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Discrete areas of SO2 adsorption from the atmosphere onto iron
Corrosion Science, 1973, Vol. 13, pp. 69 to 72. Pergamon Press. Printed in Great Britain
SHORT COMMUNICATION
DISCRETE AREAS OF SOs A D S O R P T I O N F R O M THE ATMOSPHERE ONTO IRON* J. R. DUNCAN and D. J. SPEDDING Chemistry Department
and E. E. WHEELER Pathology Department, University of Auckland, Auckland, New Zealand
RECENT reports x,2 have discussed the presence of discrete areas of 35SO~- adsorption from aqueous solution onto steel samples. One report 1 concluded that there was no clear primary reason for the uneven distribution found, while the other group s attributed it to specific adsorption at anodic areas of the surface in the electrolytic corrosion cells set up on their samples, which had been precorroded in the atmosphere for several months. We have studied the adsorption of B5SO2 from the atmosphere onto iron surfaces which were not visibly rusted, and have found a similar effect. Thin iron films (0.012 mm thick, 99-95%Fe) were fixed to atuminium studs used for electron microscopy (10 mm diameter). These were exposed for several hours in a flow system to an atmosphere containing a known concentration (0.1-0.2 ppm) of 35SO2 in air. Relative humidity was maintained at 70 or 95 per cent r.h. The humidity inside the exposure vessel was assessed from readings of wet and dry bulb thermometers, asses concentrations were determined by flowing part of the effluent gas stream at a known rate through a wash bottle containing 1%H2Os solution in water. This was counted in a liquid scintillation spectrometer at the end of the exposure period. The sample was then left in the apparatus with the humid air stream (containing no SOs) passing over it for a period ranging from a few minutes to 14 d, to provide a variety of post-exposure conditions. Control exposures were performed using an identical system, but omitting the BsSO~ from the air stream. The samples were kept in closed containers in the presence of silica gel until they could be examined under the scanning electron microscope (Cambridge Scanning Electron Microscope, Mark II, at 20 kV and 45 ° orientation). Figure 1 shows that no differences in the physical structure of the surfaces of the iron samples exposed to different atmospheric conditions could be detected by electron microscopy. No surface heterogeneities were found on these surfaces. The samples were then autoradiographed by exposure to X-ray film (Agfa Gevaert Curix FW Screen film), for 2 weeks. This showed major variations in the degree of uptake of SOs over the metal surface, with most of the asSO~ activity centred in specific areas up to 0.5 mm in diameter (Fig. 2b). Surface features large enough to cause these *Manuscript received 16 May 1972. 69
70
J . R . DUNCAN and D . J. SPEDDING
differences might have been expected to show under the electron microscope, which could differentiate to 0.5 ~tm, but none was found. Neither could they be correlated with surface phenomena visible under a light microscope (Fig. 2a). There was greater uptake on those samples exposed at 95 per cent r.h. than on those exposed at 70 per cent r.h., and there was increased uptake at the edges, where the metal film had been trimmed to size. The size of the spots found is similar to that found by Barton e t al. I and that found on some of the samples used by Matsushima and Ueno, 2 who suggested in a discussion of their results that adsorption occurs at anodic areas because of ion migration in solution. However, even at 95 per cent r.h., the electrolyte film on the surfaces we have studied will be so thin 8,4 that it may not act as an ordinary electrolyte, 5 hence an alternative mechanism is likely. Hedvall e t aL e have shown that a dipolar molecule such as HsS may be preferentially adsorbed at a copper rod which is at potential + 100 V compared with one at -- 100 V, with respect to earth (zero). Since the SOs molecule is also dipolar, with the sulphur atom as the positive pole, it might also be expected to show a similar behaviour. We have attempted to verify this, but have not found any measurable difference in 35SO2 adsorption onto iron electrodes at + 1.0, -- 1.0 and zero (earth) volts. Similar differential uptake to that described above was again found in this experiment, on all three electrodes. In an analogous experiment in solution containing asSO]-, using an unspecified potential, Matsushima and Ueno produced considerably more adsorption on their anode than on their cathode, s The iron films used in all our experiments were undoubtedly coated with an oxide film of unknown composition. I f the SO2 molecules were to be preferentially adsorbed directly from the gas phase onto anodic areas of the surface, by analogy with asSO]adsorption from solution s, the oxygen atoms of the SOs molecule would be required to form loose bonds to the oxide surface, since they are the negative end of the dipole and the surface anode will carry a nominal positive charge. It is intuitively more favourable that the sulphur atom should be presented to the surface, where it could form more stable (i.e. stronger) bonds with the oxygen atoms of the oxide layer, and hence be easily oxidized to sulphate, which has been shown to be present on surfaces onto which SOs has been adsorbed. 