Effect of Oxygen
on
the
Corrosion of Steels
Ind. Eng. Chem. 1938.30:83-85. Downloaded from pubs.acs.org by KAROLINSKA INST on 01/23/19. For personal use only.
FRANK G. FRESE1 Massachusetts Institute of Technology, Cambridge, Mass.
it
Stainless steel tested under different in distilled water, in pressures neutral sodium chloride solutions, and in a mixture of sodium chloride and hydrochloric acid, and mild steel under in distilled different oxygen pressures of corroshow a maximum rate water, sion below one atmosphere oxygen pressure. At oxygen pressures above those represented by the maxima, the corrosion rate rapidly decreases. This behavior is explained on the assumption of the formation of a more protective film at high oxygen pressures. That a more protective film was probably formed was evidenced by the observation with mild steel in distilled water, that as the corrosion rate decreased from the maxithe corrosion mum, products became hard and compact. The corrosion rate of stainless steel in sodium chloride solution rose with increased salt concentration. This was probably due to the well-known filmdestroying action of the chloride ion, the action increasing with rise of salt oxygen
intended to investigate the corrowater. Since the corrosion generally slow under most conditions, it was thought that increasing the oxygen pressure over the neutral salt solution not only might shorten the test periods, but also avoid the weakness of many accelerated corrosion tests in which the character of the solution is made quite different from that found in service conditions. Preliminary work showed, however, that high oxygen pressure did not increase the corrosion rate appreciably but did disclose behavior of such interest that the problem was enlarged to the study of the effect of oxygen pressure on the corrosion of both stainless steel and mild steel in distilled water and in several concentrations of sodium chloride solutions. Several tests were also carried through to study the corroding effect on stainless steel of a mixture of sodium chloride and hydrochloric acid under various oxygen pressures. was
sion of stainless steel in sea ORIGINALLY of this material is
Literature Not much work has been done on the study of the effect of oxygen concentration on corrosion rate. Walker, Cederholm, and Bent (4) tested the effect of oxygen concentration on the rate of corrosion of pure iron wire in distilled water. The tests were of the stagnant type and continued for about 4 days. Over this period the corrosion rate was approximately proportional to the oxygen pressure throughout the range of pressures employed. The data from the two series of tests indicate no tendency for the corrosion rate to attain a maximum value and then decrease. Speller (S) determined the effect of variations of oxygen concentration of McKeesport, Pa., city water on the corrosion of chrome irons and of ordinary sheet steel. A year’s test at 49° C. showed that the corrosion of the Bessemer steel and of the two low-chrome irons was proportional to the oxygen concentration of the water; the corrosion of the two highchrome irons, however, was independent of the oxygen con-
content.
The corrosion rate of mild steel in 3.5 per cent sodium chloride solution was roughly proportional to the oxygen pressure. This is the expected behavior when no protective film is present.
centration. Herzog and Chaudron (1) tested the corrosion resistance of Duralumin in sea water and in 3.5 per cent sodium chloride solution under pressures ranging from 30 to 120 atmospheres. The tests were of the stagnant type. Up to 90 atmospheres the attack appeared to be proportional to the oxygen pressure, but beyond this pressure the increase in corrosion rate was slight with horizontal samples. With vertical samples there was a substantial increase in the corrosion rate in passing from 90 to 120 atmospheres. Lee (2) supported steel disks on three glass points below the solution level in glass vessels. The latter were placed one above the other in a steel bomb. A series of 10-day corrosion1
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time curves was obtained for these samples in 0.1 N potassium chloride solution with a number of oxygen pressures up to 25 atmospheres. The corrosion-time curves are approximately straight lines up to 15 atmospheres. At 20 atmospheres the curve departs from a straight line, and beyond this pressure the corrosion rate falls off continuously, the curve tending to become horizontal. This change in the slope of the curve corresponded with a change in the nature of the corrosion products, part of which at 25 atmospheres assumed a hard, granular form. Lee remarks that this compact material adhered very strongly to the test specimens and was probably responsible for the slowing up of the corrosion rate.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Experimental Procedure The following description of the method of preparing the samples for testing applies to all specimens unless otherwise stated:
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rosion loss was substantial and any cleaning loss would probably affect the total loss very slightly. The work reported here was carried out on one kind of stainless steel and on one kind of mild steel. Table I gives the average change in weight of the samples. The results of duplicate tests with stainless steel in distilled water and in neutral salt solutions never differed by more than 0.2 mg. For stainless steel in a mixture of sodium chloride and hydrochloric acid and for mild steel the variations were never greater than 50 per cent.
