Aluminum and aluminum alloys as a substitute for cadmium in the aerospace industry

Due to legal requirements, the development of high-performance corrosion protection coatings to replace environmentally harmful cadmium coatings is of great interest. Aluminum alloys are interesting materials for this purpose, but can only be electroplated from aprotic electrolytes such as ionic liquids. The identification of alloys that offer reliable cathodic corrosion protection and an understanding of the deposition process are essential for the implementation of corresponding technical processes.

Introduction:

Every year, the elimination of corrosion damage devours enormous sums of money. In 2017 alone, the corrosion of metallic structures caused damage of around 1.4 trillion euros and thus around 3.8 % of the global gross national product [4]. Corrosion can not only lead to complete material failure and thus cause high costs, but can also pose risks to people and the environment [5]. This underlines the importance of efficient corrosion protection measures and their continuous further development from both an economic and social point of view.

In order to meet high technical requirements, new materials are constantly being developed, but these often have insufficient corrosion resistance. Proven corrosion protection strategies include coatings with metallic (e.g. zinc) and non-metallic (e.g. paints) coatings, which can be produced using physical, chemical and electrochemical methods [6,7]. The cathodic corrosion protection of steel is particularly important in many sectors, such as the construction, automotive and aerospace industries [6]. Classic cadmium coatings and modern alloys such as zinc-nickel play a central role here [8-10].

Fig.1: Corrosion processes in damaged (left) cadmium and (right) aluminum coatings using the example of oxygen corrosion [1]

Due to the REACH regulation [11] enacted by the European Union (EU), the demand for environmentally friendly substitutes for substances of very high concern (SVHC9 ) such as cadmium [12,13] is increasing. In this context, aluminum is seen as a promising alternative and possible 1:1 replacement for cadmium [12-14]. Aluminum offers advantages such as high environmental compatibility and recyclability [15] as well as a high specific power content (2982 Ah kg-1) [16]. However, the oxide layer that forms on contact with atmospheric oxygen limits the cathodic corrosion protection effect [17]. Cadmium layers do not form a stable insulating top layer, which enables a local current flow that contributes to the protection of the substrate (Fig. 1 left). The natural oxide layer of aluminum, on the other hand, interrupts this circuit (Fig. 1 right), which can lead to considerable corrosion damage and failure of the base material if the coating is damaged. Alloying aluminum with other metals can inhibit the formation of the natural oxide layer and thus improve the cathodic protection effect, but this also increases the inherent corrosion of the material [18,19]. Potentially suitable alloying elements are zinc [19-23], chromium [24-27] and tin [19]. Both AlCr [24-30] and AlZn [25,31,32] were successfully deposited from ionic liquids (ILs) and high-temperature salt melts (HTSM). Sn was deposited on Au, Pt and glassy carbon from Lewis-basic and Lewis-acidic mixtures of AlCl3 and 1-ethyl-3-methylimidazolium chloride ([EMIm]Cl) [33]. Alloy deposition was not investigated. However, AlSn was successfully realized from organic solvents [34] and HTMS (High Temperature Molten Salts) [28,35]. Detailed investigations of the deposition, the layer properties and the corrosion behavior, especially with regard to cathodic corrosion protection, have not yet been carried out for these alloys.

Electroplating of aluminum from ILs

Due to its negative Nernst potential (-1.66 V vs. SHE [36]), aluminum cannot be deposited from aqueous electrolytes, which is why various non-aqueous electrolyte systems have been developed [37]. Some of these processes have reached industrial maturity (e.g. SIGAL process [38-40] and REAL process [41,42]), but have serious disadvantages such as high flammability of the solvent used.

Ionic liquids are promising media in this context. However, high chemical costs have so far slowed down their industrial use [43]. As the prices for these chemicals are falling steadily due to great interest in their use for various applications, their economic use is becoming increasingly attractive. The deposition of metals and alloys that can be deposited from aqueous electrolytes (e.g. Zn, Cu, Cr, Pd, Ag, Pt), but also those that can only be deposited from aprotic systems due to their base character, from ILs has been extensively investigated [43]. However, the specific investigation of the cathodic protection effect is a neglected aspect and has so far only been investigated using accelerated test methods, such as the neutral salt spray test, and primarily for pure aluminum coatings [44-46]. The durable cathodic corrosion protection of a coating such as aluminum under mild atmospheric conditions cannot be adequately evaluated using such test methods due to its tendency to self-passivate. The use of alternative test methods is therefore imperative.

