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Electrodeposition and characterization of tin-zinc alloy coatings
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surface science ELSEVIER
Applied Surface Science 103 ( 1996)
Electrodeposition
and characterization O.A. Ashiru
Research Institute,
I S9- 169
King Fahd Unicersir_v
Received
*,
ofPetroleum
13 September
of tin-zinc
alloy coatings
J. Shirokoff
and Minerals,
1995: accepted
Bo.r %c). Dhnhrm
31261. Strudi Arcdk
I8 February 1996
Abstract This paper reports the electrodepostion of tin-zinc alloy coatings from a non-cyanide alkaline stannate/‘zincate bath formulation containing a complexing/stabilizing agent. The bath also comprises addition agents which considerably improve the coating properties and give a semi-bright finish. The effects of variables of the process on deposit composition were studied. The 25 + 5% zinc (balance tin) alloy coatings offer excellent corrosion protection with no obvious white corrosion product. The deposit offers protection which is better than cadmium, zinc and zinc-nickel alloy coatings for equal thicknesses. The tin-zinc deposit obtained from the stannate/zincate bath is fine grained, semi-bright, and shows considerable improvements over the tin-zinc deposit from the previously used cyanide plating process. Electron microscopy and X-ray methods reveal microstructural information and structure-corrosion properties of these alloy coatings. Kewords:
Tin-zinc
alloy coating:
Electroplating
bath; Corrosion;
1. Introduction Tin-zinc electrodeposits offer outstanding corrosion protection for steel by combining the barrier protection of tin with the galvanic protection of zinc, without the bulky corrosion product associated with wholly zinc deposits. The alloy coating also gives a whiter color and superior solderability properties. It is therefore not surprising that a number of applications of tin-zinc alloy deposits have been known and well recognized [1,2]. Tin-zinc coatings have been used on the chassis of electrical and electronic appa-
_ Corresponding author. Tel.: + 966-3-86043 8603996; e-mail: [email protected]. 0169.4332/96/$15.00 Copyright f/I SO169-4332(96)00466-7
18; fax: + 966-3.
Microstructure:
Electrodeposition:
Non-cyanide
hath
rates and on critical automotive parts such as fuel and brake line components. Other possible applications include car body panels, where the trend is towards lengthening corrosion warranties, and the aerospace industry where the absence of corrosion is of paramount importance. The patent filed by Marino [31 in 19 IS covering the deposition of tin-zinc and other alloys from solutions containing phosphoric and sulphanilic acids is probably the first known significant publication on tin-zinc coatings. From 1915 to 1930 there was little further development, but during the 1930s several patents [4- IO]on tin-zinc alloy plating were granted. Despite these early patents, there has been limited work on industrial deposition of tin-zinc alloy coating. There were also no reports of the industrial applications and technical properties of electrode-
0 1996 Elsevier Science B.V. All rights reserved.
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O.A. Ashiru, J. Shirokoff/AppEied
posited tin-zinc. The deposition of tin-zinc alloys became a practicable process when the sodium stannate/cyanide tin-zinc plating electrolyte using polarized tin-zinc alloy anodes was developed at the International Tin Research Institute (then Tin Research Institute) during the later stages of the Second World War [ 11,121, together with the corresponding potassium bath, which was developed a few years later in the USA 1131. At the present time, this is the only well-known industrial process for plating tinzinc alloys. However, since the mid-1960s the use of tin-zinc had declined considerably and the coating is now not used as widely as expected despite the outstanding corrosion protection properties. This may be partly because of the unpopularity of cyanide solutions and also probably due to the fact that the tin-zinc cyanide plating baths are difficult to operate and require constant cumbersome monitoring and control. For example, after plating for a few ampere hours, the percentage of zinc co-deposited with tin starts to drop dramatically and this necessitates constant maintenance of the bath. Another probable reason for the decline in tin-zinc plating may be the fact that the matte tin-zinc finish was considered less attractive than a number of bright finishes which are now available. Finally, there was also the cost disadvantage, because prior to 1985 tin was a relatively expensive and coded as precious commodity. The problems encountered in tin-zinc plating from cyanide baths necessitated the effort by several researchers to develop non-cyanide plating systems capable of producing bright deposits [14-221. Unfortunately, these baths have operational problems associated with limited solubility and long-term stability. Thus, further work is pertinent in view of the current thinking for a replacement of the toxic cadmium coatings, and the fact that the price of tin has fallen dramatically. It therefore seems likely that the turn of this century will see a revival of interest in tin-zinc alloy coatings as a substitute for cadmium coatings. In this publication, the technical improvements achieved in the electrodeposition of tin-zinc from a newly developed alkaline non-cyanide bath [23,24] and the structure-corrosion property of the deposit will be discussed in the light of the fact that structure has been linked in numerous other studies to a materials corrosion response for different crystallographic planes and grain orientations [25-281.
Surface Science IO3 (1996) 159-169
2. Experimental 2.1. Electroplating
work process
The electroplating bath is mainly a mixture of sodium stannate, sodium zincate, and free sodium hydroxide dissolved in de-ionized water. In addition, a complexing agent, a brightener, and a grain refiner were also added to the bath. The full details of the bath formulation and preparation are described in patents [23] on the invention of the plating system. The list below is a typical bath formulation used to obtain 70/30 to 80/20 tin/zinc alloy compositions, with plating carried out at 1.2 A/dm2 and a bath temperature of 65°C: Zinc metal (added as sodium zincate) Tin metal (added as sodium stannate) Sodium hydroxide Potassium sodium tartrate (complexant) Hexamine (brightener) Trisodium phosphate (grain refiner)
1.0-1.6 g/e 40-60 g/e 16-26 g/L 60-80 g/L 5- 14 g/e 3-8 g/L
The pH of the plating solutions is usually between 12 and 13. The bath was operated from anodes made from rolled tin-zinc alloy plates. The anodes used for each electrodeposition process was suitably selected to have the same composition as the alloy deposit that is expected from the electroplate. In order to ensure dissolution of tin in the stannic form, the tin-zinc anodes were maintained in filmed condition, as in the deposition of tin from an alkaline stannate bath [22]. The film was established by slow insertion of the anodes into the solution with the current already flowing and after the cathode is already connected up in the bath and the plating circuit is complete. The electroplating experiments were carried out at temperatures between 40°C and 80°C and the operating current densities ranged from 0.5 to 2 A/dm2, depending on the test. The electroplating parameters were varied in the experiments in order to establish the effect of such variables on tin-zinc alloy deposition. The corresponding plating conditions are reported with the results. Deposition was carried out for sufficient time to give about 7 to 9 pm thick deposits. The two destructive techniques used to analyze the composition and thickness of the coat-
O.A. Ashiru, J. Shirokoff/Applied
ings were spark source mass spectrometry (SSMS) and optical microscopy of metallographically prepared samples in cross-section. In the present study, the tin-zinc alloy coatings were electrodeposited on two types of cathode substrates, which are mild steel and high purity copper panels. In both cases, the size of the panels was 15 cm X 10 cm. Prior to electrodeposition, the mild steel panels were cleaned by electrolytic degreasing in 3% &i-sodium phosphate solution at 70°C cathodically for 30 s and then anodically for 5 s. After rinsing in water at 25°C the steel panels were pickled for 30 s in 5% hydrochloric acid solution, rinsed again, and immersed immediately in the plating solution, while the copper panels were first cleaned by abrasive blasting with alumina and solvent degreasing in 1,l , 1-trichloroethane, before being given the same degreasing and pickling treatment as the mild steel panels. The electrodeposition process was carried out in a 25 / plating tank, made from a heat resistance material and equipped with a thermostat controlled immersion heater. The volume of the bath was maintained at 20 / for all of the electrodeposition and the pH of the bath was monitored regularly by an electronic pH meter. After electroplating, the tin-zinc plated samples were thoroughly rinsed in running distilled water, and stored in a dry atmosphere to prevent rusting.
