Composition, Morphology, and Topography of Galvanic Coatings Fe-Co-W and Fe-Co-Mo
© The Author(s). 2017
Received: 17 March 2017
Accepted: 4 May 2017
Published: 15 May 2017
Ternary coatings Fe-Co-W with an iron content of 40–55 at.%, cobalt 39–44 at.%, and tungsten 4–12 at.% and Fe-Co-Mo with an iron content of 40–55 at.%, cobalt 39–44 at.%, and tungsten 4–12 at.% were obtained by galvanostatic and pulse electrolysis on the mild steel substrate from iron(III) citrate-based electrolyte. The influence of electrolysis mode and parameters on composition of deposited alloys was studied. The competing reduction of iron and tungsten in Fe-Co-W coatings as well as the competitive deposition of iron and cobalt in Fe-Co-Mo coatings at various current densities were defined. Simultaneously, the alloy enrichment with molybdenum is more marked at a pulse mode. Atomic force microscope analysis of the Fe-Co-W alloy coating morphology and surface topography indicates their globular structure with spherical grains in the range of 2.5–3.5 μm. The surface of Fe-Co-Mo is characterized by parts of a globular structure with an average conglomerate size of 0.3–0.5 μm and singly located cone-shaped hills with a base diameter of 3 μm. Sites with a developed surface were detected within the same scan area which topography is identical to the crystal lattice of cobalt with the crystalline conglomerate sizes in the range of 0.2–1.75 μm.
KeywordsAFM analysis Composition Electrolyte bath Fe-Co-W (Mo) coating Pulse mode Topography
The researchers’ and technologists’ increased interest to multicomponent galvanic alloys of iron triad metals with refractory components (W, Mo, and etc.) [1, 2] is caused by several reasons. The first is creation of new technology of coatings with a unique set of functional properties such as wear resistance and corrosion resistance, increased catalytic activity and microhardness, and magnetic properties [3, 4]. This allows to replace toxic chromium plating and to create effective catalytic materials, more available compared to traditional platinum-based systems [5, 6].
Secondly, the scientific interest is connected with the solution of theoretic problems determining the mechanism of “induced co-deposition”  and physical and chemical sense, of which is the conjugate electrochemical reduction of two or more metals. However, the mechanism of induced deposition is the subject of much debate up to date .
Obviously, in each individual case, the formation of the coating depends on the qualitative and quantitative composition of the electrolyte and on the synthesis conditions, wherein the modes and parameters of the electrolysis will determine in a particular way the concentration ratio of the alloy components and phase composition of the coatings .
The functional properties of the coatings are structurally dependent, so both their composition and surface morphology will determine the performance characteristics of materials. The roughness and surface friction are the main characteristics of the surface quality of the coatings. Previously, it was shown  that binary and ternary coatings obtained in a pulse mode are characterized by higher microhardness and wear resistance due to their smooth surface. The roughness of electrolytic deposits can be significantly decreased using pulse mode . Since the structure of the electrolytic alloys determines both the properties and application of coatings, study of their morphology and topography remains relevant.
In spite of a sufficient number of works devoted to the binary alloy Fe (Co, Ni)-W (Mo) [12–14] patterns concerning the electrolysis mode influence on composition of ternary alloys, their topography and morphology require detailed investigation.
The Fe-Co-W coatings were formed on a mild steel substrate and on a copper substrate from electrolytic bath of composition: (M) iron(III) sulfate 0.1–0.15, cobalt sulfate 0.15–0.2, sodium tungstate 0.04–0.06, sodium citrate 0.3–0.4, sodium sulfate 0.1, and boric acid 0.1; the pH value was adjusted within the range of 4.0–4.5 by addition of sulfuric acid or sodium hydroxide. The Fe-Co-Mo coatings were formed on a mild steel substrate and on a copper substrate from the same electrolyte but containing the sodium molybdate 0.06–0.08 M instead of sodium tungstate; the pH value was adjusted within the range of 3.5–4.5. Rectangular samples with a surface area of 2 × 10−2 dm2 were used as working electrodes.
