The selection of trivalent chromium hard chromium base working solution is also the key to successfully preparing chromium-nickel composite coating. After comparing various trivalent chromium working fluids on the market, BSC16 trivalent chromium hard chromium plating was adopted. Because the plating bath is composed of chemical reagents with excellent stability, the flushing water of the parts after electroplating can be directly added to the bath, and even zero discharge of hard chromium electroplating can be achieved, which not only saves water and metal materials, but also protects the environment. Under the best electroplating conditions, the current efficiency can reach more than 30%, which is about three times the efficiency of conventional hexavalent chromium plating, and the deposition rate is about five times that of hexavalent chromium plating [6] - [9] . To reduce a metal chromium ion, the hexavalent chromium ion requires six electrons, and the trivalent chromium ion only requires three electrons, so the production of the same hard chromium coating, deducting the higher solution resistance to the current consumption, electroplating the same thickness of hard chromium plating, trivalent chromium hard chromium plating at least saves about 30% energy consumption than the hexavalent chromium hard chromium plating. This type of hard chromium plating technology is stable, insensitive to impurities such as iron, nickel, and hexavalent chromium ions, and easy to maintain, and can be added to the plating solution to prepare chromium nickel and other composite coatings, significantly improving the toughness and corrosion resistance of the coating. The coating produced by this type of hard chromium electroplating technology has good polishing performance, reduces the friction coefficient of copper-chromium friction pair, and improves the wear resistance. The comprehensive cost of this type of hard chromium plating technology is lower than that of hexavalent chromium plating, about 50% of that of hexavalent chromium plating, especially reducing the cost of wastewater treatment, environmental maintenance (such as no need to install chromium fog absorption recovery tower, strong exhaust at the plating site), and greatly improving the operating environment of the plating site. With the popularization and use of this technology and scale effect, the production cost has the prospect of reducing. The product conforms to ROSH law and relevant environmental protection regulations at domestic and abroad and belongs to energy-saving green environmental protection and safety electroplating technology [10] . The trivalent chromium concentrates working liquid developed by Beijing Blue Chemical Centre was used, the content of metal chromium ion was 25 g/l, and 2 g/l metal nickel ion was added. The Hull experiment was conducted on the brass Hull cell test plate with a special chrome-plated Hull chamber. The electroplating was conducted for one minute at a current density of 10 amps, a temperature of 45 degrees Celsius and a pH2. See Figure 1 for the Hull slot test piece.

Electroplating equipment that can modulate current is used to prepare chrome-nickel composite coatings with different thickness ratios according to different current modulation methods.

Metal content measurement of composite coating the chromium layer and the metal content in the nickel layer of the Cr-Ni composite coating were measured by X-ray respectively at different current densities.

Electron metallography the fracture and surface metallography of multi-layer structures were observed by electron microscope.

Mechanical properties the brittleness of the coating is measured by indentation method. Mechanical bending method was used to check the brittleness and fracture morphology of the coating.

Standard Hull cell test, the temperature is 45 degrees Celsius, the current is ten amps, plating time is 1 minute.

It can be seen from the above picture of the Hall Chamber experiment that there is basically no chromium metal precipitation in the current density interval below 10 amps/cm², but nickel metal can be precipitated in the interval of 10 amps/cm², so the chrome-nickel composite metal deposit layer can be prepared by periodically controlling the deposition current density.

The modulated current electrodeposition experiment was conducted after 20 g/L nickel sulfate was added into the base bath and fully dissolved.

We used 35 A/dm² per minute for chromium and 8 A/dm² for nickel, respectively (see Figure 2 ).

Chromium 40 A/dm² for 30 seconds, nickel 5 A/dm² for one minute (see Figure 3 ).

Multi-layer chromium-nickel deposition experiments were conducted by period-modulated current.

From the metallographic diagram ( Figures 2-3 ), it can be found that the chrome-nickel multilayer structure is indeed prepared, but it is still not enough to reduce the brittleness of the chromium coating, and the internal stress still causes penetration cracks, but the cracks are less than that of single-layer chromium plating.

The prepared chromium nickel composite coating was subjected to destructive bending experiments, and the fracture morphology was observed with an electron microscope, and it was found that the layered fracture was helpful to reduce the fracture brittleness ( Figures 4-6 ).

By X-ray diffraction analysis, the content of metal chromium in the chromium layer is more than 90%, and the content of metal nickel in the nickel layer is also more than 90%, indicating that the preparation of chromium-nickel composite electrodeposition layer is indeed realized.

Previous experiments show that the thickness of the nickel layer is still not enough to relieve the electrodeposition stress of chromium. Therefore, the following micro-cracked chromium-nickel deposition layer is obtained by adjusting the modulated current electrodeposition parameters ( Figure 7 ).

By adjusting the modulated current parameters and the composition of the solution, the chromium-nickel composite metal of the rod-like material (30CrMnSi) was electrodeposited ( Figure 8 ), and the chromium-nickel composite coating without cracks was obtained ( Figure 9 ). The indentation experiment shows that it has excellent toughness. The indentation test of single-layer trivalent chromium coating showed many cracks ( Figure 10 ). It is proved that good toughness of chromium-nickel composite coating can be obtained by careful selection of modulated current parameters and other process parameters.