2.2. Corrosion test The salt spray exposure test was conducted on the steel panels that were coated with the tin-zinc electroplate and other comparable coatings according to ASTM B-l 17 to determine corrosion resistance and cathodic protection. The following five sets of samples were used for the salt spray exposure test: 80/20 tin/zinc 90/ 10 zinc/nickel cadmium zinc (cyanide bath) zinc (acid bath) The 80/20 tin/zinc were deposited as described above, while the other four coatings were supplied by a commercial electroplater job shop. The objective of this part of the research program is to com-
Swjace Science 103 (19%) 159-169
161
pare the performance of tin-zinc coating with other well established protective coating systems. The salt spray test was performed on six of each of the panel’s five sets of coating systems. The panel dimensions used for all the salt spray exposure test were 15 cm X 10 cm. The ASTM B- 117 salt spray exposure test was performed according to the following test conditions: 5Yc sodium chloride in Salt solution distilled water 7 pH of collected solution 20 psi Air supply 35 * 2°C Chamber temperature Placement 30” from vertical and at least 10 cm apart to permit free settling of fog The state of the corrosion of the samples was noted almost on a daily basis - the percentage of rusted surface and the appearance of the samples were recorded. The samples were also photographed at reasonable intervals to keep a visual record of their appearances and were removed from the salt spray cabinet when their rust condition became too severe for more data to be collected. All the samples were removed from the salt spray chamber after 1500 h of testing.
2.3. X-ray d$fraction and mic’rostructurcrl studies Coatings of 80/20 tin-zinc on copper and steel substrates were analyzed by X-ray diffraction (XRD), an X-ray pinhole camera, X-ray lluorescence spectrometry (XRF) and spark source mass spectrometry (SSMS). The XRD analysis was used to identify the crystalline phases present in the coatings and substrates, preferred orientations present in these phases and provide some comments on stress, although stress analysis by XRD was not a focus of the work. X-ray pinhole camera exposures were used to determine fibre textures in the coatings while XRF and SSMS were employed to determine the elemental composition of the coatings after metallographic removal of the substrate. The steel substrate removal was required in order to avoid overlapping Debye rings in the X-ray pinhole pattern. The XRD was performed on a Philips PW 1700 automated diffractometer with a monochromator and
162
O.A. Ashiru, J. Shirokoff/Applied
spinner. Diffraction patterns were generated on a vertical goniometer attached to a broad focus X-ray tube with a copper target operating at 45 kV and 30 mA. This analysis was computer-assisted so that the interplanar spacing values could be corrected for the instrument error function by analyzing a silicon standard and subsequent quantitative analysis performed by matching the X-ray pattern of the sample with a data base of patterns for reference compounds. X-ray pinhole camera measurements were conducted on electropolished samples and exposed in transmission and back reflection using a fine focus X-ray tube of copper target material. Bulk chemical analysis was obtained from XRP measurements on a Philips PV9500 energy dispersive fluorescence spectrometer. Samples were analyzed using a rhodium target operating at 20 kV and 300 mA for 200 s and detection from a Silicon(Lithium) detector which had a resolution of 163 eV at 5.9 kV and an active area of 10 mm’. Morphological study was carried out by examining the as-plated tin-zinc samples under a scanning electron microscope. Further microstructural studies of the tin-zinc coatings was accomplished by preparing 3 mm discs, jet-polished to perforation for scanning transmission electron microscopy (STEM) observation on a Philips CM-20 operating at 200 kV. The electrolyte used for jet-polishing was a mixture of 30% nitric acid in methanol with experimental conditions of 10 V and 0.1 A at - 30°C in a Struers Tenupol 2 Polipower.
Surface Science 103 (1996) 159-169
compared to zinc it can be asserted that zinc is preferentially deposited because a tin: zinc ratio in solution of typically 30 : 1 will produce deposits with a ratio of approximately 5 : 1. Fig. 3 shows the effect of free NaOH concentration of the plating bath on the zinc content of the deposited tin-zinc alloy coating, with plating carried out at 1.4 A/dm*, 70°C and pH = 13. An increase in the free alkali content of the bath leads to an increase in the zinc content of the deposited tin-zinc alloy as shown in Fig. 3, the graph shows a region of fairly constant alloy composition when the free sodium hydroxide is between 16 and 20 g/Y’. The concentration of the potassium sodium tartrate (complexant) in the bath has very little effect on the amount of zinc co-deposited with tin. As shown in Fig. 4, over a concentration range of 40 to 120 g/Z there is a decrease of less than 5 wt% of zinc in the coating. As seen in Fig. 5, the zinc content of the deposit decreases with temperature. There is a more or less constant deposit composition within the temperature range of 60-70°C. Fig. 6 shows the effect of current density on alloy composition. At very low current density, the deposit is rich in zinc, but above 0.7 A/dm2 consistent alloy compositions are produced. Thus it is possible to obtain a consistent alloy composition from the bath within a flexible operating condition if the temperature and current density are
3. Results and discussion
3.1. Esfects of variable on tin-zinc composition
alloy coating
With all other operating conditions kept constant (current density 1.3 A/dm’, temperature 65°C and pH = 131, an increase in the tin content of the bath led to an increase in the tin content of the deposited alloy as shown in Fig. 1. An increase in the zinc content of the bath at constant operating conditions (current density 1.2 A/dm*, temperature 65°C and pH = 13), gave a marked and almost linear increase in the zinc content of the deposit (Fig. 2). Since the electrolyte contains a large excess of tin
28
32
36
40
44
48
52
56
Sn(g/l)
Fig. 1. The effect of tin concentration in the stannate-zincate bath on the tin-zinc deposit composition. Concentration of other bath constituents are: Zn metal 1.3 g/k’, NaOH 20 g/k’, potassium sodium tartrate 70 g/e, hexamine 8 g/e, and trisodium phosphate 5 g/e. The operating conditions of the bath were 1.3 A/dm*, 65°C and pH = 13.