Pretreatment of sample surface included mechanical polishing, polishing, degreasing, chemical etching in a mixture of 10% hydrochloric acid and 10% sulfuric acids, thorough washing with distilled water, and drying.
The coatings were formed in two modes: (i) galvanostatic with the current density i 2–6 A dm−2 and (ii) pulsed with unipolar pulse current with the amplitude i of 2–4 A dm−2 at a pulse duration t on = 1°10−2–2°10−2 s and pause time t off = 1°10−2–5°10−2 s. As anode served plates of AISI 304 steel; the cathode-to-anode area ratio was 1:5, volume current density was kept at the level 2 A dm−3. Both the galvanostatic and pulse electrolyses were performed using dc and pulse current supply unit (ZY-100±12).
The coating time was 20 to 30 min and deposit thickness was 8–10 μm according to the electrolysis time. The coatings with thickness of 30 μm were deposited onto copper substrate only for X-ray analysis.
The chemical composition of the coatings was determined by X-ray fluorescence method using a portable spectrometer “SPRUT” with a relative standard deviation of 10−3–10−2. The error at determining the content of the components is ±1 mass percent. To verify the results, the energy-dispersive X-ray spectroscopy was performed using an electron probe micro-analyzer Oxford INCA Energy 350 integrated into the SAM system. The content of components (in terms of metal) in the coatings are presented in at.%.
The structure of the coatings was examined by X-ray diffraction analysis using a diffractometer (DRON-2.0) in the emission of iron anode.
The surface morphology of Fe-Co-W and Fe-Co-Mo thin films was studied by an atomic force microscopy (AFM) using an NT–206 microscope. The tapping mode was conducted to measure samples’ surface morphologies. Scanning was performed by using the contact probe CSC-37 with a cantilever lateral resolution of 3 nm . And the scan sizes were fixed at 39.9 × 39.9 μm and 10.0 × 10.0 μm, and the height of the surface relief was recorded at a resolution of 256 × 256 pixels. For each sample, a variety of scans were obtained at random locations on the surface of Fe-Co-W and Fe-Co-Mo thin films. In order to analyze the AFM images, all image data were converted into the Surface Explorer software. The root mean square (R q), mean particle height and its distribution, surface skewness, and particle diameter were obtained.
Results and Discussion
Previous studies  have shown that co-deposition of iron and cobalt with molybdenum and tungsten may be held both in galvanostatic and pulse modes. The current density, electrolysis duration, and pulse and pause time will influence on the process efficiency, coating quality, and refractory component content in the deposit.
Unipolar pulsed current significantly increases the efficiency of the cathode process, which is related to more complete implementation of reactant adsorption, product desorption, chemical reduction of intermediate tungsten oxides by hydrogen ad-atoms, and ligand removal in a pause period.
The current efficiency increases almost twice compared with a stationary mode and is 70–75 and 63–68% at a current density of 3 and 4 A dm−2 respectively.
Increasing pause duration at a current density of 4 A dm−2 and pulse duration of 10 ms leads to competitive reduction of cobalt and tungsten with iron decreasing the content of the latter (Fig. 2c).
Varying the ratio of the pulse/pause allows to precipitate coating with extended range of alloy components and therefore with a different level of functional properties. This will significantly expand the scope of the ternary alloy Fe-Co-W.
On the contrary, more uniform deposits with globular structure with singly located crystallites are formed by the non-stationary mode (Fig. 3b).
It should be noted that the current efficiency for Fe-Co-Mo coating deposition in the galvanostatic mode is higher than for the Fe-Co-W alloy and is in the range of 43–65%.
The unipolar pulse current provides a Fe-Co-Mo coating with a wider range of alloy components. The iron content in the coating varies in the range of 44–52 at.%, cobalt 37–40 at.%, and molybdenum 11–16 at.%. The trend to the iron content decreasing accompanied with some cobalt content increasing is saved with rising pulse current amplitude, as seen from Fig. 4b, c. Enrichment alloy with molybdenum is observed simultaneously to the competitive reduction of iron and cobalt. The current efficiency C e of alloy deposition at a current density of 3 A dm−2 is of 45–60% and is lower as compared to C e = 70–82% for coatings obtained at i = 2 A dm−2.
