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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.

Ce-based conversion coatings (CeCCs) are a promising alternative to toxic chromate coatings on the metal substrates. In this work the CeCCs were electrodeposited on aluminium alloy AA6060 from aqueous solution of Ce(NO3)(3) at different potentials (-0.95 V, -1.2 V and -1.4V). Effect of deposition potential and post-treatment in the phosphate solution on morphology and protective properties of CeCCs with top cataphoretic epoxy coating was studied. To assess the differences between the protective systems, originating from the different CeCCs pre-treatments, electrochemical impedance spectroscopy (EIS), polarization measurements, AFM and SEM/EDS analysis were used. The EIS study was undertaken to follow the evolution of corrosion behaviour of epoxy coating/CeCCs protective systems over prolonged time of exposure to the chloride environment (3 wt.% NaCl). Results suggest significantly improved corrosion stability of epoxy coating on AA6060 with as-deposited CeCCs sub-layers with respect to the same epoxy coatings with phosphate post-treated CeCCs. The far best protective properties, i.e., the greatest value of pore resistance and the lowest value of corrosion current density were provided by the epoxy coating/CeCC protective system with CeCC deposited at -1.2V and without post-treatment.

Among potential replacements for chromates, cerium-based conversion coatings (CeCCs) have been identified as leading candidates, showing a good promise as environmentally compliant corrosion inhibitors on Al alloys. In this work, CeCCs were synthesized on M6060 by electrolytic deposition (ELD), a well-suited method for the fabrication of films with controllable properties, at low processing temperature and cost. The CeCCs were deposited at room temperature from Ce(NO3)(3) solution, applying different cathodic potentials (-1.1, -1.4 and -1.6 V vs. SCE) for different deposition times between 5 and 20 min. Structural, morphological and compositional properties of the films were characterized by X-ray diffraction, Raman spectroscopy and scanning electron microscopy coupled with energy dispersive spectroscopy. Ceria films were nanostructured, exhibiting a cubic fluorite structure. From corrosion perspective, the quality of CeCCs was evaluated in 0.5 M NaCI solution via electrochemical impedance spectroscopy and linear sweep voltammetry. As shown, the coating corrosion quality is the interplay between deposition potential and deposition time. The highest potential led to highly cracked films of poor adherence, even at the shortest deposition time, which disqualified them from any corrosion protection application. The lowest potential (-1.1 V) produced crack-free coatings, but longer times ( gt = 10 min) were needed to provide the protective layer formation. The best corrosion protection offered CeCCs prepared at -1.4 V. The highest R-p (582 k Omega cm(2)) and the smallest CPE (12.6 . 10(-6) s(n) Omega(-1) cm) were obtained at 20 min deposition time. However, from economical point of view, 10 min deposition is a promising choice. These CeCCs also improved pitting corrosion resistance owing to homogeneous microstructure and sufficient thickness.

Among potential replacements for chromates, cerium-based conversion coatings (CeCCs) have been identified as leading candidates, showing a good promise as environmentally compliant corrosion inhibitors on Al alloys. In this work, CeCCs were synthesized on M6060 by electrolytic deposition (ELD), a well-suited method for the fabrication of films with controllable properties, at low processing temperature and cost. The CeCCs were deposited at room temperature from Ce(NO3)(3) solution, applying different cathodic potentials (-1.1, -1.4 and -1.6 V vs. SCE) for different deposition times between 5 and 20 min. Structural, morphological and compositional properties of the films were characterized by X-ray diffraction, Raman spectroscopy and scanning electron microscopy coupled with energy dispersive spectroscopy. Ceria films were nanostructured, exhibiting a cubic fluorite structure. From corrosion perspective, the quality of CeCCs was evaluated in 0.5 M NaCI solution via electrochemical impedance spectroscopy and linear sweep voltammetry. As shown, the coating corrosion quality is the interplay between deposition potential and deposition time. The highest potential led to highly cracked films of poor adherence, even at the shortest deposition time, which disqualified them from any corrosion protection application. The lowest potential (-1.1 V) produced crack-free coatings, but longer times (>= 10 min) were needed to provide the protective layer formation. The best corrosion protection offered CeCCs prepared at -1.4 V. The highest R-p (582 k Omega cm(2)) and the smallest CPE (12.6 . 10(-6) s(n) Omega(-1) cm) were obtained at 20 min deposition time. However, from economical point of view, 10 min deposition is a promising choice. These CeCCs also improved pitting corrosion resistance owing to homogeneous microstructure and sufficient thickness.

