The Analytical Approach
Dale E. Newbury Center for Analytical Chemistry National Bureau of Standards Washington, D.C. 20234
Edited by Jeanette G. Grasselli
What Is Causing Failures of AluminumWire Connections in Residential Circuits? Have you ever seen an electrical connection that resembles Figure 1? Many people have because over heating at the screw connection in the junction assemblies used for duplex electrical outlets leads to "glow" fail ures. The "glow" is a transient condi tion that occurs during the develop ment of an arc or arc discharge, and "glow" failures of electrical connec tions in residential branch circuits constructed with aluminum wire have been recognized for several years ( / 3). "Glowing" connections will not perceptibly affect the electrical per formance function of lights, appli ances, or other electrical loads, and will not blow fuses, trip circuit breakers, or operate ground fault circuit interrupt ers. But the "glow" failures, of course, are safety hazards, since combustible material in contact with glowing con nections can ignite and cause fires. Therefore, laboratory investigations were undertaken at the National Bu reau of Standards to determine quan tifiable conditions for "bad" electrical connections and to identify physicalchemical mechanisms and environ ments that cause them to fail. Figure 1 shows a junction assembly tested to glow failure under simulated conditions in the laboratory. The fail ure produced sufficient heat to melt the plastic housing of the junction as sembly and the insulation on the alu minum 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 not subject to U.S. Copyright Published 1982 American Chemical Society
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 na ture of the connection and the diffi culty in preparing samples of the ap propriate region. To provide a new starting point for the study, a simple apparatus was designed to produce an electrical contact between the alumi num wire and steel screw at a single controlled location. Moreover, the pressure on the connection could be adjusted to simulate a loose connec tion. With this simple apparatus, fail ures could be 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 speci mens, then attack the much more
thermal cycling due to changes in tem perature, 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 Ω. Strategy
While the engineering characteris tics of the glow failure in aluminum wire connections have been well de fined, the underlying physical-chemi cal mechanism(s) that causes the fail ure has not been understood. To eluci date this mechanism, scanning elec tron microscopy and X-ray microanal ysis were used to study the current-
Fe Screw
Tab AI Wire
1 cm
Figure 1. Duplex electrical outlet assembly after undergoing glow failure in a labo ratory test ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982 · 1059 A
Fe
5
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50 μΓΠ
50 μνη (b)
Figure 2. (a) SEM image of region on aluminum wire dam 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 To reach an understanding of the mechanism for the failure of the alu minum wire connections, it is neces sary to combine chemical analysis with microstructural information. The electron microprobe, in which scan ning electron microscopy and optical microscopy (for high-resolution mor phological information) are combined with X-ray spectrometry (for elemen tal 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 pro vides continuous monitoring of the X-ray spectrum for all elements with atomic number equal to or greater than 11 (sodium). Quantitative com positional values have been derived by comparing the EDS spectra to pure el ement spectra with the NBS theoreti cal matrix correction procedures, FRAME C (6) for flat samples and FRAME Ρ (7) for rough surfaces. The characteristic X-ray lines of alumi num, iron, copper, and zinc are suffi ciently well separated in energy so that peak overlaps are either insignifi cant or are accurately calculated by the FRAME C procedure. By using EDS spectrometry and the FRAME C procedure, a full four-element quanti tative analysis could be calculated during the accumulation of the next
5 0 μ,ΓΤΐ
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 gen erally total between 98.5% and 101.5%. Totals below 98% suggest the presence of an undetected element, e.g., oxygen. The FRAME Ρ procedure utilizes measurements of the X-ray peak-tobackground ratio to eliminate geomet rical effects on X-ray intensities. This matrix correction procedure is still in the process of development and pro duces a wider error distribution. In those cases where FRAME Ρ has been tested on topographically rough, com· positionally homogeneous standards, the relative errors have been generally less than ±20% in situations where conventional analysis without correc tions for sample topography produced relative errors of ±100% or more (7). X-ray intensity area scans provide a powerful means of obtaining a qualita tive compositional map that shows the
1060 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
distribution of the constituents of in terest (5). To obtain statistically valid information in the X-ray area scan, a large number of X-ray pulses of a par ticular energy must be processed. Such area scans are best prepared with a WDS for two main reasons: 1) The peak-to-background ratio mea sured 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 50 000 cps or more at a single X-ray energy as compared to the EDS which is limited to about 5000 cps (at opti mum resolution) integrated across the full energy range excited by the beam. In the present study, electron and light imaging, quantitative EDS anal ysis, and qualitative WDS area scan ning were used to characterize the alu minum 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. The samples ob-
tained from glow failures produced in the simple apparatus were examined by stereo optical microscopy and scan ning electron microscopy. The struc tures observed on the wire and screw are shown in Figures 2a and 2b, re spectively. The structures can be de scribed as craters from which material has been removed with evidence of surface melting, as shown by the solid ified droplets and spatter structures marked in Figure 2c. EDS X-ray mi croanalysis revealed that some of the material in the droplets and spatters outside the crater on the steel screw was nearly pure iron, which indi cates that temperatures in excess of 1550 °C, the melting point of iron, were reached in the crater during the glow failure. Furthermore, quantitative X-ray microanalysis with FRAME Ρ of the various numbered locations in Figures 2a and 2b revealed that significant material transport had occurred be tween the wire and screw components during the glow failure. Thus, alumi num was transferred to the damaged crater on the iron screw and vice versa. This combination of high tempera ture and intimate mixing of the metal lic elements in the craters produced during the glow failure should provide an ideal environment for the forma tion of intermetallic compounds, which are strongly favored in the alu minum-iron binary system (9). To check this hypothesis and to deter mine how far beneath the surface the intermetallic compound(s) might form, a metallographic section was prepared through the crater perpen dicular to the surface of the wire. This
