Electrochemical deposition (ECD) is of great importance in particular for highly durable long-term corrosion protection in various areas such as automotive bodywork, construction industry and several outdoor and off-shore applications under harsh environments. In addition to corrosion protection, other functional properties such as decorative features (e. g. color and gloss) and mechanical properties (resistance to mechanical use) are of interest. The crystal structure of electrodeposited metals depends on several process-related parameters such as current density, electrolyte concentration, types of additives (inhibitors), pH-value, electrolyte temperature and hydrodynamics (flow characteristics). A distinct process parameter such as current density may affect both microstructure, i. e. the type of deposit structure and texture, and mechanical properties, e. g. indentation hardness and indentation modulus. Zinc was electrodeposited on steel sheets from a cyanide-free alkaline electrolyte by using different additives. The current efficiency and the deposition rate remarkably depend on the electrolyte conditions. Different crystal structures of zinc were observed in dependence on current density and electrolyte composition. As a result, mechanical properties and roughness of the coatings are affected by the crystal structure. The process-related interdependence of microscopic features and macroscopic properties of zinc deposits was studied by X-ray diffraction (XRD) regarding microstructure, by scanning electron microscopy (SEM) with respect to morphology and by instrumented indentation testing (IIT) for determination of indentation hardness, modulus and creep. It has been shown that the interdependence of process-related deposition parameters, deposit microstructure and macroscopic coating properties is a prerequisite of a model-based process simulation.
Einfluss der Prozessparameter auf Mikrostruktur und mechanische Eigenschaften von galvanischen Zinkschichten
Die elektrochemische Abscheidung ist ein wichtiges Verfahren zur Herstellung von hochbeständigen Korrosionsschutzschichten für unterschiedliche Anwendungen, beispielsweise den Automobilbau, Maschinenbau oder verschiedene Außen- und Offshore-Anwendungen mit hohen Umweltbelastungen. Darüber hinaus müssen Korrosionsschutzschichten auch funktionelle oder dekorative Eigenschaften (z. B. Farbe oder Glanz) und mechanische Eigenschaften (Verschleißbeständigkeite) aufweisen. Die Kristallstruktur von abgeschiedenen Metallen hängt von unterschiedlichen Prozessparametern bei der Abscheidung ab, wie der Stromdichte, der Metallionenkonzentration, den Arten der Zusatzstoffe (Inhibitoren), dem pH-Wert, der Elektrolyttemperatur oder der Hydrodynamik (Strömungscharakteristik). Die Stromdichte beispielsweise beeinflusst sowohl die Mikrostruktur einer Schicht (Art der Kristalle und deren Textur) als auch deren mechanische Eigenschaften (Härte). Für die Untersuchung wurde Zink aus einem cyanidfreien Elektrolyten mit verschiedenen Zusätzen auf Stahlblech abgeschieden. Die Stromausbeute und damit auch die Abscheidegeschwindigkeit hängen von den Elektrolytparametern ab. Die Zinkschichten wiesen in Abhängigkeit von Stromdichte und Elektrolytzusammensetzung verschiedene Kristallstrukturen auf. Diese Unterschiede machten sich auch bei den mechanischen Eigenschaften und der Rauheit bemerkbar. Die prozessabhängige Struktur wurde mittels Röntgenbeugung (Mikrostruktur), REM (Morphologie) und instrumentierte Eindringprüfung (Härte, Eindringmodul, Kriechverhalten) untersucht. Die Kenntnis dieser Kennwerte ist für die Durchführung von Simulationen der galvanischen Abscheidung notwendig.
1 Introduction
The crystallographic and mechanical characterisation of zinc coatings is a prerequisite for the development and the application of a model-based simulation strategy describing the application-oriented interdependence of processes parameters and layer properties. Zinc coatings are used as model system because they are firstly important for corrosion protection and secondly well-understood [1]. Of course, a simulation approach should be extendable to other metals, alloys, pulse plating or the incorporation of particles.
The process conditions have an effect on the electrochemical deposition. On the one hand, there are chemical parameters like electrolyte type, pH-value, type of additive as well as electrolyte and additive concentrations in the electrolyte bath. On the other hand, there are physical parameters such as current density, hydrodynamics and bath temperature. Substrate features like material, shape and roughness have also to be considered. The number of substrates and their position within the rack are also of importance.
Current density, electrolyte and additive concentrations of two different electrolyte types were varied around a standard reference process (SP) in order to simulate a change of process conditions and a modification of layer properties. Investigations are focused on pure zinc layers without top-coating to reveal the interdependence of micro- and macroscopic properties of the coating as grown. The mechanical quantities indentation hardness and indentation modulus serve as fingerprint of the zinc coating as deposited.