The electrochemical deposition of aluminum from ILs is only possible from Lewis acid electrolytes. The mixture of AlCl3 and [EMIm]Cl with a molar excess of AlCl3 is an established system. Heptachloroaluminate ions (_Al2Cl7-) are formed in the electrolyte, which can be reduced to aluminum (equation 1) [47-49].

4 _Al2Cl7-+3 e- → Al + 7 AlCl4- <1>

In addition, the addition of further metal salts (e.g. _ZnCl2, _SnCl2) or the anodic dissolution of the respective metal in the electrolyte allows the deposition of aluminum alloys. As the effect of complexing agents for ILs has only been investigated to a very limited extent so far, the use of multi-anodes represents a promising approach for stabilizing the metal ion concentration in the electrolyte in this context. This is essential for ensuring the deposition of reproducible alloy layers [50].

Results of the tests

The suitability of various aluminum alloys for the cathodic corrosion protection of steel was investigated. For this purpose, the deposited coatings were comprehensively characterized with regard to their deposition, morphology and composition as well as the crystallographic structure [3] (not shown). Their corrosion properties were then examined in detail. All potentials discussed in the following are related to the Ag/AgCl reference electrode.

Potentiodynamic polarization

The corrosion potentials_ECorr were determined from polarization curves in 3.5 wt% NaCl solution. For this purpose, the resting potential _EOCP was first measured for 5 min to 120 min and then polarized in the range _EOCP ± 200 mV at a feed rate of 0.1 mV ^s-1._ECorr was determined from the Tafel plot of the polarization data or, as for the highly reactive AlSn alloys, from the _EOCP measurements.

For a cathodic corrosion protection effect, the coating must have a corrosion potential below the potential of the steel substrate to be protected (-510 mV to -630 mV) (Fig. 2). Technical cadmium coatings, which have a corrosion potential of -700 mV to -730 mV, meet this requirement. Commercial sacrificial anode alloys, referred to here as 40Al and 60Al, are also suitable with their corrosion potential of -950 mV to -1020 mV. The Tafel plot of the polarization curves for binary aluminium alloys with Cr, Zn and Sn shows the shift of the corrosion potential_ECorr with the alloy content. The corrosion potential of all alloys approaches a value of approx. -780 mV to -800 mV, which corresponds to the value for pure aluminum in 3.5 wt% NaCl solution (Fig. 2 right, black arrow) [51,52]. This value, which is comparable to cadmium, justifies the potential suitability of aluminum and its alloys as a cadmium substitute.

Fig. 2: (left) Tafel plot of potentiodynamic polarization curves (0.1 mV s-1, 3.5 wt% NaCl, 25°C) for AlCr (red), AlZn (green) and AlSn alloys (violet) as well as Cd (orange) and the steel substrate (black) and (right) corrosion potential ECorr vs. concentration of Cr (red), Zn (green) and Sn (violet) in binary alloys AlMe and comparison to corrosion potential ranges of the steel substrate (black), Cd (orange) and the sacrificial anode alloys 40Al and 60Al (blue). The dashed lines represent the qualitative trend, but have no physical significance. The corrosion potential of the alloys for c(Me) → 0 wt% (black arrow, blue dot) corresponds to the value for Al [1, 51,52]

If chromium is added, the corrosion potential increases and exceeds the lower value for steel at an alloy content of around 5 wt% to 10 wt%. While corrosion potentials comparable to cadmium and uniform corrosion can be observed below these chromium contents and thus cathodic protection of steel is conceivable, higher contents lead to pitting and red rust formation. Therefore, pores and cracks extending to the substrate can be assumed, suggesting that the measured potentials correspond to those of the steel substrate.

The corrosion potential decreases with increasing zinc content. This is surprising insofar as the standard potential of zinc is significantly more positive than that of aluminum [36]. At an alloy content of 20% by weight, a minimum of around -1050 mV is reached, indicating the effective disruption of the natural oxide layer of the aluminum and the associated activation. As the zinc content increases further, the corrosion potential increases, indicating the increasing dominance of zinc with respect to the measured potential. These values are close to those for sacrificial anode alloys, which in combination with uniform surface corrosion of the AlZn alloys suggests cathodic corrosion protection of steel. Differences in corrosion behavior between the deposited alloys and the sacrificial anode materials are due to other elements (e.g. Ti, In) in the latter.

