- Email: [email protected]
Rhenium plating
by Terence Jones, M.I.M.F. Gloucester,
England,
U.K.; E-mail,
[email protected]
henium offers more than a trivial confusion of its chemical symbol with rhodium (rhodium is Rh, whereas rhenium is Re). As some of the more interesting uses of rhenium are in electroless alloy deposits, the title reflects the more general aspect of deposition, not necessarily electrolytic alone. Rhenium should be interesting for wear resistance and electrical/electronic contact purposes, but seems to have lost out in the glamour stakes. Since it may cost considerably less than gold, but has not so far attracted the jewelry trade’s interest, presumably it is likely to remain an interesting oddity to the world at large, and no more. Rhenium is generally described as a silvery metal, extremely hard, which is not attacked at all by hydrochloric acid. As few metallic materials are so completely resistant in this way, rhenium may have some practical applications for this property alone, for the platinum group metals are slightly susceptible to this acid. Rhenium is reasonably resistant to other chemicals but is rapidly attacked in oxidizing media such as HNO,, HClO,, H,O,, and also fused alkalis. Although not considered a member of the platinum group, it is certainly a rare metal and this is reflected in its cost - currently in the region of $3 to $10 per gram of metal, $15 to $30 per gram as potassium or ammonium perrhenate.l Compared with gold, rhenium is likely to be one third the cost. Chemical compounds may show valencies from -1 to +7, but the most stable and important valency is +7. (A valency of -1 is surely unique.) The orbital structure is lXe1 4f14 5d5 6s2. An extract from the periodic table, shown in Table I, illustrates the relationship of Re and the adjacent elements. Since properties tend to run within periods as well as groups, we find that although elemental Re resembles osmium and tungsten, the chemistries of the compounds have more group resemblances. Thus, Re compounds are similar to those of technetium, and to a lesser extent those of manganese, within Group VII a. Setting Tc aside (a fission product of uranium from nuclear reactors, it is used only in medicine as a radioactive imaging isotope), we find that analogies between Mn and Re compounds are closer than those of OS and Re. All useful plating electrolytes are based on the perrhenate ion ReO, as discussed later. The physical properties of Re include: the most
86
refractive metal after tungsten, with a melting point of 3,lBO“C; most dense after osmium and tungsten, with density = 21.2, and a rather high electrical resistivity of about four times that of tungsten at 18.7 micro ohm/cm (20°C). The linear coefficient of expansion is 6.6 x 10e6/“K. It has interactions with the ferromagnetic metals when codeposited with one or more of them, affecting coercivity and remanence very markedly. Nominally, rhenium has had only minority uses; e.g., alloyed with tungsten in electric lamp filaments for improved ductility and to reduce reactivity of tungsten heating elements.2 As a catalyst, it is of some importance in catalytic reforming of aromatic hydrocarbons. It has been used for electrical contacts, in silver-rhenium alloy for self-lubricating bearing surfaces, and as bearings for analog instruments.3 The extremely high melting point of rhenium, and its unusual resistance to recrystallization, has led to its use in high-temperature environments as a constituent of the newer generation of “superalloys” for gas turbines (indeed this is said to be the largest market for rhenium).3 Because Re does not form carbides, another use is as an electroplated lining of Re applied to graphite vessels and furnaces, for use above 1,5Oo”C, so avoiding carbon contamination of the workload.5 Rhenium also has possible uses as a barrier layer 6
alar erties Rhenium has several properties that are less than “noble” in character - the electrodeposited metal tarnishes rapidly in humid conditions, it easily dissolves in oxidants, and oxidation of the metallic powder above 150°C (in this respect similar to osmium) indicate a surface rather less than inert. As mentioned above, rhenium runs the full gamut of valencies in its compounds, but Re VII is the most stable form: the Re III and IV compounds are quite stable, but Re V and VI are rare and tend to disproportionate. Re VII compounds include the yellow heptoxide Re207 obtained by heating Re in air or oxygen. It dissolves in water to form perrhenic acid, HRe04, which has a second form H,ReO, (cf. the periodic acid forms HIO, and H,IO,). The normal perrhenates exist in dilute solution, shifting Metal Finishing
:ble Group Via
Period 4 Period 5 Period 6
Cr MO W
GroupVIIa
Mn Te Re
Group VIII - transitional (Blue indicates precious metals group)
Fe Ru OS
to the tribasic form shown above in higher basicity solutions. This is associated with a color shift from colorless to yellow to red. The perrhenate ion, which is classed as a strong acid (along with periodic and perchloric acids) is quite stable and, despite a plausible analogy with permanganate and also chromate ions, perrhenates do not enter into redox reactions.7 This stability explains why perrhenate is the basis of electroplating solutions, though the need for 7 electrons per ion reduced is not an attractive aspect. It is puzzling that, although apparently without oxidizing power, potassium, sodium, and ammonium perrhenates are classified as an oxidizing hazards1 Perhaps our computer systems equate “per” with hazard regardless of chemical nomenclature. E~ect~o~yte~ for ~~e~~u~ D~~os~ti~n These were summarised briskly by Meyers as being based on potassium or ammonium perrhenate, usually with ammonium sulfate and sulphuric acid added, and the process parameters within very wide limits of : PH temperature current density
0.3 to 10 25-90°C 8 to 200 Aldm2 (see also ref. 8)
He found that ammonium sulfate increased conductivity and cathode efficiency, a low pH of 1 to 1.5 gave improved brightness, and adding magnesium sulfamate reduced stress thus permitting heavier deposits at higher current densities. The reported cathode efficiencies for all rhenium solutions is rather low, typically 5 to 15%, and Meyer suggests the very low hydrogen overvoltage is responsible: shifting to an alloy deposit (ReNi for instance) gives a remarkable increase to approximately 80% cathode efficiency. (This should not be confused with the relatively large amount of electricity needed to deposit Re even if 100% CE were attainable, due to the necessity of using a 7-valent compound. The formulation and parameters recommended by Meyer in this reference are :
June 2003
-co Rh Ir
NiPd Pt
Group Ib
_
Group IIb
Zn Cd
CU
Ag Au
Hg 10 g/L l-l.5 10 g/L 60430°C 30 g/L 10 to 15 A/dm2 25 g/L moderate
KFieO, PH H,SO, temperature (NH,SO,EMg current density (NH4),SO, agitation
Optima of 60°C and 10 A/dm2 are indicated, but unfortunately thicknesses obtained are not cited. In his paper, Meyer offered ideas as to routes from the credible ReOa anion to Re deposition: in an interesting discussion following, J.M. West presented partial polarization plots, derived from Meyer’s data, for Re electrodeposition and allied hydrogen evolution. From these he drew conclusions as to the Re species in solution from which the metal is deposited, suggesting that Meyer’s arguments might be erroneous. DePew and Larson9 used a proprietary process detailed below to obtain heavy Re deposits of 13 to 100 pm, for wear resistance tests. This deposit gave very nodular deposits, perhaps not surprisingly, since the cathode efficiency was only 2 to 3%, even with 10 g/L of Re present, and current densities of 39 to 77 A/dm2 used. This process (ACR “Rhenplate”) was described by CamplO as containing: Re NH,OH conc’d. PO, (no data)
The parameters PH temperature current density
It mix 5.7, urn
10 g/L 80 ml/L 200 g/L
were: 5.7 60°C 40 A/dm2 (although 31 Aldm2 is also given in the text
seems likely the base electrolyte was effectively a of ammonium phosphates buffered to pH around say 260 g/L of a 1 + 1 mix of mono- and diammoniphosphates, plus 14.3 g/L ammonium perrhen-
87
arguably, by using process of Camp.
