either in most points. I n glacial acetic acid it resists like copper, while it is better than nickel in hydrochloric acid. TIN-The corrosion of tin by weak nonoxidizing acids is also determined primarily by dissolved oxygen. The few
0.485 9.4 1.14 1.18
0.0142 0,0147
0.0086
0.025 0,0281 0.0222 0,107 0.025 0.032 0,338 0.157 0.0239 1.18 0.490 0.046 4.50 1.67 0.2i0
2.191 6 .(
0.447 0.465 0.0056 0.0058 0,0034 0,010 0,0082 0,0087 0,042 0.010 0.012 0.133 0,062 0.0094 0.464 0.193 0.018 1.77 0.658 0.106
0z:H:
1.0
1.1 3.6 11.1 0.9
1.1 5.3 2.3
1.0 0.8 1.7 1.0 7.3 23.4 43.8 4.9
25.0
43.8
2,g
9., 36.2 0.8
1.0 3.5 1.0 1.2 3.2 1.0 1.2 1.l
--Or
Cm. -0.007 0.060 0.495 0.778 -0.045 0.17 0.72 0.57 -0.01s -1.5 0.83 -0.05
- H-
Inch -0.003 0.024 0.195 0.306 -0.018 0.066
0,090 0.323
0.283 0.224 -0,007 -n- . -H 0.326 -0.022 0.035 0.129
0.100 0.497 0.943 0.107 0.221 1.07 -0,054 -0,007 0.062 0.000 0,090 0.101 -0.05 0.59 0.197
0,039 0.199 0.371 0.042 0.087 0.422 -0.021 -0.003 0.024 0.000 0.035 0.040 -0.02 0.230 0.078
0.367
0.145
data on this metal show over a hundredfold increase with three different acids when saturated with oxygen. EFFECTOF TEMPERATURE-The experiments with varying temperatures in sulfuric acid (Table 11) show that the corrosion under hydrogen increases with temperature in all cases. With steel the increase caused by oxygen is somewhat erratic, b u t a t 50" C. oxygen increases corrosion only with 20 per cent acid. Oxygen is much less important a t the higher temperatures, both because of its lowered solubility and because of the more rapid increase in corrosion with hydrogen gas evolution. With copper the oxygen effect is still predominating a t 50" C. because corrosion in the absence of oxygen is only slight. Thus, oxygen increases the rate 36-fold in 6 per cent acid, even a t 50" C. With aluminium the effect is small a t any temperature, although the tendency is certainly to minimize the importance of oxygen as temperature goes up. Conclusions
1-The presence of dissolved oxygen usually increases the corrosion of metals by acids. 2-The effect of dissolved oxygen is not important with oxidizing acids because its oxidizing action is overshadowed by that of the acid itself. 3-Under conditions of passivity and film formation induced by oxidizing conditions-aluminium and chromium steels, and often in strong nitric and sulfuric acids-dissolved oxygen may actually decrease the corrosion rate. 4-The corrosion due to dissolved oxygen becomes more important as the corrosion due to other causes is diminished; thus, (a) In the case of anodic metals such as iron, which evolve hydrogen gas in acids, the oxygen effect is more important in weaker acids where hydrogen evolution is slow. ( b ) Cathodic metals such as copper and nickel, which do not evolve hydrogen from acids, are quite resistant to nonoxidizing acids in the absence of oxygen, but may be rapidly attacked when oxygen is present. (c) Oxygen corrosion of iron in acids becomes less important as the temperature is raised, chiefly because the corrosion with hydrogen gas evolution increases a t a much more rapid rate than does the oxygen corrosion.