7 Such an attack is more feasible at cathodic areas, where the negative charge can attract the positive end of the dipole, the sulphur atom. However, our results indicate that a potential difference of 2 V is not sufficient to cause differential adsorption. The likely reactions at the surface (excluding those involving sulphur dioxide) are: 1,8
Anode
(a) Fe
--+
E = + 0.47 (b) Fe 2+
~
Fe 2+ + 2e0.059 --log 2
(1) (Fe 2+) V (nhe)
Fe a+ + e -
E = -- 0"77 - - 0"058 log Fe3+ V (nhe)
(2)
Fie;. I. Electron micrographs of iron surfaces. (a) Exposed to 0-01 p p m SO~ in air for 280 rain followed by 60 h at 95 per cent r.h. (b) Exposed to 0.11 ppna SO,_>in air for 100 rain during 22 h exposure at 95 per cent r.h. (c) Exposed to clean air at 95 per cent r.h. for 150 h,
FIG. 2. (a) Micrograph o f same area of metal surface as in autoradiograph in (b). (b) Autoradiograph of iron surface after exposure to 0.11 ppm ~SOa in air for 100 rain during 22 h exposure at 95 per cent r.h. The metal sample used here is that shown in more detail in Fig. I b.
sch adsorption from atmosphere onto iron Cathode (a) Os + 2HsO q- 4e-
~
40H-
71 (3)
E = + 0.40 -- --0"058log (OH-)4 V (nhe) 4 Po2 (b) Reverse of (2). The total potential of the corrosion cell is therefore unlikely to exceed the 2 V used in our experiment, and hence some different explanation of the results is again necessary. A third hypothesis is that there is differential adsorption onto different types of oxide surface. Much has been written on the types of oxide formed on iron in various controlled atmospheres. 9,x° It appears that different modifications of Fe2Oz are the major final products under most conditions; thus Meise111 has found only the 7 forms of Fe2On and FeOOH, and no trace of the ct and ~ forms on an iron surface corroded at 25°C and 100 per cent r.h. At higher temperatures, the a forms predominate. 12 However, Ramasubramanian and his co-workers 12 found a layer of 3'-Fe2On intermediate between layers of FenO4 and ct-FesOa even at 350°C, suggesting that the 7 form is formed from FenO4 (which has a similar structure) and then converted to a-FesOn. They further showed cross-sections of the oxide layers on the base irtm, and commented that the layer arrangement was different on different crystal faces. The theory of cracking of oxide layers due to electrostriction and film surface tension is well developed, as and it is known that anion adsorption decreases the thickness critical for breakdown. It might then be suggested that the points at which the SOs is adsorbed are points where there are inconsistencies in the oxide layer, whether due to grain boundaries of the base metal or simply cracking of the oxide layer. These would have to be very small (less than 0.5 ~zm)since they were not detected by electron microscopy. There appears to have been no data published on comparative SOs uptake by ct-FesOn, 3'-FesOa and FenO4; however, Ishikawa and Iouye x4 have found that a and 2, forms of FeOOH adsorb SOs to approximately the same degree, but ~-FeOOH takes up considerably more, and rather faster. All three forms have different structures, though they can be interconverted; ~-FeOOH requires presence of anions such as C1- to stabilize its structure, and is converted to ~-FesOn via 7-FenO3 on heating. 15 A possible reason for the formation of the discrete areas of adsorption observed might then involve (a) Fracture of the oxide layer at a submicroscopic point, allowing adsorption of the SO2 at a different type of oxide form from that on the outside surface, (b) Repair of the oxide layer during normal growth, obscuring the imperfection from microscopic detection. Acknowledgement--We wish to thank the New Zealand University Grants Committee for financial
assistance. REFERENCES 1. K. BARTON,D. KtrCm'NgA, Z. BARTONOVAand E. BERA~q~K,Corros. Sci. 11, 937 (1971). 2. L MA~USmMAand T. U~NO, Corros. Sci. 11, 129 (1971). 3. K. BARTONand Z. BARTONOVA,Werkstoffe Korros. 21, 85 (1970).
