A novel system for atmospheric corrosion experiments

Corrosion Science, Vol. 23, No. 1, pp. !-8, 1983 Printed in Great Britain.

0010-938X/831010001-.08 $03.00100 Pergamon P r o s Ltd.

A NOVEL SYSTEM FOR ATMOSPHERIC CORROSION EXPERIMENTS* J. P. FRANEY Bell Laboratories, Murray Hill, NJ 07974, U.S.A. Abstract--Atmospheric corrosion testing in the laboratory requires the capability of achieving stable and realistic exposure conditions, exposure of multiple samples, wide ranges of concentrations of corrosive gases, and exposure times varying from seconds to weeks. This paper describes a system that meets these requirements. It utilizes polymer permeation as a source of corrosive gases, a hookand-loop fastening system for sample manipulation, a muitiport chamber for sample exposure, and a dedicated desktop computer system for sample monitoring and data reduction. System characteristics include the generation of corrosive trace gases from a low level of 8 + 1 ppb to a high level of 5 =i= 0.1 ppm, exposure times of less than 1 rain to 5 weeks, and simulta~eoos exposure of 60-90 samples. INTRODUCTION IN ~ECENT years the interest in interactions between gases and surfaces has increased rapidly. The interest is not only fundamental, but has much practical value. This is due in part to the volume of sensitive microcircuits being manufactured, and to the increased cost o f materials on convehtional piece parts. The corrosion testing of materials is used to derive corrosion rates and perhaps to provide enough data to ascertain a field lifetime for the products. Laboratory systems for atmospheric corrosion experiments have undergone significant changes in the past decade. Many early systems were designed to detect porosity or gas tightness of fittings and were in no way representative of actual field conditions. The tests generally involved SO~ and used concentrations of (2.5-10) x 104 parts per million (ppm)) "4 (Typical atmospheric concentrations of SOs are 1-100 parts per billion (ppb) or lower, s.s) Modern production testing of thin films and platings often uses similar approaches, with exposure to 0.5-2 ppm Cls 7.s or 0 - 2 0 ) × 10~ ppm HCP being found useful. Since atmospheric concentrations of gaseous chlorine compound do not exceed a few ppb, 5 the tests are diagnostic rather than realistic. More recent tests often involve mixed corrosive gases, and often achieve concentrations more representative o f those actually encountered by materials and equipment. Among the corrosive gases involved are C12l°.n, HCI, n H~S~°-~4, NO~,~°.n,~4,~" O~n. ~5 and SO2. ~°.n,~s'~s These gases are generally present in the atmosphere in concentrations of a few ppb, although H~S, Nee, Os, and SO2 concentrations between 0.1 and 1.0 ppm are occasionally encountered. Exposures of samples to these gases have been useful in evaluating environmental ruggedness, but the systems are complex and the corrosion mechanisms are not well understood. Corrosive gases for laboratory exposures have been generated in a variety of ways. The original technique ~ consisted of mixing a desired volume of corrosive gas into *Manuscript received 15 March 1982. 1

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a volume of dilution gas, generally prepurified air. Gas mixing can be routinely used to achieve concentrations as low as a few ppm~.16; some workers have made mixtures by this method containing a few tenths of a ppm.19,s° A novel gas generation method is used in the "British Post Office test"?, sl The test environment is created by the combustion of coal gas to which carbon disulfide has been added; the result is an atmosphere containing 25 ± 5 ppm SOs and 3000 -1- 500 ppm COs. The technique has not been widely accepted for research use, partly because of the high SOs concentrations that are generated and partly because of continuous large variations in the SO2 and CO~ concentrations. Ozone is too reactive to be stable in any container; it is generated by ultraviolet irradiation of oxygen, followed promptly by mixing and dilution as desired, ix To achieve low concentrations of corrosive gases having relatively high vapor pressures, generation by gas permeation through polymers is used. Permeation tubes for a number of gases are commercially available; they consist of tubing with permeable walls into which the temporarily liquified gas is placed. The tube is then capped by impervious end plugs. Permeation tubes are generally suitable for Cls,S.~°,11 HsS,~°-~4 and SOsxo,n;ls.x4,xv, and are sometimes used for NO2.a°,14 The rate of permeation must be individually determined for each tube, permeation rates cannot be varied widely, and the supply of gas in the tube is limited. In addition, the strength of the materials from which the tubes are made limits them to gases whose vapor pressures are less than about two atmospheres. An alternative is to use a permeable material inserted into a flow line that is fed from a pressurized and regulated tank,iS, 2s an option which minimizes the undesirable characteristics of the tubes while retaining the attractive features of a permeation source. Asbestos plugs have proven satisfactory for this purpose; polymer membranessa are less susceptible to water vapor absorption problems and their permeant flows are more responsive to modification by pressure and temperature variation. It is often possible to calculate the corrosive gas concentration expected in exposure chambers. The loss of corrosive gases to chamber walls and sample surfaces is an ever-present possibility, however, and such loss will vary with the gas species, the type of samples, the experiment temperature, the relative humidity, etc. Such losses have been noted for C12,s HsS,x3,~4 and SO2.~8,s4 Convenient and continuous measurement of the concentrations of corrosive gases and of other experimental parameters has thus been shown to be desirable. Other concerns in corrosion experiment system design include the stability of the system, especially during sample removal,~s and the homogeneity of the gas concentrations within the test chamber? s SYSTEM DESIGN The new exposure system developed for use in corrosion testing is shown schematically in Fig. 1. The corrosive gas supply is a pressurized cylinder which supplies gas through a corrosion-resistant adjustable regulator which feeds 0.25 in. diameter Teflon tubing. The tubing is placed in a I 1. Erlenmeyer flask and capped. Permeation through the tubing walls serves as the source of the corrosive gas. The amount of corrosive gas flow into the mixing chamber is controfied by the regulator pressure and the length of the tubing. A carrier gas, generally prepurified air, is injected into

