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In this study, corrosion protection efficiency of undoped epoxy and epoxy coating doped with cerium on aluminum alloy AA6060 is evaluated over prolonged exposure to 3wt% NaCl solution by electrochemical impedance spectroscopy (EIS). Different amounts of Ce(NO3)(3) are added into the epoxy suspension used for the cataphoretic deposition. Suspension stability is characterized by zeta potential and particle size values. Water content and thermal stability of the coatings were determined by thermogravimetric analysis. Insufficient corrosion stability, very close to that of undoped epoxy, is revealed for the coating with 1mM Ce. Higher cerium doping contributes to a significant increase in corrosion stability (2-3 orders of magnitude greater charge-transfer resistance). Epoxy coating doped with 5mM Ce displays a pronounced self-healing ability and the best corrosion protection and adhesion, owing to evenly distributed cerium precipitates on the aluminum surface. In the case of 10mM Ce-doped coating, the formation of Ce clusters and higher porosity causes a certain lowering of the protective ability. EIS measurements on epoxy coating with artificial defect confirm these findings.

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 study, corrosion protection efficiency of undoped epoxy and epoxy coating doped with cerium on aluminum alloy AA6060 is evaluated over prolonged exposure to 3wt% NaCl solution by electrochemical impedance spectroscopy (EIS). Different amounts of Ce(NO3)(3) are added into the epoxy suspension used for the cataphoretic deposition. Suspension stability is characterized by zeta potential and particle size values. Water content and thermal stability of the coatings were determined by thermogravimetric analysis. Insufficient corrosion stability, very close to that of undoped epoxy, is revealed for the coating with 1mM Ce. Higher cerium doping contributes to a significant increase in corrosion stability (2-3 orders of magnitude greater charge-transfer resistance). Epoxy coating doped with 5mM Ce displays a pronounced self-healing ability and the best corrosion protection and adhesion, owing to evenly distributed cerium precipitates on the aluminum surface. In the case of 10mM Ce-doped coating, the formation of Ce clusters and higher porosity causes a certain lowering of the protective ability. EIS measurements on epoxy coating with artificial defect confirm these findings.

Protective coatings should provide good barrier to ingress of corrosive species, sufficient adhesion to the substrate as well as good compatibility with additional top coats. The presence of pigments and/or functional agents in the coating composition could be beneficial in the corrosion protection. The doping with rare earth elements, like cerium, has shown to enhance the corrosion stability of organic coatings due to their self-healing properties. In this work cerium doped nanometric epoxy primers were deposited cataphoretically with different amounts of cerium nitrate (1, 5 and 10 mM) with the aim of optimizing the cerium content in the epoxy emulsion with respect to adhesion and corrosion stability of deposited primer coating. Cerium-free epoxy coating was also analyzed as a reference. The long term protection was evaluated in the chloride environment (3 wt.%NaCl) using electrochemical impedance spectroscopy (EIS) and polarization measurements. Coatings were also analyzed by scanning electron microscopy (SEM) coupled with EDX, Fourier transform infrared spectroscopy and thermogravimetric measurements. Size and zeta potential of epoxy emulsion were measured by dynamic light scattering and Laser Doppler velocimetry. Adhesion was determined both by a standardized pull-off method and by swelling in N-methyl pyrrolidone. The ceria content, as well as its distribution in the epoxy coating, was determined by SEM cross-sectional analysis. The homogeneity of epoxy coatings was evaluated and related to corrosion stability. The results showed that cerium-doping can promote the formation of compact epoxy coatings with enhanced protectiveness in the early stage of immersion in the corrosive media. Increased barrier properties of epoxy primers suggest good compatibility of ceria particles with polymeric epoxy layer. However, zeta potential measurements indicated that higher amounts than 10 mM Ce(NO3)3 added into the epoxy emulsion resulted in unstable emulsion. The water content in the epoxy coatings and their thermal stability were also determined. In addition, a detailed EIS study was undertaken to follow the evolution of corrosion stability of all samples with time of exposure to corrosive media. It was shown that all examined ceria containing epoxy emulsions resulted with the enhanced adhesion and corrosion stability of epoxy primers. However, the smallest loading of 1 mM showed only improvement during the initial exposure to NaCl solution. EIS and polarization measurements confirmed that higher ceria loadings also provide long term protection in chloride solution.The optimal ceria content in the epoxy emulsion, with respect to adhesion strength, thermal stability and long-term corrosion stability of protective systems was 10 mM. June 07-11, 2015, Pravets, Bulgaria

