Bronze coatings on copper
G. Moretti, A. Gambirasi, A. Zingales Università di Venezia - Dipartimento di Chimica
G. Capobianco Università di Padova - Dipartimento di Chimica - Fisica
G. Ottaviani Università di Modena - Dipartimento di Fisica
M. Gajo Elsy Research, Conegliano (TV)
keywords: “White” bronze coatings on copper, ion implantation, Ar ion beam mixing, electrochemical corrosion tests
Nickel is know to be the most frequent cause of contact allergy in Europe, where 10 to 20% of female population is allergic to nickel (1).
The adsorption of Ni ions released from some Ni-containing materials in direct and prolonged contact with the skin generally causes sensitisation: further exposition to Ni salts results in allergies as eczemas, dermatitis, etc. (2, 3).
These Ni effects seem to exclude that in the future the coatings containing high percentages of Ni and used as bright decorative substrate in object in direct contact with the skin can still be used. Thus the Industries (galvanic decorative industries and others: jewellery, watches, etc.) producing objects containing Ni in contact with the human body will must substitute this metal in those final treatments which deposit it on a substrate.
Among the various coatings alternative to Ni, some nickel-free alloys (as Cu-Sn and Cu-Sn-Zn bronzes) are known for their aesthetic effect and for the good corrosion resistance especially when the layer thickness is some m (4, 5).
The objective of the present work was to investigate the corrosion behaviour of copper treated with two different techniques: electroplating and ion beam mixing technique. Copper has been electroplated with a coating of “White Bronze” (Cu66 wt% Sn23 wt% Zn10 wt% Pb1wt%). The properties of this coating are the high brightness (as a noble metal) that allow to use it for decorative industries.
Other important characteristic are:
1) the high passivity of the layer, because of the high contents of tin (35%) that should allow a good corrosion resistance of the material;
2) the absence of Nickel that is an allergy-causing factor. The bronze coating was also ion beam mixed by bombardment with an Argon ion beam. The corrosion behaviour of bronze coatings, compared with that of pure copper, was investigated a) using the usual electrochemical techniques, to study the corrosion mechanism in a alkaline solution (pH 12.4);
3) surface analysis as Rutheford Backscattering Spectroscopy (RBS) and Scanning Electron Microscopy (SEM), to verify thickness, homogeneity and composition of coatings.
Implanted Bronze-electroplated copper showed the best corrosion behaviour. The electrochemical tests showed small corrosion rates and a corrosion mechanism similar to that of more noble coatings.
In all experimental tests the specimens were of commercial Cu (99.99% purity) obtained from the same 0.5 mm thick plate with an exposed surface of almost 10.5x10.5 cm. As the result of preliminary tests on commercial copper plates which revealed the non homogeneity of this material, a coating (20m) of Cu was electrodeposited over the commercial plate.
The plate of coppered Cu was then electro-coated by the “white bronze” (Cu 55 wt%; Sn 35 wt% and Zn 10 wt%) alloy layers. The specimens were painted with a strippable varnish to preserve them from wear during the cutting and handling. Discs of 11.3 mm of diameter were then cut from the plates.
Before the galvanic treatments all the plates of commercial Cu were degreased in an ultrasonic bath at 60°C (30-120 s). After being rinsed in deionized water, the plates were electrolytic etched to remove the oxides, using a solution containing K2CO3, NaOH, silicates, phosphates, ionic and non-ionic biodegradable surfactants (operating conditions: 30-60 °C; immersion time: 30-120 s; current density: from 1 to 4 A/dm2, voltage: 3-5 V). After being rinsed again in deionized water, the plates were activated in acid salts to remove every basic residual product of the electrolytic etching from the metal surface.
As previously mentioned, an electrolytic copper coating (20m thick) was deposited to eliminate surface irregularities and obtain a good substrate before alloy deposition. The galvanic bath was composed of inorganic salts and some additives (see Tab. 1). The operating conditions were as following: Temperature: 24-26°C; Cathodic current density: 1-4 A/dm2; Voltage: 1-4 V; Deposition rate: 0.7 m/min at 3 A/dm2; Air blow: 12-20 m3/h for meter of cathodic bar; Anodes: copper/phosphorus 0.03-0.06%; Anodic sacks: Meraklon; Filtration: continuous 2-3 Volume/hour; Filters: polypropylene 5 m; Efficiency: about 100 %.
Le condizioni operative sono le seguenti:
Copper Sulphate penta-hydrate
Sulphuric acid (r = 1.84 g/ml)
Tab. 1: Composition of bath for acid coppering.