Stainless steel samples averaged in dimensions Vs X 1 X l5/s inches (0.32 X 2.5 X 4.1 cm.); those of the mild steel, 3/ie In preparing the 1 X l6/s inches (0.48 X 2.5 X 4.1 cm.). samples, specimens were cut from sheets of the material, ground on emery paper, and finished on No. 1 metallographic paper. The polished specimens were cleaned by scrubbing three times with benzene and a soft brush, dipped in alcohol, then scrubbed twice with carbon tetrachloride, and finally dried with a clean towel. The stainless steel as received was in sheet form containing about 18 Table I. Corrosion in Rocking Pressure Vessel after per cent chromium and 8 per cent nickel with the 8-Day Exposure usual small amounts of carbon, silicon, sulfur, /—Av. Loss in Weight of Stainless Steel— Av. Loss in Weight of Mild Steel and The had sheets been manganese. phosphorus, 3.5% NaCl + hot-rolled, annealed, and pickled. The mild steel Distd. 3.5% Oxygen Distd. 12% 20% 0.05 N 3.5% 12% 20% was common boiler plate. Pressure NaCl NaCl NaCl HC1 water NaCl NaCl NaCl The high-pressure reaction chamber employed Aim. MgA Mg. Mg. Mg. Mg. Mg. Mg. Mg. Mg. in much of the work consisted of a thick-walled i 0.0 0.0 0.0 0.0 0.15 7.2 1.1 2.8 1.6 steel cylinder with an inner bore 23/2 inches (6.4 0.04 0.0 0.1 0.3 0.45 8.6 0.1 0.0 cm.) in diameter and 14 inches (35.6 cm.) long. 0.25 0.4 0.3 15.8 0.2 0.25 0.4 0.35 0.5 13.36 25,7 25.7 A brass rack, carrying five 50-cc. beakers in which 16.0 8.3 0.5 0.2 0.15 0.8 0.45 5 1 the test specimens together with the proper solu1.0 0.0 0.05 0.2 1 5 111.0 0.25 19.2 tion were placed, fitted into the bore of the pres9.66 0.0 11.2 1020.0 61.3 0.05 8.3 3570 sure vessel. Escape of gas from the pressure vessel a lead gasket bewas prevented by compressing 0 To calculate the average penetration in inches per year, multiply the tabulated loss in tween the lid and the wall of the vessel. Oxygen milligrams by 9 X 10 -5 for stainless steel and by 8.4 X 10 "5 for mild steel. 6 Two-day exposure for one of the was led from a steel cylinder through copper tubing specimens. to the reaction chamber. In the copper tubing line All was placed a gage to register the pressures. connections were made in the usual way, using bolts At pressures of one atmosphere or below, a mercury and cones. Discussion manometer was substituted for the gage. The nature of the The data show that for stainless steel in neutral solutions tests requires that the solutions in the small beakers be kept saturated with oxygen at all times during the test period. This or in distilled water, the corrosion is slight in all cases. In all was achieved by rocking the reaction cylinder through an angle four a point of maximum corrosion which solutions there is of 60° and with a period of about 40 seconds. Whether under gas occurs at an oxygen pressure below one atmosphere. At 10 pressure or in vacuo, all test samples were rocked during the test and 61 atmospheres pressure the corrosion of stainless steel is period. Samples tested in vacuo were placed in glass tubes containing very low in 3.5 per cent sodium chloride solution. The apthe corroding solution, the tubes were evacuated by an aspiraparent slight increase in corrosion rate in passing from 10 to to active the solution was warmed boiling, produce tor, the 61 is probably without significance. The data the atmospheres for and continued about 15 tubes was minutes, finally boiling likewise show that the corrosion rate increases with salt conwere sealed. Tests made at 0.2 atmosphere of oxygen pressure were carried centration. This probably finds explanation in the fact that out in open beakers. Tests carried out at pressures between 0.2 chloride solutions have a destructive action on protective made in vessel the under and vacuum were pressure atmosphere films—the greater the concentration of the salt the more dethe necessary air pressure. Initially the air was removed from structive the action of the solution. the aspirator, thus removing a large part of the the vessel gases dissolved in the solution. Air was then let in to the reThe fact that high oxygen pressures do not accelerate but quired pressure. Twice daily air was allowed to enter the vessel reduce the corrosion rate, indicates definitely that actually to one atmosphere pressure and then partially removed to the the high oxygen pressure increases the protectiveness of the required pressure, the purpose being to keep the partial pressurface film. It is fairly well agreed that with mild steel and sure of oxygen over the samples constant during the test period. At oxygen pressures of one atmosphere and below, allowance iron, higher oxygen pressures bring about increased corrosion. Above one was made for the vapor pressure of the solution. With stainless steel, however, the general opinion is that this atmosphere pressure this correction was neglected. material is very resistant to corrosion, once a slight oxide At the conclusion of a test (lasting 8 days in most instances) film is formed on the surface. The high oxygen concentrathe samples were removed from the corroding medium, scrubbed with a soft brush in tap water, allowed to stand 3 minutes in a tion probably intensifies the protective action of the film, solution of hydrochloric acid (2 cc. of concentrated acid in 100 cc. presumably by causing the film to be formed more closely and then in distilled water washed in water, dipped tap water), and adherently to the metal surface. in alcohol, and dried. The samples were then placed in a desiccator for 24 hours and finally weighed. In most instances the The data on the corrosion of stainless steel in a mixture of tests were made in duplicate (sometimes in triplicate). All sodium chloride and hydrochloric acid show that the losses the tests were made at room temperature which varied from 22° This means that the are greater than in neutral solutions. to 25° C. experimental accuracy is probably also greater. Here, too, the maximum corrosion occurs at an oxygen pressure below Since in many tests with stainless steel