The inter- and intra-sample homogeneity of zinc coatings and the reproducibility of the deposition process were discussed earlier [2]. In case of the standard reference process, thickness distribution mainly depends on the substrate position in the rack and the hydrodynamics in the electrolyte. The hydrodynamics within a uniform plating tank was investigated by means of computational fluid dynamics [3].
A fundamental classification of electrodeposited metal was developed by Fischer [4]more than 50 years ago. Four main types of deposit structures can be distinguished with rising inhibition intensity:
- field-oriented isolated crystals type (FI)
- basis-oriented reproduction type (BR)
- field-oriented texture type (FT)
- unoriented dispersion type (UD)
The FI-type is caused by random growth for electrolyte temperatures above the recrystallization temperature of the electrodeposited metal. These crystals are long and isolated like whiskers, dendrites or loose crystalline powder. Consequently, the corrosion resistance and the hardness are low and hence the FI-type is not useful for technical applications. The BR-type is a coherent deposit. Grain size and surface roughness increase with deposit thickness. The FT-type is also a coherent deposit with small grain size almost homogeneously distributed throughout the deposit structure. This type of structure crystallizes in the form of a two-dimensional nucleation. The deposit structure of the UD-type is also a coherent one with small grain sizes, however crystals are generated with a three-dimensional nucleation. Other intermediate types and hybrids can also be observed.
Winand [5, 6] described the electrical field stability of the main deposit structures in dependence on the ratio between the apparent cathodic current density and the diffusion-limited current density (J/Jdl) in dependence on the inhibition intensity. In general, the ratio J/Jdl is directly correlated with the mass transfer to the electrode.
The basis of each deposit structure is the crystal orientation of the electrodeposited metal. Further important deposition parameters are the potential and the overvoltage. Pangarov [7] described the crystal orientation in dependence on the overvoltage of zinc (acidic electrolyte) and other metals. The study compared the theoretical and experimental results for preferred orientation of deposits at low, intermediate and high overvoltage. The work required to form a two-dimensional nucleus of the type (hkl) was calculated in dependence on the overvoltage. The expected orientations are (001) and (101) for low overvoltage, (110) for intermediate overvoltage and (100) for high overvoltage in h.c.p. crystal lattice like zinc.
Vlassak et al. [8] discussed indentation hardness and indentation modulus of zinc single crystals. The mechanical properties vary because zinc is an anisotropic material. Indentation hardness and indentation modulus could be correlated with the crystal orientation of zinc single crystals. However, electrodeposited zinc remarkably differs from a zinc single crystal.
2 Experimental
2.1 Substrate features
The substrate is a commercially cold-rolled steel sheet DC01 (material no.: 1.0330) in deep-drawing quality. The dimensions of the substrates are 50 mm x 50 mm x 1 mm with three holes to fix them onto the electroplating rack. A fully loaded rack contains 25 substrates. The steel sheets display roll-marks of a bump-like structure with a bump height of about 6 µm.
2.2 Process chemicals
The degreased substrates are deposited by using cyanide-free alkaline zinc electrolytes inclusive additives (Tab. 1). For the standard reference process, two different industrial processes were used designated as ZnA and ZnB. The reproducibility of the standard reference process and the current density variation were carried out under high-throughput conditions. These process parameters are presented in Table 2. After deposition, samples were brightened up with diluted nitric acid.
The variations
- zinc content (ZIN)
- basic additive (BAS)
- brightener (BRI)
- current density (jSP = 2 A/dm2, j++ = 8 A/dm2)
were carried out in uniform plating tanks (TA) of a volume of 200 litres with Venturi nozzles for a controlled electrolyte flow. The electrolyte composition in the standard reference process was defined as 100 %. For process variation one electrolyte component were either decreased (–) or increased (+) even beyond concentrations of the technological process window (Tab. 3). In any case, the hydroxide concentration was adapted to the zinc content.
2.3 Measurement techniques
The preferred orientation of deposits was determined by X-ray diffraction in Bragg-Brentano configuration (Seifert XRD 7) and Cu Kα radiation. The texture coefficients Tc(hkl) of zinc deposits were calculated from the X-ray intensity data. Morphology and surface topography were examined by scanning electron microscopy (Hitachi S-4100). Layer thickness of roughly 10 µm was determined by means of energy dispersive X-ray fluorescence (XRF) with a measurement area of (3 x 3) cm2 (Fischerscope X-Ray XDAL). Roughness data were measured by mechanical profilometry (XP-2 Ambios).
