A Platinized Titanium Anode for Chromium Electroplating

Abstract

There are two types of electrolyte for chromium electroplating: the Sergent bath and the chromate solution containing fluorine compound. The Sergent bath is an acidified chromate solution consisting of some 250 g/liter CrO~, and easy to operate. However, the current efficiency for chromium deposition is low, about 10% or less. Metal deposition no longer takes place if the electric circuit is once interrupted. This is probably due to rapid passivation of the metal surface, and it is troublesome. Therefore, the Sergent bath is inadequate for some applications, such as the barrel plating.Of the modified solutions containing fluorine compound, the fluosilicate bath, consisting of CrO~, H~SO4, and sodium hexa-fluosilicate Na2SiFs, is most popular, although other electrolytic solutions, such as those containing fluoborate, for example, are also used. The current efficiency for metal deposition from the fluosilicate bath (FS bath, hereafter) is somewhat large compared to that from the Sergent bath because of the difference of the catalytic activity in both solutions. Metal deposition from the FS bath is able to continue even if the electric current is switched off during operation. It is of importance from the practical viewpoint.A number of scientific papers on the chromium electroplating have been published since Mfiller proposed the mass transport through the oxide film formed on the cathode surface (1). However, the mechanism on the chromium electrodeposition has not yet been fully established.Lead and its alloys, such as Pb-Sn and Pb-Sb, are employed as the anode in both the Sergent bath and the FS bath (2, 3). A lead dioxide layer formed on the surface prevents the anode from corrosion. At the open-circuit potential or at zero current, however, the oxide layer changes to lead chromate, and the cell voltage becomes high. This is caused by an extremely high resistance of the PbCrO4 layer. The consecutive formation of PbO, Pb(OH)2, PbCrO4, and PbO2 on the lead anode immersed in the chromium electroplating bath has been pointed out on the respective potentials before oxygen evolution, but no significant effect of addition of sulfuric acid to the chemical composition of the oxide layer has been found (4).The corrosion rate of the Pb alloy anode in the Sergent bath is in the range 1-10 mg/Ah, depending on the alloy composition, the casting, and the operating conditions, but the anode consumption in the FS bath is great. According to the laboratory test, the corrosion rates of a Pb-2% Sn alloy anode in the Sergent bath and the FS bath were 5 and 12 mg/Ah, respectively, under normal conditions. With the plant experiences, the grain boundaries of *Electrochemical Society Active Member.alloy are attacked, fine particles go away into the solution, and a part of these solids precipitate on the cell bottom which is troublesome for the cell operation.The lead alloy anode is consumed by two factors: the chemical attack and the physical degradation. The anode surface is dissolved gradually by electrolysis or by the faradaic curre...

Electrodeposition of Chromium

“…This gives good results but is not always practical. Platinized titanium anodes with thermally deposited indium dioxide are proposed [125]. Titanium anodes coated with PtO 2 are recently recommended for chromium plating [126].…”

The history of progress in dimensionally stable anodes

“…19 .20 McIntyre2 1 reported that iridium oxide coatings tested for use as anodes during cathodic electrodeposition of organic coatings showed no degradation and an estimated service life of four years, while in similar tests, ruthenium oxide coatings were marginal and cobalt oxide coatings were unsatisfactory. Hine et al 22 tested a platinized titanium sheet loaded with thermally deposited iridium oxide for use in chromium electroplating. Hardee et al 23 have reviewed the use of DSAtype anodes for high-speed electroplating and electrogalvanizing.…”

Studies of the Surface Reaction Mechanisms of Pb-3 wt%Sn-0.5 wt%Ag Anode in CrO3Solutions

“…In addition, corrosion mechanism of the lead-alloy has been intensively investigated in sulphuric acid solutions. [13][14][15] For anode in solution containing chromium, some researches indicate that the alloying additions in lead base alloys anode affect the microstructure and electrochemical characteristics of active materials, 16 corrosion rate 17 and layers structure 18 on the electrode. However, few reports regarding anode surface action in the more aggressive (strong oxidation and high acidity) environment of CrO 3 solution at high current densities of above 10 A/dm 2 are available in literature.…”