Investigation of brass corrosion process in sodium sulfate solution

Abstract

Copper is included to a group of the most important non-ferrous metals, which has a wide commercial (technical) use. It has been known since ancient times, one of the first elements that the man met. The importance of copper and its alloys is reflected in the names of whole epochs in development of mankind. Archaeological findings of copper objects in the Bor vicinity, originated from the Bronze Age, and indicate the use of native copper in making the copper tools, jewelry, weapons, and as a special rarity of that age (the Eneolithic period) the copper ax is mentioned. Thanks to its very good constructional characteristics, especially physical, and then also to the significant proportion of distribution in nature (although the amount of copper in the earth's crust is only 0.01%), copper is one of the metals that has found the widest use, immediately after iron. The main area of copper use is the electrical industry; less copper is used in the form of products obtained by the processing in plastic, both in technically pure form and in the form of alloys. Copper and its alloys, first of all brass, are widely used due to their physical and chemical properties. Corrosion presents a specific process that involves the loss or degradation of metal components and it is usually of electrochemical nature, with extensive use for galvanic cells. Corrosion is a disintegration of metals by unintentional chemical or electrochemical reactions, starting from the surface. All metals have a tendency to oxidation, some more easily than the others. A tendency to oxidation forms a galvanic series. Knowledge of metals in a galvanic series is an important information for understanding and its possible use in the construction and contacts of different metals. Quality engineering requires the understanding of material compatibility. There are often requirements that different metals are in contact, and then the galvanic compatibility leads to a surface patination of metal and protecting it from further corrosion. The zinc amount ranges from 5-45% in the brass alloys. Generally, corrosion resistance of brass decreases with increasing amount of zinc. It is a common difference between those alloys containing less than 15% zinc (better corrosion resistance) and those with higher amount of zinc. The main problem in the alloys with high concentration of zinc is the processes of dezincification and stress corrosion cracking - SCC. This work presents the interpretation of corrosion behavior of two types of brass (CuZn-28 and CuZn-42), with five deformation degree (0, 20, 40, 60 and 80%), in a variety of corrosive environments. Tests were carried out using the electrochemical procedure, the potentiostatic method. The potential dependences on time and corrosion potentials were determined, as well as the dependence of corrosion current densities on potential. The measured values of corrosion potentials and corrosion current densities were observed as the characteristics of dezincification process and corrosion resistance of tested samples of cold-deformed samples of brass CuZn-42 and CuZn-28. As the working electrolyte, 0.1M solution of Na2SO4 was used, in which chloride ions were added in concentrations of: 5•10-4M, 5•10-3M, 5•10-2M, 1•10-1M, 5•10-1M and 1.0M and copper(II)-ions in concentrations of: 1•10-3M, 5•10-3M, 1•10-2M and 5•10-2M. Also, the effect of different corrosion inhibitors was tested, with different concentrations, on increase in corrosion resistance of brass. Corrosion inhibitors, whose effect on corrosion behavior of brass was tested, were the followings: benzotriazole (BTA), thiourea (TU), ethylene-diamine-tetra-acetic acid (EDTA), 2-butin-1.4-diol (DS-3) and hydrazine-sulfate (HS), in concentrations of 1•10-2% and 1•10-1% (by volume). Based on the obtained results, the effects of deformation degree and zinc content, the concentration of corrosive agents, pH-value of solutions and corrosion inhibitors on corrosion behavior of brass were determined. By understanding the mechanisms, and based on the obtained results of electrochemical measurements, the answer is obtained to the question on interaction between the inhibitors and metal surface. The obtained results of corrosion behavior of deformed brass samples showed that in all tested solutions, with increasing deformation degree, the values of corrosion current density (corrosion rate) also increase. Deformation degree of 80%, in certain cases, shows the inhibition effect on corrosion process, while brass with deformation degree of 60% is the most susceptible to corrosion damage. Brass with higher copper content (CuZn-28), shows better corrosion resistance than brass CuZn-42, in all tested solutions. Inhibitory effect of increased concentrations of chloride ions was observed for concentrations of 5•10-2M and partly 1•10-1M. Based on the appearance of tested brass samples, the certain conclusions were made regarding to their susceptibility in the dezincification process. The highest values corrosion current densities for all brass samples are present in solutions of copper(II)-ions, at its highest concentration of 5•10-2M. Corrosion inhibitors, used in these tests, are known as corrosion inhibitors of copper, copper and steel alloys. Tests were carried out in acidic solution of sodium sulfate. Sulfate solution and acidic media were selected after thoroughly studied literature data, as well as based on conditions of production and exploitation of copper and copper alloys. In the solutions of copper(II)-ions, all tested inhibitors have shown the certain degree of inhibition, with the best level of protection provided by inhibitor BTA. In the chloride ion solutions, at concentration of inhibitors of 1•10-2%, only inhibitor thiourea shows the inhibition effect. If the concentration of corrosion inhibitor is increased to 1•10-1%, the inhibition effect is shown by the inhibitors BTA and thiourea. The degree of inhibition of the inhibitor BTA is in the range 94-99% and thiourea 40-63%. Based on the presented, it can be concluded that the corrosion behavior of tested types of brass, CuZn-42 and CuZn-28, with five degrees of deformation, depends on concentration of copper in brass, brass deformation degree, concentrations of Cu(II)-ions and Cl--ions, pH-value of solution and type and concentration of the used corrosion inhibitors. Data obtained during development of this doctoral dissertation were published in the form of scientific papers from SCI list.