Figure 1. Rhenium: CE vs pH and temperature
(from Ref. 8).
ate. Interestingly, DePew and Larson found that an addition of sodium perrhenate greatly reduced porosity, which suggests they had introduced a beneficial as well as additional cation. Camp refers to the motive for his work as being to avoid the darkening of rhenium deposits from various electrolytes, in a timespan from hours to months after plating. This he ascribed to oxide film; it did not seem to relate to process pH, for rhenium deposits from a citric bath at pH 1.5, and a citrate bath at pH 5.7, developed darkening at the same delay time of 4 to 7 months. Deposits from the weakly acidic phosphate bath described above remained fully bright for over a year. Greco,ll although mainly concerned with alloy deposits, refers to “a well-documented characteristic behavior of shiny as-plated rhenium in its instability in moisture (turning dark),” which is corrected by heating in hydrogen at 950 to 1,OOO”C for 15 to 60 minutes - or
Figure 2. Rhenium: CE vs CD, 60°C, pH 1 (from Ref. 8).
88
the weakly
acidic
phosphate
er In a very thorough doctoral thesis by Meyer,12 he gives a tabulated list of processes showing that few earlier workers went above 17 Afdm2, but those cases cited seem to be very high at 50,100, and 200 Aldm2.13 For his own work on rhenium deposition, Meyer used an electrolyte very similar to that given in his later publication4 but here using magnesium sulfate (not sulfamate); it was: The data he acquired on cathode efficiency vs. pH, current density, and temperature are illustrated in Figures 1, 2, and 3. While noting the great number of variables involved in measuring hydrogen overvoltage, his own results confirmed that on Re, as on Pt, H was readily discharged. KReO H2SO% (NH4),SO, MgSO,,7H,O Starting pH
10 g/L 10 g/L 25 g/L 25 g/L 1.0
In a later survey of hydrogen evolution in electrolysis, and in particular of results published by workers in this field, Bond and Kuhn et a1.14 give exchange current densities [- log (i, /A cm-2)1 for a wide range of metals, including: Re (5.1) Au (median 5.1) Pd (median 3.3) Pt (median 2.9)
Ru (4.2) Ir (4.2) OS (4.0) Rh (median 3.5)
Figure 3. Rhenium: CE vs temperature, Ref. 8).
pH 1, 10 A/dm2
(from
Metal Finishing
This suggests the hydrogen overvoltage on Re is significantly higher than on Pt, as against Meyer’s comments, and thus other causes for the very low cathode efficiency may exist. A direct comparison with Meyer’s own figures is difficult, but his observations surely relate more directly to “in bath” conditions. Throwing power is very low at low current densities, improving as current density is raised and ionic concentration lowered, which is to be expected in a very low cathode efficiency ambience where most of the current goes to release hydrogen. Interestingly, there is no reference to darkening of Re deposits, or its avoidance or causes, such as oxide or hydride films or other mechanisms.6>7 A review of effects of electrolyte components includes these observations: (NH,)SSO, increases the cathode efficiency at lower current density by the buffering effect of (NH,)+ ions reducing H+ ion activity and so increasing their discharge voltage. Mg++ ions reduce internal stress considerably, this effect is more pronounced at higher current densities. Other empirical trials showed that surface-active agents tend to improve cathode efficiency, presumably by increasing the hydrogen overvoltage; that chrome alum [C!r(SO,),~K,S0,~24H,Ol makes Re deposits very bright; that sulfamic acid reduces internal stress; that good deposits are obtainable from sulfate-free electrolytes based on pyrophosphate, or on ammonia - pH approximately 10 (this last item seems to have been overlooked since). Confirmation that a wide range of compounds may be useful is in a generalised formulation for a lowstress rhenium or alloy bath, containing Al, sulfonates, and perchlorates, in addition to those cited here.15 Alloy deposits are readily formed with Re and one or more of Ni, Co, In, Cr, Pd, Rh, Fe, Ag, Au, associated with a dramatic increase in cathode efficiency to 80% or more; it is equally valid to say due to an increase in H+ discharge potential. ~~~~~~ a.8 er Rhenium as a barrier to diffusion is not the first option one is likely to think of. Its very high melting point is an interesting start though, and Meyer presents some interesting results to illustrate rhenium has capabilities. Diffusion across interfaces and its control is an area of great importance in many fields yet remains less than fully understood. One example, prevention of interfacial diffusion of Au into Si wafers, is vital in the manufacture of semiconductor devices. Another is the use of Re or other refractory metals as a barrier to copper diffuJune 2003
sion during brazing of electrical contact assemblies. l6 Table II summarizes the results of heat treatment of samples at 450” C, which Meyer shows as microsections. With reference to #37 in Table II, Meyer comments that rhenium deposits tend to develop cracks at these higher temperatures, due to the differences in linear expansion rates; the fissures so formed allow Au to penetrate and diffuse freely. To illustrate this effect, he calculates that for the sample coupons used, the linear growth at 450°C is 240 pm for brass, 170 pm for Au, and 80 pm for Re - the mismatch is substantial. On the other hand, the usual purpose of heat treatment of this sort is to accelerate intermetallic diffusion and permit rapid evaluation of different systems. As practical application conditions will likely be far less stringent than 45O”C, Re may be acceptable as an effective barrier to diffusion having regard to service temperature and mechanical deformation.
An electrolyte based on fluoborate is described by Tsuru et all7 offering a notably higher CE than, or ammoniacal-citrate-based elece.g., sulfate trolytes. The components are NH,ReO,, HBF,, and NaBF4, to give: ReO; BF,-
0.00373 moles (i.e. 1 g/L NH,ReO,) 0.66 moles (i.e. 84 ml/L HBF,, 50% w/w)
The adjustment of pH requires, not surprisingly, NaOH and HBF,, and as in all other rhenium processes, this bath employs a perrhenate compound as the source of Re. The suggested optima are a current density of 10 A/dm2 (1,000 A/m2), pH value 1.3, and temperature of 60°C; a cathode efficiency of 14 to 16% is attainable, according to the graphical data provided (see Figs. 4-6). The cathode efficiency vs. current density is almost flat from < 1 A/dm2 to 10 A/dm2, which may relate to the high conductivity and mobility of fluoborates: however, the highly acidic nature of the fluoborate ion is unattractive for many purposes. In view of the very successful use of the less aggressive methane sulfonic acid for tin and tin-lead processes, it may be worth investigation as equally suitable for Re plating. 89 .-
Brass plus Figure # in Meyer
Time at 450” C (min)
Layer 1 thickness
Layer 2 thickness
(pm)
(pm)
23
Au
Nil
15
24
Au
5
25
Comments
Brass diffuses into Au layer
Au
25
30
Brass diffusesi
45
into Au layer Au
26
60
Brass diffusesi
55
into Au layer 27
Ni&Au
Nil
10
20
28
Ni&Au
15
10
38
Brass diffusing through nickel into Au
Ni&Au
29
30
55
10
Brass diffusing through nickel into Au
Ni&Au
30
60,
50
10
Brass diffusing through nickel into Au
Ni&Au
31
120
50
10
Inter-diffusion almost completec
Pd&Au
32
15
20
15
Pd is hardly visible; has dis solved in Au
33
Re&Au
Nil
10
20
34
Re&Au
15
10
20
No change
35
Re&Au
30
10
20
No change
36
Re&Au
60
10
18
No change
37
Re&Au
180
10
12-15 external to Re layer
18-25 urn inside Re layer
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information
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Finishing
40
50
1.3
1
60
pki value
temperature o C
Figure 4. Re fluoborate
solution. pH at 1.3 (from Ref. 16).
