Influence of temperature on elektrochemical and anomalous behaviour of chromium and chromium alloys in acid solutions

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 vodonika apsorbovanog tokom katodne polarizacije. Ukupna anodna gustina struje na potencijalu otvorenog kola je suma anodne pasivacione struje i odgovarajuće struje oksidacije apsorbovanog vodonika, tako da korozioni potencijal postaje pozitivniji kako se smanjuje koncentracija apsorbovanog vodonika i jH → 0. Katodno izdvajanje vodonika se odvija na pasivnom filmu. Korozioni potencijal elektrode od čelika 304 koji nije bio zasićen vodonikom je potencijal koji se formira u saglasnosti sa elektrohemijskim Vagner-Traudovim modelom sa dve suprotne reakcije (katodno izdvajanje vodonika na pasiviranom čeliku i anodno rastvaranje metala kroz pasivni film). Katodnom aktivacijom i dugotrajnijom katodnom polarizacijom do značajno negativnih potencijala i vrlo velikih katodnih struja nije bilo moguće da se pasivni sloj ukloni i postigne aktivno elektrohemijsko rastvaranja metala bez prisustva pasivnog sloja, tj. da se dobije slobodna metalna površina.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 activated, and then only one corrosion potential Ecorr.2 can be noticed. The observed anodic peak on the anodic potentiodynamic curve for 304 stainless steel electrode in deaerated sulphuric acid solutions appears not to be the real anodic passivation peak, as it was observed for a number of metals in similar conditions, but peak which arises from the anodic oxidation of hydrogen absorbed during previous cathodic polarization. The total anodic current density at the open circuit potential is the sum of the anodic passivation current density and the corresponding anodic current density for the oxidation of the previously absorbed hydrogen, so that the corrosion potential becomes positive as the absorbed hydrogen concentration decreases and jH → 0. The cathodic reaction of hydrogen evolution is occurring on the stainless steel surface with a passive film. Corrosion potential (for 304 stainless steel electrode without absorbed hydrogen) is established according to the electrochemical Vagner-Traud model, with two opposite reactions: cathodic reaction of hydrogen dissolution on a passive surface of steel and an anodic reaction of dissolution of metal through a passive film. It was impossible to remove a passive film and starts the active electrochemical dissolution without the passive layer, i.e. at a bare metallic surface, with the cathodic activation or a longer cathodic polarization to more negative potentials

Hemijsko i elektrohemijsko rastvaranje hroma na sobnoj i na povišenim temperaturama

The influence of temperature on electrochemical and chemical dissolution of chromium was studied in the acidic sulphuric solutions. The hydrogen evolution reaction, as well as the anodic dissolution and chemical dissolution of chromium from activated surfaces in sulphuric acid solutions pH 1 follow the 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 of the chemical dissolution of chromium leads to the signifycantly noticeable chemical corrosion on elevated temperatures in comparison with the electrochemical corrosion.Ispitivan je uticaj temperature na elektrohemijsko i hemijsko rastvaranje hroma u kiselim rastvorima sulfata. 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. Velika energija aktivacije hemijskog rastvaranja hroma dovodi na povišenim temperaturama do znatno izraženije hemijske korozije u odnosu na elektrohemijsku koroziju

Determination of susceptibility to intergranular corrosion of stainless steels type X5CrNi18-10 in field

In this paper, the DL EPR method (electrochemical potentiokinetic reactivation with double loop) was modified and used to study the susceptibility to intergranular corrosion and stress corrosion cracking of a stainless steel type X5CrNi18-10. The tests were performed in a special electrochemical cell, with the electrolyte in the gel form. Modified DL EPR method is characterized by simple and high accuracy measurements as well as repeatability of the test results. The indicator of susceptibility to intergranular corrosion (Qr/Qp)GBA obtained by modified DL EPR method is in a very good agreement with the same indicator obtained by standard DL EPR method. The modified DL EPR method is quantitative and highly selective method. Small differences in the susceptibility of the stainless steel type CrNi18-10 to intergranular corrosion and stress corrosion cracking can be determined. Test results can be obtained in a short time. The cost of tests performed by modified DL EPR method is much lower than the cost of tests by conventional chemical methods. Modified DL EPR method can be applied in the field on the stainless steels constructions

Determination of susceptibility to intergranular corrosion of stainless steels type X5CrNi18-10 in field

In this paper, the DL EPR method (electrochemical potentiokinetic reactivation with double loop) was modified and used to study the susceptibility to intergranular corrosion and stress corrosion cracking of a stainless steel type X5CrNi18-10. The tests were performed in a special electrochemical cell, with the electrolyte in the gel form. Modified DL EPR method is characterized by simple and high accuracy measurements as well as repeatability of the test results. The indicator of susceptibility to intergranular corrosion (Qr/Qp) GBA obtained by modified DL EPR method is in a very good agreement with the same indicator obtained by standard DL EPR method. The modified DL EPR method is quantitative and highly selective method. Small differences in the susceptibility of the stainless steel type CrNi18-10 to intergranular corrosion and stress corrosion cracking can be determined. Test results can be obtained in a short time. The cost of tests performed by modified DL EPR method is much lower than the cost of tests by conventional chemical methods. Modified DL EPR method can be applied in the field on the stainless steels constructions