O.A. Ashiru, J. Shirokoff/Applied
Surface Science 103 (19961 159-169
163
45 40 2
35
ii
30
3
25 20 15
I
101 0.4
I
I
0.8
I
I
1.2
I
I
I
1.6 Zn ( g/l )
2
I
I
I -1
2.4
2.8
Fig. 2. Effect of zinc concentration in the stannate-zincate bath on the tin-zinc deposit composition. Concentration of other bath constituents are: Sn metal 50 g/e, NaOH 20 g/e, potassium sodium tartrate 70 g/J, hexamine 8 g/k’, and trisodium phosphate 5 g/e. The operating conditions of the bath were 1.2 A/dm’. 65°C and pH = 13.
kept within the ranges of 60-70°C and 0.7-2 A/dm’, respectively. 3.2. Corrosion
resistance
and coating characteriza-
tion
Fig. 7 and Fig. 8 show comparative results of salt spray resistance of 8 pm coatings on mild steel of semi-bright 80/20 tin-zinc, 90/ 10 zinc-nickel,
I55 14
16
18
20
22
24
26
28
101 40
I
I 60
I 80
h
I 100
I IlO
COMPLEXANT ( g/l ) Fig. 4. Effect of the concentration of potassium sodium tartrate on the composition of the tin-zinc alloy deposit. The concentration of the other constituents of the stannate-zincate plating bath are: Sn metal 50 g/k’, Zn metal 1.3 g/e, NaOH 20 g/e, hexamine 8 g/e, and trisodium phosphate 5 g/k’. The plating was carried out at 1.4 A/dm*, 70°C and pH = 13.
cadmium and zinc plated from both acid and cyanide baths. It may be clearly seen that the best protection is offered by the 80/20 tin-zinc alloy coating. The tin-zinc electro-deposited coatings on both the steel and copper cathodes were separated from their respective cathodes by metallographic polishing and then chemically analyzed by XRF and SSMS. The major elements of the 80% tin/20% zinc, 90% zinc/ 10% nickel, cadmium and zinc were confirmed to be present in these concentrations. Also a minor impurity element of sulfur was present in the tin-zinc coatings. Sulfur is expected since it is a key ingredi-
30
Fig. 3. The effect of free NaOH concentration of the stannatezincate plating bath on the zinc content of the deposited tin-zinc alloy coating, with plating carried out at 1.4 A/dm’, 70°C. and pH = 13. Concentration of other bath constituents are: Sn metal 50 g/t”, Zn metal 1.3 g/L, potassium sodium tartrate 70 g/k’, hexamine 8 g/e, and trisodium phosphate 5 g//.
01 40
I
50
60 TEMPERATURE
70
0
Fig. 5. Effect of plating temperature on the tin-zinc alloy composition. The plating bath used for the test was the same as reported in the experimental section.
O.A. Ashiru, J. Shirokoff/Applied
164
200
0.5 0.7
0.9 I.1 1.5 1.7 1.3 CURRENT DENSITY A/dm2
Surface Science 103 (1996) 159-169
1.9
Fig. 6. Effect of current density on deposit composition. The plating bath used for the test was the same as reported in the experimental section.
80/20
Sn/Zn
Cd
ent, co-deposited from an additive package used to refine the grain size. It is well known that the orientation distribution of crystalline films grown on crystalline substrates is strongly influenced by preferred orientations. The origin of preferred orientations has been discussed and debated in the scientific literature since it can be the result of several controlling factors [29]. For example, preferred orientations may be due to: (1) the atomic (lattice) mismatch and the associated interface defect structure between the film and substrate, (2) anisotropy in crystal nucleation and growth rates of the film and/or (3) the interfacial energy versus misorientation curve for a given interfacial system. However, since we are not studying the various mechanisms of orientation development in
90/ 10 Zn/Ni
Zn (CN)
Fig. 7. Typical photograph
of samples exposed to salt spray test (ASTM B 117)after 725 h. (Dark regions are red in color.)
O.A. Ashiru.
APPEARANCE TIME ( HOURS
.I. Shirokoff/Applied
Surface Science 103 (fY96)
165
159-169
OF RED RUST - 8 km THICKNESS
) % RUST !z4 0.5% mllm 5% B 10%
Cd Sn/Zn
Zn/Ni
Cyanide -Zn
Fig. 8. Percentage of rust with time during exposure (ASTM B 117).
Acid Zn
Fe
the salt spray test
‘0.0
10.0
20.0
~0.0
40.0
5x0
60.0
3J.o
m.0
28 Fig 9. X-ray diffraction pattern of 80/20 tin-zinc coating steel substrate and reference patterns for tin, zinc and iron
this work, but rather using the X-ray data to identify structure and preferred orientations in our film/substrate interface systems the results will be discussed accordingly. Fig. 9 is the XFW pattern obtained from a 80/20 tin-zinc coating on a steel substrate. The phases identified in the pattern are also shown in the figure and they correspond to tetragonal tin, hexagonal close packed zinc and body centered cubic iron. Inherent in this pattern is a stronger than random X-ray intensity for (101) tin planes and this result is expected since it is the close packed plane of tin which is known to be the preferred direction of
Fig. 10. X-ray pinhole camera patterns of 80/20 (b).
tin-zinc
coatings
on a
growth for tin in this form. It is also seen here that little if any preferred orientation exists for the zinc phase in the coating and iron (steel) substrate, the later of which is a different result than observed for tin-zinc coatings on copper having displayed a stronger than random X-ray intensity for the (200) planes in the copper sheet [24]. The observation that there is a (200) preferred orientation in the copper substrate used but none in the steel (iron) substrate is
peeled off a steel substrate
in (a) and attached
to a copper substrate
in
166
O.A. Ashiru, .I. Shirokofl/Applied
likely due to a strong rolling texture in the copper substr; ate (copper panel cathodes). This texture is the result of mechanical deformation, grain re-orienta-
S&ace
tion and sometimes re-crystallization and grain growth through processing. The rolled mild steel panel cathodes appear to be untextured or at least less textured than the copper since their corresponding XRD pattern lacks any preferred orientations which can be a sign of texturing in rolled metal panels. Once again there is a (101) preferred orientation in the tin phase of the tin-zinc coating on both the copper and iron panels. This (101) tin preferred orientation is the close packed plane of tin and it is also the preferred growth plane of tin. This result would suggest that the atomic packing anisotropy in the crystal growth of the coating may play a role in the development of this orientation. The strongest X-ray intensity peak in the pattern which corresponds to the (101) tin plane appears to be fairly narrow in width and it is also well aligned with the theoretical line position for tin in Fig. 9. Low residual stress in the film deposit can be related to the X-ray peak width in the pattern for the (101) tin planes [30-321. These characteristics may indicate that the tin phase is of low residual stress, a feature desirable in coatings applied to flexible engineering components. Transmission pinhole camera patterns are found in Fig. 10(a) for a tin-zinc coating removed from a steel cathode substrate and in (b) for a tin-zinc coating on copper. The pattern in (a> consists of four Debye rings: an inner set of two rings which corresponds to the (200) and (101) planes of tin and an outer set which corresponds to the (220) and (211) planes of tin. The brightest ring on the pattern is that of the (101) plane indicating that it has the strongest X-ray intensity due to more (101) planes diffracting X-rays. All four rings contain a uniform distribution of many smaller spots for each diffracting grain. This tells us that the tin phase is fine grained and has no directional orientation within the plane of the coating. These orientation relationships can be described as follows: A B C D
Fig. II. Scanning electron micrographs (2000 X ) of 8 pm thick coating: ; of 80% tin-20% zinc deposited from (a) the conventional cyanide bath, (b) the stannate-zincate bath without additives, (c) the Stan nate-zincate bath with additives.