2
1.5
Figure 5. Re fluoborate
solution at 6OT (from 16).
the connector engineer’s is not widely known.
repertoire,
but it seemingly
There is a dearth of information about the physical properties of rhenium electrodeposits;
Greco (in ref. 7) says the hardness is 494 HV (electrolytically), as compared to 525 for chromium. Camp (see Table III taken from ref. 6) gives hardness values versus current density and includes wrought rhenium metal as well. As can readily be seen, higher current density appears to confer higher hardness, which thus may be linked with H evolution/adsorption. A single figure of about 800 VHN (kg/mm21 is given by Meyer for the wrought metal. The use of rhenium for electrical contacts is proposed by Camp (ref. 7) who cites indefinite wear on slip rings and switching contacts on motor commutators; it certainly appears worth investigation as a low-cost alternative to the traditional “hard” contact metals, Pt and Rh. One might think Re should be a useful addition to
June 2003
In an interesting and detailed account of his work on rhenium alloys with the iron group metals, Greco (ref. 7) used additions of KReO, to a variety of orthodox nickel baths, including sulfamate, and cobalt analogs of two of them. For iron, he used the following bath formulation: FeS04,7H,0 Citric Acid KRe04 Re content
59.4 g/L (Fe++= 12 g/L) 6.6 g/L 10 g/L (Re7+ = 6.4 &) 35% of total metal ions
Replenishment with the more soluble NH,Re04 was found preferable for all baths. The rhenium content as proportion of total metal in the bath varied from 0.84 to 70%, for Ni-Re, and 0.84 to 60.5% for Co-Re; deposit analyses showed a linear increase in Re/Co as Re concentration increased, and a linear reduction as cathode current density was increased 91
Figure 8. Deposit composition
Figure 6. Re fluoborate
and other solutions (from Ref. 16).
vs cathode CD (from Ref. IO).
if heat treated, e.g., 400” C for 1 hour. Dispersion hardening with submicron alumina and heat treatment was studied as well (Figs. 9 & 10). A silver-rhenium alloy plating process was developed by Turnslslg to give a nominal composition of 97% Ag, 3% Re, for use as a high-temperature and extreme load-bearing solid film lubricant. The formulation was:
(Fig. 7). In the Ni-Re system, Re content decreased sharply, perhaps exponentially, as CD increased (Fig. 8; note that x axis is nonlinear). Deposits of Ni-Re were not considered satisfactory, being rough, but the cobalt and especially iron-rhenium deposits were sound, smooth, and thick; the colour becoming more silvery as Re content increased. Microsections of Ni-Re deposits showed structure varying with CD, type of bath, and Re content. In contrast, Co-Re and Fe-Re give reproducible results regardless of bath type or parameters. The deposit growth habits of Ni-Re, and especially Co-Re, show some curious combinations of laminar and conical structures; Greco speculates that adhering gas bubbles might be one initiating cause. However, the writer has found this structure to be typical of Ni-Co deposits plated with high solution flow rate and adequate wetting agent present, so other causes may fit better. For Co-Re, the deposit hardness (Knoop) falls in range 450-750, with increases up to 1,100 available
Turns established that the most effective control method was to plate at an applied voltage of 1.32 to 1.4, relative to the saturated calomel electrode (SCE). It is evident from photomicrographs in Turns’ paper, that the very porous electrodeposit performed two functions - high retention of the MoS, lubricant grease applied, and following compaction under load, the Ag-Re film continued to give excellent lubrication. A major application for this finish was on aircraft wing pivot pins, clearly a demanding and critical function. Other suc-
Figure 7. Re in Co-Re alloy vs Re concentration
Figure 9. Co-Re hardness vs heat treatment
92
(from Ref. IO).
NaRe04 KLW3 KCN AgCN
1 g/L 15 g/L 75 g/L 5 gfIJ
(from Ref IO).
Metal Finishing
Wrought Rhenium: Mechanical work 10%
20% 30% Electroplated
Rhenium
At 13.5 Afdm2 At 40 A/dm2
tions, with Re content in the alloy deposited remaining constant with further increases of Re(VI1) in solution. In this case the partial efficiency for Re peaks at about 85% Re in the alloy, i.e., an atomic ratio of 1:1.7 for Ni:Re - compare the 1:l ratio for ammoniacal citrate. The authors suggest that a stable intermetallic compound Ni3Re4 is present at the electrode surface and propose surface complexes of nickel-rhenium acetate as explaining their results.
At 54 A/dm2 cessful applications included elimination of fretting corrosion and contacts for rotating switches with very long lifetimes. Rhenium-nickel alloys are the subject of two papers by Fukushima and Akiyama et al. In the first one”” the behavior of both acid sulfate (pH 1.75) and ammoniacal citrate (pH 8) based electrolytes was examined. In both cases the CE for Re alone was extremely low, as was that for Ni alone in the acid electrolyte - not so surprising at pH 1.75 and Ni concentration of 1.5 g/L - although in the citrate solution, at pH 8 and Ni 6 g/L, the CE was 60 to 85%. More interesting by far was the GE of approximately 80% in both solutions when Ni and Re were present, with the partial current efficiency of Re peaking at an alloy composition of about 75 wt. % Re (which is 50 at. %) in the alloy deposited. Synergy of some sort was occurring, and the second paper21 reported on, and discussed at length, polarization curves in the citrate solution. The authors suggested that “in the potential region more noble than -O.SV, adsorbed perrhenate ions on the cathode serve as an ion bridge . .. and electrocatalytically enhance Ni deposition.” Where the potential is less noble than -0.8V the direct discharge of Ni ions as well as electrocatalytic reduction increases the rate of Ni deposition: however, the mechanisms which regulate Re deposition remain obscure. Berezina et al., in a more recent paper,22 reported work on Ni-Re alloys from nickel acetate based electrolytes, extending their work on the co-deposition of nickel and rhenium.23 Using O.lM nickel acetate plus O.OlM sodium perrhenate, the cathode efficiencies of about 40 to 50% obtained at pH 4 (higher CD; lower CE) fell to about 10% when the pH was reduced with acetic acid to 2.8 due to an increase in H+ ion reduction competing with alloy deposition. Curiously, when the pH is raised again using NaOH, the CE falls further, perhaps due to common ion effects from acetate and nickel. The cathode efficiency reaches a maximum when Ni and Re are at approximately equal concentra-
June 2003
eresting patents for electroplated rhenium alloys are: A rhodium-rhenium alloy electrolyte for bright “flash” plating, with Re at 0.1 to 2 g/L as perrhenate and Rh at 1 to 2 g/L typically, in a sulfate or sulfamate or phosphate type base electrolyte.24 Deposition of a nickel-rhenium alloy with >15% Re, which will remain nonmagnetic during prolonged service. Proposed use is to repair steam tubes in pressurized water reactor.25 Electroplated Re-Ni or Re-Co deposit on nickel or iron contact substrate, for long lifetime and low contact resistance. Re content range may be 5 to 70% wt.26 For producing magnetic recording media, two electrolytic process patents for Re-containing alloys are: (a) vertical magnetic recording, using Co-Re bath for tape or disk 27 and (b) use of a Co-Mn-Ni-Re electrolyte for rigid or floppy disks, with, e.g., Hc value of 1,050 Oe (Oersteds).28
The only significant mention of electroless rhenium is a Russian patent2g of 1974, using NH,ReO, and formaldehyde sulfoxylate. Almost all the interest has been in electroless rhenium alloys, mostly quinquenary for magnetic recording, or ternary for electrochemical research, enhanced corrosion and mechanical properties. si Pearlstein and Weightman30 examined the magnetic properties of electroless cobalt deposits with various alloy metal additions. They described the result of adding Re (as KRe04) as giving a high ratio of coercivity to remanence. The rhenium concentration was 0.52 g/L plus 7.43 g/L of cobalt. The same authors later3i reported on the effects of adding various metal salts to electroless nickel baths. They concluded that acidic electroless Ni baths were not very promising but by adding KRe04 obtained up to 46% Re in Ni-Re alloy deposits from
93
Pulse plating can yield surprisingly high cathode efficiencies and much improved deposit or distribution properties, and can also be most disappointing should the processes and parameters chosen be unsuitable. So wide is this field that only a paraphrase of one significant paper can be given here. With so low a cathode efficiency and questionable deposit properties in many cases, rhenium plating processes are evidently, perhaps desperately, in need of the improvements that pulse plating can offer! Crack formation in Re deposits was addressed by Hosokawa et a1.,36 (more data and discussion are given in Ref. 37) using a simple bath of: Figure 10. Microhardness vs temperature Co-Re plus alumina (from Ref. IO).
of heat treatment
for
a citrate-ammonia bath at a pH of 9. This very high rhenium content is noteworthy since it resulted from a rhenium concentration of only 0.99 g/L plus 7.8 g/L of nickel in the bath. The 46% Re alloy had a very high melting point of about 1,700”C. No comment was made as to the mechanical properties of these materials.
eltic Work on the production of magnetic layers with vertical anisotropy from electroless baths indicates that rhenium is virtually essential as an alloy component for this preferred type of high-density recording medium, which has a typical composition of CoNiReMnP. See Takano & Matsuda,32 Takano et a1.,33 and Homma & Osaka.35
The type Ni-Me-P(B), obtained by the electroless route, are mentioned by Vaskelis, who quotes NiRe50%-P, and Ni-Re20%-B as the generally accepted maximum attainable Re levels.35
Figure 11. Pulse length vs CE (from Refs. 25 and 26).