Cerium-based conversion coatings (CeCCs) are one of the most prospective alternatives to the widely used chromate conversion coatings (CCCs) due to their anticorrosion efficiency, environmentally friendly nature and low cost. In this work, the CeCCs on AA6060 were prepared by immersion into aqueous cerium salt solutions at room temperature, and subsequently post-treated in heated phosphate solution. The effect of counter ion (nitrate and chloride) on the coating properties was studied testing CeCCs as sole or conversion layers for the top polyester coating. Since the 60 mu m thick polyester coating was applied, an artificial defect of 0.8 mm hole was introduced to faster assess the differences between pretreatments. The system with CCC pretreatment was used as reference. Corrosion properties were investigated in 0.5 M NaCl solution by electrochemical impedance spectroscopy while the adhesion strength was measured by NMPR (N-methyl-2-pyrrolidone) and pull-off tests. As shown, the post-treated chloride-based CeCC offered better protection than crack-free thin nitrate-based CeCC, when used as sole coatings. On the other hand, it was brought to evidence that in combination with top powder polyester coating, the CeCC deposited from nitrate solution exhibited better protection compared to protective system pretreated with chloride-based one. Excellent polyester coating adhesion was found independently on aluminium surface pretreatment.

Cerium-based conversion coatings (CeCCs) are one of the most prospective alternatives to the widely used chromate conversion coatings (CCCs) due to their anticorrosion efficiency, environmentally friendly nature and low cost. In this work, the CeCCs on AA6060 were prepared by immersion into aqueous cerium salt solutions at room temperature, and subsequently post-treated in heated phosphate solution. The effect of counter ion (nitrate and chloride) on the coating properties was studied testing CeCCs as sole or conversion layers for the top polyester coating. Since the 60 mu m thick polyester coating was applied, an artificial defect of 0.8 mm hole was introduced to faster assess the differences between pretreatments. The system with CCC pretreatment was used as reference. Corrosion properties were investigated in 0.5 M NaCl solution by electrochemical impedance spectroscopy while the adhesion strength was measured by NMPR (N-methyl-2-pyrrolidone) and pull-off tests. As shown, the post-treated chloride-based CeCC offered better protection than crack-free thin nitrate-based CeCC, when used as sole coatings. On the other hand, it was brought to evidence that in combination with top powder polyester coating, the CeCC deposited from nitrate solution exhibited better protection compared to protective system pretreated with chloride-based one. Excellent polyester coating adhesion was found independently on aluminium surface pretreatment.

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 non-homogeneous 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.

The influence of different types of conversion phosphate coatings on the corrosion stability of top powder polyester coating on steel was investigated. The morphology of phosphate coatings was investigated using scanning electron microscopy (SEM), composition by energy dispersive X-ray spectroscopy (EDS) and surface coverage by voltametric anodic dissolution (VAD) technique. The corrosion stability of polyester coating on steel pretreated by phosphate coatings was investigated by electrochemical impedance spectroscopy (EIS). It was shown that small crystal nuclei of iron-phosphate coating have sporadically emerged on the steel substrate while zinc-phosphate coating had more compact structure with better surface coverage. After prolonged exposure, there was a significant decrease in Rp values for polyester/zinc-phosphate protective system, as a result of the disbondment of polyester coating, while Rp values for polyester coating on steel pretreated with iron-phosphate coating remained almost unchanged, pointing to a greater stability of this protective system.