Science 103 (1996) 159-169
(200)tin (10l)tin (220)tin (2 11 )tin
I]steel ]Isteel ]Isteel I]steel
surface, surface, surface, surface,
[without [without [without [without
direction] direction] direction] direction]
Note that no preferred orientation is given to the steel cathode substrate for this system since it was
O.A. Ashiru, J. Shirokoff/Applied
removed in sample preparation. The tin-zinc sample was detached from the steel substrate by mechanically polishing the steel after it was punched into a 3 mm disc. The disc was further thinned by jet-polishing for characterization by TEM and a transmission pinhole camera. For the tin-zinc films on copper substrates the TEM samples were prepared the same way but for the pinhole camera characterization the copper substrate was not removed and the film was exposed in back reflection. It can also be said that the orientation relationship labeled B is highly preferred. Fig. 10(b) contains additional information from the copper phase to which it was plated. There are two additional Debye rings which correspond to the (111) and (200) planes of copper. On closer inspection one sees that the two rings are actually composed of several bright spots. These bright spots are from larger coarse grained copper and their location is a function of the directional alignment of the copper grain distribution. The specific grain orientation is most likely the result of a deformation texture from rolling the copper cathode substrate sheet or a subsequent re-crystallization texture if it was annealed. Also present in the pattern are the overlapping Debye rings for (111) copper planes and (2201 tin planes. The following orientations were observed for tin-zinc on copper cathode substrates:
Fig. 12. Bright field electron micrograph (20000 pattern of zinc (B = (0001)) and tin in (b).
167
Surface Science 103 (1996) 159-169
E F G H I J
(2OO)tin (lOlltin (220)tin (2OO)tin (10 1)tin (220)tin
I](11 l)copper, ]I(1 1 l)copper, ]I(1 1 ljcopper, ]I(200)copper, I](200)copper, I((200)copper,
[without [without [without [without [without [without
direction] direction] direction] direction] direction] direction]
Once again, orientation relationships F and I for (101) tin planes parallel to copper are highly preferred. Deposits from the non-cyanide tin-zinc plating bath are fine grained as can be seen in Fig. 11 which compares the microstructure of matte and semi-bright 80% tin-20% zinc with the cyanide tin-zinc coating of comparable thickness and the same composition of tin and zinc. Fig. 1 l(a) is the matte finish from the conventional cyanide bath without additives, Fig. 1 l(b) is the semi-bright finish from the stannate/zincate bath without additives and Fig. 1 l(c) is the semi-bright (brighter) finish from the stannate/zincate bath with additives. Presented in Fig. 12(a) is a bright field electron micrograph of the zinc phase in a tin matrix. The zinc phase appears as dark rods or plates which are of small grain size (< 5 pm> and uniformly distributed. This arrangement suggests that the distribution of tin-zinc galvanic cells on a microscopic scale contributes to the material’s enhanced anti-corrosion
X ) of the zinc phase in a tin matrix in (a) and the corresponding
selected area diffraction
168
O.A. Ashiru, J. Shirokoff/Applied
behavior. The corresponding selected area diffraction pattern shows us that the second phase zinc precipitate in view is of the c-axis B = (0001) orientation, while the tin matrix surrounding it is of high index orientation in this example. However, many other orientations were also observed. The tin phase (matrix) has not been indexed because only one tin spot appears in the diffraction pattern. In general, the zinc phase appeared not to have any preferred directional alignment.
4. Conclusions
(1) Under the condition studied, it has been proven that a deposit of 80% tin/20% zinc can be obtained from alkaline electrolyte. m - zmc electroplated coating (2) The (80/20) t’ provided superior corrosion protection when compared to zinc, (90/10) zinc/nickel, and cadmium coatings, without developing a white corrosion product and requiring considerably longer time (750 h) to develop red rust in a salt spray (ASTM Bll7) environment. (3) The microstructure of a (80/20) tin-zinc electroplated coating consists of small particles of zinc in a tin matrix. The tin phase has a highly preferred orientation for ( 10 1kin 11cathode substrate, without directional preference. The ( 10 I) preferred orientation is the close packed plane of tin and it is also the preferred growth plane of tin which may have contributed to the superior structure-corrosion properties of the coating.
Acknowledgements The work reported in this paper was initiated at the International Tin Research Institute (ITRI), Uxbridge, UK, as part of a visiting research fellowship. One of the authors (O.A.A.) is grateful to the ITRI for the award of the fellowship. The Research Institute of King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia is gratefully acknowledged for provision of support for further study on tin-zinc coatings. The authors would also like to thank Dr. U. Erb and Mr. P. Nolan for technical assistance with the STEM work which was finan-
Surface Science 103 (1996) 159-169
cially supported by the Natural Sciences neering Research Council of Canada.
and Engi-
References [l] P.J. Miller and J.W. Cuthbertson, Met. Finish. J. 47 (1949) 44. [2] F.A. Lowenheim and R.M. Macintosh, J. Electrodep. Tech. Sot. 27 (1951) 115. [3] P. Marino, British Patent 10, 133 (1915). [4] Q. Marino, British Patent 405. 433 (1932). [5] M.M. Thompson and J.C. Patten, U.S. Patent 1, 876 (1932) 156. 161 B.R. Haueisen, U.S. Patent 1, 904 (1930) 732. [7] Mead Research Engineering Co., British Patent 407, 670 (1931). [8] S.O. Cowper-Coles and Ferbrite Ltd., British Patent 420, 103 (1933). [9] S.W. Baier and D.J. Macnaughton, British Patent 525, 364 (1939). [lo] V.A. Lowinger and S.W. Baier, British Patent 533, 610 (1939). [ll] R.M. Angles, J. Electrodep. Tech. Sot. 21 (1946) 268. [12] J.W. Cuthbertson and R.M. Angles, J. Electrochem. Sot. 94 (1948) 73. [13] F.A. Lowenheim, U.S. Patent 2, 675 (1954) 347. 1141 R.M. Angles, J. Electrodep. Tech. Sot. 94 (1948) 73. 1151 A.E. Davies and R.M. Angles, Trans. Inst. Met. Finish. 33 (1956) 277. [16] J. Vaid and T.L. Rama Char, J. Sci. Ind. Res. India 16a (1957) 324. [17] R.D. Srivastava and R.C. Muckergee, Metallobemlche 30 (1976) 408. [18] O.A. Ashiru, private communication, 1990. [19] A.E. Davies, R.M. Angles and J.W. Cuthbertson, Trans. Inst. Met. Finish. 29 (1953) 227. [20] N. Dohi and K. Obata, J. Met. Finish. Sot. Jpn. 24 (1973) 674. [21] C.J. Raub, W. Pfeiffer and M. Vetter, Galvanotechnik 70 (1979) 7. [22] J.W. Price, Tin and Tin Alloy Plating (Electrochemical Publications Ltd.) p. 69. [23] O.A. Ashiru and S.J. Blunden, U.S. Patent 5, 378 (1995) 346; O.A. Ashiru and S.J. Blunden, Int. Patent WO 92/04485 (1992). [24] J. Shirokoff and O.A. Ashiru, in: Proc. 6th Middle East Corrosion Conf., Manama, 1994 (Bahrain Society of Engineers, Manama, 1994) p. 465. [25] R. Guo, F. Weinberg and D. Tromans, Corros. Sci. 51 (1995) 356. 1261 G. Palumbo and K.T. Aust, in: Materials Interfaces: Atomic Level Structure and Properties, Eds. D. Wolf and S. Yip (Chapman and Hall, London, 1992) p, 190. [27] K.T. Aust and G. Palumbo, Trans. Jpn. Inst. Met. 27 (1986) 995.