Comparable to the AlZn alloys, but significantly stronger, the corrosion potential also decreases with increasing tin contents. Even small amounts of tin below 1 wt% lead to values of up to -1400 mV and to strong activation. The wetting of such alloys with NaCl solution leads to spontaneous gas evolution, which makes it difficult to record polarization curves and causes very noisy measurements (Fig. 2 left). Assuming complete inhibition of oxide layer formation, it can be assumed that the gas is hydrogen. By increasing the tin content, the corrosion potential increases, as in the case of AlZn alloys, but remains well below -1200 mV. The strong activation of the material causes rapid dissolution in the electrolyte, which requires extremely low tin contents for technically viable coatings. This is difficult to guarantee due to the deposition behavior of this alloy [3], especially for geometrically complex components, which severely limits the use of such an alloy.

Neutral salt spray test

The neutral salt spray test (NSS test) was carried out in accordance with DIN EN ISO 9227 [53]. NaCl solution (5% by weight) was sprayed into the chamber at intervals (10 min spraying, 50 min break) at a temperature of 35 °C. Bare steel substrates were used as a reference as well as samples coated with Al, AlCr (approx. 3 wt% Cr), AlZn (approx. 4 wt% Zn) and AlSn (approx. 2 wt% Sn) (coating thickness approx. 10 µm). The edges and uncoated areas of the samples were covered with PVC paint. The corrosion progress was evaluated by daily visual inspection with regard to the first appearance of red rust.

Fig. 3: Time to red rust formation tRR during the NSS test and the EE test in the scribe and on the entire exposed surface for uncoated substrates as well as Al, AlCr (approx. 3 wt% Cr), AlZn (approx. 4 wt% Zn) and AlSn coatings (approx. 2 wt% Sn). The values for the NSS test represent the time until the first appearance of red rust and those for the EE test represent the time until red rust coverage of approx. 5 % [1]

After 24 hours, the steel reference already showed severe red rust formation over the entire surface (Fig. 3). In contrast, aluminum offers effective protection in the NSS test, which is also evident from the literature [44-46]. Over a period of approximately two weeks, discoloration of the coating was observed without any signs of red rust formation. After 33 days (>790 h), red rust appeared in places, indicating the cracking of the coating and the failure of the protective effect. AlCr alloy coatings also showed discoloration, which, in contrast to pure aluminum, occurred within one day and after three days led to the isolated formation of red rust, which spread over the entire surface within a few more days. AlZn alloys behaved similarly to pure aluminum in terms of discoloration and red rust formation. The latter was observed in isolated cases after 30 days (>720 h). Due to their high reactivity, as discussed above, strong discoloration and staining within two days and rapid red rust formation and spread from the fourth day were observed in the case of AlSn alloys.

Outdoor weathering

Environmental exposure tests (EE test) [54] were carried out in accordance with DIN EN ISO 2810 [55] and DIN 55665 [56] on the roof of the IMN MacroNano at the Technical University of Ilmenau (GPS: 50°41'03.4 "N, 10°56'23.2 "E). The samples were aligned to the south at an inclination of (15 ± 2)° to the horizontal. Analog samples were used for the NSS test. In addition to covering the sample edges, an artificial defect was introduced in the form of a scribe extending to the substrate. In the period from June 2019 to July 2020, the corrosion progress was observed by visual inspection. The inspection intervals were daily inspections for six weeks, twice a week for the following four weeks and then weekly inspections. Based on photographs and image analysis using ImageJ/FIJI v.146, the red rust coverage was quantified and a value of 5 % was determined as the onset of corrosion. While the NSS test leads to an acceleration of the corrosion processes due to increased salt concentration and temperature, outdoor weathering makes it possible to observe the corrosion under realistic conditions, but also depending on the geographical location and civilization and seasonal environmental conditions. This leads to more reliable results, but requires significantly longer test periods (months to years).