The influence of steel surface pretreatment with different types of iron-phosphate coatings on the corrosion stability and adhesion characteristics of polyester coatings on steel was investigated. The phosphate coating was chemically deposited either from the simple novel plating bath, or with the addition of NaNO2, as an accelerator in the plating bath. The morphology of phosphate coatings was investigated using atomic force microscopy (AFM). The corrosion stability of polyester coatings on steel pretreated by iron-phosphate coatings was investigated by electrochemical impedance spectroscopy (EIS) in 3% NaCl solution, while "dry" and "wet" adhesion were measured by a direct pull-off standardized procedure. It was shown that greater values of pore resistance. R-p, and smaller values of coating capacitance of polyester coating, C-c, on steel pretreated with iron-phosphate coating were obtained, as compared to polyester coating on steel phosphated with accelerator, and on the bare steel. The surface roughness of phosphate coating deposited on steel from the bath without accelerator is favorable in forming stronger bonds with polyester coating. Namely, the dry and wet adhesion measurements are in accordance with EIS measurements in 3% NaCl solution, i.e. lower adhesion values were obtained for polyester coating on steel phosphated with accelerator and on the bare steel, while the iron-phosphate pretreatment from the novel bath enhanced the adhesion of polyester coating on steel.

The influence of deposition temperature and concentration of NaNO2 in the phosphating bath on the surface morphology and coverage of iron-phosphate coatings on low carbon steel was investigated. The phosphate coatings were chemically deposited on steel from phosphate bath at different temperatures (30-70 degrees C) and with the addition of different amounts of accelerator, NaNO2 (0.1, 0.5 and 1.0 g dm(-3)). The morphology of phosphate coatings 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). Surface coverage was evaluated by the voltammetric anodic dissolution (VAD) technique. It was shown that the increase in temperature of the NaNO2-free phosphating bath up to 70 degrees C caused an increase in surface coverage. The addition of NaNO2 in the phosphating bath significantly increased the surface coverage of phosphate coatings deposited at temperatures lower than 50 degrees C. The phosphate crystals were of laminated and needle-like structures for deposits obtained at temperatures lower than 50 degrees C, while at higher temperatures needle-like structure was transformed to laminated structure. The increase in NaNO2 concentration in the phosphating bath from 0.1 to 1.0 g dm(-3) did not significantly increase the surface coverage, but decreased the crystals size, consequently favouring the phosphate nucleation and better packing of the crystals.

The effect of different type of iron-phosphate coatings on corrosion stability and adhesion characteristic of top powder polyester coating on steel was investigated. Iron-phosphate coatings were deposited on steel in the novel phosphating bath with or without NaNO2 as an accelerator. The corrosion stability of the powder polyester coating was evaluated by electrochemical impedance spectroscopy (EIS), adhesion by pull-off and NMP test, while surface morphology of phosphate coatings were investigated by atomic force microscopy (AFM). The adhesion and corrosion stability of powder polyester coatings were improved with pretreatment based on iron-phosphate coating deposited from NaNO2-free bath.

One of the most important factors in corrosion prevention by protective coatings is the coating adhesion loss under environmental influence. Thus, adhesion strength is often used when characterizing protective properties of organic coatings on a metal substrate. In order to improve the adhesion of organic coating the metal substrate is often pretreated in some way. In this work, the adhesion of polyester coatings on differently pretreated aluminium surface (by anodizing, with and without sealing, by phosphating and by silane film deposition) was examined. The dry and wet adhesion of polyester coatings were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test. It was shown that under dry test conditions all polyester coatings showed very good adhesion, but that aluminium surface pretreated by silane film showed superior adhesion. The overall increase of wet adhesion for polyester coating on aluminium pretreated by silane film was maintained throughout the whole investigated time period. The different trends in the change of adhesion of polyester coatings were observed for different aluminium pretreatments during exposure to the corrosive agent (3% NaCl solution). The highest adhesion reduction was obtained for polyester coating on aluminium pretreated with phosphate coating. The corrosion stability of polyester coated aluminium was investigated by electrochemical impedance spectroscopy in 3% NaCl solution. The results confirmed good protective properties of polyester coating on aluminium pretreated with silane film, i.e. greater values of pore resistance and smaller values of coating capacitance were obtained in respect to other protective systems, whereas charge-transfer resistance and double-layer capacitance were not measurable during 2 months of exposure to a corrosive agent.

