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

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

Naponska korozija metala i legura i njihovih zavarenih spojeva, Deo I: uslovi nastanka naponske korozije

Opisani su osnovni mehanizmi naponske korozije različitih metala i legura i njihovih zavarenih spojeva. Detaljnije su razmotreni metalurški i korozioni uslovi nastanka naponske korozije ugljeničnih i niskolegiranih čelika, a posebno njihovih zavarenih spojeva. Najveća pažnja u ovom radu je posvećena razmatranju naponske korozije nerđajućih čelika (austenitnih, martenzitnih, feritnih i dupleks nerđajućih čelika) i posebno njihovih zavarenih spojeva. Značajna pažnja je takođe posvećena naponskoj koroziji aluminijumskih legura serije 2000 (Al-Cu-Mg), 5000 (Al-Mn, Al-Mg-Mn) i 7000 (Al-Zn-Mg-(Cu)), dok je naponska korozija zavarenih spojeva kod legura bakra, legura titana i legura nikla samo ukratko razmotrena

Naponska korozija metala i legura i njihovih zavarenih spojeva, Deo II: metode ispitivanja

U radu su razmotrene metode ispitivanja naponske korozije metala i legura i njihovih zavarenih spojeva. Opisani su principi tih metoda, postupci izvođenja, oblasti primene, kao i prednosti i nedostaci pojedinih metoda. Shematski su prikazani tipični rezultati koji se dobijaju njihovom primenom. Posebna pažnja je posvećena metodama ispitivanja naponske korozije zavarenih spojeva. Opisane su klasične metode ispitivanja statički opterećenih glatkih uzoraka (Cprstenovi, uzorci na savijanje itd.), kao i savremene metode kao što je metoda male brzine deformacije (SSRT) i metode zasnovane na principima mehanike loma. Navedeni su odgovarajući standardi (ISO i ASTM) za njihovo izvođenje, kao i preporuke za sprečavanje naponske korozije zavarenih spojeva različitih metala i legura

Corrosion of Archaelogical Artefact made of Forged Iron

One artefact of the archaeological cultural heritage from Roman period (IV century) that was found near Šabac in Serbia, was analysed. In corrosion products of the artefact (knife), dominant phases were goethite (α-FeOOH) and magnetite (Fe3O4). Presence of these types of corrosion products explains a good preservation of the base metal (iron) over the centuries and stability after excavation. Also, the less stable lepidocrocite (γ-FeOOH) and the phases that come from rocks and land surrounding environment (like SiO2) were identified in the corrosion products. Phases containing chloride ions (i.e. akaganéite) have not been detected in the corrosion products. This indirectly indicates that the amount chloride ions were rather low in underground exploitation conditions

Termografsko ispitivanje energetske efikasnosti zgrade Vojnog muzeja

Infrared (IR) thermography, as a diagnostic technique, is used to find anomalies in the thermal signature of the Military museum building in Belgrade, to identify irregularities or deficiencies, such as wet materials, voids, or missing insulation and to inspect energy efficiency of the museum building and microclimatic indoor conditions. It is very important to perform preventative maintenance and stop undesirable environment influences that induce structural damage, modification of materials and agglomeration of pollutants and microorganisms on the cultural heritage artifacts, stored in the museum depot or exhibited in the galleries. The main causes for corrosion in historical buildings and museum artifacts are moisture and changeable temperature conditions. This paper deals with the results obtained in the application of IR thermography in determination of these conditions in the Military museum building, where very important metal artifacts are exhibited and deposed. The results show poor thermal insulation, wet walls and, generally, low energy efficiency.Infracrvena termografija, kao dijagnostička tehnika, služi za ispitivanje energetske efikasnosti zgrada i unutrašnjih mikroklimatskih uslova. Korišćena je za otkrivanje nepravilnosti toplotnih karakteristika zgrade Vojnog muzeja u Beogradu, s ciljem da se pronađu nepravilnosti ili nedostaci, kao što su vlažni materijali, šupljine, oštećenja ili nedostatak toplotne izolacije. Ova ispitivanja su veoma važna za preventivno održavanje zgrade, kao i za sprečavanje nepovoljnih uticaja iz okruženja na objekte istorijskog i kulturnog nasleđa, koji izazivaju oštećenja na strukturi i materijalu i dovode do nagomilavanja štetnih zagađivača na objektima koji su smešteni u muzejskim depoima ili izloženi u galerijama. Na primer, vlaga i promenljivi temperaturni uslovi glavni su uzroci pojave i razvoja korozije u zgradama od istorijskog značaja i muzejskim eksponatima. Ovaj rad prikazuje rezultate termografskih ispitivanja zgrade Vojnog muzeja, gde se nalaze veoma značajni metalni eksponati. Rezultati pokazuju da postoji veoma loša toplotna izolacija, vlaga u zidovima i slaba energetska efikasnost