72
J . R . DUNCANand D. J. SPF~DING
4. V. V. SKORCHELLETFIaIld S. E. Ttr~c)~Nslcn, Z appl. Chem. USSR 28, 453 (1955). 5. J. L. R.OSENFELD, T. J. PAVLUTSKAJA, K. A. ZHIGALOVA and T. A. AKIMOVA, Trudy Inst. Fiz. I~im. Akad. Nauk. SSSR 7, 22 (1959); ref. Chem. Abstr. 55, 4192 (1961). 6. J. A. t'I~VALL, N. G. VAN)C~BERGand P. O. ]3LOMQVIST,Acta Chem. Scand. 22, 363 (1968). 7. K. W. GLASSand R. A. Koss, Can. J. Chem. 49, 2832 (1971). 8. H. KAESCHE, Werkstoffe Korros. 15, 399 (1964). 9. I~ C. LOGANIand W. W. SMELTZER,Can. Metal. Quart. 10, 149 (1971). 10. K. HAUF~, Oxidation of Metals. Plenum, New York (1965). I1. W. MEISEL,Z. Chem. 11, 238 (1971); ref. Chem. Abstr. 75, 66668 (1971). 12. N. R.AMASUBRAMANIAN,P. B. SE'vVELLand M. COHEN, J. electrochem. Soc. 115, 12 (1968). 13. N. SATO, Electrochim. Acta 16, 1683 (1971). 14. T. ISmY,AWA and K. Iou'tE, J. chem. Soc., Japan 91, 935 (1970); ref. Chem. Abstr. 74, 80246 (1971). 15. A. L. MACKAY,Mineralog. Mag. 32, 545 0960).
SHORT COMMUNICATION
DISCRETE AREAS OF SOs A D S O R P T I O N F R O M THE ATMOSPHERE ONTO IRON* J. R. DUNCAN and D. J. SPEDDING Chemistry Department
and E. E. WHEELER Pathology Department, University of Auckland, Auckland, New Zealand
RECENT reports x,2 have discussed the presence of discrete areas of 35SO~- adsorption from aqueous solution onto steel samples. One report 1 concluded that there was no clear primary reason for the uneven distribution found, while the other group s attributed it to specific adsorption at anodic areas of the surface in the electrolytic corrosion cells set up on their samples, which had been precorroded in the atmosphere for several months. We have studied the adsorption of B5SO2 from the atmosphere onto iron surfaces which were not visibly rusted, and have found a similar effect. Thin iron films (0.012 mm thick, 99-95%Fe) were fixed to atuminium studs used for electron microscopy (10 mm diameter). These were exposed for several hours in a flow system to an atmosphere containing a known concentration (0.1-0.2 ppm) of 35SO2 in air. Relative humidity was maintained at 70 or 95 per cent r.h. The humidity inside the exposure vessel was assessed from readings of wet and dry bulb thermometers, asses concentrations were determined by flowing part of the effluent gas stream at a known rate through a wash bottle containing 1%H2Os solution in water. This was counted in a liquid scintillation spectrometer at the end of the exposure period. The sample was then left in the apparatus with the humid air stream (containing no SOs) passing over it for a period ranging from a few minutes to 14 d, to provide a variety of post-exposure conditions. Control exposures were performed using an identical system, but omitting the BsSO~ from the air stream. The samples were kept in closed containers in the presence of silica gel until they could be examined under the scanning electron microscope (Cambridge Scanning Electron Microscope, Mark II, at 20 kV and 45 ° orientation). Figure 1 shows that no differences in the physical structure of the surfaces of the iron samples exposed to different atmospheric conditions could be detected by electron microscopy. No surface heterogeneities were found on these surfaces. The samples were then autoradiographed by exposure to X-ray film (Agfa Gevaert Curix FW Screen film), for 2 weeks. This showed major variations in the degree of uptake of SOs over the metal surface, with most of the asSO~ activity centred in specific areas up to 0.5 mm in diameter (Fig. 2b). Surface features large enough to cause these *Manuscript received 16 May 1972. 69