Indentation hardness, indentation modulus and indention creep data were derived from the instrumented indentation test (Picodentor® HM500, Fischer). The maximum force of 40 mN corresponds to an indention depth of the Vickers indenter of about 1 µm. Hence, indentation depth does not exceed 10 % of layer thickness to avoid any major substrate influence [9].
The elemental depth distribution of the zinc coatings was determined by means of glow discharge optical emission spectrometry (GDOES; GDA 750 Analyser, Spectruma Analytic).
3 Results
3.1 Current efficiency
The cathodic current efficiency is the percentage of the current contributing to metallic deposition. It is almost 100 % in acid zinc electrolytes but far below 100 % in alkaline zinc electrolytes. The current efficiency decreases strongly but in a different way with increasing current density due to side reactions, mainly hydrogen evolution for ZnA and ZnB and is less depended on the current density for ZnA (Tab. 4). A lower zinc content (ZIN) remarkably decreases the current efficiency shown for ZnA in Table 3. However, there is a decrease of the current density for BAS and an increase for BRI.
3.2 Crystal structure
The preferred growth of the (hkl) planes is expressed in terms of the texture coefficient Tc(hkl) [10, 11]. Quantitative information concerning the preferential crystallite orientation can be obtained from the texture coefficient Tc(hkl) defined as:
I(hkl) are the XRD intensities of diffraction peaks and n is the number of diffraction peak considered. The XRD reference intensities I0(hkl) (JCPDS card 4-831) refer to randomly oriented grains. If Tc(hkl) ≈ 1 for all (hkl) planes considered, a randomly oriented crystallite structure similar to the JCPDS reference is present whereas values higher than 1 indicate the abundance of grains in a given (hkl) direction. Values 0 < Tc(hkl)< 1 indicate a lack of grains oriented in that direction. The greater Tc(hkl) the more pronounced the preferential growth of the crystallites in the direction perpendicular to the considered (hkl) plane. The eight most important zinc crystal orientations [(002), (100), (101), (102), (110), (112), (200), (201)] were used to characterize the zinc crystal structure. Therefore, the maximum value of Tc(hkl) is 8. That means 100 % of the crystallites are in the orientation of the (hkl) plane perpendicular to the substrate plane.
The diffraction pattern of ZnA and ZnB coatings at different current densities is shown in Figures 1 and 2. The corresponding texture coefficients are given in Table 5. Most of the zinc coatings have a preferred orientation in the (110) plane. But there are differences in the crystal orientation if the zinc is electrodeposited at lower current densities (0.5 A/dm2). The zinc deposits ZnA have no or a rather weak preferred orientation of lattice planes (100), (101) and (110). Hence, Tc(hkl) equals to or less than 2, i. e. crystals are orientated more similar to the zinc powder (XRD reference). However, the ZnB coating at lower current density have a preferred orientation in the (100) plane.
The texture coefficients of the zinc coatings ZnA are given in Table 6 and of ZnB in Table 7. A very low concentration of the basic additive already results in powder-like coatings in some areas. The crystal structure is randomly or weakly oriented. In both cases (ZnA and ZnB), a higher current density (j++) leads to an increase of the (100) texture coefficient. The (100) texture coefficient increases (> 2) for ZIN+ and BRI– at ZnA coatings too. The (101) texture coefficient also increases corresponding to no or a rather weak preferred orientation of lattice planes. The variations ZIN+ and BRI– show a similar behaviour because a higher zinc content corresponds to a lower concentration of the brightener.
The SEM micrograph (Fig. 3) shows a typical vertical crystal growth along the field lines (sample: ZnA, 2 A/dm2) that can be visualized by a cross-section etched with diluted alcoholic nitric acid. A vertical crystal growth like (100), (110) and (200) planes forms deposit structures of the field-oriented texture type (FT). In comparison, Figure 4 (cross-section, SEM) illustrates the random crystal growth of the sample ZnA at 0.5 A/dm2 which is in agreement with the crystallographic results.
The surface of the zinc coating (ZnA, 2 A/dm2) without brightening consists of the crystal plates (Fig. 5). The thickness of the crystal plates is partially less than 50 nm with a quite different length below 1 µm. The crystal plates are uniformly distributed over the surface. Usually, zinc coatings are brightened up after the deposition and a dense microstructure without crystal plates can be observed by SEM.
3.3 Roughness of the grinded substrates
The grinded substrates have an average roughness of Ra = (0.05 ± 0.01) µm. The average roughness decreases after the zinc deposition for ZnA ((0.027 ± 0.003) µm) and ZnB ((0.026 ± 0.003) µm) at standard reference conditions (2 A/dm2) and also at higher current density (3.5 A/dm2, Ra ≈ 0.03 µm). The grooves are levelled-off. Then the average roughness increases as a result of deposition at lower current density (0.5 A/dm2): ZnA ((0.13 ± 0.04) µm) and ZnB ((0.17 ± 0.06) µm). Hence, both the grain size and the crystal structure have an effect on the roughness.