94
ReO, H2S04
Operating
parameters
15 g/L 5mlL 20°C and pH 0.8 (for the effect of variations from these values, see below)
This solution operates with up to 10% cathode efficiency using direct current (DC) at 10 A/dm2, and pulse plating trials were conducted at the same average CD (i.e., averaged over the whole cycle time), using square pulses, the pulse time decreasing from 104 pses to 1 psec, and the pulse current density (PC-CD) correspondingly increasing from 1 to 200 A/cm2 (ASD figures are rather unwieldy, being 100 to 20,000 A/dm2). The cathode efficiency peaked a little above 50% (5 x the dc value), for 200 A/cm2, with very sharp maxima for all the curves plotted (Fig. 11). Cracking in the deposits was studied, for 2 pm thick deposits only (this being the objective set), after heat treatment in vacuum at 1,OOO”C. The DC deposits were substantially cracked, and the cracking diminished as the PC-CD increased, a crack-free deposit being obtained at 100Alcm2. An interesting variation in appearance from steel gray to black to bright silver was obtainable by using pulse times of 1 to 10 psec, and 7 to 100 A/cm2, in general corresponding to conditions on the left of the maxima in Figure 11. At the optimum for fully bright Re (100 psec and 100 A/cm2), deposits of >10 pm thickness were obtainable but this aspect was not pursued. Those deposits plated at long “on” times (and DC deposits), became black in a few weeks after plating, if kept in room atmosphere. Conversely, deposits plated at the optima indicated - 100A/cm2, T of 100 psec, mean CD of 10 A/dm2, giving CE of 12% - stayed fully bright for three months without heat treatment.
Metal Finishing
Mason Corporation
Effects of other variables were: pH value - the pH vs. CE response was flat from pH 0.5 to 1.0, above 1.1 only grey deposits, and above pH 2 the efficiency was negligible. Temperature - a linear response to increasing temperature, as shown in Table IV It can be seen that quite narrow parameters apply for specific desired properties, and it may not be possible to get all of them in one package. This is typical of pulse plating, despite its many virtues.
This survey of rhenium plating, limited to some extent though it is, shows there is a good deal of data available on processes, and on alloy plating in terms of deposit composition. There is not much information on the properties and appropriate uses of the resulting deposit, with the exception of the complex alloys used as magnetic recording media. Rhenium deposits are hard and wear resistant (perhaps due to its forming an oxide coating) yet retain good electrical performance, but they need to be heat treated in vacuum or hydrogen, unless from just the right solution or are plated with just the right pulse settings. If, however, Pd, Pt, and Rh prices continue to be so volatile as a result of automotive, environmental, and political demands and pressures, rhenium may well be worth trying. Indeed, as we may be forced into it despite the reservations noted above, some pre-emptive action really should be in hand by now!
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
13.
Sigma-Aldrich catalogue, UK edition; 2000-2001 U.S. Patent 6,162,552; 2000 Britannica.com; 2001 see, e.g., Rhenium Alloys, Inc.; www.rhenium.com Japan Patent 70,006,150-B; 1970 U.S. Patent 5,817,371; 1998 Mackay & Mackay, “Modern Inorganic Chemistry”, 4th edition; Blackie/Prentice & Hall: 1989. (For N. America the ISBN is O-13-488487-6) Meyer, A.R., Trans. I.M.F., 46:209-212; 1968 DePew, JR. and TL. Larson, Plating, May, pp475-478; 1970 Camp E.K., Plating, May 1965, ~~413-416 Greco VP, Plating, Feb 1972, ~~115-125 Meyer, A.R., PhD thesis, University of Geneva (in French); also, in French, in Oberflaeche-Surface, 11, 1970; Heft (issue) 3, pp 84-86; 4, pp 117-122; 6, pp 208-213; 7, pp 231-232; 8, pp 248-251 Gvozdeva, I.I., Elekhomet. Tsuetnykt Metal, 188, 212, (1957), & Sklyarenko S I et al, Akad. Nauk.
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June 2003
card 95
SSSR,Inst. Met. 100; 1958 Bond, G.C. et al., J. Electroanal. Chem. 34:1-2: 1972 U.S. Patent 3,668,083-A; 1970 U.S. Patent 5330,088; 1994 Tsuru, T., et al., J; Surface Finishing Sot. Japan; 471883-4; 1996 (in Japanese) 18. Turns, E.W., Plating, 2:127-131; 1971 19. Turns, E.W., et. al, U.S. Patent 3,342,708; 1967 20. Fukushima, H., et al., Metal Finishing, 83(3):37-41; 1985 21. Fukushima, H., et al.,Meti Finishing, 84(12):15-Q 1986 22. Berezina, S.I., et al., Metallou, 29:106-110; 1993 23. Berezina, S.I., et al., Metallou, 28:282; 1992 24. U.S. Patent 3,890,210 -A; 1975 25.European Patent 913,501-A; 1999 26. Japan Patents 52,052,106-A, 84,042,066-B; 1977 27. Japan Patent 62001122-A, 1987 28. Japan Patent 63111195-A, US Patent 4,778,573-A, 1988 29. Russian Patent SU396438-A; 1974 30. Pearl&n, E & RF Weightman, Plating, 6714716; 1967 31. Pearlstein, F. & R.F. Weightman Electrochemical Technology, Nov.-Dec.:42’7-430; 1968 22. Takano, 0, & H. Matsuda, Metal Finishing, 84(6):6064; 1986 33. !I’akano, 0, et al., Japan Met. Fin. Sot., 85(9):440-44,1984 34. Homma, T. & T.J. Osaka, Electrochem. Society, 139(10):2925-2929; 1992 35. Vaskelis, A, Galvanotechnik, 87(2); 1996 36. Hosokawa, et al., Plating & Surface Finishing, 10:5255; 1980 37. Hosokawa K. et al., Proceedings of INTERFINISH 80, pp 58-62 MF 14. 15. 16. 17.
Terry Jones’first contact with metal finishing was in 1954 under Tony Such at Wilmot Breeden in Birmongham, England. He held a number of jobs in works laboratories, a barrel-plating facility, and jobbing shops. From 1987 until retiring in 1998 he was a service chemist at Englehard Industries. Since then he has done consulting work and some part-time work in R&D at AT Poeton in Gloucester. He authored the chapter “Plating Finishes”in Handbook of Electronic Connectors, co-authored “Formation of Cobalt Cyanide Complexes in Hard Acid Gold Baths,”which was presented at Interfmish 1996, and has written two articles for Metal Finishing.