70
J . R . DUNCAN and D . J. SPEDDING
differences might have been expected to show under the electron microscope, which could differentiate to 0.5 ~tm, but none was found. Neither could they be correlated with surface phenomena visible under a light microscope (Fig. 2a). There was greater uptake on those samples exposed at 95 per cent r.h. than on those exposed at 70 per cent r.h., and there was increased uptake at the edges, where the metal film had been trimmed to size. The size of the spots found is similar to that found by Barton e t al. I and that found on some of the samples used by Matsushima and Ueno, 2 who suggested in a discussion of their results that adsorption occurs at anodic areas because of ion migration in solution. However, even at 95 per cent r.h., the electrolyte film on the surfaces we have studied will be so thin 8,4 that it may not act as an ordinary electrolyte, 5 hence an alternative mechanism is likely. Hedvall e t aL e have shown that a dipolar molecule such as HsS may be preferentially adsorbed at a copper rod which is at potential + 100 V compared with one at -- 100 V, with respect to earth (zero). Since the SOs molecule is also dipolar, with the sulphur atom as the positive pole, it might also be expected to show a similar behaviour. We have attempted to verify this, but have not found any measurable difference in 35SO2 adsorption onto iron electrodes at + 1.0, -- 1.0 and zero (earth) volts. Similar differential uptake to that described above was again found in this experiment, on all three electrodes. In an analogous experiment in solution containing asSO]-, using an unspecified potential, Matsushima and Ueno produced considerably more adsorption on their anode than on their cathode, s The iron films used in all our experiments were undoubtedly coated with an oxide film of unknown composition. I f the SO2 molecules were to be preferentially adsorbed directly from the gas phase onto anodic areas of the surface, by analogy with asSO]adsorption from solution s, the oxygen atoms of the SOs molecule would be required to form loose bonds to the oxide surface, since they are the negative end of the dipole and the surface anode will carry a nominal positive charge. It is intuitively more favourable that the sulphur atom should be presented to the surface, where it could form more stable (i.e. stronger) bonds with the oxygen atoms of the oxide layer, and hence be easily oxidized to sulphate, which has been shown to be present on surfaces onto which SOs has been adsorbed. 7 Such an attack is more feasible at cathodic areas, where the negative charge can attract the positive end of the dipole, the sulphur atom. However, our results indicate that a potential difference of 2 V is not sufficient to cause differential adsorption. The likely reactions at the surface (excluding those involving sulphur dioxide) are: 1,8
Anode
(a) Fe
--+
E = + 0.47 (b) Fe 2+
~
Fe 2+ + 2e0.059 --log 2
(1) (Fe 2+) V (nhe)
Fe a+ + e -
E = -- 0"77 - - 0"058 log Fe3+ V (nhe)
(2)
Fie;. I. Electron micrographs of iron surfaces. (a) Exposed to 0-01 p p m SO~ in air for 280 rain followed by 60 h at 95 per cent r.h. (b) Exposed to 0.11 ppna SO,_>in air for 100 rain during 22 h exposure at 95 per cent r.h. (c) Exposed to clean air at 95 per cent r.h. for 150 h,
FIG. 2. (a) Micrograph o f same area of metal surface as in autoradiograph in (b). (b) Autoradiograph of iron surface after exposure to 0.11 ppm ~SOa in air for 100 rain during 22 h exposure at 95 per cent r.h. The metal sample used here is that shown in more detail in Fig. I b.