Otpornost na opštu koroziju serije legura cink-aluminijum koje su modifikovane silicijumom i stroncijumom

The resistance to corrosion of the series of zinc-aluminum casting alloys modified with silicon and strontium was studied in a sodium chloride solution. It was shown that general corrosion was the main form of corrosion of Zn25Al-Si-Sr alloys in the test solution. The corrosion process takes place through the anodic dissolution of the alloys, while the cathodic reaction is controlled by diffusion of oxygen in the test solution. Three different electrochemical techniques were applied to determine the corrosion current density, which is the electrochemical indicator of corrosion rate. The value of corrosion current density for each Zn25Al-Si-Sr alloy was calculated using the value of polarization resistance, obtained by electrochemical impedance spectroscopy or by linear polarization resistance technique. The value of corrosion current density was also determined directly from Tafel plots, which were recorded by linear sweep voltammetry. As the content of silicon and strontium in the tested alloys increases, the value of corrosion current density increases gradually. The results obtained by different electrochemical techniques are in good agreementOtpornost na koroziju serije legura cink-aluminijum za livenje, koje su modifikovane silicijumom i stroncijumom, ispitana je u rastvoru natrijum hlorida. Pokazano je da je opšta korozija glavni oblik korozije Zn25Al-Si-Sr legura u navedenom rastvoru. Proces korozije odvija se kao anodno rastvaranje legura, dok je katodna reakcija kontrolisana difuzijom kiseonika u rastvoru. Tri različite elektrohemijske tehnike primenjene su za određivanje gustine struje korozije, koja je elektrohemijski pokazatelj brzine korozije. Vrednost gustine struje korozije za svaku Zn25Al-Si-Sr leguru izračunata je na osnovu vrednosti polarizacione otpornosti, koja je određenai spektroskopijom elektrohemijske impedanse ili tehnikom linearne polarizacije. Vrednost gustine struje korozije je takođe određena direktno iz Tafelovih dijagrama, koji su registrovani pomoću voltametrije sa linearnom promenom potencijala. Sa povećanjem sadržaja silicijuma i stroncijuma u ispitivanim legurama vrednost gustine struje korozije postepeno raste. Rezultati dobijeni različitim elektrohemijskim tehnikama su u dobroj saglasnos

Ispitivanje piting korozije nerđajućeg čelika AISI 304 u rastvorima hlorida

Stainless steels and their welded joints are often susceptible to pitting corrosion in solutions containing chloride ions. Testing of pitting corrosion of stainless steels is performed using chemical and electrochemical methods. In this work pitting corrosion of the stainless steel AISI 304 was tested in chloride solutions, without the presence and in the presence of sulphate or nitrate ions. The tests were performed in order to determine the efficiency of these ions to prevent pitting corrosion, as well as to present the capabilities of the methods applied. Electrochemical methods were used to determine the value of pitting potential (Epit) and the critical pitting temperature (Tpit). The values of these parameters represent a measure of the resistance of stainless steels and their welded joints to pitting corrosion.Nerđajući čelici i njihovi zavareni spojevi često podležu piting koroziji u rastvorima koji sadrže hloridne jone. Ispitivanje piting korozije nerđajućih čelika se vrši primenom hemijskih i elektrohemijskih metoda. U ovom radu vršeno je ispitivanje piting korozije nerđajućeg čelika AISI 304 u rastvoru hlorida, bez prisustva i u prisustvu sulfatnih, odnosno nitratnih jona. Ispitivanja su izvedena u cilju određivanje efikasnosti navedenih jona u sprečavanju piting korozije, kao i prezentacije mogućnosti primenjenih metoda. Korišćene su elektrohemijske metode za određivanje piting potencijala (Epit) i određivanje kritične temperature pitinga (Tpit). Vrednosti navedenih parametara predstavljaju merilo otpornosti nerđajućih čelika i njihovih zavarenih spojeva prema piting koroziji

Sprečavanje i uklanjanje termičkog oksida (heat tint)