White bronze electroplating
“White bronze” alloy (theoretic: Cu 55 wt%; Sn 35 wt% Zn 10 wt% Pb traces) electrodeposition was performed in a bath the composition of which is reported in Tab. 2. The bath also contains brighteners, biodegradable surfactants, conductive salts.
Cu (as metal)
Sn (as metal)
Zn (as metal)
Tab. 2: composition of the bath for deposition of bronze alloys.
The operating conditions were as follows:
temperature: 40°C; voltage: 2V; cathodic agitation: 2-5 m/s; Filtration: continuous, PP filters 1or 5m; Anodes: graphite; deposition rate: 1 m in 5.5 min; current density: 0.2 A/dm2; bath density: 15 Bè; deposit density: 8.0 g/cm3.
The homogeneity and composition of the alloys obtained in these experimental conditions were tested by Scanning Electron Microscopy (SEM), Rutherford Back Scattering (RBS) (see below) and chemical analyses. The effective percentage of the elements in the alloy resulted Cu (66 wt%) Sn (23 wt%) Zn (10 wt%) Pb (1wt%). Two kinds of samples of different thickness were prepared: 0.3 and 0.04 m.
The samples of bronze coatings on copper (0.04 m) were implanted with Ar+ at an energy of 110 keV and a dose of 6x1015 at/cm2 and with Ar+ at an energy of 95 keV and a dose of 6 x1015at/cm2 respectively. The implantation was done at room temperature.
The implantation energy was chosen to place the maximum of Ar profile at the interface of the films and is evaluated with a Montecarlo simulation program (SRIM98) (6).
RBS measurements, used to measure the film composition, were performed with a 3 MeV 4He+ beam with a detector scattering angle of 160 º.
This technique is usually considered non-destructive in the sense that the interaction of the probe beam and the sample is so mild that no changes are induced by the beam itself.
Fig. 1 shows the RBS spectra of bronze and the film composition is Cu (66 wt%) Sn (23 wt%) Zn (10 wt%) Pb (1wt%). The thickness of the film is 450Å.
Fig. 1: RBS spectrum of Bronze sample obtained with 3 MeV of 4He+ particles. The simulation spectrum [using a RUMP code (7)] is also shown. the film composition is Cu (66 wt%) Sn (23 wt%) Zn (10 wt%) Pb (1wt%).
All the electrochemical tests were carried out in alkaline medium [borate/boric acid led to pH=12.4 with NaOH (10%)], the test solution already used in literature to evaluate the corrosion behaviour on copper and other metallic materials (8, 9). All the solutions were obtained using “pure for analysis” chemicals. The working electrode, a reference saturated calomel electrode (sce, to which all the potentials reported hereafter refer), inside a Haber-Luggin's capillary probe, and two platinum counter electrodes were placed inside a Pyrex glass ASTM cell.
After the free corrosion potential (Ecorr) vs time tests (30 min), potentiodynamic tests were also carried out (“Tafel curves”) in the range greater than (Ecorr ± 250 mV) to obtain further electrochemical data as reported in literature (9). From these curves it is possible to draw out Ecorr, free corrosion potential; iL, limit current density of the O2 diffusion; icorr, corrosion current density (directly correlated to the corrosion rate, Rcorr); Epp, potential of primary passivation; Ep, potential of passivation; ip, passivation current density; Et, transpassivation potential.
The specimens were used as working electrodes inside a Teflon holder that left a 0.50 cm2 surface exposed. Anodic and cathodic polarisation curves were carried out at 25°C. The temperature was adjusted to ±0.1°C of the required value using a thermostatically controlled water bath. The corrosion cell was always filled with a known amount (700 ml) of solution, aerated by prolonged bubbling with pure air (12 h).
The potentiodynamic tests were carried out as follows: the specimen was immersed for about a half an hour; when the Ecorr was reached, the scan was started from around Ecorr until a potential ranging around E = - 1000 mV. The potential scanning rate was 500 mV/s. The sample in the Teflon holder was then taken out, substituted with a homologous one, washed and degreased with methanol, and then re-immersed inside the electrochemical cell in the same solution. After Ecorr was reached (30 min), the anodic scan was started from Ecorr until a potential around E = + 600 mV. Each test was repeated at least three times on different specimens in different solutions at the same concentration.
RESULTS AND DISCUSSION
ll the electrochemical tests, carried out on the bronze samples, aimed at verifying the duration on time of the very thin layers of bronze in the standard solution.
