In this communication, we present a generalization of our investigations concerning current state in the development of trivalent chromium baths as an environmentally friendly alternative to hazardous electroplating baths containing extremely toxic hexavalent chromium compounds. The main technological properties of sulfate trivalent chromium baths containing carbamide and formic acid as organic additives are described and compared to those typical of common Cr(VI) plating bath. It is shown that thick nanocrystalline chromium–carbon deposits may be obtained from the Cr(III) electrolytes under study, with some physicochemical and service properties of such coatings exceeding those of “usual” chromium deposits. The proposed trivalent chromium baths are distinguished by their high current efficiency and electrodeposition rate. The covering power of the Cr(III) baths is poorer than in case of hexavalent chromium baths; it may be improved by applying basic chromium sulfate (chrome tanning agent) instead of chromium sulfate as a source of Cr(III) ions in solution. There is no need to utilize toxic lead anodes in Cr(III) deposition processes. It is noted that a grave shortcoming of trivalent baths is their low conductivity in comparison with the hexavalent ones.

Chromium electroplating from trivalent chromium baths as an environmentally friendly alternative to hazardous hexavalent chromium baths: comparative study on advantages and disadvantages

The effect of formic acid concentration on the conductivity and corrosion resistance of chromium carbide coatings electroplated with trivalent chromium

Unusual "chemical" mechanism of carbon co-deposition in Cr-C alloy electrodeposition process from trivalent chromium bath

A favorable chromium coating electrodeposited from Cr(III) electrolyte reveals anti-wear performance similar to conventional hard chromium

Uniform and flat thick Cr coating (CR3 coating) was electrodeposited from a Cr(III) electrolyte containing chromium chloride and urea under the optimized process conditions. The microstructure of the coating and its tribological properties under various loads was examined. Furthermore, the performance comparison of the CR3 coating with the hard Cr coating (CR6 coating) fabricated by conventional Cr(VI) electrodepositing process, and the heat-treated thick Cr coating (CR3H coating) from the same Cr(III) electrolyte, was performed. The result shows that the main phase of the CR3 coating is amorphous, with the nanocrystals of body-centered cubic structure chromium (bcc Cr) and graphite embedded in it. The existence of the nanocrystals of bcc Cr and graphite in Cr coatings is probably the common feature of all amorphous Cr coatings from Cr(III) electrolytes. In the scope of the bearable load of the CR3 coating, its tribological properties were a little worse than those of the CR6 coating. After annealing at 180 °C for 2 h, the wear resistance of the coating was improved to a certain extent. We found the interfacial carbon film between the CR3 coating and steel substrate, which caused poor bonding and consequently insufficient load-bearing capability of the coating. There is a need to further improve the load-bearing capability and wear resistance of the CR3 coating. In view of the great environmental advantage of Cr(III) electrodeposition process and no difference of wear resistance in order of magnitude between the CR3 coating and CR6 coating, Cr(III) electrodeposition technology is still a developing direction with promising prospect.

Structure characterization and tribological properties of thick chromium coating electrodeposited from a Cr(III) electrolyte

Effect of pulsed electrodepostion on the nanocrystal size, composition, hardness, coefficient of friction and wear resistance was investigated for the Cr–C electrodeposits obtained from a trivalent chromium bath. The electrodeposits were shown to contain about 9% of carbon. Pulsed electrodeposition does not virtually affect the carbon content. At the same time, an increase in the off time duration leads to a decrease in the nanocrystals size. The hardness and wear parameters of the electrodeposits may be sufficiently improved when using pulsed current. For instance, at ton = toff = 1 s, the hardness reaches the values of ~ 1200 ÷ 1300 HV (meanwhile, it is close to 850 ÷ 950 HV at a steady-state electrolysis).

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Improving hardness and tribological characteristics of nanocrystalline Cr–C films obtained from Cr(III) plating bath using pulsed electrodeposition

The effect of the parameters of pulsed electrolysis on the current efficiency, nanocrystal size, composition, hardness, friction coefficient, and wear resistance of nanocrystalline Cr-C coatings obtained from electrolyte based on Cr(III) sulfate containing carbamide and formic acid is studied. It is shown that coatings contain ∼9 wt % of carbon; the current density and pulse ratio produce practically no effect on their composition. It is found that a maximum appears on the dependence of current efficiency on pulse ratio in the case of the pulse ratio of ∼1.05–1.1. Thereby, current efficiency greatly exceeds the value implemented in the steady-state current mode. It is shown that, if the microhardness of Cr-C deposits obtained at constant current is close to 850–900 HV, the microhardness may increase to ∼1200−300 HV when pulsed electrolysis is used in certain modes. It is found that application of pulsed electrolysis allows a significant decrease in the friction coefficient of chromium-carbon coating (steel counterbody) both under the dry friction conditions and in the case of boundary lubrication and also results in an increase in the deposit wear resistance.

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V. V. Artemchuk

Papers

Improving hardness and tribological characteristics of nanocrystalline Cr–C films obtained from Cr(III) plating bath using pulsed electrodeposition

Electrodeposition of nanocrystalline chromium-carbon alloys from electrolyte based on trivalent chromium sulfate using pulsed current