Dip-immersion is a simple and cost-effective method for the preparation of Ce-based conversion coatings (CeCCs), a promising alternative to toxic chromate coatings, on metal substrates. In this work, CeCCs were prepared on Al-alloy AA6060 from an aqueous solution of cerium chloride at room temperature. The effect of immersion time and post-treatment in phosphate solution on the microstructure and corrosion properties of the coatings was studied. The longer the immersion time was, the thicker but more nonhomogeneous and cracked were the CeCCs. The post-treatment contributed to a sealing of the cracks, as proven by an increase in the corrosion resistance compared with the as-deposited coatings. The CeCCs prepared at longer deposition times and post-treated showed much better corrosion protection than those prepared at shorter deposition times. A detailed electrochemical impedance spectroscopy (EIS) study was undertaken to follow the evolution of the corrosion behaviour of the CeCCs with time of exposure to an aggressive chloride environment (3.5 % NaCl). For the sake of comparison, the EIS properties of bare AA6060 were also investigated. Linear voltammetry was performed to complete the study. The results confirmed the formation of protective CeCCs on the surface of AA6060. However, even CeCCs prepared at longer deposition times and post-treated provided short term protection in the aggressive environment, due to the small thickness of the coating. Nanošenje prevlaka potapanjem u rastvor je jednostavna i ekonomski prihvatljiva metoda za pripremu konverzionih prevlaka na bazi cerijuma na metalnim supstratima, koje su dobra zamena za toksične hromatne prevlake. U ovom radu prevlake na bazi cerijuma su nanesene na aluminijumsku leguru AA6060 iz vodenog rastvora cerijum-hlorida na sobnoj temperaturi. Ispitivan je uticaj vremena potapanja i uticaj naknadnog tretmana u fosfatnom rastvoru na mikrostrukturu i zaštitna svojstva dobijenih prevlaka. Dužim potapanjem su dobijene deblje ali nehomogene prevlake. Naknadni tretman je povoljno uticao na zatvaranje prskotina, što je u saglasnosti sa izmerenom većom korozionom otpornošću u poređenju sa prevlakama bez tretmana. Prevlake dobijene dužim nanošenjem su pokazale bolju korozionu zaštitu od prevlaka dobijenih kraćim nanošenjem. Koroziona stabilnost prevlaka je ispitivana u agresivnoj hloridnoj sredini (3,5 % NaCl) metodama spektroskopije elektrohemijske impedancije (SEI) i linearne voltametrije. Detaljno je praćeno koroziono ponašanje prevlaka sa vremenom delovanja korozionog agensa. Poređenja radi, SEI analiza je sprovedena i za leguru AA6060 bez prevlake. Rezultati istraživanja potvrđuju da dobijene prevlake na bazi cerijuma predstavljaju dobru korozionu zaštitu leguri AA6060. Međutim, čak i kvalitetnija prevlaka (20 min potapanja i naknadno tretirana u fosfatnom rastvoru) obezbeđuje leguri kratkoročnu zaštitu u agresivnoj sredini usled svoje male debljine.

In this study, the effect of deposition time and concentration of NaNO2 in the phosphate bath on the surface morphology of iron-phosphate coatings on low carbon steel was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition of ironphosphate coatings was determined using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), while surface coverage was evaluated by the voltammetric anodic dissolution (VAD) technique in the borate solution. The addition of NaNO2 to the phosphate bath significantly increased the surface coverage since better packed crystals of smaller size, which favour the phosphate nucleation, were obtained. It was also shown that prolonged deposition time increased the surface coverage, coating roughness and crystal size in the lateral direction, altering also the crystal shape from large platelets non-uniformly distributed on the steel surface during the initial time to better packed laminated and needle-like structures during prolonged exposure. U ovom radu je, primenom skenirajuće elektronske mikroskopije (SEM) i mikro- skopije međuatomskih sila (AFM), ispitivan uticaj vremena taloženja i koncentracije NaNO2 u rastvoru za fosfatiranje na morfologiju gvožđe-fosfatnih prevlaka na nisko-ugljeničnom čeliku. Sastav gvožđe–fosfatnih prevlaka je određen primenom energetske disperzione atomske analize (EDS) i difrakcijom H-zraka (XRD), dok je pokrivenost površine određivana anodnim rastvaranjem u boratnom rastvoru primenom linearne voltametrije (VAD). Pokazano je da dodatak NaNO2 u rastvoru za fosfatiranje značajno povećava pokrivenost površine čelika, jer se formiraju bolje pakovani kristali, manje veličine. Duže vreme taloženja takođe povećava pokrivenost površine, kao i hrapavost prevlake i širinu kristala. Oblik kristala se tokom dužeg vremena taloženja menja od kristala u obliku većih ploča neuniformno raspoređenih na površini čelika, ka bolje pakovanim laminarnim i igličastim kristalima.

