Search

In this paper, phenomena of chemical corrosion of metals and alloys in electrolyte solutions are analyzed. It is shown that iron, chromium and other metals and alloys dissolve much faster than is the corrosion rate determined by electrochemical methods. This means that the chemical dissolution takes place simultaneously with the electrochemical dissolution. The chemical dissolution does not depend on electrode potential. Under some conditions, chemical dissolution of metals is the dominant process of dissolution. Several mechanisms of chemical corrosion are described. Also, the consequences of chemical dissolution of various metals and alloys are discussed, as well as the hydrogen evolution during the chemical dissolution. The process of hydrogen evolution during the chemical corrosion is not subjected to the laws of electrochemical kinetics., U ovom preglednom radu analizirana je hemijska korozija metala i legura u rastvorima elektrolita. Pokazano je da se gvožđe, hrom i neki drugi metali i legure rastvaraju mnogo brže nego što je njihova brzina korozije određena elektrohemijskim metodama. To znači da se istovremeno sa elektrohemijskim rastvaranjem odvija i hemijsko rastvaranje, koje ne zavisi od potencijala. U nekim uslovima hemijsko rastvaranje metala je dominantan proces rastvaranja. Opisano je više mehanizama hemijske korozije. Takođe, razmatrane su posledice hemijskog rastvaranja različitih metala i legura, kao i pojava izdvajanja vodonika pri hemijskom rastvaranju. Proces izdvajanja vodonika pri hemijskoj koroziji ne podleže zakonitostima elektrohemijske kinetike.

In this work we studied the electrochemical behavior of chromium white cast irons in deaerated aqueous sulphuric acid solutions of the pH 1-3. The measurements of concentrations of dissolved iron and chromium in the solution, as well as the volume of hydrogen evolved were carried out. By comparing the results, it was found that besides the electrochemical corrosion, another, chemical, process occurs at the same time. This process takes place at the surface of the metal, in the contact with the water molecules or hydrogen ions, and produces hydrogen evolution. These results show, that the usual electrochemical determination of the metal corrosion rate must be very critically applied., U ovom radu proučavano je elektrohemijsko ponašanje visokohromnog belog gvožđa u dearisanim rastvorima sumporne kiseline u oblasti pH 1-3. Provedena su merenja koncentracije rastvorenog gvožđa i hroma u rastvoru, kao i zapremine izdvojenog vodonika. Poređenjem rezultata konstatovano je da pored elektohemijske korozije jednovremeno teče i drugi hemijski proces koji se odigrava na površini metala u kontaktu sa molekulima vode ili vodoničnim jonima uz izdvajanje vodonika. Ovi rezultati ukazuju da se uobičajene metode elektrohemijskog određivanja brzine korozije metala moraju primenjivati vrlo kritički, uz prethodno dokazivanje da su ovakvi hemijski procesi u odnosu na elektohemijske na ispitivanom metalu relativno zanemarljivi.

The electro deposition of the Co-Ni-Mo-O alloy powders from alkaline ammonium electrolytes and at constant current density was investigated. Metals of iron group, Co and Ni induce deposition of Mo, which alone can not be deposited from water electrolytes. Electrodeposited powders were investigated by SEM analysis where it is possible to determine the particle size, as well as the morphology of powders. The quantitative mass and chemical composition of Co-Ni-Mo-O alloy powders was determined by EDX analysis., Alkalni, amonijačni elektrolit je korišćen za dobijanje prahova Co-Ni-Mo-O legure. Prahovi su taloženi pri konstantnoj gustini struje, kao sporedna reakcija pri reakciji intenzivnog izdvajanja vodonika. Metali grupe gvožđa Co i Ni indukuju taloženje Mo, koji sam ne može da se taloži iz vodenih elektrolita. Dobijeni prahovi su ispitani EDX, X-ray i SEM analizom i na osnovu dobijenih rezultata može se zaključiti da se promenom odnosa koncentracija soli u elektrolitu za taloženje, kao i gustinom struje taloženja može uticati na hemijski sastav i veličinu čestica prahova Co-Ni-Mo-O legure. Rezultati rendgenske analize ukazuju na amorfnu strukturu dobijenih prahova.

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.