Dale E. Newbury Center for Analytical Chemistry National Bureau of Standards Washington. D.C. 20234
Edited by Jeanette G. Grasselli
What Is Causing Failuresof AluminumWire Connections in Residential Circuits? Have you ever seen an electrical connection that resembles Figure l? Many people have because overheating a t the screw connection in the junction assemblies used for duplex electrical outlets leads to “glow” failures. The ‘‘glow’’ is a transient condition that occurs during the development of a n arc or arc discharge, and “glow” failures of electrical connections in residential branch circuits constructed with aluminum wire have been recognized for several years (13).“Glowing” connections will not perceptibly affect the electrical performance function of lights, appliance,or other electrical loads, and will not blow fuses, trip circuit breakers, or operate ground fault circuit interrupters. But the “glow” failures, of course, are safety hazards, since combustible material in contact with glowing connections can ignite and cause fires. Therefore, laboratory investigations were undertaken a t the National Bureau of Standards to determine quantifiable conditions for “had” electrical connections and to identify physicalchemical mechanisms and environments that cause them to fail. Figure 1 shows a junction assembly tested to glow failure under simulated conditions in the laboratory. The failure produced sufficient heat to melt the plastic housing of the junction assemhly and the insulation on the aluminum wire 5 cm from the screw. Also, a copper color on the surface revealed that the brass components near the screw had been heated sufficiently to result in dezincification. The general characteristics of glow failures are: 1)They can be initiated after wire creep and the lack of “springiness” of the assembly causes the aluminum wire-screw connection to become mechanically loose. These loose connections can also result from expansion and contraction during This article iwt oubjen Io U.S. wy~i@t Publishad 1982 American Chemical Society
thermal cycling due to changes in temperature, particularly in outside walls; 2) the failure is always accompanied by the evolution of heat and often by a visible glow; and 3) the heat evolved during the glow failure can raise the temperature of the junction assembly significantly. In the laboratory tests ( 4 ) , temperatures as high as 360 “C were measured at the tab shown in Figure 1. The function resistance of a connection during glow failure is on the order of 0.1 0. Slrategy While the engineering characteristics of the glow failure in aluminum wire connections have been well defined, the underlying physical-chemical mechanism(s) that causes the failure has not been understood. Toelucidate this mechanism, scanning electron microscopy and X-ray microanalysis were used to study the current-
carrying interfaces in junctions tested to failure in the laboratory. Earlier studies in our laboratory of similar junctions were unsuccessful because of the complex, three-dimensional nature of the connection and the difficulty in preparing samples of the appropriate region. To provide a new starting point for the study, a simple apparatus was designed to produce an electrical contact between the aluminum wire and steel screw at a single controlled location. Moreover, the pressure on the connection could be adjusted to simulate a loose connection. With this simple apparatus, failures could he produced that exhibited the general characteristics of the glow failures observed in actual residential junction assemblies. The failures were induced by subjecting the circuit to a series of “on-off‘ current cycles. We could first study these simple specimens, then attack the much more
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1 cm Figure 1. Duplex electrical outlet assembly after undergoing glow failure in a laboratory test ANALYTICAL CHEMISTRY, VOL. 54. NO. 9. AVGUST 1982
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Figure 2. (a) SEM image of region on aluminum wire d a m aged during glow failure. (b) Damaged region on screw head, which corresponds to the damaged region on the aluminum wire. (c)Detail of damaged region on screw, which shows evidence of solidification structures
complicated specimens represented by Figure 1.
Analytical Methods T o reach an understanding of the mechanism for the failure of the aluminum wire connections, it is necessary to combine chemical analysis with microstructural information. The electron microprobe, in which scanning electron microscopy and optical microscopy (for high-resolution morphological information) are combined with X-ray spectrometry (for elemental analysis), is ideal for such a study (5). Both energy-dispersive X-ray spectrometry (EDS) and wavelengthdispersive X-ray spectrometry (WDS) were used in the study-EDS for quantitative analysis and WDS for X-ray area scans. The use of EDS is particularly attractive since it provides continuous monitoring of the X-ray spectrum for all elements with atomic number equal to or greater than 11 (sodium). Quantitative compositional values have been derived by comparing the EDS spectra to pure element spectra with the NBS theoretical matrix correction procedures, FRAME C ( 6 )for flat samples and FRAME P (7) for rough surfaces. The characteristic X-ray lines of aluminum, iron, copper, and zinc are sufficiently well separated in energy so that peak overlaps are either insignificant or are accurately calculated by the FRAME C procedure. By using EDS spectrometry and the FRAME C procedure, a full four-element quantitative analysis could be calculated during the accumulation of the next 1060A