In this study, the effect of deposition time and concentration of NaNO2 in the phosphate bath on the surface morphology of iron-phosphate coatings on low carbon steel was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition of iron-phosphate coatings was determined using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), while surface coverage was evaluated by the voltammetric anodic dissolution (VAD) technique in the borate solution. The addition of NaNO2 to the phosphate bath significantly increased the surface coverage since better packed crystals of smaller size, which favour the phosphate nucleation, were obtained. It was also shown that prolonged deposition time increased the surface coverage, coating roughness and crystal size in the lateral direction, altering also the crystal shape from large platelets non-uniformly distributed on the steel surface during the initial time to better-packed laminated and needle-like structures during prolonged exposure.

In this study, the effect of deposition time and concentration of NaNO2 in the phosphate bath on the surface morphology of iron-phosphate coatings on low carbon steel was investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The composition of ironphosphate coatings was determined using energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD), while surface coverage was evaluated by the voltammetric anodic dissolution (VAD) technique in the borate solution. The addition of NaNO2 to the phosphate bath significantly increased the surface coverage since better packed crystals of smaller size, which favour the phosphate nucleation, were obtained. It was also shown that prolonged deposition time increased the surface coverage, coating roughness and crystal size in the lateral direction, altering also the crystal shape from large platelets non-uniformly distributed on the steel surface during the initial time to better packed laminated and needle-like structures during prolonged exposure. U ovom radu je, primenom skenirajuće elektronske mikroskopije (SEM) i mikro- skopije međuatomskih sila (AFM), ispitivan uticaj vremena taloženja i koncentracije NaNO2 u rastvoru za fosfatiranje na morfologiju gvožđe-fosfatnih prevlaka na nisko-ugljeničnom čeliku. Sastav gvožđe–fosfatnih prevlaka je određen primenom energetske disperzione atomske analize (EDS) i difrakcijom H-zraka (XRD), dok je pokrivenost površine određivana anodnim rastvaranjem u boratnom rastvoru primenom linearne voltametrije (VAD). Pokazano je da dodatak NaNO2 u rastvoru za fosfatiranje značajno povećava pokrivenost površine čelika, jer se formiraju bolje pakovani kristali, manje veličine. Duže vreme taloženja takođe povećava pokrivenost površine, kao i hrapavost prevlake i širinu kristala. Oblik kristala se tokom dužeg vremena taloženja menja od kristala u obliku većih ploča neuniformno raspoređenih na površini čelika, ka bolje pakovanim laminarnim i igličastim kristalima.