11-Effect of Velocity Scope of Investigation
For the investigation of the effect of velocity it was planned to make one series of runs a t varying velocities on the corrosion of copper by different strengths of sulfuric, acetic, and
INDUSTRIAL AND ElVGINEERING CHEMISTRY
April, 1925
3-Velocity up to 120 cm. (4 feet) per second increases corrosion in air-saturated hydrochloric acid almost 13-fold and more than doubles the attack by sulfuric acid. It does not, however, exert an appreciable effect with acetic acid. At 120 cm. (4 feet) per second the hydrochloric acid is twenty times as corrosive as the sulfuric and four hundred times the acetic acid in aerated solutions. 4 - 1 ~ ~the ~concentration ~ ~ i ~of aerated ~ sulfuric acid from 6 to 30 per cent causes a 25 per cent decrease in corrosion rate.
The results of the investigation of the effect of velocity on the corrosion of steel in concentrated sulfuric acid are given in Table IV. Each of the corrosion rates reported repre-
353
ACID
Per cent by weight
Cm./vear under Oz
Cm./year under air
H2SO4 CtH4OZ HCI
15to20 6 6 4
0,343 0.376 0.058 3.53
0.091 0.102 0.0114 0.33
Ratio: Cm./year under 0 2 Cm./year under air 3.8 3.7 5.1 10.7
The ratio of corrosion in an oxygen-saturated Solution to that in a solution saturated by air should be about 5 if the rate is proportional by oxygen concentration. The ratios given above confirm this assumption reasonably for the sulfuric and acetic acids, although the figure is somewhat high for hydrochloric acid. Hom-ever, since corrosion by
Table IV-Corrosion
of Steel in S u l f u r i c Acid (Average penetration per year)
Acid concentration Per cent -At by weight Cm. I
80.5 80.5 80.5 89.0 91.9 94.8 97.8
restInch
0.036 0.018
0.014 0.007
0.147 0.112 0.046 0,053
0.058 0,044 0.018 0,021
FIRST 48 HOURS . 7 -At 9 cm. (0.3 ft)/sec. 6 1 cm (2 ft.)/sec. Cm.
0.079 0.56
Inch
Cm.
Inch
0.031 0.220
0.109 0.030 0.231 0.064 0.498 0.198 0.46
0.043 0.012 0.091 0.025 0.196 0.078 0.18
Cm. 0,058 0,089 0,084 0.00 0.028
SECOND 48 HOURS rest9 cm.(0.3 ft.)/sec. 6 1 cm (2 f t )/see Inch Cm. Inch Cm. Inch 0.0058 0 , 0 0 2 3 0.023 0.035 0.033 0.00 0.011
0.00 1.36
0.043 0,112
0.017 0.044
0,084 0.33
0.00 0.536 0.033 0.13
REMARKS Air-saturated Illuminating gas-saturated
61 cm./sec. sample viouslv -. -_., at - - reqt ....
pre-
No previous treatment a t rest 100.0 0.51 0.20 0.58 0.23f 1.75 0.69 0.00 0.00 0.00 0.00 2.36 0.93 6 1 cm./sec. sample previously a t rest 100.0 7.6 3.0 No previous treatment a t rest 100.0 2.13 0.54 Previously pickled in 95 per cent acid for 72 hours Mole-Hydrogen gas evolution observed a t start of all runs. Samples in 100 per cent acid runs a t 61 cm./sec. had polished, vitreous appearance after run and in places had lost over half their thickness. 97.8
2.67
sents the average of the results obtained with four test pieces. Figure V gives the effect of acid concentration on corrosion rate at a velocity of 61 cm. (2 feet) per second. The results of these experiments are somewhat erratic, but the more outstanding features are: 1-At rest, none of the acids between 80 and 100 per cent show severe corrosion, although the concentrations around 90 per cent seem to be most reactive. 2-Saturation with air seems to decrease corrosion in the only acid where air saturation was employed (80.5 per cent). Saturation with illuminating gas, conversely, increases corrosion in this acid. 3-At 61 cm. ( 2 feet) per second, corrosion is low, from 80 to 95 per cent, except for a maximum point a t 92 per cent. At this same velocity 98 per cent acid is quite corrosive and 100 per cent acid has a very severe action on steel. (See Figure V) 4-The corrosion by 98 per cent and 100 per cent acids a t 61 cm. ( 2 feet) per second is much less if the sample has been previously exposed to strong acid in quiet solution. Discussion of Results
ACCURACY OF DaTA-The reported results are probably not reproducible to within better than 25 per cent. This is due to a number of factors, such as (1) nonuniformity of the metal, including effect of previous treatment; (2) the presence of corrosion products in the solution, either dissolved or as suspended solids which might cause erosion of protective films in velocity runs; (3) effect of changing corrosion rate with time, which is particularly important when protective films of corrosion products may be formed; and (4) possibilities of setting up velocity cells due to variable velocities over the surface of the metal. Despite the variability in results, these experiments do give a semiquantitative idea of the corrosion to be expected in the acids studied and also show the effect of velocity. RESULTSWITH COPPER-The results given in Table 11 for copper in air-free 6 and 15 per cent sulfuric and acetic acids indicate that the corrosion of copper in these acids depends on the presence of dissolved oxygen. It is interesting to compare the results for copper given in Tables I and 111.