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spectrum (spectral accumulation time, 200 s). Experience in the analysis of known multielement samples with the FRAME C procedure suggests that the error distribution is characterized by a relative standard deviation of 3.5% ( 8 ) .When all constituents are analyzed, the FRAME C results generally total between 98.5% and 101.5%. Totals below 98%suggest the presence of an undetected element, e.g., oxygen. The FRAME P procedure utilizes measurements of the X-ray peak-tobackground ratio to eliminate geometrical effects on X-ray intensities. This matrix correction procedure is still in the process of development and produces a wider error distribution. In those cases where FRAME P has been tested on topographically rough, compositionally homogeneous standards, the relative errors have been generally less than 420% in situations where conventional analysis without corrections for sample topography produced relative errors of f10% or more (7). X-ray intensity area scans provide a powerful means of obtaining a qualitative compositional map that shows the
ANALYTICAL CHEMISTRY. VOL. 54. NO. 9. AUGUST 1982
distribution of the constituents of interest ( 5 ) .To obtain statistically valid information in the X-ray area scan, a large number of X-ray pulses of a particular energy must be processed. Such area scans are best prepared with a WDS for two main reasons: 1)The peak-to-background ratio measured with the WDS spectrometer is higher by a factor of 10 than that for the EDS spectrometer; 2) the WDS can process photons at count rates of 50 OOO cps or more at a single X-ray energy as compared to the EDS which is limited to about 5000 cps (at optimum resolution) integrated across the full energy range excited by the beam. In the present study, electron and light imaging, quantitative EDS analysis, and qualitative WDS area scanning were used to characterize the aluminum wire connections. Each mode contributed valuable information, and no one mode alone was adequate for a complete understanding of the problem. Results Model Aluminum Wire-Steel Screw Connections. "be samples ob-
tained from glow failures produced in the simple apparatus were examined by stereo optical microscopy and scanning electron microscopy. The structures observed on the wire and screw are shown in Figures 2a and 2b. respectively. The structures can be described as craters from which material has been removed with evidence of surface melting, as shown by the solidified droplets and spatter structures marked in Figure 2c. EDS X-ray microanalysis revealed that some of the material in the droplets and spatters outside the crater on the steel screw was nearly pure iron, which indicates that temperatures in excess of 1550 O C , the melting point of iron, were reached in the crater during the glow failure. Furthermore, quantitative X-ray microanalysis with FRAME P of the various numbered locations in Figures 2s and 2b revealed that significant material transport had occurred between the wire and screw components during the glow failure. Thus, aluminum was transferred to the damaged crater on the iron screw and vice versa. This combination of high temperature and intimate mixing of the metal. lic elements in the craters produced during the glow failure should provide an ideal environment for the formation of intermetallic compounds, which are strongly favored in the aluminum-iron binary system ( 9 ) .To check this hypothesis and to determine how far beneath the surface the intermetallic compound(s) might form, a metallographic section was prepared through the crater perpendicular to the surface of the wire. This
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Figure 3. Optical micrograph of metallographic section taken through duplex electrical outlet assembly that had undergone glow failure. Note presence of reaction zones (circled) between alumi. num wire and screw and between aiuminum wire and brass plate
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cross section revealed a subsurface wne in compositional contrast in a hackscattered electron SEM image. Quantitative EDS microanalysis of thii region indicated the composition of the region corresponded closely to F e d l , with a minor amount of copper. Residential Connections. Armed with these observations from the model experiment, we were prepared to attack the actual residential wiring junctions that had undergone glow failures. Our metallographer prepared a metallographic section through the *ew-brass plate assembly by successively grinding and polishing the sample, which was embedded in epoxy, and examining the interface until the reaction zone was located. An example of this work is shown in Figure 3, where extensive reaction wnes have been exposed simultaneously at the wire-screw interface and the w i r e plate interface. An eledron image of the reaction zone at the wirwcrew interface is shown in Figure 4a. X-ray area scnns for iron and aluminum, Figures 4b and 4c,reapctively, reveal the in-
trusion of aluminum into the screw. Compositional contrast in the SEM image reveals two distinct regions in this reaction zone. A series of quantitative EDS analyses at the locations marked in Figure 4a confirms the presence of these regions of slightly different composition (Table I), which correspond approximately to the intermetallic communds FeAl. - " and Fetus. The wir-date reaction zone is shown in an klectron image in Figure 5 4 again revealing a region of the braas plate in which the composition has been modified by the reaction. X-ray area w s foy a l w i n u m (Figure 5b), zinc (Figure 5 4 , and copper (Figure 5d) suggest a replacement of zinc in the brass by aluminum with relatively little modification of the copper concentration. Thii ohservation is confmed by the quantitative EDS analy8es (Table 11) at the locations indicated in Figure 5a, which show that the reaction region has copper and aluminum as major constituents with a minor amount of zinc remaining.