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[email protected]
henium offers more than a trivial confusion of its chemical symbol with rhodium (rhodium is Rh, whereas rhenium is Re). As some of the more interesting uses of rhenium are in electroless alloy deposits, the title reflects the more general aspect of deposition, not necessarily electrolytic alone. Rhenium should be interesting for wear resistance and electrical/electronic contact purposes, but seems to have lost out in the glamour stakes. Since it may cost considerably less than gold, but has not so far attracted the jewelry trade’s interest, presumably it is likely to remain an interesting oddity to the world at large, and no more. Rhenium is generally described as a silvery metal, extremely hard, which is not attacked at all by hydrochloric acid. As few metallic materials are so completely resistant in this way, rhenium may have some practical applications for this property alone, for the platinum group metals are slightly susceptible to this acid. Rhenium is reasonably resistant to other chemicals but is rapidly attacked in oxidizing media such as HNO,, HClO,, H,O,, and also fused alkalis. Although not considered a member of the platinum group, it is certainly a rare metal and this is reflected in its cost - currently in the region of $3 to $10 per gram of metal, $15 to $30 per gram as potassium or ammonium perrhenate.l Compared with gold, rhenium is likely to be one third the cost. Chemical compounds may show valencies from -1 to +7, but the most stable and important valency is +7. (A valency of -1 is surely unique.) The orbital structure is lXe1 4f14 5d5 6s2. An extract from the periodic table, shown in Table I, illustrates the relationship of Re and the adjacent elements. Since properties tend to run within periods as well as groups, we find that although elemental Re resembles osmium and tungsten, the chemistries of the compounds have more group resemblances. Thus, Re compounds are similar to those of technetium, and to a lesser extent those of manganese, within Group VII a. Setting Tc aside (a fission product of uranium from nuclear reactors, it is used only in medicine as a radioactive imaging isotope), we find that analogies between Mn and Re compounds are closer than those of OS and Re. All useful plating electrolytes are based on the perrhenate ion ReO, as discussed later. The physical properties of Re include: the most
86
refractive metal after tungsten, with a melting point of 3,lBO“C; most dense after osmium and tungsten, with density = 21.2, and a rather high electrical resistivity of about four times that of tungsten at 18.7 micro ohm/cm (20°C). The linear coefficient of expansion is 6.6 x 10e6/“K. It has interactions with the ferromagnetic metals when codeposited with one or more of them, affecting coercivity and remanence very markedly. Nominally, rhenium has had only minority uses; e.g., alloyed with tungsten in electric lamp filaments for improved ductility and to reduce reactivity of tungsten heating elements.2 As a catalyst, it is of some importance in catalytic reforming of aromatic hydrocarbons. It has been used for electrical contacts, in silver-rhenium alloy for self-lubricating bearing surfaces, and as bearings for analog instruments.3 The extremely high melting point of rhenium, and its unusual resistance to recrystallization, has led to its use in high-temperature environments as a constituent of the newer generation of “superalloys” for gas turbines (indeed this is said to be the largest market for rhenium).3 Because Re does not form carbides, another use is as an electroplated lining of Re applied to graphite vessels and furnaces, for use above 1,5Oo”C, so avoiding carbon contamination of the workload.5 Rhenium also has possible uses as a barrier layer 6
alar erties Rhenium has several properties that are less than “noble” in character - the electrodeposited metal tarnishes rapidly in humid conditions, it easily dissolves in oxidants, and oxidation of the metallic powder above 150°C (in this respect similar to osmium) indicate a surface rather less than inert. As mentioned above, rhenium runs the full gamut of valencies in its compounds, but Re VII is the most stable form: the Re III and IV compounds are quite stable, but Re V and VI are rare and tend to disproportionate. Re VII compounds include the yellow heptoxide Re207 obtained by heating Re in air or oxygen. It dissolves in water to form perrhenic acid, HRe04, which has a second form H,ReO, (cf. the periodic acid forms HIO, and H,IO,). The normal perrhenates exist in dilute solution, shifting Metal Finishing
:ble Group Via
Period 4 Period 5 Period 6
Cr MO W
GroupVIIa
Mn Te Re
Group VIII - transitional (Blue indicates precious metals group)
Fe Ru OS
to the tribasic form shown above in higher basicity solutions. This is associated with a color shift from colorless to yellow to red. The perrhenate ion, which is classed as a strong acid (along with periodic and perchloric acids) is quite stable and, despite a plausible analogy with permanganate and also chromate ions, perrhenates do not enter into redox reactions.7 This stability explains why perrhenate is the basis of electroplating solutions, though the need for 7 electrons per ion reduced is not an attractive aspect. It is puzzling that, although apparently without oxidizing power, potassium, sodium, and ammonium perrhenates are classified as an oxidizing hazards1 Perhaps our computer systems equate “per” with hazard regardless of chemical nomenclature. E~ect~o~yte~ for ~~e~~u~ D~~os~ti~n These were summarised briskly by Meyers as being based on potassium or ammonium perrhenate, usually with ammonium sulfate and sulphuric acid added, and the process parameters within very wide limits of : PH temperature current density
0.3 to 10 25-90°C 8 to 200 Aldm2 (see also ref. 8)
He found that ammonium sulfate increased conductivity and cathode efficiency, a low pH of 1 to 1.5 gave improved brightness, and adding magnesium sulfamate reduced stress thus permitting heavier deposits at higher current densities. The reported cathode efficiencies for all rhenium solutions is rather low, typically 5 to 15%, and Meyer suggests the very low hydrogen overvoltage is responsible: shifting to an alloy deposit (ReNi for instance) gives a remarkable increase to approximately 80% cathode efficiency. (This should not be confused with the relatively large amount of electricity needed to deposit Re even if 100% CE were attainable, due to the necessity of using a 7-valent compound. The formulation and parameters recommended by Meyer in this reference are :
June 2003
-co Rh Ir
NiPd Pt
Group Ib
_
Group IIb
Zn Cd
CU
Ag Au
Hg 10 g/L l-l.5 10 g/L 60430°C 30 g/L 10 to 15 A/dm2 25 g/L moderate
KFieO, PH H,SO, temperature (NH,SO,EMg current density (NH4),SO, agitation
Optima of 60°C and 10 A/dm2 are indicated, but unfortunately thicknesses obtained are not cited. In his paper, Meyer offered ideas as to routes from the credible ReOa anion to Re deposition: in an interesting discussion following, J.M. West presented partial polarization plots, derived from Meyer’s data, for Re electrodeposition and allied hydrogen evolution. From these he drew conclusions as to the Re species in solution from which the metal is deposited, suggesting that Meyer’s arguments might be erroneous. DePew and Larson9 used a proprietary process detailed below to obtain heavy Re deposits of 13 to 100 pm, for wear resistance tests. This deposit gave very nodular deposits, perhaps not surprisingly, since the cathode efficiency was only 2 to 3%, even with 10 g/L of Re present, and current densities of 39 to 77 A/dm2 used. This process (ACR “Rhenplate”) was described by CamplO as containing: Re NH,OH conc’d. PO, (no data)
The parameters PH temperature current density
It mix 5.7, urn
10 g/L 80 ml/L 200 g/L
were: 5.7 60°C 40 A/dm2 (although 31 Aldm2 is also given in the text
seems likely the base electrolyte was effectively a of ammonium phosphates buffered to pH around say 260 g/L of a 1 + 1 mix of mono- and diammoniphosphates, plus 14.3 g/L ammonium perrhen-
87
arguably, by using process of Camp.
Figure 1. Rhenium: CE vs pH and temperature
(from Ref. 8).
ate. Interestingly, DePew and Larson found that an addition of sodium perrhenate greatly reduced porosity, which suggests they had introduced a beneficial as well as additional cation. Camp refers to the motive for his work as being to avoid the darkening of rhenium deposits from various electrolytes, in a timespan from hours to months after plating. This he ascribed to oxide film; it did not seem to relate to process pH, for rhenium deposits from a citric bath at pH 1.5, and a citrate bath at pH 5.7, developed darkening at the same delay time of 4 to 7 months. Deposits from the weakly acidic phosphate bath described above remained fully bright for over a year. Greco,ll although mainly concerned with alloy deposits, refers to “a well-documented characteristic behavior of shiny as-plated rhenium in its instability in moisture (turning dark),” which is corrected by heating in hydrogen at 950 to 1,OOO”C for 15 to 60 minutes - or
Figure 2. Rhenium: CE vs CD, 60°C, pH 1 (from Ref. 8).