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are shown in Fig. 2. The chamber, constructed of Pyrex, has closely spaced but randomly placed holes. It is 5 cm in diameter and 32 cm long; the holes are 2 cm in diameter. The hook-and-loop fastening consists of mated 1.5 cm diameter discs with adhesive backing. (The system uses Minnesota Mining and Manufacturing Co. "Scotchmate" brand fasteners, though others are commercially available and would probably be equally suitable.) Prior to inserting a sample into the chamber a tapered stopper with a hook fastener on the smaller end is pressed against a loop fastener attached to the back of the sample, The stopper is then inserted into one of the ports of the chamber. All of the unused ports are closed with blank stoppers. The commercially-available stoppers are made of low sulfur content natural rubber with inorganic f-fliers. Mass spectroscopic experiments indicate that outgassing from the stopper material is negligible to temperatures at least as high as 80°C. Since the samples are held securely to the stoppers by the hook-and-loop fasteners, they may be positioned at a variety of locations within the flowing gas stream, thus allowing positional inhomogeneities to be investigated. At gas flows of 0.5--7 lpm in the chamber, no such effects have been noted. DISCUSSION

A wide range of gas concentrations can be achieved: the current range of the system is from l0 i 1 ppb to 5 d- 0.1 ppm. To achieve this dynamic range, the source gas pressure is varied from 5 to 30 psi and the length of the Teflon tubing from 0.33 to 1.0 m. Typical carrier gas flow is 5 lpm. Since the pressurized gas is regulated prior to being fed to the permeable tubing, gases with high vapor pressures (i.e. CO2) can be introduced into the system, an option not possible with standard permeation tube sucees.

In Table I, the minimum concentration results achieved with this system are compared with those reported by other experimenters. For H~S and SO~, the concentrations are the lowest thus far reported. For OCS, thus far not studied by other workers, the 30 ppb minimum is among the lowest trace gas concentrations listed. Together with the wide dynamic range, the system permits the insertion and removal of samples from the exposure chamber without disequilibration of the existing stable atmosphere. This rapid sample handling affords very accurate short exposures (as short as a few seconds). The hook-and-loop fastening system is made of non-corrodible polymeric materials which introduce no trace contaminants into the system. Sample handling is facilitated both inside and outside the multiport chamber (Fig. 3). Two examples of the stability of trace gas concentrations within the chamber are shown in Figs 4 and 5. In Fig. 4, the H2S concentration during a period of frequent sample removal is plotted; the concentration fluctuations were not distinguishable in the data, placing an upper limit of ± 1 % on the variability. Figure 5 shows the H~S concentration over a period of more than two weeks during which no adjustments to improve stability were made. The concentration variation around the mean value for the period was within the range ± 4 %; the variation appears to be a consequence of changes in ambient air temperature within the laboratory. Relative stability of this magnitude is suitable for most laboratory corrosion experiments; it can be improved if desired by minor adjustments to gas flows during multiday exposures.

FI~. 2. The multiport exposure chamber and the hook-and-loop fastening system for sample exposure.

FIG. 3. Use of the hook-and-loop fastening system for sample manipulation outside the exposure chamber. The round sample on the left is unexposed copper. The square sample at the top is unexposed 725 alloy (89Cu-9Ni-2Sn); that at the bottom is 725 alloy exposed to 2.5 ppm H~S for 24 h to produce a sulfide film of average thickness 6.1 nm. The round sample at right center is copper exposed to 34 ppb OCS for 7 h to produce a sulfide film varying in thickness from 0.2 n m at the edge to 2.0 at the center. The round sample at bottom right is copper exposed to 33 ppb OCS for 24 h to produce a sulfide film of average thickness 1.7 nm.

