EFFECT OF FORMIC-ACID ON THE CHROMIUM ELECTRODEPOSITION - HYDRIDE FORMATION

Hydrogen content in chromium and phase structure of deposits are studi ed in relation to the time t(i) elapsed after introduction in formic a cid into the chromic acid electrolyte. It is found that the hydrogen c ontent of chromium increases with the increasing t(i), reaching a maxi mum corresponding to the hydrogen amount in chromium hydride. The high er the HCOOH additive concentration, the longer t(i), at which the hyd rogen content in chromium reaches a maximum. The X-ray diffraction ana lysis confirms the conversion of bcc chromium into hcp chromium at an increase in the hydrogen content in the deposit and a decrease in the fraction of hcp chromium at t(i) corresponding to a decrease in the hy drogen content in chromium and to a decrease in the current efficiency after passing a maximum. The fact that the dependences of hydrogen co ntent in chromium and the current efficiency on t(i) are similar is ex plained by variation in the thickness and density of the cathodic film . The latter is associated with the redox interaction between HCOOH an d CrO3 and the concentration changes in the near-cathode layer. This e xplanation is supported by the effect of hydrodynamic conditions and t he role of other factors affecting the HCOOH oxidation in the electrol yte and the diffusion limitations of the electrode process.