sch adsorption from atmosphere onto iron Cathode (a) Os + 2HsO q- 4e-
~
40H-
71 (3)
E = + 0.40 -- --0"058log (OH-)4 V (nhe) 4 Po2 (b) Reverse of (2). The total potential of the corrosion cell is therefore unlikely to exceed the 2 V used in our experiment, and hence some different explanation of the results is again necessary. A third hypothesis is that there is differential adsorption onto different types of oxide surface. Much has been written on the types of oxide formed on iron in various controlled atmospheres. 9,x° It appears that different modifications of Fe2Oz are the major final products under most conditions; thus Meise111 has found only the 7 forms of Fe2On and FeOOH, and no trace of the ct and ~ forms on an iron surface corroded at 25°C and 100 per cent r.h. At higher temperatures, the a forms predominate. 12 However, Ramasubramanian and his co-workers 12 found a layer of 3'-Fe2On intermediate between layers of FenO4 and ct-FesOa even at 350°C, suggesting that the 7 form is formed from FenO4 (which has a similar structure) and then converted to a-FesOn. They further showed cross-sections of the oxide layers on the base irtm, and commented that the layer arrangement was different on different crystal faces. The theory of cracking of oxide layers due to electrostriction and film surface tension is well developed, as and it is known that anion adsorption decreases the thickness critical for breakdown. It might then be suggested that the points at which the SOs is adsorbed are points where there are inconsistencies in the oxide layer, whether due to grain boundaries of the base metal or simply cracking of the oxide layer. These would have to be very small (less than 0.5 ~zm)since they were not detected by electron microscopy. There appears to have been no data published on comparative SOs uptake by ct-FesOn, 3'-FesOa and FenO4; however, Ishikawa and Iouye x4 have found that a and 2, forms of FeOOH adsorb SOs to approximately the same degree, but ~-FeOOH takes up considerably more, and rather faster. All three forms have different structures, though they can be interconverted; ~-FeOOH requires presence of anions such as C1- to stabilize its structure, and is converted to ~-FesOn via 7-FenO3 on heating. 15 A possible reason for the formation of the discrete areas of adsorption observed might then involve (a) Fracture of the oxide layer at a submicroscopic point, allowing adsorption of the SO2 at a different type of oxide form from that on the outside surface, (b) Repair of the oxide layer during normal growth, obscuring the imperfection from microscopic detection. Acknowledgement--We wish to thank the New Zealand University Grants Committee for financial
assistance. REFERENCES 1. K. BARTON,D. KtrCm'NgA, Z. BARTONOVAand E. BERA~q~K,Corros. Sci. 11, 937 (1971). 2. L MA~USmMAand T. U~NO, Corros. Sci. 11, 129 (1971). 3. K. BARTONand Z. BARTONOVA,Werkstoffe Korros. 21, 85 (1970).
72
J . R . DUNCANand D. J. SPF~DING
4. V. V. SKORCHELLETFIaIld S. E. Ttr~c)~Nslcn, Z appl. Chem. USSR 28, 453 (1955). 5. J. L. R.OSENFELD, T. J. PAVLUTSKAJA, K. A. ZHIGALOVA and T. A. AKIMOVA, Trudy Inst. Fiz. I~im. Akad. Nauk. SSSR 7, 22 (1959); ref. Chem. Abstr. 55, 4192 (1961). 6. J. A. t'I~VALL, N. G. VAN)C~BERGand P. O. ]3LOMQVIST,Acta Chem. Scand. 22, 363 (1968). 7. K. W. GLASSand R. A. Koss, Can. J. Chem. 49, 2832 (1971). 8. H. KAESCHE, Werkstoffe Korros. 15, 399 (1964). 9. I~ C. LOGANIand W. W. SMELTZER,Can. Metal. Quart. 10, 149 (1971). 10. K. HAUF~, Oxidation of Metals. Plenum, New York (1965). I1. W. MEISEL,Z. Chem. 11, 238 (1971); ref. Chem. Abstr. 75, 66668 (1971). 12. N. R.AMASUBRAMANIAN,P. B. SE'vVELLand M. COHEN, J. electrochem. Soc. 115, 12 (1968). 13. N. SATO, Electrochim. Acta 16, 1683 (1971). 14. T. ISmY,AWA and K. Iou'tE, J. chem. Soc., Japan 91, 935 (1970); ref. Chem. Abstr. 74, 80246 (1971). 15. A. L. MACKAY,Mineralog. Mag. 32, 545 0960).