Fig. 4: Red rust coverage vs. exposure time of the uncoated steel substrate (black) as well as Al (blue), AlCr (red), AlZn (green) and AlSn coatings (purple) in relation to (left) the scribed area and (right) the entire exposed surface. The continuous lines represent the qualitative trend of red rust coverage over the exposure time, but have no physical significance. The horizontal dashed lines (gray) mark a red rust coverage of 5 %. [1]

Clear traces of red rust could already be detected on the steel references after an exposure time of two days (Fig. 3 and Fig. 4). The area scored on the steel samples for comparison purposes was completely covered with red rust after two days (Fig. 4 left), while the remaining exposed area showed a coverage of 20% during this time (Fig. 4 right). Subsequently, the coverage remained almost constant until it increased continuously after approx. 20 weeks until the sample was completely covered. In contrast to the NSS test, Al coatings showed considerable signs of corrosion after just two days, which spread over the coated area in the following weeks. This proves that no significant cathodic corrosion protection can be achieved by pure aluminum under comparatively mild environmental conditions. AlCr alloys were able to prevent the onset of red rust formation in the scribe for a period of six days. The subsequent red rust formation suggests that there is only a marginal improvement in cathodic corrosion protection compared to pure aluminum. In the following weeks, corrosion also occurred on the remaining surface, indicating defects in the coating. After 35 to 40 weeks, the entire scribe was covered with red rust and after 50 weeks a total coverage of 75-80 % was recorded. With a red rust-free surface in the scribe after more than 20 to 55 weeks with slight discoloration, AlZn alloys are promising alloys. The variation in the occurrence of the first red rust can be explained by fluctuations in the Zn content and thus the corrosion potential of the coatings. In all samples, red rust coverage of less than 25 % was found in the scribe after 55 weeks. In addition, the remaining exposed area showed no signs of red rust over the entire outdoor exposure period. These results in combination with the results of the NSS test indicate a significant improvement in the cathodic protection effect and make these alloys extremely interesting for further research. Despite the strong reactivity of the AlSn alloy layers, red rust formation in the scribe accompanied by a clear discoloration of the layer could only be observed after five to ten weeks for these samples. After approx. 15 weeks, a rapidly progressing red rust formation set in, which led to a coverage in the scribe of 60 % after 40 weeks. Isolated red rust spots appeared on the remaining exposed area after 25 weeks, which spread over approx. 20 % of the total area after 55 weeks. This demonstrates an increase in the cathodic corrosion protection effect compared to pure aluminum as well as AlCr alloys. However, the formation of red rust in the undamaged area indicates a non-closed morphology of the coating, which makes technical application appear impractical.

Summary and outlook

The cathodic protection effect of aluminium and aluminium alloys with chromium, zinc and tin was investigated using potentiodynamic polarization, the neutral salt spray test and outdoor weathering tests.

It was shown that the ability of AlCr alloys to protect steel against corrosion is extremely low and does not meet technical requirements. A corrosion potential that increases with the chromium content counteracts the cathodic protection effect. In addition, uneven corrosion in the form of pitting was observed. Nevertheless, red rust formation was delayed compared to pure aluminum in both the NSS and EE tests. AlZn alloys exhibit high performance in both the NSS test and the EE test and are therefore suitable coatings for replacing cadmium. Uniform corrosion as well as a high protective effect and effective retardation of red rust formation are the best basis for this. The decrease in corrosion potential with increasing zinc content indicates an effective inhibition of the natural oxide layer. AlSn alloys react with a significant reduction in corrosion potential even at low tin contents and consequently with strong activation. The requirement for low tin contents represents a technical challenge for the reproducibility of the deposition. Nevertheless, an increase in the cathodic protection effect was demonstrated in the NSS test and the EE test.

These results indicate the suitability of AlZn alloys and the conditional suitability of AlSn alloys for steel. Due to process-related challenges, however, the former have a higher application potential. Progressive research coupled with falling prices for ILs is bringing the industrial implementation of IL-based aluminum alloy deposition closer. Continuous optimization of the promising AlZn alloys in terms of galvanic process control, layer morphology and composition can ensure the reliable performance of such coatings. In addition, the application-related development of the necessary system technology (anode technology, material compatibility, encapsulation) plays a decisive role in industrialization. The adaptation of the SIGAL process and the associated system technology should be considered for this purpose. An industrial application of encapsulated electrolyte systems has already been successfully realized.