the loss in weight one atmosphere, and the corrosion at one atmosphere is during the corrosion period was quite small, it was important to know how much, if any, of the loss was due to the treatslight. The significant amount of corrosion under vacuum conditions was probably accompanied by hydrogen evolution. ment following the corrosion. Four cleaned and weighed The maximum corrosion is only double that experienced in samples were carried through the treatment given corroded vacuo, and the decrease after passing the maximum is rapid. samples with the following results: Three samples showed no Again it would appear that the film becomes more protective change in weight (the balance was accurate to 0.0001 gram) with increasing oxygen pressure. and one showed a loss of 0.0001 gram. From these results it The rates of corrosion of mild steel are naturally much is fairly certain that any loss sustained by stainless steel durhigher than those of stainless steel in neutral solutions. In ing corrosion testing is actually due to corrosion. Parallel distilled water the mild steel shows a definite maximum in tests with mild steel were not made, since in this case the cor______
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JANUARY,
1938
INDUSTRIAL
AND ENGINEERING
corrosion rate, again occurring at an oxygen pressure below one atmosphere. This result is surprising and contradicts the opinion generally held. In 3.5 per cent sodium chloride solution the corrosion rate increases steadily with the oxygen pressure and is semiquantitatively proportional to the pressure over a considerable range. The small amount of corrosion taking place in vacuo may have resulted from the incomplete removal of oxygen dissolved in the solution or adsorbed on the metal surface. However, theoretically it would be possible for a little corrosion to occur even if all the oxygen were removed, the action being accompanied by evolution of hydrogen gas. The corrosion rate of mild steel under 0.2 atmosphere oxygen pressure decreases as the salt concentration rises. This behavior agrees roughly with the fact that the solubility of oxygen decreases as the salt concentration increases. The results of corrosion of mild steel in 3.5 per cent sodium chloride solution are what might be expected if no protective film is formed. In such a situation the corrosion rate will be proportional to the oxygen concentration in the solution, which in turn is proportional to the oxygen pressure in the gas phase.
With mild steel in distilled water, however, there is good evidence of decreasing corrosion with high oxygen pressure. This behavior can be most reasonably explained on the assumption of the formation of a protective film. At oxygen pressures below that corresponding to the maximum corrosion rate, the corrosion products were loose and fluffy; at pressures above this the products were so hard they had to be removed with a steel probe. It is this change in the character of the corrosion products that suggests the formation of a more protective film at higher oxygen pressures. These protective films also probably tend to form in chloride solutions but are partly destroyed by the action of the chloride ion. There is some evidence for this assumption, for at high oxygen pressures in 3.5 per cent sodium chloride solution some of the corrosion products were extremely hard.
Summary Stainless steel in neutral solutions or in distilled water corroded slightly and in all cases showed a maximum corrosion rate at oxygen pressures below one atmosphere. 1.
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2. The definite increase in corrosion of stainless steel in neutral salt solutions with increase of salt concentration was explained from the known fact of the destructive action of the chloride ion on protective films. 3. The decrease in the corrosion rate of stainless steel at high oxygen pressures was explained on the assumption that at elevated pressures a more adherent and protective film is
formed. 4. The significant corrosion of stainless steel in a mixture of sodium chloride and hydrochloric acid under vacuum was probably of the hydrogen evolution type. Here, too, there is a maximum in the corrosion rate which occurs below one atmosphere pressure. The rapid decrease in the corrosion rate after passing the maximum points to the formation of a protective film. 5. Mild steel in distilled water shows a definite maximum in corrosion below one atmosphere. At higher pressures the corrosion rate decreased and was accompanied by a pronounced hardening of the corrosion products. This hardening of the corrosion products suggests the formation of a more protective film at the higher oxygen pressures. 6. In 3.5 per cent sodium chloride solution the corrosion of mild steel increases steadily with oxygen and is roughly proportional to the pressure for a considerable range. This is the anticipated behavior when no protective film is present. 7. With mild steel under 0.2 atmosphere oxygen pressure the corrosion decreases as the salt concentration rises. This is roughly parallel with the fact that the solubility of oxygen decreases as the salt concentration increases.
Acknowledgment The writer wishes to express his indebtedness to W. G. Whitman, of the Chemical Engineering Department, . I. T., for his significant help and keen interest in the work.
Literature (1) (2) (3) (4)
Cited
Herzog, E., and Chaudron, G., Compt. rend., 190, 1189 (1 30). Lee, A. R., Tro,ns. Faraday Soc., 28, 707 (1932). Speller, F. N., private communication. Walker, W. H., Cederholm, A. M., and Bent, A. L., J. Am. Chem Soc., 29, 251 (1907).
Received April 30, 1937. Chemical Engineering of .
The author was I. T. in 1935-36.
a
Fellow in the Department of
Installing a ForsterLining in the Burning Zone of a Rotary Cement Kiln ite
The paper between rings represents an expansion allowance and will burn out. Individual bricks in the rings are separated by steel plates which fuse to them and produce a monolithic effect. (See article by Harvey and Birch, page 27.)