Intensification of competitive reduction of the alloying metals at varying the time parameters of pulsed electrolysis (on time t on, ratio on/off time t on/t off) should also be noted.
Analysis of the results shows that ternary alloys Fe-Co-W and Fe-Co-Mo deposition by a unipolar pulse mode is more effective and allows to obtain coatings with target content of the components in the alloy.
Traditionally, in materials science, the roughness is an indicator of surface quality and depends on the material processing. The roughness of galvanic coatings is the result of the alloy deposition and may serve as an additional indicator of the surface development as well as topography .
The coating samples Fe-Co-W and Fe-Co-Mo containing refractory component of 10–12 at.% obtained on mild steel were used for AFM analysis.
The parameters R a and R q for Fe-Co-W alloy were defined as 0.3 which is much higher than those for the substrate and shows substantial development of the surface.
It was established earlier  that globular structure of the surface is caused by the refractory metals present in the alloy. We can expect the increased microhardness and catalytic properties of the resulting Fe-Co-W coatings in this case as shown in the results of previous researchers .
Part A is characterized by the even structure with an average size of conglomerates 0.3–0.5 μm and singly located cone-shaped hills with a base diameter of ~3 μm and a height of 0.6 μm as one can see from Fig. 10c. As appears from 2D and 3D map topography of the surface (Fig. 10a, b), the cone-shaped hills are formed of the smaller spheroids.
Site B is characterized by more developed surface compared with site A. The hexagonal crystal lattice of cobalt with sufficiently sharp hills alternating by valleys is visualized at the 2D and 3D maps of the coatings’ surface (Fig. 11a, b). The cross-section of profile between markers 1 and 2 indicates that the crystalline conglomerate sizes are in the range of 2.0–4.0 μm, wherein the surface of larger crystalline size of 2.0–4.0 μm is formed with a smaller grain size of 0.5–1.0 μm as one can see from Fig. 11c.
The parameter R q for part A and part B was defined as 0.35 and 0.30, respectively, reflecting the greater roughness of part A caused by availability of the high hills. However, values of R q for parts of different morphology have no significant effect on the average roughness of the coatings R a = 0.25. Accordingly, to the R a and R q, the Fe-Co-Mo coating has a roughness class surface of 8–9.
The ternary coatings Fe-Co-W obtained by galvanostatic and pulse electrolysis modes are characterized by a more ordered structure than the substrate and the presence of agglomerates of spherical grains in the range of 2.5–3.5 μm. The results indicate the competing processes of recovery of iron and tungsten when forming Fe-Co-W coatings.
The ternary coatings Fe-Co-Mo obtained by galvanostatic and pulse electrolysis modes are characterized by developed surface containing sites with globular structure and hexagonal crystalline conglomerate sizes in the range of 2.0–4.0 μm lattice of cobalt. The results of elemental analysis of Fe-Co-Mo coatings obtained on a substrate of mild steel 08KP at various current densities demonstrate competition process of recovery of iron and cobalt.
The unipolar pulse mode is more effective and allows to obtain coatings with the specified content of the components in the alloy at the electrodeposition of ternary alloys Fe-Co-W and Fe-Co-Mo.
Synthesized Fe-Co-W and Fe-Co-Mo coatings with average roughness of 0.25 can be attributed to 8th to 9th roughness class.
The authors acknowledge the Karpenko Physico-Mechanical Institute of the NAS of Ukraine, Department of Physics of metals and semiconductors of National Technical University “Kharkiv Polytechnic Institute,” for providing all the support during the study period.
IY assisted in carrying out the experiments, analyzed the data, and wrote the paper. MV and NS planned the study and assisted in the interpretation, preparation, and proof-reading of the manuscript. YS carried out the experiments. All authors read and approved the manuscript.
The authors declare that they have no competing interests.
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