Dip-immersion is a simple and cost-effective method for the preparation of Ce-based conversion coatings (CeCCs), a promising alternative to toxic chromate coatings, on metal substrates. In this work, CeCCs were prepared on Al-alloy AA6060 from an aqueous solution of cerium chloride at room temperature. The effect of immersion time and post-treatment in phosphate solution on the microstructure and corrosion properties of the coatings was studied. The longer the immersion time was, the thicker but more nonhomogeneous and cracked were the CeCCs. The post-treatment contributed to a sealing of the cracks, as proven by an increase in the corrosion resistance compared with the as-deposited coatings. The CeCCs prepared at longer deposition times and post-treated showed much better corrosion protection than those prepared at shorter deposition times. A detailed electrochemical impedance spectroscopy (EIS) study was undertaken to follow the evolution of the corrosion behaviour of the CeCCs with time of exposure to an aggressive chloride environment (3.5 % NaCl). For the sake of comparison, the EIS properties of bare AA6060 were also investigated. Linear voltammetry was performed to complete the study. The results confirmed the formation of protective CeCCs on the surface of AA6060. However, even CeCCs prepared at longer deposition times and post-treated provided short term protection in the aggressive environment, due to the small thickness of the coating. Nanošenje prevlaka potapanjem u rastvor je jednostavna i ekonomski prihvatljiva metoda za pripremu konverzionih prevlaka na bazi cerijuma na metalnim supstratima, koje su dobra zamena za toksične hromatne prevlake. U ovom radu prevlake na bazi cerijuma su nanesene na aluminijumsku leguru AA6060 iz vodenog rastvora cerijum-hlorida na sobnoj temperaturi. Ispitivan je uticaj vremena potapanja i uticaj naknadnog tretmana u fosfatnom rastvoru na mikrostrukturu i zaštitna svojstva dobijenih prevlaka. Dužim potapanjem su dobijene deblje ali nehomogene prevlake. Naknadni tretman je povoljno uticao na zatvaranje prskotina, što je u saglasnosti sa izmerenom većom korozionom otpornošću u poređenju sa prevlakama bez tretmana. Prevlake dobijene dužim nanošenjem su pokazale bolju korozionu zaštitu od prevlaka dobijenih kraćim nanošenjem. Koroziona stabilnost prevlaka je ispitivana u agresivnoj hloridnoj sredini (3,5 % NaCl) metodama spektroskopije elektrohemijske impedancije (SEI) i linearne voltametrije. Detaljno je praćeno koroziono ponašanje prevlaka sa vremenom delovanja korozionog agensa. Poređenja radi, SEI analiza je sprovedena i za leguru AA6060 bez prevlake. Rezultati istraživanja potvrđuju da dobijene prevlake na bazi cerijuma predstavljaju dobru korozionu zaštitu leguri AA6060. Međutim, čak i kvalitetnija prevlaka (20 min potapanja i naknadno tretirana u fosfatnom rastvoru) obezbeđuje leguri kratkoročnu zaštitu u agresivnoj sredini usled svoje male debljine.

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.

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.

Chromium dissolution in aqueous sulfuric acid solution of pH 1 was studied electrochemically on chromium electrodes with different crystallographic structures. A slow potentiodynamic method was used for the electrochemical measurements in deaerated solutions (purged with nitrogen),while the Cr(III) ions in the solution after the corrosion were determined by atomic absorption spectrometry. Three electrode materials with a very dominant crystallite orientation resembling single crystal structures (i.e., 111 and 110) confirmed by the electron backscattering diffraction (EBSD), were used in the experiments. The (111) structures were somewhat more active electrochemically (both anodic and cathodic) than the (110) structure. However, Cr electrochemically deposited in standard plating bath, assumed from literature data to has also the (111) structure, was more than 4 times active for anodic dissolution and, by the same number, less active for cathodic hydrogen evolution. The concentrations of Cr(III) ions determined in the solution after definite times of corrosion of all the materials showed almost two times larger dissolution rates than observed electrochemically by three different electrochemical methods (Wagner-Traud, Stern-Geary, electrochemical impedance spectroscopy). This is explained by the simultaneous occurrence of potential independent chemical dissolution of Cr, by a direct reaction of metallic Cr with H2O molecules, proposed a long time ago by Kolotyrkin and coworkers. . Proučavano je elektrohemijsko rastvaranje hroma u vodenim rastvorima sumporne kiseline (pH 1) sa elektrodama od hroma različite kristalografske strukture. Primenjena je spora potenciodinamička metoda u deaeriranim rastvorima (uz provođenje azota) na 25°C, a joni Cr(III) u rastvoru posle određenog vremena korozije određivani su atomskom apsorpcionom spektroskopijom. U eksperimentima su upotrebljena tri elektrodna materijala sa dominantnim kristalnim strukturama koje podsećaju na mono kristalne (tj. 111 i 110), a što je potvrđeno EBSD metodom. Nađeno je da je struktura (111) elektrohemijski aktivnija (i anodno i katodno) od strukture (110). Međutim, elektrolitički istaložen Cr iz standardnog kupatila za hromiranje, a koji na osnovu literaturnih podataka ima strukturu (111) bio je oko 4 puta aktivniji u anodnoj reakciji i isto toliko manje aktivan za katodnu reakciju izdvajanja vodonika. Analitički određivane koncentracije Cr(III) jona u rastvoru posle određenog vremena spontane korozije pokazivale su dva puta veće koncentracije nego što bi se očekivalo na osnovu brzine elektrohemijske korozije, određivanih metodama Wagner-Traud, Stern-Geary i elektrohemijskom impedansnom spektroskopijom. Ovo je objašnjeno jednovremenim odigravanjem i elektrohemijske reakcije i hemijske reakcije direktnog reagovanja metalnog Cr sa molekulima vode, po mehanizmu predloženom od Kolotirkina i saradnika. .