With his work on the electrochemical deposition of aluminum and aluminum alloys [1], Dr. Böttcher won the Nasser-Kanani Prize 2024. Alongside him, Frank Simchen also received the award with a publication on the corrosion protection of magnesium. This article builds on [1] and has already been published in English in two scientific journals [2,3]. A further article by the author on the subject of anode passivation will appear in a forthcoming issue.

Literature

[1]R. Böttcher, Electrochemical deposition of aluminum and aluminum alloys from ionic liquids. Dissertation, Ilmenau, 2021.
[2]R. Böttcher, A. Valitova, A. Ispas, A. Bund, Electrodeposition of aluminum from ionic liquids on high strength steel, Trans. Inst. Met. Finish. 97 (2019) 82-88. https://doi.org/10.1080/00202967.2019.1573941.
[3]R. Böttcher, A. Ispas, A. Bund, Binary Aluminum Alloys from 1-ethyl-3-methylimidazolium-based Ionic Liquids for Cathodic Corrosion Protection, Metals 13 (2023) 377. https://doi.org/10.3390/met13020377.
[4]B. Chico, D. de La Fuente, I. Díaz, J. Simancas, M. Morcillo, Annual Atmospheric Corrosion of Carbon Steel Worldwide. An Integration of ISOCORRAG, ICP/UNECE and MICAT Databases, Materials 10 (2017). https://doi.org/10.3390/ma10060601.
[5]R.W. Revie, H.H. Uhlig, Corrosion and corrosion control: An introduction to corrosion science and engineering, John Wiley & Sons, Hoboken, New Jersey, 2008.
[6]C.H. Hamann, W. Vielstich, Elektrochemie, 4th ed., Wiley-VCH-Verl., Weinheim, 2005.
[7]J. Sander, L.L. Kirmaier, M. Manea, D. Shchukin, E. Skorb, Korrosionsschutz durch Beschichtungen: Fundamentals and new concepts, Vincentz Network, Hannover, 2011.
[8]V.C. McMillan, Electrodeposited Zinc-Nickel as an Alternative to Cadmium Plating for Aerospace Application, 1991.
[9]K.R. Baldwin, C.J.E. Smith, M.J. Robinson, Cathodic Protection of Steel by Electro-deposited Zinc-Nickel Alloy Coatings, CORROSION 51 (1995) 932-940. https://doi.org/10.5006/1.3293569.
[10]C. Schade, H. Käszmann, Corrosion protection of the top class - Electro-deposited zinc-nickel on ferrous materials, WoMag (2013).
[11]European Parliament and Council of the European Union, Regulation concerning the Registration, Evaluation, Authorization and Restriction of Chemicals: REACH, 2006.
[12]K. Legg, Cadmium replacement alternatives for the Joint Strike Fighter, Rowan Technology Group, Libertyville, IL, USA, Report (2000).
[13]R. Mason, M. Neidbalson, M. Klingenberg, P. Khabra, C. Handsy, Update on alternatives for cadmium coatings on military electrical connectors, Metal Finishing 108 (2010) 12-20. https://doi.org/10.1016/S0026-0576(10)00012-7.
[14]M. Bielewski, Replacing cadmium and chromium, Research and Technology Organization and NATO, RTO-AG-AVT-140 (2011) 1-22.
[15]C. Vargel, Corrosion of aluminum, Elsevier, Rotterdam, 2004.
[16]D. Orbakh (Ed.), Nonaqueous electrochemistry, 1st ed., Marcel Dekker, New York, 1999.
[17]Friedrich Ostermann (Ed.), Application Technology Aluminum, 2nd ed., Spring-er, Berlin, Heidelberg, 2007.
[18]W. von Baeckmann, W. Schwenk, W. Prinz (Eds.), Handbuch des kathodischen Korrosionsschutzes: Theorie und Praxis der elektrochemischen Schutzfahren, 3rd ed., VCH, Weinheim, 1989.
[19]W. von Baeckmann, Taschenbuch für den kathodischen Korrosionsschutz, 6th ed., Vulkan, Essen, 1996.