3.4 Elemental depth profiling
The depth profile of zinc coatings was determined by means of glow discharge optical emission spectrometry (GDOES). The homogeneous incorporation of organic additives can be proved with this method. Using the example of the zinc coating ZnA (Fig. 6) was plain that besides zinc (Zn) also carbon (C), nitrogen (N), oxygen (O) and hydrogen (H) are even incorporated in the coating. The determination of any hydrogen content in deposited zinc coatings was not possible. Hodoraba et al. [12] carried out a feasibility study on potential candidates of certified reference material for determination of hydrogen concentration.
The element depth profiling without hydrogen is given in Table 8 for ZnA coatings. The values are standardised on 100 weight percent. The share of the foreign atoms (C, N, O) is each below one percentage by weight in the depth profile from 1 µm to 8 µm of the standard reference process (SP-TA). The additives were adsorbed on the deposited crystals, enlarge their surface and make the formation of new nuclei possible.
The zinc coatings with less basic additive (BAS–) include clear more oxygen and nitrogen but less carbon. The zinc coating of variation ZIN+ contains also more oxygen compared to the standard zinc coating (SP).
3.5 Mechanical properties
In this study, indentation hardness (HIT), indentation modulus (EIT) and indentation creep (CIT) were used as macroscopic coating fingerprint. Figure 7 presents the different mechanical properties of ZnA and ZnB as a function of current density. Indentation hardness of ZnB is approximately 1500 N/mm2. There are marginal differences in dependence on current density. However, the indentation hardness of ZnA (at 0.5 A/dm2) is lower than the hardness of other zinc deposits (ZnA and ZnB). The indentation modulus of ZnA is approximately 100 GPa whereas the values of ZnB are slightly higher. The indentation modulus of ZnA and ZnB increases at lower current densities (0.5 A/dm2) but the standard deviation as well. That is an indication for increasing inhomogeneity and/or roughness. The indentation creep of zinc deposits is between 6 % and 7 % but increases for ZnA at lower current densities (0.5 A/dm2). This is also represented by different mechanical properties in comparison to the other zinc deposits.
The instrumented indentation test of the zinc coatings BAS– is not possible on account of the powdery surface. The average indentation hardness of the zinc coatings ZnA are approximately between 950 N/mm2 and 1500 N/mm2 (Tab. 9). The zinc coatings ZnA of the standard reference process (SP-TA) and higher additive concentration (BAS+ and BRI+) show higher indention hardness and for ZIN+ and BRI– a lower indentation hardness. The indentation modulus decreases below 100 GPa in the following sequence: j++, BRI– and ZIN+. The indentation creep is significantly increased for ZIN+ and BRI– (Tab. 9).
The indentation hardness of ZnB is generally higher than ZnA with average values between 1350 N/mm2 and 1900 N/mm2 (Tab. 10). The indentation modulus varies between 101 GPa and 116 GPa. The indentation creep of zinc coatings ZnB is approximately in between 5 % and 6 % with the exception of ZIN+.
4 Discussion
The crystal structure of zinc deposits has an effect on the mechanical properties of the coating. A rise of hardness can be observed in case of:
- the incorporation of impurity atoms [13]
- a decrease of the grain size [14, 15]
- a variation of crystal orientation [16].
A study of Vlassak et al. deals with indendation hardness and indentation modulus of zinc single crystals as a function of the crystal orientation. As a result of the above mentioned additives (grain decreaser), hardness of zinc single crystals (between 0.54 GPa and 0.65 GPa) is much lower than for electrodeposited zinc. The theoretical indentation moduli vary between 68 GPa (basal plane) and 130 GPa (prismatic plane). The indentation modulus (or modulus of elasticity) of the zinc deposits ZnA and ZnB is less than the theoretical values for the zinc single crystals in the prismatic plane.
Although the crystal structure of ZnB coatings changes from plane (100) to (110), there are no differences in the mechanical properties because the crystal growth is vertical to the substrate corresponding to the field-oriented texture type of the deposit structure. However, the indentation hardness of ZnA coatings decreases by the transition from the (110) preferred orientation to the random orientation. This is obviously an intermediate structure between the preferred growth direction vertical (FT-type) and parallel (BR-type) to the substrate plane. Twin crystals grow in oblique directions and frequently appear in these phases. The type of the deposit structure is termed twin intermediate type (Z) which consists of diagonally layered deposits. A Z-type like that was observed in a coherent deposit already 1923 of Blum et al. [17].
