Trivalent Chromium Electroplating Baths—The Inner-Sphere Complex Hypothesis

There are 880 studies focused on trivalent chrome baths, and several studies suggest the formation of Cr(III)L(H2O)52+, where L is an additive such as oxalate. The literature suggests that this compound decreases the energy needed in the electrodeposition process. We call this approach the inner-sphere complex hypothesis because these complexes are suggested, such as principal intermediate compounds. There are several disadvantages of this postulate, which are numbered in our study. This hypothesis was tested via Fourier transform infrared spectroscopy performed in attenuated total reflectance (ATR) mode. In addition, the potassium bis(oxalato) diaqua chromate (III) dihydrate (K[CrC2O42OH22]·2H2O) compound was selected as a probe molecule because it contains bridging C−O−Cr bonds, which are supposedly the largest number of bonds in the inner-sphere complexes in bath solutions. There is strong evidence of numerous bridging C−O−Cr bonds in the solid sample; conversely, in solution, Cr(III) prefers to form terminal bonds (Cr−O). These results suggest that the concentration of the inner-sphere complex is lower in solution. In solutions containing chromium (III) sulfate and oxalate anions, the concentrations of these complexes are much lower. Although some inner-sphere complexes are formed, their concentration does not seem to be relevant to the electrodeposition process. Otherwise, at high ionic strengths, the formation of ion pairs and hydrogen bonds between Cr(III) and additives is probable. Our research highlights the importance of vibrational spectroscopy in resolving the mechanics of the trivalent chrome electrodeposition process. This is the first study reporting a band of Cr−O bonds in trivalent chrome baths.