Ispitivan je uticaj temperature na elektrohemijsko i hemijsko rastvaranje tri vrste hroma u neutralnim i kiselim rastvorima sulfata, sa i bez dodatka hloridnih jona (krupnozrni liveni hrom, sitnozrni valjani hrom i prevlaka hroma). Takođe su ispitivane elektrohemijske i korozione karakteristike nerđajućeg čelika 304 (18Cr-9Ni).Brzine elektrohemijske i hemijske korozije su međusobno slične kod sve tri vrste hroma, iako prevlaka hroma ima 5 puta bržu anodnu reakciju i skoro 5 puta sporiju katodnu reakciju izdvajanja vodonika. Hrom sa teksturom (110) je manje elektrohemijski aktivan (anodno i katodno) u odnosu na hrom sa teksturom (111). Kristalna ravan (110) je najgušće pakovana ravan kod hroma.Reakcije izdvajanja vodonika, anodnog rastvaranja i hemijskog rastvaranja hroma sa aktivirane površine hroma u rastvoru sumporne kiseline pH 1 slede Arenijusovu zavisnost sa prividnom energijom aktivacije 35 kJ mol-1, 58 kJ mol-1 i 62 kJ mol-1, respektivno. Veća energija aktivacije za anodni proces, nego za katodni, dovela je do pomeranja korozionog potencijala u smeru negativnih potencijala za približno 0,8 mV K-1. Velika energija aktivacije hemijskog rastvaranja hroma dovodi na povišenim temperaturama do znatno izraženije hemijske korozije u odnosu na elektrohemijsku koroziju.Pri koncentracijama Cl- jona manjim od 3 M, korozioni potencijal je posledica simultanog odvijanja reakcije izdvajanja vodonika na oksidom prekrivenoj površini hroma sa reakcijom anodnog rastvaranja hroma kroz pasivni film (Ekor.1) ili katodnim izdvajanjem vodonika i anodnim rastvaranjem ogoljene površine kada se površina hroma depasivira (Ekor.2). Za koncentracije Cl- jona veće od 3,5 M površina pasivnog hroma se spontano aktivira i tada se primećuje samo jedan korozioni potencijal Ekor.2.Primećeni anodni pik na elektrodama od nerđajućeg čelika 304 u sumpornoj kiselini nije realni anodni pasivacioni pik, kao što je primećeno za veći broj metala u sličnim uslovima, već potiče od anodne oksidacije vodonik, The influence of temperature on electrochemical and chemical dissolution of three types of chromium (cast chromium, metallic fine-grained chromium, and electroplated coating of chromium on steel coupons) was studied in neutral and acidic sulphuric solutions, with and without the addition of chloride ions. Also, electrochemical and corrosion characteristics of the stainless steel 304 (18Cr-9Ni) electrode were studied.The electrochemical and chemical corrosion rate of all types of chromium was similar, although chromium coating has 5 times faster anodic reaction, and almost 5 times slower hydrogen evolution cathodic reaction. The chromium with texture (110) was somewhat less active electrochemically (both anodic and cathodic), than the chromium with texture (111). Crystalline plane (110) is the most dense packed chromium plane.The hydrogen evolution reaction, as well as the anodic dissolution and chemical dissolution of chromium from activated surfaces in sulphuric acid solutions pH 1 follows Arrhenius law, with the apparent activation energies of 35 kJ mol-1, 58 kJ mol-1 and 62 kJ mol-1, respectively. The higher activation energy for the anodic process, than for the cathodic one resulted in shifting of corrosion potential in the negative direction for approximately 0.8 mV K-1. The higher activation energy for chemical dissolution than for the electrochemical chromium corrosion at higher temperatures increases considerably the part of chemical corrosion in the overall corrosion of chromium.When the concentration of Cl- ions is smaller than 3 M, corrosion potential is the result of simultaneous cathodic reaction of hydrogen evolution on chromium oxide covered surface and anodic dissolution of chromium through the passive film (Ecorr.1), or by the cathodic hydrogen evolution and anodic dissolution on bare chromium, when the chromium surface is activated (Ecorr.2). When the concentration of Cl- ions is higher than 3.5 M, the passive chromium surface is immediately activa