Zinc crystal plates with a thickness below 50 nm could be detected by SEM. Youssef et al. [18] reported of nanocrystalline zinc deposits (51 nm), although they have been formed at highest overpotential [crystal orientation (100) and (110)] from a zinc sulphate electrolyte with pulse plating. Saber et al. [19] produced nanocrystalline zinc deposits by pulse plating technique using additives. The hardness varied due to the grain size corresponding to the Hall-Petch-relation [20]. Jelinek [21] reported of fine needles on zinc surfaces that are colloidal zinc. These reach of the passivity anode by means electrophoresis to the cathode.
Further important deposition parameters are the potential and the overvoltage. Pangarov [7] described the crystal orientation in reference to the overvoltage of zinc (acid electrolyte) and other metals. The crystal orientations of ZnA and ZnB (except for the deposition at lower current density (0.5 A/dm2) are examples for intermediate overvoltage because the crystals have a (110) preferred orientation. The crystal orientation of ZnA turns into a random orientation at lower current density. There is a weak orientation in the plane (100), (101) and (110) whereas the peak (101) has the highest intensity (Fig. 1). This would have been expected if the overpotential decreases. Because the main part of the overpotential is used to the hydrogen formation, the overpotential decreases at lower current density. An indication of the hydrogen formation at the deposition is the current efficiency. That is the part of the current which is actually used for the zinc deposition.
However, the preferred orientations of ZnB change from (110) to (100) at lower current density. The (100) crystal orientation indicates a higher overvoltage during deposition, most probably as a result of the additives.
The indentation creep is increased and the indentation hardness is decreased at the random oriented ZnA coatings ZIN+ and BRI– similar to j+. In this case, the influence on impurity atoms (Oxygen) is negligible. Both zinc coatings deposited at higher current density (j++), show an increased texture coefficients Tc(100) (> 2). But that does not affect the mechanical properties because both planes (100) and (110) are vertically oriented.
5 Conclusions
Zinc was electrodeposited on steel sheets from a cyanide-free alkaline electrolyte with different additives. The crystal structure of the standard reference processes ZnA and ZnB usually has a (110) preferred orientation. This may change either to a random orientation (ZnA) or to a (100) preferred orientation (ZnB) at lower current density (0.5 A/dm2), affected by the overpotential during deposition. The zinc crystals of ZnA and ZnB which have a (110) or (100) preferred orientation form the field-oriented texture type of the deposit structure. These zinc coatings show similar mechanical properties (indentation hardness, indentation modulus and indentation creep). However, the indentation hardness decreases at a random zinc crystal orientation whereas the indentation creep increases.
The variations of the zinc electrolyte composition are particularly borderline cases on the possibility in the uniform plating tank (TA). The lowering of the basic additive concentration to 5 % leads to powdery coatings with low adhesion without any preferred orientation and higher oxygen and nitrogen content. In general, a surplus of additives in the electrolyte composition is not as important as a lack in crystallographic and mechanical properties.
Zinc-based coatings represent the huge majority of applications for the protection of steel against corrosion. As number, shape and size of components to be coated are varying, rack loading and bath parameters (hydrodynamics, current density, electrolyte composition and state, pH-value, temperature) have to be considered.
It has been shown that process-related parameters (e. g. current density) interdepend on the coating microstructure (e. g. texture coefficient) interdepend on the mechanical properties (e. g. EIT, HIT, CIT). This is a prerequisite for a model-based simulation of electrochemical processes as shown for zinc deposition [22].
Acknowledgments
This work was supported by the Federal Ministry of Education and Research under the contract number 01R/0711B. The investigations were accomplished in the project Anwendungsorientierte Simulation zur Planung und Produktion maßgeschneiderter elektrolytisch erzeugter Oberflächen (abbreviation: AnSim). The authors thank the DLR as funding agency and the project partners, in particular the industrial partners SurTec and Dr. Hesse responsible for the zinc deposition. The authors would like to acknowledge B. Strauß (SEM micrographs), Dr. V.-D. Hodoraba (GDOES, H2 content) and the Research Institute Precious Metals & Metals Chemistry (fem) for the glow discharge optical emission spectrometry (GDOES).
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Corresponding author
Jennifa Baier; Tel.: +49-30-8104 2594; fax: +49-30-8104 1827; e-mail: jennifa.baier@bam.de
DOI: 10.7395/2013/Baier1
Tc(hkl) =
I(hkl)/I0(hkl)
∑n(Ii(hkl)/I0i(hkl))
i=1
1
n
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