Chromium galvanic coating modified by graphene oxide
The microhardness of galvanic chromium coating obtained from a standard electrolyte nanomodified by graphene oxide has been studied experimentally. It was observed that when graphene oxide is added to the electrolyte at a concentration of 10 mg/l, the microhardness of the chromium coating increases to 1,064 kg/mm2 (as compared with the chromium coating obtained from the standard chromium electrolyte without additives; the increase in microhardness is 19.8%).
Previous studies have shown that the use of nanoparticles, single-walled carbon nanotubes , multi-walled carbon nanotubes (MWCNTs) of the Taunit brand  and nanodiamonds  in chromium galvanic processes enables to obtain positive results in terms of increasing the functional properties of galvanic coatings and, in particular, increasing the microhardness and wear-resistance of coatings.
It was observed that the use of single-walled carbon nanotubes concentration of 50 mg/l increases the chromium coating microhardness to 858 kg/mm2 , at multi-walled carbon nanotubes concentration of 80 mg/l – to 1,024 kg/mm2  and when nanodiamond concentration is equal to 12 g/l the chromium coating microhardness grows to 1,050 kg/mm2 . In comparison with the chromium coating obtained from a standard electrolyte without additives the microhardness arises to 16, 20 and 23%, correspondingly.
Interesting results were obtained when using a comparatively new nanomaterial in the galvanotechnics, so-called grapheme oxide (GO). The significant increase of polarization resistance and decrease of the corrosion rate have been observed in case of Zn–GО composite coatings as compared with a pure Zn-coating. The corrosion rate gradually drops with an increase in the amount of graphene oxide in the coatings. Adding of graphene oxide at different concentrations (0.125, 0.25, 0.375 и 0.5 g/l) to the electrolyte leads to accretion of the coating up to 6.41 µ. The microstructure and morphology of the electrically deposited composite Zn–GО coatings were changed by adding of the graphene oxide.
It is interesting to investigate the influence of graphene oxide on the galvanic process characteristics, particularly, on microhardness of the chromium coatings (see a photo of graphene oxide obtained using an atomic-force microscope in Fig.1) on the basis of a positive experience of applying the nanoparticles, SWCNTs, MWCNTs of the Taunit brand and nanodiamonds in galvanotechnics.
The technology of graphene oxide production is based on the improved method of Hummer – Offerman, where natural graphite is oxidized with potassium permanganate in the concentrated sulfuric acid medium with followed dilution of the reaction mixture with water. After that the reaction mixture is treated with another oxidizer (hydrogen peroxide) and the prepared carbon-contained oxidation product is washed first with an acid solution then with water. Graphite is used as a carbon-contained material. Before being treated with potassium permanganate, the hydrogen peroxide sulfuric acid solution is applied to wash graphite. In this case hydrogen peroxide should be applied in the amount of 0.15 to 0.30 mass units per 1 mass unit of graphite in terms of 100% hydrogen peroxide . The final product is water dispersion with a concentration of dry substance (graphene oxide) equal to 1% after washing out acids and manganese salts. NanoTechCentre LLC produces two types of graphene oxide: standard graphene oxide and graphene oxide of deep oxidation. The graphene oxide of deep oxidation differs from the standard one by smaller lateral size of flakes (nearby 1–5 µ) and improved colloidal stability of aqueous solutions. The aim of the work is to study microhardness of the precipitate obtained when producing the nanomodified chromium galvanic coating by doping electrolyte with graphene oxide.
Galvanic chromium coating was achieved with the aid of a widely used industrial product – the standard sulphate electrolyte for chroming of the following composition: chromium anghydride CrO3 – 250 g/l; sulphuric acid H2SO4 – 2,5 g/l.
Square plates made of St3 grade steel of 0.1 dm2 (30 × 30 mm) were used as the cathode in the experimental research. Only the side of the plate facing the anode was coated, the other side was isolated.
A lead plate of the following composition: 10% tin and 90% lead was used as the anode. The ratio of the area of the anode to cathode was 1 : 1.
Plating of the chromium coating was performed after the preparation and ageing of the chromium electrolyte without additives.
Then, a chromium coating doped with graphene oxide of deep oxidation at various concentrations (from 7 to 52 mg/l) produced in NanoTechCenter LLC, Tambov city, was applied. Aqueous dispersion of graphene oxide of deep oxidation was treated by ultrasonic before adding it into the working electrolyte in order to decrease the aggregation of the particles. Coated were 5 parts for every combination of concentrations. Microhardness Нμ of the obtained coating was measured using of PMT-3M tester.
The PMT-3M microhardness tester is designed to measure microhardness of materials by pressing a Vickers diamond tip with a square base of a tetrahedral pyramid into the test material, which provides geometric and mechanical similarity of prints as the indenter deepens under the action of the load. Diagonals of the prints were measured using a FOM-1-16 photoelectric ocular micrometer with automatic processing of the measured results. The measurement error was 2%.
Microhardness of each sample was measured at 5 points where the prints were symmetrical, after which the results were averaged. Further averaging was carried out for all 5 parts of each experiment.
The results are presented in Table 1 and Fig.2 (Fig.2 shows average value and dispersion of the experimental data)
As a result of the experiments, it was found that the microhardness of the chromium coating doped with the graphene oxide of deep oxidation increases, while the highest microhardness (1,064 kg / mm2) was obtained at 10 mg/l concentration of graphene oxide of deep oxidation (as compared to the chromium coating obtained from standard chromium electrolyte without additives, the increase in microhardness is 19.8%).
An experimental study of microhardness of the obtained chromium plating from a standard electrolyte nanomodified with the graphene oxide of deep oxidation was carried out.
It was experimentally established that addition of the graphene oxide of deep oxidation increases microhardness of the chromium coating. It was found that the highest microhardness was obtained by adding graphene oxide at a concentration of 10 mg/l, the chromium coating microhardness increases from 888 kg/mm2 to 1,064 kg/mm2 (as compared with the chromium coating obtained from the standard chromium electrolyte without additives, the increase in microhardness is 19.8%).
A chrome coating of increased microhardness (and, as a result, of increased wear resistance) is interesting for use on parts subject to dynamic loads in a friction mode. Lifetime of the chromium-coated parts obtained from the electrolyte with graphene oxide is significantly longer than with a conventional chromium coating. ■