Uticaj strukture metalnog hroma na njegovu elektrohemijsku i hemijsku koroziju

It was shown that metallic chromium in aqueous solution of sulfuric acid dissolves in accord with the electrochemical mechanism proposed by Wagner and Traud, but also by a direct chemical reaction of metal with water molecules from the electrolyte, as proposed by Kolotyrkin and coworkers. The experiments in this study were carried out with three different chromium samples: large grain size cast chromium obtained from Merck Co., Germany fine grain metallic chromium rods, obtained from Goodfellows, Pa, USA, and a 121 џт thick electroplated layer of chromium on steel. It was shown that the rates of electrochemical and chemical corrosion are close to each other and similar on all three materials, while on the electroplated chromium surfaces anodic dissolution was ca. 5 times faster than on other chromium surfaces while catholic hydrogen evolution was ca. 5 times slower than on other chromium surfaces. These differences are obviously the consequences of the differences in the structure of electroplated chromium as contrasted to the other chromium samples. This conclusion is supported by the results obtained with the samples with two large crystal grains of different structure and orientation.Pokazano je da se u vodenim rastvorima sumporne kiseline metalni hrom rastvara elektrohemijskim mehanizmom u skladu sa modelom Vaguera i Trauda ali da se hrom paralelno rastvara i hemijskim mehanizmom po modelu Kolotirkina i saradnika direktnim reagovanjem metala sa molekulima vode iz rastvora. Kao materijal za ispitivanje služio je krupnozrni liveni hrom sitnozrni kovani i valjani hrom i elektrolitički istaložena hromna prevlaka. Pokazano je da su brzine elektrohemijske i hemijske korozije međusobno slične kod sva tri materijala, dok je elektrohemijska prevlaka imala oko 5 puta bržu anodnu reakciju i skoro 5 puta sporiju katodnu reakciju izdvajanja vodonika. Ove razlike posledica su svakako razlika u strukturi i kristalografskoj orijentaciji hromne prevlake prema rastvoru, što se u izvesnoj meri uočava i na primeru krupnozrne livene strukture, gde dva različita velika zrna imaju određene razlike u brzinama ovih elektrohemijskih reakcija, iako u znatno manjoj meri