Ispitivan je uticaj temperature na elektrohemijsko i hemijsko rastvaranje tri vrste hroma u neutralnim i kiselim rastvorima sulfata, sa i bez dodatka hloridnih jona (krupnozrni liveni hrom, sitnozrni valjani hrom i prevlaka hroma). Takođe su ispitivane elektrohemijske i korozione karakteristike nerđajućeg čelika 304 (18Cr-9Ni). Brzine elektrohemijske i hemijske korozije su međusobno slične kod sve tri vrste hroma, iako prevlaka hroma ima 5 puta bržu anodnu reakciju i skoro 5 puta sporiju katodnu reakciju izdvajanja vodonika. Hrom sa teksturom (110) je manje elektrohemijski aktivan (anodno i katodno) u odnosu na hrom sa teksturom (111). Kristalna ravan (110) je najgušće pakovana ravan kod hroma. Reakcije izdvajanja vodonika, anodnog rastvaranja i hemijskog rastvaranja hroma sa aktivirane površine hroma u rastvoru sumporne kiseline pH 1 slede Arenijusovu zavisnost sa prividnom energijom aktivacije 35 kJ mol-1, 58 kJ mol-1 i 62 kJ mol-1, respektivno. Veća energija aktivacije za anodni proces, nego za katodni, dovela je do pomeranja korozionog potencijala u smeru negativnih potencijala za približno 0,8 mV K-1. Velika energija aktivacije hemijskog rastvaranja hroma dovodi na povišenim temperaturama do znatno izraženije hemijske korozije u odnosu na elektrohemijsku koroziju. Pri koncentracijama Cl- jona manjim od 3 M, korozioni potencijal je posledica simultanog odvijanja reakcije izdvajanja vodonika na oksidom prekrivenoj površini hroma sa reakcijom anodnog rastvaranja hroma kroz pasivni film (Ekor.1) ili katodnim izdvajanjem vodonika i anodnim rastvaranjem ogoljene površine kada se površina hroma depasivira (Ekor.2). Za koncentracije Cl- jona veće od 3,5 M površina pasivnog hroma se spontano aktivira i tada se primećuje samo jedan korozioni potencijal Ekor.2. Primećeni anodni pik na elektrodama od nerđajućeg čelika 304 u sumpornoj kiselini nije realni anodni pasivacioni pik, kao što je primećeno za veći broj metala u sličnim uslovima, već potiče od anodne oksidacije vod, The influence of temperature on electrochemical and chemical dissolution of three types of chromium (cast chromium, metallic fine-grained chromium, and electroplated coating of chromium on steel coupons) was studied in neutral and acidic sulphuric solutions, with and without the addition of chloride ions. Also, electrochemical and corrosion characteristics of the stainless steel 304 (18Cr-9Ni) electrode were studied. The electrochemical and chemical corrosion rate of all types of chromium was similar, although chromium coating has 5 times faster anodic reaction, and almost 5 times slower hydrogen evolution cathodic reaction. The chromium with texture (110) was somewhat less active electrochemically (both anodic and cathodic), than the chromium with texture (111). Crystalline plane (110) is the most dense packed chromium plane. The hydrogen evolution reaction, as well as the anodic dissolution and chemical dissolution of chromium from activated surfaces in sulphuric acid solutions pH 1 follows Arrhenius law, with the apparent activation energies of 35 kJ mol-1, 58 kJ mol-1 and 62 kJ mol-1, respectively. The higher activation energy for the anodic process, than for the cathodic one resulted in shifting of corrosion potential in the negative direction for approximately 0.8 mV K-1. The higher activation energy for chemical dissolution than for the electrochemical chromium corrosion at higher temperatures increases considerably the part of chemical corrosion in the overall corrosion of chromium. When the concentration of Cl- ions is smaller than 3 M, corrosion potential is the result of simultaneous cathodic reaction of hydrogen evolution on chromium oxide covered surface and anodic dissolution of chromium through the passive film (Ecorr.1), or by the cathodic hydrogen evolution and anodic dissolution on bare chromium, when the chromium surface is activated (Ecorr.2). When the concentration of Cl- ions is higher than 3.5 M, the passive chromium surface is immediately act