Figure 4. (a)scanning electron microwmh of reaction zone atiiuminum wire-steel screw interface; backscatter4 eieo tron signal showing compositional contrast. (b)X-ray area scan for aluminum. (c)X-ray area scan for iron
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The previous observations were made on connections that had exhibited the macroscopic characteristics of the glow failure (emission of visible light, evolution of heat, and development of resistance across the junction). As a further step, the incipient stage of the glow failure was studied by interrupting a current cycle teat after only a few cycles and before any macroscopic glow failure characteristics could he observed. Again, reaction mnes showing the formation of intermetallic compounds were observed at the w i r w r e w interface and at the wire-plate interface. What is remarkable a b u t these structures is their extraordinarily small size. The careful metallographic preparation of appropriate samples was absolutely critical to the detection of these incipient failure structures.
lnterpretatlon The discovery of intermetallic compound formation at the current-carrying interfaces in aluminum wire connections is vital in developing an understanding of the underlying catma
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Figure 5. (a) Scanning electron micrograph of reaction zone at aluminum wire-brass plate interface. Backscattered electron signal reveals compositional contrast. (b) X-ray area scan for aluminum. (c)X-ray area scan for zinc. (d) X-ray area scan for copper of the failure. Intermetallic compounds have an electronic structure such that the outermost electrons of the metallic atoms are localized in bonds, thus reducing the availability of free conduction band electrons and increasing the resistivity. For example, the intermetallic compound FesAl has a resistivity of approximately 100 &an, a faetor of 40 higher than pure aluminum and a factor of 10 higher than pure iron. The resistivity of the intermetallic compounds is sufficient to provide resistance for Joule heating at the junction, but the resistance is not 80 high 89 to completely attenuate the flow of current. From these observations the sequence of events of a glow failure thus seems to be: 1) The connection becomes mechanically loose, leading M arcing between the various components, wire-screw and wire-plate; 2) the arcing leads to high temperatures locally, which causes evaporation and mutual transfer of the constituents
between the circuit components: 3) the wmbmation of high temperature and intimate mixing of the elements provides an ideal environment for the formation of intermetallic compounds; 4) the resistivity of the intermetallic compounds provides sufficient resistance to produce Joule heating; and 5 ) the process operates with positive feedback, that is, additional formation of intermetallic compounds leads to more Joule heating, which favors addi-
tional formation of the intermetallic compounds. Eventually, temperatures may reach a sufficiently high level so that the reaction proceeds in the liquid phase over an extensive region, as evidenced by the dendrite structure formed during solidification, shown in Figure 5a The observations of small regions of intermetallic compound formation in the incipient stages of failure are more difficult to explain. Perhaps these
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ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
areas are sites of small localized arcs, or perhaps these regions are small asperities a t which contact between the components first occurs. Because of the relatively small cross section of these asperities, the current density passing through them is extremely high. The high current density may stimulate intermetallic compound formation through local small-scale heating or material transport by electromigration. These observations of intermetallic compounds formation at the currentcarrying fractions make the choice of substitute materials difficult. Clearly, the aluminum wire should not be in contact with metals with which it can form intermetallic compounds. Unfortunately, examination of the phase diagrams of aluminum reveals that many binary and ternary systems contain extensive sequences of intermetallic compounds. More work is needed on the details of the interactions of components a t current-carrying interfaces.
Acknowledgment I would like to express special thanks to Charles Brady, whose metallographic sections made this work possible, Sidney Greenwald for the design of the simple laboratory aluminum wire connection, and Joy Shoemaker for preparation of the manuscript. References (1) Meese, W. J.; Beausoliel, R. W. “Ex-
ploratory Study of Glowing Electrical Connections”; Nat. Bur. Stand. (U.S.) Build. Sci. Ser. 103, October 1977. (2) Rabinow, J. “Some Thoughts on Electrical Connections”; Nat. Bur. Stand. U.S. NBSIR 78-1507, August 1978. (3) Rabinow, J. “Special Report on Aluminum Wire”; Report for U.S. Consumer Product Safety Commission, September 1974. (4). Newbury, D.; Greenwald, S. “Observations on the Mechanisms of High Resis-
tance Junction Formation in Aluminum Wire Connections,” J. Res. Nat. Bur. Stand. U.S. 1980,85,429-40. (5) Goldstein, J. I.; Yakowitz, H.; Newbury, D. E.; Lifshin, E.; Colby, J. W.; Coleman, J. R. “Practical Scanning Electron Microscopy”; Plenum: New York, 1975. (6) Myklebust, R. L.; Fiori, C. E.; Heinrich, K. F. J. “FRAME C: A Compact Procedure for Quantitative Energy-Dispersive Electron Probe X-ray Analysis”; Nat. Bur. Stand. U.S. Tech. Note 1106, September 1979. (7) Small, J. A.; Newbury, D. E.; Myklebust, R. L. “Analysis of Particles and Rough Samples by FRAME P, a ZAF Method Incorporating Peak-to-Background Measurements.” In “Microbeam Analysis-1979”; Newbury, D. E., Ed.; San Francisco Press, 1979; pp 243-46. (8) Myklebust, R. L.; Newbury,.D. E. “The Use and Abuse of a Quantitative Analysis Procedure for Energy-Dispersive X-ray Microanalysis.”In “Microbeam Analysis-I 979”; Newbury, D. E., Ed.; San Francisco Press, 1979; pp 231-37. (9) Hansen, M. “Constitution of Binary Alloys”; McGraw-Hill: New York, 195% pp 84-95.