Potential vs time tests
Fig. 2 reports the curves E vs time obtained with bronze coatings compared with that of electrolytic copper, Tab 3 the results. The trend of the curves obtained with the bronze samples show that only the bronze 0.3µ seems to have a behaviour similar to the Cu specimens even if the Ecorr is more negative (-286/-288 mVsce for 0.3µ, -217 mVsce for Cu). In fact, both bronzes 0.04µ show very negative initial Ecorr (around -560 mVsce for 0.04µ compared with -480 mVsce 0.04µ I.I.): the potentials stabilize after around 10 min but only the I.I. samples reach values slightly more noble than copper.
-217 ± 3
-217 ± 3
-288 ± 7
-286 ± 6
-274 ± 8
-274 ± 8
Bronze 0.04m I.I.
-206 ± 7
-206 ± 6
Tab. 3 Comparison between the average Ecorr obtained with the different samples (from E-time and potentiodynamic tests).
Fig. 2: Some typical E-time curves obtained with bronze coatings compared with that of electrolytic copper (pH=12.4, T 25°C, aerated solution).
Potentiodynamic tests allow drawing out different electrochemical data, in particular the corrosion current density, icorr, directly correlated with the corrosion rate of the different surface treatments.
Fig. 3 reports some typical potentiodynamic curves obtained with the different bronze coatings, compared with a typical one of electrolytic Cu in the same experimental conditions.
Tab. 4 reports the more significant electrochemical data.
15.7 ± 1.9
1.8 ± 0.1
1.8 ± 0.2
5.0 ± 0.6
17.9 ± 0.8
2.3 ± 0.1
0.19 ± 0.05
0.5 ± 0.1
17.4 ± 0.5
2.3 ± 0.1
0.30 ± 0.07
0.8 ± 0.1
Bronze 0.04m I.I.
19.0 ± 1.1
1.8 ± 0.1
0.15 ± 0.05
0.4 ± 0.1
Tab. 4 Electrochemical data obtained from the potentiodynamic (and E-time) curves with different surface treatments. Rcorr is in mdd, mg/dm2/die (pH=12.4, T 25°C, aerated solution).
Fig. 3: Some typical potentiodynamic curves obtained with different Bronze coatings compared with that of electrolytic copper (pH=12.4, T 25°C, aerated solution).
The similar trends of the Bronze 0.3m and Bronze 0.04m curves substantially differ from that obtained with the Cu. In both cases the passivation capacity of the surface treatments is clear and the corrosion rate, Rcorr, reaches values one order of magnitude lower than that of Cu.
From a comparison with Cu, one can point out that the Rcorr of Bronze coatings are significantly lower than that of copper even at thickness of 0.04m .
Similar trend shows the I.I. specimen: even if the passivation current density ip is equal to that of Cu, the Rcorr is even lower than those obtained with the other bronze coatings. This means that the I.I. treatment has a positive influence on the corrosion behaviour of the back-mixed coating.
The average iL of bronze layers results higher than that of Cu (18-19 towards 16 mA/cm2). The ip of non-implanted specimens are appreciably higher than that of Cu, even if the anodic characteristics of the bronzes compared with that of Cu are significantly more tending to a rapid passivation of the metallic surface.
In any case the I.I. specimens show a behaviour similar to that of Bronze (0.3m) coatings (0.4 and 0.5 mdd respectively), seeing the Rcorr of bronze 0.04m (0.8 mA/cm2).
Further analyses are underway to investigate the degree of modification induced by the I.I. process.
Seeing that the European law 1811 (June 1998) fixes the limit of Ni-release in 0.5 mg/cm2/week, in the future the galvanic industries have to substitute the coatings containing this metal. The coatings “Ni-free” are a good alternative. This paper demonstrates the good corrosion performance of “white bronze” thin layers and of the I.I. treatments. One can conclude that:
It is possible to deposit an alloy of “white bronze” (Cu66 wt% Sn23 wt% Zn10 wt% Pb1wt%) with high efficiency: the coating are homogeneous and brilliant.
Electrochemical tests carried out in a standard solution [pH=12.4 (borate-boric acid + NaOH (10%)); T 25°C; aerated solution] showed that thin galvanic coatings (0.04-0.3m) of “white bronze” reduce the Rcorr of more than one order of magnitude even if they do not ennoble the Ecorr of the substrate (Cu electrolitically coppered - 20m);
the ion implanted bronze 0.04m specimens behaves as the Bronze 0.3m ones, showing that after the I.I. treatment the Rcorr of I.I. is around half of that of not implanted bronze 0.04m
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