The influence of hot-dip galvanized steel (HDG) surface pretreatment with phosphate coatings on the corrosion stability and adhesion characteristics of epoxy coatings electrodeposited on HDG steel was investigated. Phosphate coatings were deposited on hot-dip galvanized steel from baths with different concentrations of NaF (0.1, 0.5 and 1.0 g dm(-3)) and at different temperatures (50, 65 and 80 degrees C). The influence of fluoride ion concentration in the phosphating bath, as well as the deposition temperature of the bath, on the adhesion characteristics and corrosion stability of epoxy coatings on phosphated HDG steel was investigated. The dry and wet adhesions were measured by a direct pull-off standardized procedure, as well as indirectly by NMP test, while corrosion stability was investigated by electrochemical impedance spectroscopy (EIS). It was shown that under dry testing conditions all the samples exhibited very good adhesion. However, the different trends of adhesion loss of different protective systems during exposure to corrosive agent (3% NaCl solution) were observed. The lowest adhesion values were obtained for epoxy coating on hot-dip galvanized steel, while the phosphate pretreatments significantly increase the adhesion of epoxy coatings. The highest values of adhesion strength were obtained for epoxy coating on HDG steel modified by phosphate coating deposited at 65 degrees C with 0.5 g dm(-3) NaF in the phosphating bath. EIS measurements in 3% NaCl solution are in accordance with adhesion measurements, i.e. greater values of charge-transfer resistance and smaller values of double-layer capacitance were obtained, as compared to other protective systems.

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.

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. .

The influence of hot-dip galvanized steel surface pretreatment on the adhesion of epoxy cataphoretic coating was investigated. Phosphate coatings were deposited on hot-dip galvanized steel and the influence of fluoride ions in the phosphating plating bath, as well as the deposition temperature of the plating bath, were investigated. The dry and wet adhesion of epoxy coating were measured by a standard pull-off method. The surface roughness of phosphatized galvanized steel was determined, as well as the wettability of the metal surface by emulsion of the epoxy resin in water. The adhesion of epoxy coatings on phosphatized hot-dip galvanized steel was investigated in 3wt.%NaCl., U radu je ispitivana modifikacija površine pocinkovanog čelika fosfatnim prevlakama na adheziju epoksidne prevlake na fosfatiranom pocinkovanom čeliku. Površina pocinkovanog čelika je modifikovana hemijskim taloženjem fosfatnih prevlaka, pri različitim koncentracijama fluoridnog jona u rastvoru za fosfatiranje, kao i na različitim temperaturama rastvora za fosfatiranje i praćena je pramena adhezije epoksidne kataforetske prevlake na njoj, pre i tokom delovanja korozionog agensa. Ispitivana je hrapavost modifikovanih površina pocinkovanog čelika, kao i njihova kvašljivost emulzijom epoksidne smole u vodi. Adhezija epoksidih prevlaka na fosfatiranom pocinkovanom čeliku je ispitivana u rastvoru 3mas.% NaCl.

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.

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.

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.