1.05
means other than oxygen would tend to give a ratio lower than 5, the deviation with hydrochloric acid does not contradict the conclusion that the rate of corrosion of copper by a given nonoxidizing acid is about proportional to the concentration of dissolved oxygen. This is also indicated by a comparison of the results obtained in 6.1 and 30.5 per cent sulfuric acid. The ratio of oxygen solubilities in these two acids is about 1.5, while the ratio of corrosion rates observed is 1.4. The very rapid corrosion with hydrochloric acid is probably due in part to the action of dissolved chlorides of copper. Cuprous ions formed by corrosion would diffuse out into the main body of the acid and be oxidized to the cupric form. These : 3 ] m then actilike cupric o n sdissolved would oxygen in a cathode reaction ,3 &?* Cu++ = Cuf
+@
c ~ ~ w lKmW P t t
n AM i j l r m r u war
P
r-
$0
Now, if the rate of diffusion of oxygen to the cathode is an important factor in determining corrosion rate, the parallel diffusion of cupric ions will increase the total diffusion of oxidizing M r -62 ‘0 zo Qo ad iao lzo agent and make the wDcirr - MI .w ncmo acid more corrosive. The corrosion in hydrochloric acid increases almost in proportion to the velocity (Figure IV). If the reaction is controlled by diffusion processes one would expect a velocity effect similar to that of velocity in heat transfer and other diffusion phenomena-i. e., the rate would be proportional to about the 0.8 power of velocity. The few data on Figure IV approximate this relationship when it is realized that the condition “at rest” is not really “zero” velocity because of convection currents. The effect of velocity, therefore,
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INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 17, No. 4
further substantiates the idea that corrosion of copper in be eaten away by attrition against the glass rod passing hydrochloric acid is determined largely by the rate of diffu- through them. The wearing occurred only in the direction sion of oxidizing agents. where the rod exerted pressure against the metal and might The increase with velocity in sulfuric acids is definite, but amount to 0.3 cm. in 72 hours. This phenomenon further is much less than would be expected from a diffusion process. confirms the concept that the low corrosion a t rest is due to It is probable that some process involving film formation film protection and that the acceleration m t h velocity is supervenes-possibly the formation on the surface of slight largely the result of removing the film. The presence of amounts of insoluble cuprous salts. Since films on copper solid corrosion products suspended in the acid probably has are generally quite adherent, increase in velocity would not a further effect of the same general nature. The accelerating effect of velocity on the corrosion of steel destroy them and the resultant velocity effect would be slight. The results with acetic acid show even less of an by these concentrated acids has also been noted by Fawsitt increase with velocity and PainsD These authors also attribute the increased corroand may be explain- sion to the removal of a protective coating of corrosion able in a manner products (FeSOd.H*O). Since both the solution and test similar to the above. pieces were agitated in their experiments, it is impossible to The theory of film determine the velocities that they employed. f o r m a t i o n in these Conclusions cases is advanced as a 1-The corrosion of copper in solutions of sulfuric, hydrotentative proposition requiring further in- chloric, and acetic acids is dependent on the presence of dissolved oxygen. vestigation. 2-In sulfuric and acetic acids the amount of copper corroRESULTS WITH STEEL I N SULFURIC sion is practically proportional to the dissolved oxygen content. Acm-The results in This generalization probably holds true in hydrochloric acid, Table IV show only although the data do not so closely approximate the theory. 