26

Electrodeposition of Chromium Alloys

26.1 Introduction

CAST OR WROUGHT chromium alloys, containing the iron-group metals, have numerous uses in the the home and in technology. These applications are based on following properties of the alloys: good corrosion resistance; high strength and hardness; and low rate of oxidation and retention of strength at elevated temperatures. Chromium alloys, if they could be electrodeposited, would have many uses as protective coatings for steel and as electroformed objects. Investiga-tions have been made of the deposition of chromium alloys, but thus far no process has been developed to the point of commercial feasibility. The electro-deposition of chromium-iron alloys comes the nearest to being a successful process.

A priori, the deposition of chromium alloys would not seem too difficult, since chromium can be deposited alone from two types of baths, the trivalent bath and the chromic acid bath ; and the metals with which it is most frequently alloyed—iron, nickel, and cobalt—can be readily deposited individually. Furthermore, chromium forms solid solutions with these metals and, therefore, the alloys should be depositable at a potential more noble (more positive) than that of chromium deposited alone. However, as the discussion in Section 7.6* showed, a close relation between the crystal structures and the lattice parameters of two metals alone is not adequate to ensure successful codeposition.

The difficulty in depositing chromium alloys from the trivalent bath is mainly chemical, rather than electrochemical. It is probably related to the ease with which basic chromium salts precipitate in the cathode diffusion layer. Since chromium deposits with a rather low cathode current efficiency, the pH of the cathode diffusion layer increases on electrolysis and causes precipitation of basic compounds. The inclusion of the latter in the deposits causes them to be powdery, stressed, or exfoliated.

The low efficiency of chromium deposition from the trivalent bath is attribut-able to the negative (unnoble) potential of chromium with respect to hydrogen:

Cr3+ + 3e -> Cr E° = - 0 . 7 4

Measures aimed at preventing the above-mentioned precipitation of basic corn-complex pounds—measures, such as increasing the acidity of the bath or forming complex chromium ions—still further lower the efficiency of chromium deposition or may prevent the deposition entirely. Thus, we have the dilemma that those conditions which should lead to a better quality of deposit are inimical to the current efficiency.

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* See Volume I of this treatise for Chapters 1 to 21 .