O.A. Ashiru, J. Shirokoff/Applied [28] P. Lin, G. Palumbo, U. Erb and K.T. Aust, Ser. Met. Mater. 33 (1995) 1387. 1291 R. Raj and S. Sass, J. Phys. (Paris) 49 C5 (1988). [30] B.D. Cullity, Elements of X-Ray Diffraction, 2nd ed. (Addison-Wesley, Reading, MA, 1978) p. 447.
Surface Science 103 (1996) 159-169
169
[31] H.P. Klug and L.E. Alexander, X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd ed. (Wiley, New York, 1974) ch. 11. [32] C. Barrett and T.B. Massalski, Structure of Metals. 3rd ed. (Pergamon, Oxford, 1980) p. 453.
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applied
surface science ELSEVIER
Applied Surface Science 103 ( 1996)
Electrodeposition
and characterization O.A. Ashiru
Research Institute,
I S9- 169
King Fahd Unicersir_v
Received
*,
ofPetroleum
13 September
of tin-zinc
alloy coatings
J. Shirokoff
and Minerals,
1995: accepted
Bo.r %c). Dhnhrm
31261. Strudi Arcdk
I8 February 1996
Abstract This paper reports the electrodepostion of tin-zinc alloy coatings from a non-cyanide alkaline stannate/‘zincate bath formulation containing a complexing/stabilizing agent. The bath also comprises addition agents which considerably improve the coating properties and give a semi-bright finish. The effects of variables of the process on deposit composition were studied. The 25 + 5% zinc (balance tin) alloy coatings offer excellent corrosion protection with no obvious white corrosion product. The deposit offers protection which is better than cadmium, zinc and zinc-nickel alloy coatings for equal thicknesses. The tin-zinc deposit obtained from the stannate/zincate bath is fine grained, semi-bright, and shows considerable improvements over the tin-zinc deposit from the previously used cyanide plating process. Electron microscopy and X-ray methods reveal microstructural information and structure-corrosion properties of these alloy coatings. Kewords:
Tin-zinc
alloy coating:
Electroplating
bath; Corrosion;
1. Introduction Tin-zinc electrodeposits offer outstanding corrosion protection for steel by combining the barrier protection of tin with the galvanic protection of zinc, without the bulky corrosion product associated with wholly zinc deposits. The alloy coating also gives a whiter color and superior solderability properties. It is therefore not surprising that a number of applications of tin-zinc alloy deposits have been known and well recognized [1,2]. Tin-zinc coatings have been used on the chassis of electrical and electronic appa-
_ Corresponding author. Tel.: + 966-3-86043 8603996; e-mail: [email protected]. 0169.4332/96/$15.00 Copyright f/I SO169-4332(96)00466-7
18; fax: + 966-3.
Microstructure:
Electrodeposition:
Non-cyanide
hath
rates and on critical automotive parts such as fuel and brake line components. Other possible applications include car body panels, where the trend is towards lengthening corrosion warranties, and the aerospace industry where the absence of corrosion is of paramount importance. The patent filed by Marino [31 in 19 IS covering the deposition of tin-zinc and other alloys from solutions containing phosphoric and sulphanilic acids is probably the first known significant publication on tin-zinc coatings. From 1915 to 1930 there was little further development, but during the 1930s several patents [4- IO]on tin-zinc alloy plating were granted. Despite these early patents, there has been limited work on industrial deposition of tin-zinc alloy coating. There were also no reports of the industrial applications and technical properties of electrode-
0 1996 Elsevier Science B.V. All rights reserved.
160
O.A. Ashiru, J. Shirokoff/AppEied
posited tin-zinc. The deposition of tin-zinc alloys became a practicable process when the sodium stannate/cyanide tin-zinc plating electrolyte using polarized tin-zinc alloy anodes was developed at the International Tin Research Institute (then Tin Research Institute) during the later stages of the Second World War [ 11,121, together with the corresponding potassium bath, which was developed a few years later in the USA 1131. At the present time, this is the only well-known industrial process for plating tinzinc alloys. However, since the mid-1960s the use of tin-zinc had declined considerably and the coating is now not used as widely as expected despite the outstanding corrosion protection properties. This may be partly because of the unpopularity of cyanide solutions and also probably due to the fact that the tin-zinc cyanide plating baths are difficult to operate and require constant cumbersome monitoring and control. For example, after plating for a few ampere hours, the percentage of zinc co-deposited with tin starts to drop dramatically and this necessitates constant maintenance of the bath. Another probable reason for the decline in tin-zinc plating may be the fact that the matte tin-zinc finish was considered less attractive than a number of bright finishes which are now available. Finally, there was also the cost disadvantage, because prior to 1985 tin was a relatively expensive and coded as precious commodity. The problems encountered in tin-zinc plating from cyanide baths necessitated the effort by several researchers to develop non-cyanide plating systems capable of producing bright deposits [14-221. Unfortunately, these baths have operational problems associated with limited solubility and long-term stability. Thus, further work is pertinent in view of the current thinking for a replacement of the toxic cadmium coatings, and the fact that the price of tin has fallen dramatically. It therefore seems likely that the turn of this century will see a revival of interest in tin-zinc alloy coatings as a substitute for cadmium coatings. In this publication, the technical improvements achieved in the electrodeposition of tin-zinc from a newly developed alkaline non-cyanide bath [23,24] and the structure-corrosion property of the deposit will be discussed in the light of the fact that structure has been linked in numerous other studies to a materials corrosion response for different crystallographic planes and grain orientations [25-281.