3-The corrosion of copper in aerated hydrochloric acid s l i g h t corrosion at rest for the various increases rapidly with velocity and the velocity effect parallels strengths between 80 that observed in diffusion processes. It seems probable that and 100 per cent. the rate of diffusion of dissolved oxygen and of copper ions The penetrations re- controls this reaction. ported are probably higher than occur in practice, since Velocity accelerates the the rate usually decreases with time in the quiet solutions. corrosion in sulfuric acids only slightly, and N o t c A n extensive series of tests by Knietscha over the same field of the incraase with acetic HrSOa concentration studied in this paper showed maxima at 80, 90, and acid is even less. The slightly above 100 per cent. Knietsch’s corrosion rates were obtained in 72-hour beaker tests and no study was made of the effect of velocity. His explanation of protecresults are in general lower than the values for zero velocity for the 48 hours tive film action with reported in this paper. these two acids reBelow 90 per cent acid the formation of permanganates from quires further investimanganese in the steel was observed in several cases, and gation. the influence of this constituent has not yet been determined. &The corrosion of The two runs with 80.5 per cent acid, using air-saturated steel in 80 to 100 per $ ’’ and illuminating gas-saturated acids, respectively, are par- cent sulfuric acid is f ticularly interesting. These runs-which are substantiated slight when there is no S 2.4 Pe by other runs not reported in this paper-show that under agitation. certain conditions there is less corrosion in the presence of 5-Corrosion of steel 16 dissolved oxygen than in its absence. This fact seems most by concentrated sulreadily explainable on the assumption noted in the first part furic acids may be .d of this paper, that the oxidizing power of the dissolved oxygen greater in the absence increases the passivifying tendency and decreases the corro- of dissolved oxygen o >O 100 sion rate. than in air-saturated cam % RY W N G ~ I I n acid from 80 to 90 per cent, velocity does not cause a solutions, owing to the marked increase in corrosion, whereas above 90 per cent added passivifying effect of dissolved oxygen. its effect is greatly to increase the attack. I n the 90 to 100 +In sulfuric acid above 90 per cent the corrosion of per cent acids, increasing the velocity from 0 to 61 cm. (2 steel is markedly increased by increasing the velocity. This feet) per second results in increasing the corrosion rate increase is due to the removal of or prevention of formation several fold-the specific increase varying with the different of a protective film on the metal surface. acids. This effect must be mainly attributed to prevention 7-Steel first exposed to quiet acid is to a certain extent of protective film formation, because samples on which a protected from the action of the acid when the velocity is film had previous!y formed (by a former prolonged immersion increased, probably on account of the initial formation of at rest) corroded less than untreated samples. However, protective films which are not readily removed. even the treated samples corroded quite rapidly a t high veAcknowledgment locity indicating that velocity serves to remove gradually Much of the experimental work referred to in this paper any protective film which may have been formed. I n particular, the samples exposed to 98 and 100 per cent was carried out under the authors’ direction in thesis inacids at 61 cm. (2 feet) per second showed very severe attack vestigations by W. A. Wilson, C. F. Daley, and E. F. Britt. and had a peculiar vitreous appearance similar to a glass or The experiments on the effect of velocity in the corrosion of slag. I n such cases the drilled holes in the test piece would steel were run by J. W. Poole. 8
B n . , $4, 4069, 4090 (1801).
8
J . Proc. Roy. SOC.N . S. Wales, 63, 396 (1919).