Investigation of intergranular corrosion welded joint of austenitic stainless steel by electrochemical methods

Sensitization degree of the austenitic stainless steel welded joints was investigated by electrochemical methods: the double loop electrochemical potentiokinetic reactivation (DL EPR) in H2SO4 + KSCN solution and by of the corrosion potentials measurement of the steel in the solution drop of HNO3 + FeCl3 6H2O + HCl. The welded joints, by an X-ray radiographic method on possible presence of the weld defects, were tested. Grain size of the base metal and the welded joints, by applying an optical microscopy were determined. The existence of compatibility between the results for different electrochemical methods is shown. Heat affected zone (HAZ) of the austenitic stainless steel welded joints has shown significant degree of sensitization. The corrosion potentials measurement method is simple, nondestructive method, that gives qualitative informations about degree of the sensitizations of the stainless steel. The double loop electrochemical potentiokinetic method gives quantitative evidence about susceptibility of the stainless steel to intergranular corrosion. This method can be applied with some adaptation in field environments

Laser cleaning of corrosion, efficient and environmentally friendly method

In recent years, lasers have been used more and more in different areas of human life, science, industry, medicine, military, agriculture, and show business, protection of environmental and culturalheritage objects, in the arts and so on. Laser technology has some advantages in many areas over classical methods. This paper presents the results of laser application for cleaning corrosion of metal objects, cooper and brass. The first sample was oxidised cooper plate and the second ashtray made of brass. Nd: YAG laser was used as a source of radiation to clean surface corrosion deposits. Removal of layers from the surface occurs through the process of laser ablation. Three wavelengths were used with different fluences. The cleaning results, i.e. the morphological and chemical changes were investigated by optical microscopy, XRF analysis and colorimetry. The obtained results show that lasers are efficient corrosion cleaning and environmentally friendly method

STRESS CORROSION CRACKING OF AN AL-ZN-MG-CU ALLOY AFTER DIFFERENT PRECIPITATION HARDENING TREATMENTS

Stress corrosion cracking (SCC) was tested in a high strength 7000 series aluminium alloy applied for weapons and military equipment. The effect of one step- and two-step precipitation hardening (aging) on SCC was investigated. The slow strain rate test (SSRT) and the fracture mechanics (FM) method were used for SCC testing. The measurements of electrical resistivity allowed evaluating an aging degree in the tested alloy. Both testing methods have shown that the alloy after two-step precipitation hardening is significantly more resistant to SCC. The critical stress intensity factor for SCC, KISCC is significantly higher, and the crack growth rate on the plateau, vpl is more than one order of magnitude lower for the alloy in this state. Processes that take place at the crack tip are proposed and the effect of copper was analyzed. The effect of the testing solution (NaCl) temperature on the vpl was determined. Two values of the apparent activation energy, Ea, were obtained; one value (46.6 kJ mol-1) refers to higher temperatures, while the other (70.4 kJ mol-1) refers to lower test temperatures. These values of Ea correspond to the different processes that control the vpl, at higher and lower temperatures