Surface Science IO3 (1996) 159-169
2. Experimental 2.1. Electroplating
work process
The electroplating bath is mainly a mixture of sodium stannate, sodium zincate, and free sodium hydroxide dissolved in de-ionized water. In addition, a complexing agent, a brightener, and a grain refiner were also added to the bath. The full details of the bath formulation and preparation are described in patents [23] on the invention of the plating system. The list below is a typical bath formulation used to obtain 70/30 to 80/20 tin/zinc alloy compositions, with plating carried out at 1.2 A/dm2 and a bath temperature of 65°C: Zinc metal (added as sodium zincate) Tin metal (added as sodium stannate) Sodium hydroxide Potassium sodium tartrate (complexant) Hexamine (brightener) Trisodium phosphate (grain refiner)
1.0-1.6 g/e 40-60 g/e 16-26 g/L 60-80 g/L 5- 14 g/e 3-8 g/L
The pH of the plating solutions is usually between 12 and 13. The bath was operated from anodes made from rolled tin-zinc alloy plates. The anodes used for each electrodeposition process was suitably selected to have the same composition as the alloy deposit that is expected from the electroplate. In order to ensure dissolution of tin in the stannic form, the tin-zinc anodes were maintained in filmed condition, as in the deposition of tin from an alkaline stannate bath [22]. The film was established by slow insertion of the anodes into the solution with the current already flowing and after the cathode is already connected up in the bath and the plating circuit is complete. The electroplating experiments were carried out at temperatures between 40°C and 80°C and the operating current densities ranged from 0.5 to 2 A/dm2, depending on the test. The electroplating parameters were varied in the experiments in order to establish the effect of such variables on tin-zinc alloy deposition. The corresponding plating conditions are reported with the results. Deposition was carried out for sufficient time to give about 7 to 9 pm thick deposits. The two destructive techniques used to analyze the composition and thickness of the coat-
O.A. Ashiru, J. Shirokoff/Applied
ings were spark source mass spectrometry (SSMS) and optical microscopy of metallographically prepared samples in cross-section. In the present study, the tin-zinc alloy coatings were electrodeposited on two types of cathode substrates, which are mild steel and high purity copper panels. In both cases, the size of the panels was 15 cm X 10 cm. Prior to electrodeposition, the mild steel panels were cleaned by electrolytic degreasing in 3% &i-sodium phosphate solution at 70°C cathodically for 30 s and then anodically for 5 s. After rinsing in water at 25°C the steel panels were pickled for 30 s in 5% hydrochloric acid solution, rinsed again, and immersed immediately in the plating solution, while the copper panels were first cleaned by abrasive blasting with alumina and solvent degreasing in 1,l , 1-trichloroethane, before being given the same degreasing and pickling treatment as the mild steel panels. The electrodeposition process was carried out in a 25 / plating tank, made from a heat resistance material and equipped with a thermostat controlled immersion heater. The volume of the bath was maintained at 20 / for all of the electrodeposition and the pH of the bath was monitored regularly by an electronic pH meter. After electroplating, the tin-zinc plated samples were thoroughly rinsed in running distilled water, and stored in a dry atmosphere to prevent rusting.
2.2. Corrosion test The salt spray exposure test was conducted on the steel panels that were coated with the tin-zinc electroplate and other comparable coatings according to ASTM B-l 17 to determine corrosion resistance and cathodic protection. The following five sets of samples were used for the salt spray exposure test: 80/20 tin/zinc 90/ 10 zinc/nickel cadmium zinc (cyanide bath) zinc (acid bath) The 80/20 tin/zinc were deposited as described above, while the other four coatings were supplied by a commercial electroplater job shop. The objective of this part of the research program is to com-
Swjace Science 103 (19%) 159-169
161
pare the performance of tin-zinc coating with other well established protective coating systems. The salt spray test was performed on six of each of the panel’s five sets of coating systems. The panel dimensions used for all the salt spray exposure test were 15 cm X 10 cm. The ASTM B- 117 salt spray exposure test was performed according to the following test conditions: 5Yc sodium chloride in Salt solution distilled water 7 pH of collected solution 20 psi Air supply 35 * 2°C Chamber temperature Placement 30” from vertical and at least 10 cm apart to permit free settling of fog The state of the corrosion of the samples was noted almost on a daily basis - the percentage of rusted surface and the appearance of the samples were recorded. The samples were also photographed at reasonable intervals to keep a visual record of their appearances and were removed from the salt spray cabinet when their rust condition became too severe for more data to be collected. All the samples were removed from the salt spray chamber after 1500 h of testing.
2.3. X-ray d$fraction and mic’rostructurcrl studies Coatings of 80/20 tin-zinc on copper and steel substrates were analyzed by X-ray diffraction (XRD), an X-ray pinhole camera, X-ray lluorescence spectrometry (XRF) and spark source mass spectrometry (SSMS). The XRD analysis was used to identify the crystalline phases present in the coatings and substrates, preferred orientations present in these phases and provide some comments on stress, although stress analysis by XRD was not a focus of the work. X-ray pinhole camera exposures were used to determine fibre textures in the coatings while XRF and SSMS were employed to determine the elemental composition of the coatings after metallographic removal of the substrate. The steel substrate removal was required in order to avoid overlapping Debye rings in the X-ray pinhole pattern. The XRD was performed on a Philips PW 1700 automated diffractometer with a monochromator and
162
O.A. Ashiru, J. Shirokoff/Applied
spinner. Diffraction patterns were generated on a vertical goniometer attached to a broad focus X-ray tube with a copper target operating at 45 kV and 30 mA. This analysis was computer-assisted so that the interplanar spacing values could be corrected for the instrument error function by analyzing a silicon standard and subsequent quantitative analysis performed by matching the X-ray pattern of the sample with a data base of patterns for reference compounds. X-ray pinhole camera measurements were conducted on electropolished samples and exposed in transmission and back reflection using a fine focus X-ray tube of copper target material. Bulk chemical analysis was obtained from XRP measurements on a Philips PV9500 energy dispersive fluorescence spectrometer. Samples were analyzed using a rhodium target operating at 20 kV and 300 mA for 200 s and detection from a Silicon(Lithium) detector which had a resolution of 163 eV at 5.9 kV and an active area of 10 mm’. Morphological study was carried out by examining the as-plated tin-zinc samples under a scanning electron microscope. Further microstructural studies of the tin-zinc coatings was accomplished by preparing 3 mm discs, jet-polished to perforation for scanning transmission electron microscopy (STEM) observation on a Philips CM-20 operating at 200 kV. The electrolyte used for jet-polishing was a mixture of 30% nitric acid in methanol with experimental conditions of 10 V and 0.1 A at - 30°C in a Struers Tenupol 2 Polipower.