88
the weakly
acidic
phosphate
er In a very thorough doctoral thesis by Meyer,12 he gives a tabulated list of processes showing that few earlier workers went above 17 Afdm2, but those cases cited seem to be very high at 50,100, and 200 Aldm2.13 For his own work on rhenium deposition, Meyer used an electrolyte very similar to that given in his later publication4 but here using magnesium sulfate (not sulfamate); it was: The data he acquired on cathode efficiency vs. pH, current density, and temperature are illustrated in Figures 1, 2, and 3. While noting the great number of variables involved in measuring hydrogen overvoltage, his own results confirmed that on Re, as on Pt, H was readily discharged. KReO H2SO% (NH4),SO, MgSO,,7H,O Starting pH
10 g/L 10 g/L 25 g/L 25 g/L 1.0
In a later survey of hydrogen evolution in electrolysis, and in particular of results published by workers in this field, Bond and Kuhn et a1.14 give exchange current densities [- log (i, /A cm-2)1 for a wide range of metals, including: Re (5.1) Au (median 5.1) Pd (median 3.3) Pt (median 2.9)
Ru (4.2) Ir (4.2) OS (4.0) Rh (median 3.5)
Figure 3. Rhenium: CE vs temperature, Ref. 8).
pH 1, 10 A/dm2
(from
Metal Finishing
This suggests the hydrogen overvoltage on Re is significantly higher than on Pt, as against Meyer’s comments, and thus other causes for the very low cathode efficiency may exist. A direct comparison with Meyer’s own figures is difficult, but his observations surely relate more directly to “in bath” conditions. Throwing power is very low at low current densities, improving as current density is raised and ionic concentration lowered, which is to be expected in a very low cathode efficiency ambience where most of the current goes to release hydrogen. Interestingly, there is no reference to darkening of Re deposits, or its avoidance or causes, such as oxide or hydride films or other mechanisms.6>7 A review of effects of electrolyte components includes these observations: (NH,)SSO, increases the cathode efficiency at lower current density by the buffering effect of (NH,)+ ions reducing H+ ion activity and so increasing their discharge voltage. Mg++ ions reduce internal stress considerably, this effect is more pronounced at higher current densities. Other empirical trials showed that surface-active agents tend to improve cathode efficiency, presumably by increasing the hydrogen overvoltage; that chrome alum [C!r(SO,),~K,S0,~24H,Ol makes Re deposits very bright; that sulfamic acid reduces internal stress; that good deposits are obtainable from sulfate-free electrolytes based on pyrophosphate, or on ammonia - pH approximately 10 (this last item seems to have been overlooked since). Confirmation that a wide range of compounds may be useful is in a generalised formulation for a lowstress rhenium or alloy bath, containing Al, sulfonates, and perchlorates, in addition to those cited here.15 Alloy deposits are readily formed with Re and one or more of Ni, Co, In, Cr, Pd, Rh, Fe, Ag, Au, associated with a dramatic increase in cathode efficiency to 80% or more; it is equally valid to say due to an increase in H+ discharge potential. ~~~~~~ a.8 er Rhenium as a barrier to diffusion is not the first option one is likely to think of. Its very high melting point is an interesting start though, and Meyer presents some interesting results to illustrate rhenium has capabilities. Diffusion across interfaces and its control is an area of great importance in many fields yet remains less than fully understood. One example, prevention of interfacial diffusion of Au into Si wafers, is vital in the manufacture of semiconductor devices. Another is the use of Re or other refractory metals as a barrier to copper diffuJune 2003
sion during brazing of electrical contact assemblies. l6 Table II summarizes the results of heat treatment of samples at 450” C, which Meyer shows as microsections. With reference to #37 in Table II, Meyer comments that rhenium deposits tend to develop cracks at these higher temperatures, due to the differences in linear expansion rates; the fissures so formed allow Au to penetrate and diffuse freely. To illustrate this effect, he calculates that for the sample coupons used, the linear growth at 450°C is 240 pm for brass, 170 pm for Au, and 80 pm for Re - the mismatch is substantial. On the other hand, the usual purpose of heat treatment of this sort is to accelerate intermetallic diffusion and permit rapid evaluation of different systems. As practical application conditions will likely be far less stringent than 45O”C, Re may be acceptable as an effective barrier to diffusion having regard to service temperature and mechanical deformation.
An electrolyte based on fluoborate is described by Tsuru et all7 offering a notably higher CE than, or ammoniacal-citrate-based elece.g., sulfate trolytes. The components are NH,ReO,, HBF,, and NaBF4, to give: ReO; BF,-
0.00373 moles (i.e. 1 g/L NH,ReO,) 0.66 moles (i.e. 84 ml/L HBF,, 50% w/w)
The adjustment of pH requires, not surprisingly, NaOH and HBF,, and as in all other rhenium processes, this bath employs a perrhenate compound as the source of Re. The suggested optima are a current density of 10 A/dm2 (1,000 A/m2), pH value 1.3, and temperature of 60°C; a cathode efficiency of 14 to 16% is attainable, according to the graphical data provided (see Figs. 4-6). The cathode efficiency vs. current density is almost flat from < 1 A/dm2 to 10 A/dm2, which may relate to the high conductivity and mobility of fluoborates: however, the highly acidic nature of the fluoborate ion is unattractive for many purposes. In view of the very successful use of the less aggressive methane sulfonic acid for tin and tin-lead processes, it may be worth investigation as equally suitable for Re plating. 89 .-
Brass plus Figure # in Meyer
Time at 450” C (min)
Layer 1 thickness
Layer 2 thickness
(pm)
(pm)
23
Au
Nil
15
24
Au
5
25
Comments
Brass diffuses into Au layer
Au
25
30
Brass diffusesi
45
into Au layer Au
26
60
Brass diffusesi
55
into Au layer 27
Ni&Au
Nil
10
20
28
Ni&Au
15
10
38
Brass diffusing through nickel into Au
Ni&Au
29
30
55
10
Brass diffusing through nickel into Au
Ni&Au
30
60,
50
10
Brass diffusing through nickel into Au
Ni&Au
31
120
50
10
Inter-diffusion almost completec
Pd&Au
32
15
20
15
Pd is hardly visible; has dis solved in Au
33
Re&Au
Nil
10
20
34
Re&Au
15
10
20
No change
35
Re&Au
30
10
20
No change
36
Re&Au
60
10
18
No change
37
Re&Au
180
10
12-15 external to Re layer
18-25 urn inside Re layer
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Finishing
40
50
1.3
1
60
pki value
temperature o C
Figure 4. Re fluoborate
solution. pH at 1.3 (from Ref. 16).