Open circuit potential measurements and cyclic voltammetry of chromium in deaerated aqueous H2SO4 solution of pH 1 without and containing NaCl in the concentration range 0.5 - 4 M revealed that chromium exhibits two stable open circuit potentials having the character of a Wagner-Traud corrosion potential. One, E corr.1, was established on the passive surface formed by exposing Cr previously to air or passivated potentiostacially in a controlled manner, and the second one, E corr,2, at the bare Cr surface obtained by prolonged cathodic activation. There is a small difference in the E corr,1 values as a function of the passive layer properties. Addition of NaCl accelerates the hydrogen evolution reaction on the passive surface to some extent, while the same reaction on the bare surface was not affected by NaCl. On the other hand, presence of NaCl accelerates the anodic reaction on the bare surface, and it activates the dissolution of the passive layer so that the passive currents increase with addition of NaCl. This effect is so large that at concentration of NaCl larger than 3 M, the destruction of the passive layer was so fast that in a matter of seconds the Cr was activated, and the only one stable corrosion potential observed was E corr.2. No pitting of Cr in the presence of NaCl was observed up to the transpassive potentials. Merenja potencijala otvorenog kola, kao i merenja primenom ciklične voltametrije u deaerisanim vodenim rastvorima H 2SO4, pH1, bez i sa dodatkom NaCl u koncentracionom opsegu 0,5-4M, otkrila su da hrom pokazuje dva stabilna potencijala otvorenog kola koji imaju osobine Wagner-Traud-ovog korozionog potencijala. Jedan korozioni potencijal, E corr.1 , se uspostavljao na pasiviranoj površini, koja se formira prethodnim izlaganjem Cr vazduhu, ili poteciostatskim pasiviranjem na kontrolisan način, a drugi, E corr.2 , se obrazovao na goloj površini Cr koja je formirana katodnom aktivacijom u dovoljno dugom periodu vremena. Postoji izvesna mala razlika u vrednostima E corr.1 kao funkcija osobina pasivnog sloja. Dodatak NaCl u određenoj meri ubrzava reakciju izdvajanja vodonika na pasiviranoj površini, dok NaCl nema uticaja na istu reakciju na goloj površini Cr. Sa druge strane prisustvo NaCl ubrzava anodnu reakciju na goloj površini hroma i aktivira rastvaranje pasivnog sloja, tako da se struja pasivacije povećava sa dodatkom NaCl. Uticaj NaCl je toliko veliki, da pri koncentracijama većim od 3M, razaranje pasivnog sloja postaje brzo, da se Cr za vreme od nekoliko sekundi aktivira i tada se uočava samo jedan stabilni korozioni potencijal E corr.2 . Nije uočena pojava pitinga u prisustvu NaCl, sve do potencijala u transpasivnoj oblasti.