Surface Science 103 (1996) 159-169
compared to zinc it can be asserted that zinc is preferentially deposited because a tin: zinc ratio in solution of typically 30 : 1 will produce deposits with a ratio of approximately 5 : 1. Fig. 3 shows the effect of free NaOH concentration of the plating bath on the zinc content of the deposited tin-zinc alloy coating, with plating carried out at 1.4 A/dm*, 70°C and pH = 13. An increase in the free alkali content of the bath leads to an increase in the zinc content of the deposited tin-zinc alloy as shown in Fig. 3, the graph shows a region of fairly constant alloy composition when the free sodium hydroxide is between 16 and 20 g/Y’. The concentration of the potassium sodium tartrate (complexant) in the bath has very little effect on the amount of zinc co-deposited with tin. As shown in Fig. 4, over a concentration range of 40 to 120 g/Z there is a decrease of less than 5 wt% of zinc in the coating. As seen in Fig. 5, the zinc content of the deposit decreases with temperature. There is a more or less constant deposit composition within the temperature range of 60-70°C. Fig. 6 shows the effect of current density on alloy composition. At very low current density, the deposit is rich in zinc, but above 0.7 A/dm2 consistent alloy compositions are produced. Thus it is possible to obtain a consistent alloy composition from the bath within a flexible operating condition if the temperature and current density are
3. Results and discussion
3.1. Esfects of variable on tin-zinc composition
alloy coating
With all other operating conditions kept constant (current density 1.3 A/dm’, temperature 65°C and pH = 131, an increase in the tin content of the bath led to an increase in the tin content of the deposited alloy as shown in Fig. 1. An increase in the zinc content of the bath at constant operating conditions (current density 1.2 A/dm*, temperature 65°C and pH = 13), gave a marked and almost linear increase in the zinc content of the deposit (Fig. 2). Since the electrolyte contains a large excess of tin
28
32
36
40
44
48
52
56
Sn(g/l)
Fig. 1. The effect of tin concentration in the stannate-zincate bath on the tin-zinc deposit composition. Concentration of other bath constituents are: Zn metal 1.3 g/k’, NaOH 20 g/k’, potassium sodium tartrate 70 g/e, hexamine 8 g/e, and trisodium phosphate 5 g/e. The operating conditions of the bath were 1.3 A/dm*, 65°C and pH = 13.
O.A. Ashiru, J. Shirokoff/Applied
Surface Science 103 (19961 159-169
163
45 40 2
35
ii
30
3
25 20 15
I
101 0.4
I
I
0.8
I
I
1.2
I
I
I
1.6 Zn ( g/l )
2
I
I
I -1
2.4
2.8
Fig. 2. Effect of zinc concentration in the stannate-zincate bath on the tin-zinc deposit composition. Concentration of other bath constituents are: Sn metal 50 g/e, NaOH 20 g/e, potassium sodium tartrate 70 g/J, hexamine 8 g/k’, and trisodium phosphate 5 g/e. The operating conditions of the bath were 1.2 A/dm’. 65°C and pH = 13.
kept within the ranges of 60-70°C and 0.7-2 A/dm’, respectively. 3.2. Corrosion
resistance
and coating characteriza-
tion
Fig. 7 and Fig. 8 show comparative results of salt spray resistance of 8 pm coatings on mild steel of semi-bright 80/20 tin-zinc, 90/ 10 zinc-nickel,
I55 14
16
18
20
22
24
26
28
101 40
I
I 60
I 80
h
I 100
I IlO
COMPLEXANT ( g/l ) Fig. 4. Effect of the concentration of potassium sodium tartrate on the composition of the tin-zinc alloy deposit. The concentration of the other constituents of the stannate-zincate plating bath are: Sn metal 50 g/k’, Zn metal 1.3 g/e, NaOH 20 g/e, hexamine 8 g/e, and trisodium phosphate 5 g/k’. The plating was carried out at 1.4 A/dm*, 70°C and pH = 13.
cadmium and zinc plated from both acid and cyanide baths. It may be clearly seen that the best protection is offered by the 80/20 tin-zinc alloy coating. The tin-zinc electro-deposited coatings on both the steel and copper cathodes were separated from their respective cathodes by metallographic polishing and then chemically analyzed by XRF and SSMS. The major elements of the 80% tin/20% zinc, 90% zinc/ 10% nickel, cadmium and zinc were confirmed to be present in these concentrations. Also a minor impurity element of sulfur was present in the tin-zinc coatings. Sulfur is expected since it is a key ingredi-
30
Fig. 3. The effect of free NaOH concentration of the stannatezincate plating bath on the zinc content of the deposited tin-zinc alloy coating, with plating carried out at 1.4 A/dm’, 70°C. and pH = 13. Concentration of other bath constituents are: Sn metal 50 g/t”, Zn metal 1.3 g/L, potassium sodium tartrate 70 g/k’, hexamine 8 g/e, and trisodium phosphate 5 g//.
01 40
I
50
60 TEMPERATURE
70
0
Fig. 5. Effect of plating temperature on the tin-zinc alloy composition. The plating bath used for the test was the same as reported in the experimental section.
O.A. Ashiru, J. Shirokoff/Applied
164
200
0.5 0.7
0.9 I.1 1.5 1.7 1.3 CURRENT DENSITY A/dm2
Surface Science 103 (1996) 159-169
1.9
Fig. 6. Effect of current density on deposit composition. The plating bath used for the test was the same as reported in the experimental section.
80/20
Sn/Zn
Cd
ent, co-deposited from an additive package used to refine the grain size. It is well known that the orientation distribution of crystalline films grown on crystalline substrates is strongly influenced by preferred orientations. The origin of preferred orientations has been discussed and debated in the scientific literature since it can be the result of several controlling factors [29]. For example, preferred orientations may be due to: (1) the atomic (lattice) mismatch and the associated interface defect structure between the film and substrate, (2) anisotropy in crystal nucleation and growth rates of the film and/or (3) the interfacial energy versus misorientation curve for a given interfacial system. However, since we are not studying the various mechanisms of orientation development in
90/ 10 Zn/Ni
Zn (CN)
Fig. 7. Typical photograph
of samples exposed to salt spray test (ASTM B 117)after 725 h. (Dark regions are red in color.)
O.A. Ashiru.
APPEARANCE TIME ( HOURS
.I. Shirokoff/Applied
Surface Science 103 (fY96)
165
159-169
OF RED RUST - 8 km THICKNESS
) % RUST !z4 0.5% mllm 5% B 10%
Cd Sn/Zn
Zn/Ni
Cyanide -Zn
Fig. 8. Percentage of rust with time during exposure (ASTM B 117).
Acid Zn
Fe
the salt spray test
‘0.0
10.0
20.0
~0.0
40.0
5x0
60.0
3J.o
m.0
28 Fig 9. X-ray diffraction pattern of 80/20 tin-zinc coating steel substrate and reference patterns for tin, zinc and iron
this work, but rather using the X-ray data to identify structure and preferred orientations in our film/substrate interface systems the results will be discussed accordingly. Fig. 9 is the XFW pattern obtained from a 80/20 tin-zinc coating on a steel substrate. The phases identified in the pattern are also shown in the figure and they correspond to tetragonal tin, hexagonal close packed zinc and body centered cubic iron. Inherent in this pattern is a stronger than random X-ray intensity for (101) tin planes and this result is expected since it is the close packed plane of tin which is known to be the preferred direction of
Fig. 10. X-ray pinhole camera patterns of 80/20 (b).