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An improved experimental system for atmospheric corrosion testing of materials has been developed. The system permits short and accurate time exposures, ease in sample handling and storage, and a very wide and stable dynamic range. Any corrosive or noncorrosive gas can be used in the system. Since construction is of glass, natural rubber, Teflon, and nylon, the materials are relatively inert and non-contaminating. The system has been used to investigate the corrosion of copper by H=S~b and OCS =~ and the sulfidation of conductive silver paste. 27 Experiments on the corrosion of copper and silver alloys by these gases and by SO= are under way; the results will be reported separately. One week

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Acknowledgements--I thank R. M. Lure and R. P. Jones for performing the mass spectrometric studies of stopper outgassing. REFERENCES 1. W. H. J. VERNON, Trans. Faraday Soc. 27, 255 (1931). 2. M. CLARKEand J. M. LEEDS, Trans. Inst. Metal Finish. 46, 81 (1968). 3. M. CI~RICEand A. J. SANSUM,Trans. Inst. Metal Finish. 50, 211 (1972). 4. P. F. PROTON, Trans. Inst. Metal Finish. 50, 125 (1972). 5. T. E. GR~DEL and N. SCHWARTZ,Mater. Performance 16, 17 0977). 6. T. E. GR~DEL, Handbook of Environmental Chemistry (ed. by O. HtYrZINGEa),Vol. 2A. Springer, Heidelberg (1980). 7. A. T. ENGLISHand P. A. TURN~R,J. electronic Mater. 1, 1 (1972). 8. N. L. SeARand L. G. FEI~S~IN, IEEE Trans. Parts, Hybrids, and Packaging, PHP-13, 208 (1977). 9. J. D. SPElatrr and M. J. BIt.L, Th/n Solid Films 15, 325 (1973). 10. W. H. Aeeo'rr, IEEE Trans. Parts, Hybrids, and Packaging, PHP-10, 24 (1974). I 1. D. W. RICE, P. PEARSON, E. B. RxGeY, P. H. P. PmvPS, R. J. CAPeELLand R. TREMOURaUX, J. electrochem. Soc. 128, 275 (1981). 12. S. P. SHARMA,J. electrochem. Soc. 127, 21 (1980). 13. J. P. B ~ o ~ , C. ARCHAMeAULT,J. GtaNEMEWr,J. Y. L~ TRAONand P. RIo. IEEE Trans. Components, Hybrids, and Manuf. Tech. CH .2, 343 0979). 14. M. ISHINO, M. IOSmMOTO,K. MA~UZ and S. MITANI,Electrical Contacts-1979, IIT Research Institute, Chicago, IL, pp. 23-28 (1979). 15. J. W. SPENCEand F. H. HAttIE, Corrosion in Natural Environments, ASTM Pub. 558, Amer. Soc. for Testing and Materials, Philadelphia, PA, pp. 279-291 (1974). 16. K. E. J o n s o n and A. F. BROMt£Y,Anti-Corrosion xd, 17 (I 973). 17. L. G. FeINSTEINand N. L. SB~, IEEE Trans. Components, Hybrids, and Manuf. Tech. CHMT-2, 159 (I 979). 18. D. L. Ft.AMM,D. D. B^co~, E. KI~'SeRONand A. T. E~GLIS,, J. electrochem. Soc. 128, 679 (1981). 19. R. EpJcsso~ and T. SYDeERGE~ Werkstoffe Korros. 28, 755 (1979). 20. F. B. MANSI~.D, Electrochemical Studies of Atmospheric Corrosion, Contract Report N0001475--C~788, Rockwell International Science Center, Thousand Oaks, CA (1979). 21. J. M. LEEDSand T. E. SUCH, Trans. Inst. Metal Finishing 49, 131 (1971). 22. B. E. SALTZMANand A. F. WAalmtnto, Analyt. Chem. 37, 1261 (1965). 23. J. P. FRA~Y, J. electrochem. Soc. 126, 2159 (1979). 24. T. E. GRAD~Land J. P. F ~ Y , unpublished results (1977), 25. J. P. FRA~EY,T. E. GRA~DELand G. W. KAMMLOTr,Proc. Int. Conf. Atmos. Corrosion (ed. by W. H. All.OR), Electrochem. Soc. Pennington, NJ (1981). 26. T. E. GRAr~3~L,G. W. KAMMLOYrand J. P. FaA~Y, Science, N. Y. 212, 663 (1981). 27. J. P. FRANEY,T. E. GRAEDEL,G. J. GU^L~ERI, G. W. KA~LOIPr, J. KeL9ER, D. L. MAt~, L. H. SHARPEand V. "I~R~Y, J. mater. Sci., 16, 2360 (1981).