Effect of Oxygen on the Corrosion of Steels FRANK G. FRESEL Massachussetts Institute of Technology, Cambridge, Mass.
Stainless steel tested under different oxygen pressures in distilled water, in neutral sodium chloride solutions, and i n a mixture of sodium chloride and hydrochloric acid, and mild steel under different oxygen pressures i n distilled water, show a maximum rate of corrosion 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 rnaximum, the corrosion 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 content. The corrosion rate of mild steel i n 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.
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RIGINALLY it was intended to investigate the corro-
sion of stainless steel in sea water. Since the corrosion of this material is 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, t h a t high oxygen pressure did not increase the corrosion rate appreciably but did disclose behavior of such interest t h a t 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.
Literature Xot 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 (3) 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 a t 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 concentration. 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
time curves was obtained for these samples in 0.1 S potassium chloride solution with a number of oxygen pressures up to 25 atmospheres. The corrosion-time curves are approximately straight lines u p 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 a t 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.
Present address, 18 West Hamilton Street, Baltimore, &Id.
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Experimental Procedure The
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 l / g x 1 x inches (0.32 X 2.5 X 4.1 cm.); those of the mild steel, '//IO X 1 X 16/s inches (0.48 X 2.5 X 4.1 cm.). In preparing the cut from sheets of the material, samples, specimens 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 TABLEI. CORROSION 13 ROCKING PRESSURE VESSEL AFTER per cent chromium and 8 per cent nickel with the %DAYEXPOSURE u s u a l s m a l l a m o u n t s of carbon, silicon, sulfur, -Av. Loss in Weight of Stainless SteelAv. Loss in JVeight of Mild phosphorus, and manganese. The sheets had been 3.5% hot-rolled, annealed, and pickled. The mild steel was common boiler plate. Oxygen Distd. 3.5% 12% 20% Distd. 3.5% 12% The high-pressure reaction chamber Pressure water NaCl hac1 NaCl HC1 water NaCl NaCl 15le
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.up. in much of the work consisted of a thick-walled 0.0 0.0 0 15 0.0 7.2 1.1 1.55 2.8 1.6 steel cylinder with an inner bore 21/2 inches (6.4 0.04 0.0 0.3 0.45 8.6 . . .. .. .. .. .. .. ... cm.) in diameter and 14 inches (35.6 cm.) long. 0.1 0.0 0.4 0.3 15.8 A brass rack, carrying five 50-cc. beakers in which 0.: 0.25 0.35 0 5 13.36 24:7 25.7 1610 S'i 0.3 0.2 0.45 0.8 5.1 ..... .. ... the test specimens together wit'h the proper solu10 0.0 0 2 0.26 1 5 19:2 111 0 .. ... tion were placed, fitted into t'he bore of the pres9.66 .. .. .. ... 11.2 1020.0 .. ... 8 . 5 3570 .. ... sure vessel. Escape of gas from the pressure vessel 61.3 .. .. .. ... was prevented by a lead gasket be,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 10-6 for stainless steel and by 8.4 x 10-5 for mild steel. b Two-day exposure for one ai the specimens. was led from a steel cylinder through copper tubing to the reaction chamber. In the copper tubing line was placed a gage to register the pressures. All connections were made in the usual way, using bolts and cones. Bt pressures of one atmosphere or below, a mercury 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 xith 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 solutions there is a point of maximum corrosion which of 6oo and with a period of about 4o seconds. Whether under gas Occurs a't an oxygen Pressure below one atmosphere. At 10 pressure or in VQCUO, all t,est samples were rocked during the test and 61 atmospheres pressure the corrosion of stainless steel is period. Samples tested in V Q C Z ~ Owere placed in glass tubes containing very Ion. 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 10to tor, the solution was warmed to produce active boiling, the 61 atmospheres is Probably Without significance. The data boiling was continued for about 15 minutes, and finally the tubes 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 &loride solutions have a destructive action on protective atmosphere and vacuum were made in the pressure vessel under defilms-the greater the concentration of the salt the the necessary air pressure. Init,ially the air was removed from structive the action of the solution. the vessel by the aspirator, thus removing a large part of the 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 actually reduce the corrosion rate, indicates definitely that to one atmosphere pressure and then partially removed to the the high oxygen Pressure increases the Protectiveness of the required pressure, the purpose being t o 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 belol%r,allowance iron, higher oxygen pressures bring about increased corrosion. was made for the vapor pressure of the solution. Above one With stainless steel, however, the general opinion is that this atmosphere pressure this correction was neglected. material is Very resistant t o Corrosion, Once a slight oxide At t,he oonclwion 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 int,ensifies the protective action of the film, solution of hydrochloric acid (2 cc. of concentrated acid in 100 cc. to be formed more presumably by causing the water), washed in tap water and t,hen in distilled water, dipped and adherently t o 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" are greater than in neutral solutions. This means that the to 25' c. experimental accuracy is probably also greater. Here, too, the maximum corrosion occurs a t a n oxygen pressure below Since in many tests with stainless steel the loss in weight one atmosphere, and the corrosion a t one atmosphere is during the corrosion period was quite small, it was important slight. The significant amount of corrosion under vacuum to h o w how much, if any, of the loss was due to the treatconditions 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 vucuo, 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 t o 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. I n 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 corAtm.
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JANUARY, 1938
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corrosion rate, again occurring at an oxygen pressure below one atmosphere. This result is surprising and contradicts the opinion generally held. I n 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. I n 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. K i t h mild steel in distilled water, however, there is good evidence of decreasing corrosion Tvith 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 vere loose and fluffy; a t 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 a t 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 a t high oxygen pressures in 3.5 per cent sodium chloride solution some of the corrosion products v-ere extremely hard.
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 a t high oxygen pressures was explained on the assumption that a t 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 a t the higher oxygen pressures. 6. I n 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 . V i t h 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.
Summary
(3) Speller, F.N.,private communication. (4) Walker, W. H.; Cederholm, A. M., a n d B e n t , A. L.,J. Am. Chem. SOC.,29,251 (1907).
1. Stainless steel in neutral solutions or in distilled water corroded slightly and in all cases showed a maximum corrosion rate a t oxygen pressures below one atmosphere.
Acknowledgment The writer wishes to express his indebtedness to W. G . Khitman, of the Chemical Engineering Department, M. I. T., for his significant help and keen interest in the work.
Literature Cited (1) Herzog, E., and Chaudron, G., Cornpt. rend., 190, 1189 (1930). (2) Lee, A.R . , Trons. Faraday Soc., 28, 707 (1932).
RECEIVED April 30, 1937. The author w&s a Fellow in the Department of Chemical Engineering of IM. I. T. in 1935-36
ISSTALLING A FORSTERLINING IN THE BURNING ZONE OF A ROTARYCEMENTKILN ITE
The paper between rings represents an expansion allowance and will burn out. Individual bricks in the rings are separated by ateel plates which fuse t o them and produce a monolithic effect. (See article by Harvey and Birch, page 27.)