2
1.5
Figure 5. Re fluoborate
solution at 6OT (from 16).
the connector engineer’s is not widely known.
repertoire,
but it seemingly
There is a dearth of information about the physical properties of rhenium electrodeposits;
Greco (in ref. 7) says the hardness is 494 HV (electrolytically), as compared to 525 for chromium. Camp (see Table III taken from ref. 6) gives hardness values versus current density and includes wrought rhenium metal as well. As can readily be seen, higher current density appears to confer higher hardness, which thus may be linked with H evolution/adsorption. A single figure of about 800 VHN (kg/mm21 is given by Meyer for the wrought metal. The use of rhenium for electrical contacts is proposed by Camp (ref. 7) who cites indefinite wear on slip rings and switching contacts on motor commutators; it certainly appears worth investigation as a low-cost alternative to the traditional “hard” contact metals, Pt and Rh. One might think Re should be a useful addition to
June 2003
In an interesting and detailed account of his work on rhenium alloys with the iron group metals, Greco (ref. 7) used additions of KReO, to a variety of orthodox nickel baths, including sulfamate, and cobalt analogs of two of them. For iron, he used the following bath formulation: FeS04,7H,0 Citric Acid KRe04 Re content
59.4 g/L (Fe++= 12 g/L) 6.6 g/L 10 g/L (Re7+ = 6.4 &) 35% of total metal ions
Replenishment with the more soluble NH,Re04 was found preferable for all baths. The rhenium content as proportion of total metal in the bath varied from 0.84 to 70%, for Ni-Re, and 0.84 to 60.5% for Co-Re; deposit analyses showed a linear increase in Re/Co as Re concentration increased, and a linear reduction as cathode current density was increased 91
Figure 8. Deposit composition
Figure 6. Re fluoborate
and other solutions (from Ref. 16).
vs cathode CD (from Ref. IO).
if heat treated, e.g., 400” C for 1 hour. Dispersion hardening with submicron alumina and heat treatment was studied as well (Figs. 9 & 10). A silver-rhenium alloy plating process was developed by Turnslslg to give a nominal composition of 97% Ag, 3% Re, for use as a high-temperature and extreme load-bearing solid film lubricant. The formulation was:
(Fig. 7). In the Ni-Re system, Re content decreased sharply, perhaps exponentially, as CD increased (Fig. 8; note that x axis is nonlinear). Deposits of Ni-Re were not considered satisfactory, being rough, but the cobalt and especially iron-rhenium deposits were sound, smooth, and thick; the colour becoming more silvery as Re content increased. Microsections of Ni-Re deposits showed structure varying with CD, type of bath, and Re content. In contrast, Co-Re and Fe-Re give reproducible results regardless of bath type or parameters. The deposit growth habits of Ni-Re, and especially Co-Re, show some curious combinations of laminar and conical structures; Greco speculates that adhering gas bubbles might be one initiating cause. However, the writer has found this structure to be typical of Ni-Co deposits plated with high solution flow rate and adequate wetting agent present, so other causes may fit better. For Co-Re, the deposit hardness (Knoop) falls in range 450-750, with increases up to 1,100 available
Turns established that the most effective control method was to plate at an applied voltage of 1.32 to 1.4, relative to the saturated calomel electrode (SCE). It is evident from photomicrographs in Turns’ paper, that the very porous electrodeposit performed two functions - high retention of the MoS, lubricant grease applied, and following compaction under load, the Ag-Re film continued to give excellent lubrication. A major application for this finish was on aircraft wing pivot pins, clearly a demanding and critical function. Other suc-
Figure 7. Re in Co-Re alloy vs Re concentration
Figure 9. Co-Re hardness vs heat treatment
92
(from Ref. IO).
NaRe04 KLW3 KCN AgCN
1 g/L 15 g/L 75 g/L 5 gfIJ
(from Ref IO).
Metal Finishing
Wrought Rhenium: Mechanical work 10%
20% 30% Electroplated
Rhenium
At 13.5 Afdm2 At 40 A/dm2
tions, with Re content in the alloy deposited remaining constant with further increases of Re(VI1) in solution. In this case the partial efficiency for Re peaks at about 85% Re in the alloy, i.e., an atomic ratio of 1:1.7 for Ni:Re - compare the 1:l ratio for ammoniacal citrate. The authors suggest that a stable intermetallic compound Ni3Re4 is present at the electrode surface and propose surface complexes of nickel-rhenium acetate as explaining their results.
At 54 A/dm2 cessful applications included elimination of fretting corrosion and contacts for rotating switches with very long lifetimes. Rhenium-nickel alloys are the subject of two papers by Fukushima and Akiyama et al. In the first one”” the behavior of both acid sulfate (pH 1.75) and ammoniacal citrate (pH 8) based electrolytes was examined. In both cases the CE for Re alone was extremely low, as was that for Ni alone in the acid electrolyte - not so surprising at pH 1.75 and Ni concentration of 1.5 g/L - although in the citrate solution, at pH 8 and Ni 6 g/L, the CE was 60 to 85%. More interesting by far was the GE of approximately 80% in both solutions when Ni and Re were present, with the partial current efficiency of Re peaking at an alloy composition of about 75 wt. % Re (which is 50 at. %) in the alloy deposited. Synergy of some sort was occurring, and the second paper21 reported on, and discussed at length, polarization curves in the citrate solution. The authors suggested that “in the potential region more noble than -O.SV, adsorbed perrhenate ions on the cathode serve as an ion bridge . .. and electrocatalytically enhance Ni deposition.” Where the potential is less noble than -0.8V the direct discharge of Ni ions as well as electrocatalytic reduction increases the rate of Ni deposition: however, the mechanisms which regulate Re deposition remain obscure. Berezina et al., in a more recent paper,22 reported work on Ni-Re alloys from nickel acetate based electrolytes, extending their work on the co-deposition of nickel and rhenium.23 Using O.lM nickel acetate plus O.OlM sodium perrhenate, the cathode efficiencies of about 40 to 50% obtained at pH 4 (higher CD; lower CE) fell to about 10% when the pH was reduced with acetic acid to 2.8 due to an increase in H+ ion reduction competing with alloy deposition. Curiously, when the pH is raised again using NaOH, the CE falls further, perhaps due to common ion effects from acetate and nickel. The cathode efficiency reaches a maximum when Ni and Re are at approximately equal concentra-
June 2003
eresting patents for electroplated rhenium alloys are: A rhodium-rhenium alloy electrolyte for bright “flash” plating, with Re at 0.1 to 2 g/L as perrhenate and Rh at 1 to 2 g/L typically, in a sulfate or sulfamate or phosphate type base electrolyte.24 Deposition of a nickel-rhenium alloy with >15% Re, which will remain nonmagnetic during prolonged service. Proposed use is to repair steam tubes in pressurized water reactor.25 Electroplated Re-Ni or Re-Co deposit on nickel or iron contact substrate, for long lifetime and low contact resistance. Re content range may be 5 to 70% wt.26 For producing magnetic recording media, two electrolytic process patents for Re-containing alloys are: (a) vertical magnetic recording, using Co-Re bath for tape or disk 27 and (b) use of a Co-Mn-Ni-Re electrolyte for rigid or floppy disks, with, e.g., Hc value of 1,050 Oe (Oersteds).28
The only significant mention of electroless rhenium is a Russian patent2g of 1974, using NH,ReO, and formaldehyde sulfoxylate. Almost all the interest has been in electroless rhenium alloys, mostly quinquenary for magnetic recording, or ternary for electrochemical research, enhanced corrosion and mechanical properties. si Pearlstein and Weightman30 examined the magnetic properties of electroless cobalt deposits with various alloy metal additions. They described the result of adding Re (as KRe04) as giving a high ratio of coercivity to remanence. The rhenium concentration was 0.52 g/L plus 7.43 g/L of cobalt. The same authors later3i reported on the effects of adding various metal salts to electroless nickel baths. They concluded that acidic electroless Ni baths were not very promising but by adding KRe04 obtained up to 46% Re in Ni-Re alloy deposits from
93
Pulse plating can yield surprisingly high cathode efficiencies and much improved deposit or distribution properties, and can also be most disappointing should the processes and parameters chosen be unsuitable. So wide is this field that only a paraphrase of one significant paper can be given here. With so low a cathode efficiency and questionable deposit properties in many cases, rhenium plating processes are evidently, perhaps desperately, in need of the improvements that pulse plating can offer! Crack formation in Re deposits was addressed by Hosokawa et a1.,36 (more data and discussion are given in Ref. 37) using a simple bath of: Figure 10. Microhardness vs temperature Co-Re plus alumina (from Ref. IO).
of heat treatment
for
a citrate-ammonia bath at a pH of 9. This very high rhenium content is noteworthy since it resulted from a rhenium concentration of only 0.99 g/L plus 7.8 g/L of nickel in the bath. The 46% Re alloy had a very high melting point of about 1,700”C. No comment was made as to the mechanical properties of these materials.
eltic Work on the production of magnetic layers with vertical anisotropy from electroless baths indicates that rhenium is virtually essential as an alloy component for this preferred type of high-density recording medium, which has a typical composition of CoNiReMnP. See Takano & Matsuda,32 Takano et a1.,33 and Homma & Osaka.35
The type Ni-Me-P(B), obtained by the electroless route, are mentioned by Vaskelis, who quotes NiRe50%-P, and Ni-Re20%-B as the generally accepted maximum attainable Re levels.35
Figure 11. Pulse length vs CE (from Refs. 25 and 26).