tin-zinc
coatings
on a
growth for tin in this form. It is also seen here that little if any preferred orientation exists for the zinc phase in the coating and iron (steel) substrate, the later of which is a different result than observed for tin-zinc coatings on copper having displayed a stronger than random X-ray intensity for the (200) planes in the copper sheet [24]. The observation that there is a (200) preferred orientation in the copper substrate used but none in the steel (iron) substrate is
peeled off a steel substrate
in (a) and attached
to a copper substrate
in
166
O.A. Ashiru, .I. Shirokofl/Applied
likely due to a strong rolling texture in the copper substr; ate (copper panel cathodes). This texture is the result of mechanical deformation, grain re-orienta-
S&ace
tion and sometimes re-crystallization and grain growth through processing. The rolled mild steel panel cathodes appear to be untextured or at least less textured than the copper since their corresponding XRD pattern lacks any preferred orientations which can be a sign of texturing in rolled metal panels. Once again there is a (101) preferred orientation in the tin phase of the tin-zinc coating on both the copper and iron panels. This (101) tin preferred orientation is the close packed plane of tin and it is also the preferred growth plane of tin. This result would suggest that the atomic packing anisotropy in the crystal growth of the coating may play a role in the development of this orientation. The strongest X-ray intensity peak in the pattern which corresponds to the (101) tin plane appears to be fairly narrow in width and it is also well aligned with the theoretical line position for tin in Fig. 9. Low residual stress in the film deposit can be related to the X-ray peak width in the pattern for the (101) tin planes [30-321. These characteristics may indicate that the tin phase is of low residual stress, a feature desirable in coatings applied to flexible engineering components. Transmission pinhole camera patterns are found in Fig. 10(a) for a tin-zinc coating removed from a steel cathode substrate and in (b) for a tin-zinc coating on copper. The pattern in (a> consists of four Debye rings: an inner set of two rings which corresponds to the (200) and (101) planes of tin and an outer set which corresponds to the (220) and (211) planes of tin. The brightest ring on the pattern is that of the (101) plane indicating that it has the strongest X-ray intensity due to more (101) planes diffracting X-rays. All four rings contain a uniform distribution of many smaller spots for each diffracting grain. This tells us that the tin phase is fine grained and has no directional orientation within the plane of the coating. These orientation relationships can be described as follows: A B C D
Fig. II. Scanning electron micrographs (2000 X ) of 8 pm thick coating: ; of 80% tin-20% zinc deposited from (a) the conventional cyanide bath, (b) the stannate-zincate bath without additives, (c) the Stan nate-zincate bath with additives.
Science 103 (1996) 159-169
(200)tin (10l)tin (220)tin (2 11 )tin
I]steel ]Isteel ]Isteel I]steel
surface, surface, surface, surface,
[without [without [without [without
direction] direction] direction] direction]
Note that no preferred orientation is given to the steel cathode substrate for this system since it was
O.A. Ashiru, J. Shirokoff/Applied
removed in sample preparation. The tin-zinc sample was detached from the steel substrate by mechanically polishing the steel after it was punched into a 3 mm disc. The disc was further thinned by jet-polishing for characterization by TEM and a transmission pinhole camera. For the tin-zinc films on copper substrates the TEM samples were prepared the same way but for the pinhole camera characterization the copper substrate was not removed and the film was exposed in back reflection. It can also be said that the orientation relationship labeled B is highly preferred. Fig. 10(b) contains additional information from the copper phase to which it was plated. There are two additional Debye rings which correspond to the (111) and (200) planes of copper. On closer inspection one sees that the two rings are actually composed of several bright spots. These bright spots are from larger coarse grained copper and their location is a function of the directional alignment of the copper grain distribution. The specific grain orientation is most likely the result of a deformation texture from rolling the copper cathode substrate sheet or a subsequent re-crystallization texture if it was annealed. Also present in the pattern are the overlapping Debye rings for (111) copper planes and (2201 tin planes. The following orientations were observed for tin-zinc on copper cathode substrates:
Fig. 12. Bright field electron micrograph (20000 pattern of zinc (B = (0001)) and tin in (b).
167
Surface Science 103 (1996) 159-169
E F G H I J
(2OO)tin (lOlltin (220)tin (2OO)tin (10 1)tin (220)tin
I](11 l)copper, ]I(1 1 l)copper, ]I(1 1 ljcopper, ]I(200)copper, I](200)copper, I((200)copper,
[without [without [without [without [without [without
direction] direction] direction] direction] direction] direction]
Once again, orientation relationships F and I for (101) tin planes parallel to copper are highly preferred. Deposits from the non-cyanide tin-zinc plating bath are fine grained as can be seen in Fig. 11 which compares the microstructure of matte and semi-bright 80% tin-20% zinc with the cyanide tin-zinc coating of comparable thickness and the same composition of tin and zinc. Fig. 1 l(a) is the matte finish from the conventional cyanide bath without additives, Fig. 1 l(b) is the semi-bright finish from the stannate/zincate bath without additives and Fig. 1 l(c) is the semi-bright (brighter) finish from the stannate/zincate bath with additives. Presented in Fig. 12(a) is a bright field electron micrograph of the zinc phase in a tin matrix. The zinc phase appears as dark rods or plates which are of small grain size (< 5 pm> and uniformly distributed. This arrangement suggests that the distribution of tin-zinc galvanic cells on a microscopic scale contributes to the material’s enhanced anti-corrosion
X ) of the zinc phase in a tin matrix in (a) and the corresponding
selected area diffraction
168
O.A. Ashiru, J. Shirokoff/Applied
behavior. The corresponding selected area diffraction pattern shows us that the second phase zinc precipitate in view is of the c-axis B = (0001) orientation, while the tin matrix surrounding it is of high index orientation in this example. However, many other orientations were also observed. The tin phase (matrix) has not been indexed because only one tin spot appears in the diffraction pattern. In general, the zinc phase appeared not to have any preferred directional alignment.
4. Conclusions
(1) Under the condition studied, it has been proven that a deposit of 80% tin/20% zinc can be obtained from alkaline electrolyte. m - zmc electroplated coating (2) The (80/20) t’ provided superior corrosion protection when compared to zinc, (90/10) zinc/nickel, and cadmium coatings, without developing a white corrosion product and requiring considerably longer time (750 h) to develop red rust in a salt spray (ASTM Bll7) environment. (3) The microstructure of a (80/20) tin-zinc electroplated coating consists of small particles of zinc in a tin matrix. The tin phase has a highly preferred orientation for ( 10 1kin 11cathode substrate, without directional preference. The ( 10 I) preferred orientation is the close packed plane of tin and it is also the preferred growth plane of tin which may have contributed to the superior structure-corrosion properties of the coating.
Acknowledgements The work reported in this paper was initiated at the International Tin Research Institute (ITRI), Uxbridge, UK, as part of a visiting research fellowship. One of the authors (O.A.A.) is grateful to the ITRI for the award of the fellowship. The Research Institute of King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia is gratefully acknowledged for provision of support for further study on tin-zinc coatings. The authors would also like to thank Dr. U. Erb and Mr. P. Nolan for technical assistance with the STEM work which was finan-
Surface Science 103 (1996) 159-169
cially supported by the Natural Sciences neering Research Council of Canada.
and Engi-
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