The potentiodynamic study of the electrochemical behavior of austenitic 304 stainless steel in deaerated aqueous sulfuric acid of pH 1 revealed that the steel achieved a stable corrosion potential of ca. - 0.350 V (SCE) independent of whether the electrode had previously been cathodically "activated" or anodically passivated. It was also shown that the experimentally observed anodic peak was not the usually obtained anodic passivation peak, as is the case with a number of metal, but an artifact due to the anodic oxidation of hydrogen absorbed during the previously employed cathodic polarization and hydrogen evolution, intended to activate the initially passive surface, or even hydrogen absorbed on the open circuit potential. It was shown that this potential establishes and electrochemical corrosion potential of the Wagner-Traud type due to the evolution of cathodic hydrogen on a passivated steel surface and anodic metal dissolution through the passive layer. It was impossible to activate 304 stainless steel in sulfuric acid of pH 1 by cathodic polarization, and the usually observed anodic peak obtained under these conditions should not be considered as an active metal dissolution process and a passivation anodic peak, but rather as an artifact due to the electrochemical oxidation of the in the steel absorbed hydrogen. Potenciodinamička ispitivanja elektrohemijskog ponašanja nerđajućeg čelika 304 u deaeriranoj sumpornoj kiselini sa pH 1 pokazala su da se na čeliku uspostavlja stabilan korozioni potencijal od - 0,350 V(ZKE) nezavisno od toga da li je elektroda prethodno katodno tretirana radi "aktivacije", ili je površina bila pasivirana. Pokazano je, takođe, da eksperimentalno dobijeni anodni maksimum na potenciodinamičkoj krivi pozitivnije od korozionog potencijala nije anodni pasivacioni maksimum koji se obično dobija pri anodnoj polarizaciji većeg broja metala u sličnim uslovima, već eksperimentalni artefakt koji nastaje zbog anodne oksidacije vodonika apsorbovanog unutar čelika tokom katodnog "aktiviranja" ili čak i pri dužem držanju na korozionom potencijalu. Pokazano je da je spontano formirani potencijal otvorenog kola zapravo korozioni potencijal koji nastaje kao mešoviti potencijal Vagner-Traudovog tipa suprotnim delovawem katodne reakcije izdvajanja vodonika na pasivnom sloju i anodnog rastvaranja čelika kroz pasivni sloj. Nije bilo mogućno katodnom "aktivacijom" i dugotrajnijom katodnom polarizacijom do značajno negativnih potencijala i vrlo velikih katodnih struja da se pasivni sloj ukloni i postigne aktivno elektrohemijsko rastvaranje metala bez prisustva pasivnog sloja, tj. slobodna metalna površina. Stoga, često eksperimentalno konstatovan anodni maksimum na ovakvim čelicima ne treba da se interpretira kao anodno rastvaranje sa pasivacionim maksimumom, već kao posledica anodne oksidacije apsorbovanog vodonika u pasivnom čeliku, a koji se pod ovim uslovima ne može elektrohemijski aktivirati, odn. depasivirati.

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.

Chromium dissolution in aqueous sulfuric acid solutions in the pH range 0.5 –3 was studied electrochemically by the potentiostatic or very slow potentiodynamic method, and by the analyses of the Cr ion concentrations in the electrolyte formed during the experiments. It was shown that the electrochemical anodic dissolution follows a common Tafel line with a slope of ca. 120 mVdec-1, independent of the solution pH and the hydrodynamics while the passivation potentials and passivation currents were independent on hydrodynamics but strongly dependent on the pH. In parallel with the electrochemical dissolution, a considerable "anomalous" or chemical Cr dissolution process occurs, as evidenced by the spectrophotometric analyses of the electrolytes for Cr ions after prolonged potentiostating of the electrodes at different potentials, as well as by measuring the electrode weight losses. All these results indicate the existence of a potential independent reaction of Cr dissolution occurring in parallel to the anodic dissolution process. Mechanisms for both the electrochemical and the chemical process are proposed. The consequences of these phenomena on the behavior of some practical systems where chromium or a chromium alloy (e.g., stainless steels) are used are discussed. Proučavano je anodno rastvaranje hroma u vodenim rastvorima sumporne kiseline u oblasti pH 0,5 – 3 potenciostatskom i vrlo sporom potenciodinamičkom metodom, kao i analitičkim praćenjem promene koncentracije Cr jona tokom eksperimenata. Pokazano je da elektrohemijsko anodno rastvaranje prati uobičajeno Tafelovo ponašanje sa nagibom Tafelove prave od oko 120 mV dec-1 kao i da je nezavisno od pH i hidrodinamike. Međutim, potencijali pasivacije i struje pasivacije takođe su nezavisni od hidrodinamike, ali vrlo zavisne od pH. Jednovremeno sa elektrohemijskim rastvaranjem odigrava se i značajno "anomalno" ili hemijsko rastvaranje hroma. Ovo je potvrđeno i spektrofotometrijskim analizama rastvora tokom dužeg potenciostatskog održavanja katodnih i anodnih polarizacija, kao i merenjem gubitka mase elektrode. Svi ovi rezultati ukazuju na odigravanje nezavisne reakcije rastvaranja hroma koja nije zavisna od potencijala, kao što je slučaj sa elektrohemijskom reakcijom. Predloženi su mehanizmi odigravanja oba navedena procesa. Razmotrene su i posledice postojanja oba navedenafenomena na ponašanje nekih praktičnih sistema u kojima se koristi hrom ili legure hroma (npr. nerđajući čelik).