94
ReO, H2S04
Operating
parameters
15 g/L 5mlL 20°C and pH 0.8 (for the effect of variations from these values, see below)
This solution operates with up to 10% cathode efficiency using direct current (DC) at 10 A/dm2, and pulse plating trials were conducted at the same average CD (i.e., averaged over the whole cycle time), using square pulses, the pulse time decreasing from 104 pses to 1 psec, and the pulse current density (PC-CD) correspondingly increasing from 1 to 200 A/cm2 (ASD figures are rather unwieldy, being 100 to 20,000 A/dm2). The cathode efficiency peaked a little above 50% (5 x the dc value), for 200 A/cm2, with very sharp maxima for all the curves plotted (Fig. 11). Cracking in the deposits was studied, for 2 pm thick deposits only (this being the objective set), after heat treatment in vacuum at 1,OOO”C. The DC deposits were substantially cracked, and the cracking diminished as the PC-CD increased, a crack-free deposit being obtained at 100Alcm2. An interesting variation in appearance from steel gray to black to bright silver was obtainable by using pulse times of 1 to 10 psec, and 7 to 100 A/cm2, in general corresponding to conditions on the left of the maxima in Figure 11. At the optimum for fully bright Re (100 psec and 100 A/cm2), deposits of >10 pm thickness were obtainable but this aspect was not pursued. Those deposits plated at long “on” times (and DC deposits), became black in a few weeks after plating, if kept in room atmosphere. Conversely, deposits plated at the optima indicated - 100A/cm2, T of 100 psec, mean CD of 10 A/dm2, giving CE of 12% - stayed fully bright for three months without heat treatment.
Metal Finishing
Mason Corporation
Effects of other variables were: pH value - the pH vs. CE response was flat from pH 0.5 to 1.0, above 1.1 only grey deposits, and above pH 2 the efficiency was negligible. Temperature - a linear response to increasing temperature, as shown in Table IV It can be seen that quite narrow parameters apply for specific desired properties, and it may not be possible to get all of them in one package. This is typical of pulse plating, despite its many virtues.
This survey of rhenium plating, limited to some extent though it is, shows there is a good deal of data available on processes, and on alloy plating in terms of deposit composition. There is not much information on the properties and appropriate uses of the resulting deposit, with the exception of the complex alloys used as magnetic recording media. Rhenium deposits are hard and wear resistant (perhaps due to its forming an oxide coating) yet retain good electrical performance, but they need to be heat treated in vacuum or hydrogen, unless from just the right solution or are plated with just the right pulse settings. If, however, Pd, Pt, and Rh prices continue to be so volatile as a result of automotive, environmental, and political demands and pressures, rhenium may well be worth trying. Indeed, as we may be forced into it despite the reservations noted above, some pre-emptive action really should be in hand by now!
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
13.
Sigma-Aldrich catalogue, UK edition; 2000-2001 U.S. Patent 6,162,552; 2000 Britannica.com; 2001 see, e.g., Rhenium Alloys, Inc.; www.rhenium.com Japan Patent 70,006,150-B; 1970 U.S. Patent 5,817,371; 1998 Mackay & Mackay, “Modern Inorganic Chemistry”, 4th edition; Blackie/Prentice & Hall: 1989. (For N. America the ISBN is O-13-488487-6) Meyer, A.R., Trans. I.M.F., 46:209-212; 1968 DePew, JR. and TL. Larson, Plating, May, pp475-478; 1970 Camp E.K., Plating, May 1965, ~~413-416 Greco VP, Plating, Feb 1972, ~~115-125 Meyer, A.R., PhD thesis, University of Geneva (in French); also, in French, in Oberflaeche-Surface, 11, 1970; Heft (issue) 3, pp 84-86; 4, pp 117-122; 6, pp 208-213; 7, pp 231-232; 8, pp 248-251 Gvozdeva, I.I., Elekhomet. Tsuetnykt Metal, 188, 212, (1957), & Sklyarenko S I et al, Akad. Nauk.
mStannic Chloride (Tin Tetrachloride), (Tin IV Chloride), (Tin Chloride) stannic Chloride Solution
??
I Stannous Chloride_AntLydrous (Tin II Chloride) i i
&a-~*ii3;i:_ I
Sodium Stannate
--
mPotassium Stannate w Potassium Stannate Solution mStannous Methane Sulfonate Stannous Sulfate Solution Concentrations Available (18Og/l, 22Og/l, 24Og/l, 3OOg/l)
??
Mason
Corporufion
chemicals’
uniquefeafures include speciulized service, the highest quulify and purityin tinmefuls, and flexibility in packaging of the chemicals to meef the cusfomers needs.
Mason Corporation P.O. Box 38 ?? Schererville, IN 46375 l-2 19-865-8040 ?? Fax: l-2 19-322-36 11 800-326-3075 ?? Email: [email protected]
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June 2003
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SSSR,Inst. Met. 100; 1958 Bond, G.C. et al., J. Electroanal. Chem. 34:1-2: 1972 U.S. Patent 3,668,083-A; 1970 U.S. Patent 5330,088; 1994 Tsuru, T., et al., J; Surface Finishing Sot. Japan; 471883-4; 1996 (in Japanese) 18. Turns, E.W., Plating, 2:127-131; 1971 19. Turns, E.W., et. al, U.S. Patent 3,342,708; 1967 20. Fukushima, H., et al., Metal Finishing, 83(3):37-41; 1985 21. Fukushima, H., et al.,Meti Finishing, 84(12):15-Q 1986 22. Berezina, S.I., et al., Metallou, 29:106-110; 1993 23. Berezina, S.I., et al., Metallou, 28:282; 1992 24. U.S. Patent 3,890,210 -A; 1975 25.European Patent 913,501-A; 1999 26. Japan Patents 52,052,106-A, 84,042,066-B; 1977 27. Japan Patent 62001122-A, 1987 28. Japan Patent 63111195-A, US Patent 4,778,573-A, 1988 29. Russian Patent SU396438-A; 1974 30. Pearl&n, E & RF Weightman, Plating, 6714716; 1967 31. Pearlstein, F. & R.F. Weightman Electrochemical Technology, Nov.-Dec.:42’7-430; 1968 22. Takano, 0, & H. Matsuda, Metal Finishing, 84(6):6064; 1986 33. !I’akano, 0, et al., Japan Met. Fin. Sot., 85(9):440-44,1984 34. Homma, T. & T.J. Osaka, Electrochem. Society, 139(10):2925-2929; 1992 35. Vaskelis, A, Galvanotechnik, 87(2); 1996 36. Hosokawa, et al., Plating & Surface Finishing, 10:5255; 1980 37. Hosokawa K. et al., Proceedings of INTERFINISH 80, pp 58-62 MF 14. 15. 16. 17.
Terry Jones’first contact with metal finishing was in 1954 under Tony Such at Wilmot Breeden in Birmongham, England. He held a number of jobs in works laboratories, a barrel-plating facility, and jobbing shops. From 1987 until retiring in 1998 he was a service chemist at Englehard Industries. Since then he has done consulting work and some part-time work in R&D at AT Poeton in Gloucester. He authored the chapter “Plating Finishes”in Handbook of Electronic Connectors, co-authored “Formation of Cobalt Cyanide Complexes in Hard Acid Gold Baths,”which was presented at Interfmish 1996, and has written two articles for Metal Finishing.
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