[20]P. Seré, M. Zapponi, C. Elsner, A. Di Sarli, Comparative corrosion behavior of 55 aluminum-zinc alloy and zinc hot-dip coatings deposited on low carbon steel substrates, Corrosion Sci. 40 (1998) 1711-1723. https://doi.org/10.1016/S0010-938X(98)00073-0.
[21]H. Shih, J. Hsu, C. Sun, S. Chung, The lifetime assessment of hot-dip 5% Al-Zn coatings in chloride environments, Surf. Coat. Tech. 150 (2002) 70-75. https://doi.org/10.1016/S0257-8972(01)01508-0.
[22]D. Laverde, J.C. Zubillaga, J. Gil-Sevillano, E. Villanueva, The influence of the primer layer on mechanical damage and loss of corrosion protection of deformed painted Zn-0.16% Al and Zn-5% Al galvanized sheet steel, Corrosion Sci. 37 (1995) 79-95. https://doi.org/10.1016/0010-938X(94)00116-N.
[23]T. Prosek, D. Persson, J. Stoulil, D. Thierry, Composition of corrosion products formed on Zn-Mg, Zn-Al and Zn-Al-Mg coatings in model atmospheric conditions, Corrosion Sci. 86 (2014) 231-238. https://doi.org/10.1016/j.corsci.2014.05.016.
[24]T.P. Moffat, Electrodeposition of Al-Cr Metallic Glass, J. Electrochem. Soc. 141 (1994) L115-L117. https://doi.org/10.1149/1.2055172.
[25]L. Simanavičius, A. Staknas, A. Šarkis, Codeposition of aluminum with some metals from AlBr3-dimethylethylphenylammonium bromide solutions containing acetylacetonate of selected metal, Electrochim. Acta 46 (2000) 499-507. https://doi.org/10.1016/S0013-4686" target="_blank">https://doi.org/10.1016/S0013-4686(00)00537-5.
[26]M.R. Ali, A. Nishikata, T. Tsuru, Electrodeposition of aluminum-chromium alloys from AlCl3-BPC melt and its corrosion and high temperature oxidation behaviors, Electrochim. Acta 42 (1997) 2347-2354. https://doi.org/10.1016/S0013-4686" target="_blank">https://doi.org/10.1016/S0013-4686(96)00418-5.
[27]M. Ueda, H. Ebe, T. Ohtsuka, Electrodeposition of Al-Cr-Ni layer by pulse electrolysis in AlCl3-EMIC molten salt, Electrochemistry 73 (2005) 739-741. https://doi.org/10.5796/electrochemistry.73.739.
[28]Y. Gu, J. Liu, S. Qu, Y. Deng, X. Han, W. Hu, C. Zhong, Electrodeposition of alloys and compounds from high-temperature molten salts, J. Alloys Compd. 690 (2017) 228-238. https://doi.org/10.1016/j.jallcom.2016.08.104.
[29]T. Tsuda, G.R. Stafford, C.L. Hussey, Review-Electrochemical Surface Finishing and Energy Storage Technology with Room-Temperature Haloaluminate Ionic Liquids and Mixtures, J. Electrochem. Soc. 164 (2017) H5007-H5017. https://doi.org/10.1149/2.0021708jes.
[30]T. Tsuda, C.L. Hussey, G.R. Stafford, Progress in Surface Finishing with Lewis Acidic Room-Temperature Chloroaluminate Ionic Liquids, ECS Transactions 3 (2006) 217-231. https://doi.org/10.1149/1.2798664.
[31]S.-J. Pan, W.-T. Tsai, J.-K. Chang, I.-W. Sun, Co-deposition of Al-Zn on AZ91D magnesium alloy in AlCl3-1-ethyl-3-methylimidazolium chloride ionic liquid, Electrochim. Acta 55 (2010) 2158-2162. https://doi.org/10.1016/j.electacta.2009.11.050.
[32]Y. Sato, K. Azumi, Al-Zn co-electrodeposition by a double counter electrode electrodeposition system from an AlCl 3-1-ethyl-3-methylimidazolium chloride ionic liquid bath, Surf. Coat. Tech. 286 (2016) 256-261. https://doi.org/10.1016/j.surfcoat.2015.10.080.
[33]X.-H. Xu, C.L. Hussey, The Electrochemistry of Tin in the Aluminum Chloride-1-methyl-3-ethylimidazolium Chloride Molten Salt, J. Electrochem. Soc. 140 (1993) 618. https://doi.org/10.1149/1.2056132.
[34]F.S. Platis, G.A. Capuano, Electrodeposition of Aluminum-Tin Alloys from Organic Electrolytes, J. Electrochem. Soc. 134 (1987) 2425-2429. https://doi.org/10.1149/1.2100215.s