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S c re w
Wire
Brass
1 mm Figure 3. Optical micrograph of metal lographic section taken through duplex electrical outlet assembly that had un dergone glow failure. Note presence of reaction zones (circled) between alumi num wire and screw and between alu minum wire and brass plate
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(b)l
AI
(c)
Fe
Figure 4. (a) Scanning electron micrograph of reaction zone at aluminum wire-steel screw interface; backscattered elec tron signal showing compositional contrast, (b) X-ray area scan for aluminum, (c) X-ray area scan for iron
Table 1. X-ray Microanalysis of Reaction Zone at Aluminum Wire-Steel Screw Interface
• • : ' • • • •
Location No. (Figure 4a)
AI (wt % )
Fe (wt % )
Cu (wt % )
1 2 3 4
62.5 57.5 57.3 55.5
38.1 43.3 43.3 44.4
0.5 0 0 0
'
cross section revealed a subsurface zone in compositional contrast in a backscattered electron SEM image. Quantitative EDS microanalysis of this region indicated the composition of the region corresponded closely to FeeAl, 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 wire-screw-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 Fig ured, where extensive reaction zones have been exposed simultaneously at the wire-screw interface and the wireplate interface. An electron image of the reaction zone at the wire-screw interface is shown in Figure 4a. X-ray area scans for iron and aluminum, Figures 4b and 4c, respectively, 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 quanti tative 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 compounds Fe2Al5 and FeAl 3 . The wire-plate reaction zone is shown in an electron image in Figure 5a, again revealing a region of the brass plate in which the composition has been modified by the reaction. X-ray area scans for aluminum (Fig ure 5b), zinc (Figure 5c), and copper (Figure 5d) suggest a replacement of zinc in the brass by aluminum with relatively little modification of the copper concentration. This observa tion is confirmed by the quantitative EDS analyses (Table II) at the loca tions indicated in Figure 5a, which show that the reaction region has cop per and aluminum as major constitu ents with a minor amount of zinc re maining.
1062 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
The previous observations were made on connections that had exhib ited the macroscopic characteristics of the glow failure (emission of visible light, evolution of heat, and develop ment of resistance across the junc tion). As a further step, the incipient stage of the glow failure was studied by interrupting a current cycle test after only a few cycles and before any macroscopic glow failure characteris tics could be observed. Again, reaction zones showing the formation of intermetallic compounds were observed at the wire-screw interface and at the wire-plate interface. What is remark able about these structures is their ex traordinarily small size. The careful metallographic preparation of appro priate samples was absolutely critical to the detection of these incipient fail ure structures. Interpretation The discovery of intermetallic com pound formation at the current-carry ing interfaces in aluminum wire con nections is vital in developing an un derstanding of the underlying causes
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(a)
(b)
Cu
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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 com pounds 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 exam ple, the intermetallic compound FeeAl has a resistivity of approximately 100 μΩ-cm, a factor 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 so high as to completely attenuate the flow of current. From these observations the se quence of events of a glow failure thus seems to be: 1) The connection be comes mechanically loose, leading to arcing between the various compo nents, wire-screw and wire-plate; 2) the arcing leads to high temperatures locally, which causes evaporation and mutual transfer of the constituents
Table II. X •ray Microanalysis of Reaction Zone at Aluminum Wire-Brass Plate Interface Location No. (Figure Sa)
Al ( w l % )
Cu (wt % )
Zn(wt % )
1 2 3 4 5
5.2 16.4 18.4 18.7 0
65.0 72.4 74.2 74.6 69.4
29.7 10.2 7.3 6.8 31.1
between the circuit components; 3) the combination 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 resis tance 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 liq uid 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
ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982 · 1063 A
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areas are sites of small localized arcs, or perhaps these regions are small asperities at 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 for mation through local small-scale heat ing 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. Unfor tunately, examination of the phase di agrams of aluminum reveals that many binary and ternary systems con tain extensive sequences of interme tallic compounds. More work is need ed on the details of the interactions of components at current-carrying inter faces. Acknowledgment
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I would like to express special thanks to Charles Brady, whose metallographic sections made this work possible, Sidney Greenwald for the de sign of the simple laboratory alumi num wire connection, and Joy Shoe maker for preparation of the manu script. References
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1064 A · ANALYTICAL CHEMISTRY, VOL. 54, NO. 9, AUGUST 1982
(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 Elec trical Connections"; Nat. Bur. Stand. U.S. NBSIR 78-1507, August 1978. (3) Rabinow, J. "Special Report on Alumi num Wire"; Report for U.S. Consumer Product Safety Commission, September 1974. (4) Newbury, D.; Greenwald, S. "Observa tions 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.; Newbu ry, D. E.; Lifshin, E.; Colby, J. W.; Cole man, 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-Dis persive Electron Probe X-ray Analysis"; Nat. Bur. Stand. U.S. Tech. Note 1106, September 1979. (7) Small, J. Α.; Newbury, D. E.; Mykle bust, 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 Analy sis Procedure for Energy-Dispersive X-ray Microanalysis." In "Microbeam Analysis—1979"; Newbury, D. E., Ed.; San Francisco Press, 1979; pp 231-37. (9) Hansen, M. "Constitution of Binary Alloys"; McGraw-Hill: New York, 1958; pp 84-95.