Heat tinting is a thickening of the naturally occurring oxide layer on the surface of the stainless steels during welding. The thickness of the oxide layer next to the weld depends on the temperature, heating time, and oxygen concentration in the shielding gas. On stainless steels chromium is drawn from below the surface of the metal to form a chromium rich oxide surface layer. During welding, the shielding gas should be used to prevent hit tint formation. If formed, heat tint must be removed in order that the full corrosion resistance of the finished product is restored after welding. Mechanical and/or chemical methods of removing heat tint, that have been usually applied, are described in this article.Obrazovanje heat tint-a predstavlja povećanje debljine prirodno formiranog oksidnog sloja na površini nerđajućih čelika, tokom zavarivanja. Debljina heat tint-a u neposrednoj blizini zavarenog spoja zavisi od temperature, vremena zagrevanja i koncentracije kiseonika u zaštitnom gasu. Kod nerđajućih čelika hromom bogat oksid formira se na površini metala, dok je podpovršinski sloj osiromašen hromom. Zavarivanje treba izvoditi u zaštitnom gasu da bi se sprečilo obrazovanje heat tint-a. Ukoliko se formira, ovaj oksid mora biti uklonjen da bi se sačuvala potpuna otpornost prema koroziji zavarenih delova. U ovom radu opisane su mehaničke i/ili hemijske metode za uklanjanje heat tint-a koje se obično primenjuju

Metode ispitivanja interkristalne korozije na konstrukcijama od nerđajućh čelika na terenu

Determination of the susceptibility of austenitic stainless steel X5CrNi18-10 to intergranular corrosion (IGC) is performed by measuring the corrosion potential Ecorr in a drop of a test solution with specific chemical composition. It has been shown that a good correlation exists between the results obtained by Ecorr measurements in a drop of the test solution and the results obtained by the double loop electrochemical potentiokinetic reactivation method (DL EPR). The results are confirmed by SEM analysis of the stainless steel surface after the IGC test. The method of Ecorr measurements is a simple non-destructive method that provides qualitative information on the susceptibility of the stainless steel to IGC. The DL EPR method is a quantitative method that can determine small differences in the susceptibility of stainless steel to IGC. Simple and cheap equipment is required to perform Ecorr measurements. The method is easy to perform on stainless steel structures in the field.Određivanje sklonosti austenitnog nerđajućeg čelika X5CrNi18-10 prema interkristalnoj koroziji izvršeno je na osnovu merenja korozionog potencijala Ekor u kapi rastvora definisanog sastava. Pokazano je da postoji dobra saglasnost dobijenih rezultata sa rezultatima ispitivanja interkristalne korozije primenom metode elektrohemijske potenciokinetičke reaktivacije sa povratnom petljom (DL EPR). Dobijeni rezultati su potvrđeni SEM analizom površine čelika posle ispitivanja interkristalne korozije navedenim metodama. Metoda merenja Ekor u kapi rastvora je jednostavna, nerazarajuća metoda koja daje kvalitativne podatke o sklonosti čelika prema interkristalnoj koroziji, dok je DL EPR metoda kvantitativna, kojom se mogu odrediti male razlike u sklonosti nerđajućih čelika prema interkristalnoj koroziji. Za izvođenje ispitivanja metodom merenja Ekor u kapi rastvora potrebna je znatno jeftinija i jednostavnija oprema. Ova metoda se lako izvodi na gotovim konstrukcijama, na terenu

Korozija visokohromnog belog gvožđa u kiseloj sredini

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

Mehanizmi izdvajanja vodonika na hromu

In this work, the mechanisms of hydrogen evolution on the metal chromium in a solution of 0.1 M Na2SO4 + H2SO4, pH 1 to pH 7 were analysed. In the pH LT 3 range, hydrogen is evolved according to three different mechanisms, depending on the experimental conditions: electrochemical evolution of H2 by reaction of H+ ions according to the Volmer-Heyrovsky mechanism, on the bare chromium surface, during chemical dissolution of chromium by the Kolotyrkin mechanism, which does not depend on the electrode potential, and electrochemical hydrogen evolution by the Volmer-Heyrovsky mechanism, but on the passive chromium. At pH values greater than 3, the fourth mechanism of hydrogen evolution is observed, that is electrochemical reaction of water molecules on the surface of oxide covered chromium surface.U ovom radu analizirani su mehanizmi izdvajanja vodonika na metalnom hromu u rastvoru 0,1 M Na2SO4 + H2SO4, pH 1 do pH 7. U oblasti pH LT 3 vodonik se izdvaja u zavisnosti od eksperimentalnih uslova po tri različita mehanizma: elektrohemijskim izdvajanjem H2 reagovanjem H+ jona po mehanizmu Folmer-Hejrovski na čistoj površini hroma, pri hemijskom rastvaranju hroma po mehanizmu Kolotirkina, a koje ne zavisi od elektrodnog potencijala i elektrohemijskim izdvajanjem vodonika po mehanizmu Folmer-Hejrovski, ali na pasivnom hromu. Pri pH vrednostima većim od 3 uočava se i četvrti mehanizam izdvajanja vodonika elektrohemijskim reagovanjem molekula vode na oksidom presvučenoj površini hroma