Bakar ubrajamo u grupu najvažnijih obojenih metala, koji ima veoma široku komercijalnu (tehničku) primenu. Poznat je od davnina, jedan od prvih elemenata koje je upoznao čovek. Značaj koji su imali bakar i njegove legure, ogleda se u nazivima čitavih epoha u razvoju čovečanstva. Arheološki nalazi bakarnih predmeta u borskoj okolini potiču iz bakarnog doba i ukazuju na korišćenje samorodnog bakra od koga su se izrađivale bakarne alatke, nakit, oružje, a kao poseban raritet tog doba (eneolitsko doba) pominje se bakarna sekira. Zahvaljujući svojim vrlo dobrim konstrukcionim osobinama, a pre svega fizičkim, a zatim i srazmerno znatnoj rasprostranjenosti u prirodi (iako količina bakra u zemljinoj kori iznosi svega 0.01%), bakar je jedan od metala koji je našao najširu primenu, odmah posle železa. Glavno područje primene bakra je elektroindustrija, nešto manje bakra se upotrebljava u obliku proizvoda dobijenih preradom u plastičnom, i to kako u tehnički čistom obliku, tako i u obliku legura. Bakar i njegove legure, pre svih mesing, nalaze široku primenu zbog svojih fizičkih i hemijskih osobina. Korozija predstavlja specifičan proces koji uključuje gubitak ili degradaciju metalnih komponenti i obično je elektrohemijske prirode, sa velikom primenom kod galvanskih ćelija. Korozija je dezintegracija metala pomoću nenamernih hemijskih ili elektrohemijskih reakcija, koja počinje sa površine. Svi metali imaju tendenciju oksidacije, neki lakše od drugih. Sklonost ka oksidaciji formira galvanski niz. Poznavanje mesta metala u galvanskom nizu važna je informacija za razumevanje i njegovu moguću upotrebu u konstruisanju i prilikom kontakata različitih metala. Kvalitetan inženjering zahteva razumevanje kompatibilnosti materijala. Često postoje zahtevi da različiti metali budu u kontaktu i tada galvanska kompatibilnost dovodi do patiniranja površine jednog od metala i njegove zaštite od dalje korozije. U legurama mesinga količina cinka kreće se od 5-45%. Uopšteno, koroziona otpornost mesinga opada sa povećanjem količine cinka. To je uobičajena različitost između onih legura koje sadrže manje od 15% cinka (bolja koroziona otpornost) i onih sa većom količinom cinka. Glavni problemi kod legura sa visokom koncentracijom cinka su procesi decinkacije i naponske korozije (Stress Corrosion Cracking-SCC). U ovom radu dato je tumačenje korozionog ponašanja dve vrste mesinga (CuZn-28 i CuZn-42), sa pet stepeni deformacija (0, 20, 40, 60 i 80%), u različitim korozionim sredinama. Ispitivanja su vršena elektrohemijskim postupkom, potenciostatskom metodom. Određene su zavisnosti potencijala od vremena i korozioni potencijali, kao i zavisnosti gustina korozionih struja od potencijala. Izmerene vrednosti korozionih potencijala i gustina korozionih struja posmatrani su kao karakteristike procesa decinkacije i korozione otpornosti ispitivanih uzoraka hladno-deformisanih uzoraka mesinga CuZn-42 i CuZn-28. Kao radni elektrolit korišćen je 0.1M rastvor Na2SO4, u koji su dodavani hloridni joni, u koncentracijama: 5•10-4M, 5•10-3M, 5•10-2M, 1•10-1M, 5•10-1M i 1.0M i bakar(II)-joni, u koncentracijama: 1•10-3M, 5•10-3M, 1•10-2M i 5•10-2M. Takođe, ispitan je i uticaj različitih inhibitora korozije, sa različitim koncentracijama, na povećanje korozione otpornosti mesinga. Inhibitori korozije, čiji je uticaj na koroziono ponašanje mesinga ispitivan, bili su: benzotriazol (BTA), tiourea (TU), etilen-diamin-tetra-sirćetna kiselina (EDTA), 2-butin-1,4-diol (DS-3) i hidrazin-sulfat (HS), u koncentracijama 1•10-2% i 1•10-1% (zapreminski). Na osnovu dobijenih rezultata određeni su uticaji stepena deformacije i sadržaja cinka, koncentracije korozionih agenasa, pH-vrednosti rastvora i inhibitora korozije na koroziono ponašanje mesinga. Razumevanjem mehanizama, kao i na osnovu dobijenih rezultata elektrohemijskih merenja, dobija se i odgovor na pitanje o interakciji između inhibitora i površine metala. Dobijeni rezultati korozionog ponašanja deformisanih uzoraka mesinga, pokazali su, da u svim ispitivanim rastvorima, sa povećanjem stepena deformacije rastu i vrednosti gustina korozionih struja (brzine korozije). Stepen deformacije od 80%, u određenim slučajevima, pokazuje inhibitorski efekat na proces korozije, dok je mesing sa stepenom deformacije od 60% najpodložniji korozionom razaranju. Mesing sa većim sadržajem bakra (CuZn-28), pokazuje bolju korozionu otpornost od mesinga CuZn-42, u svim ispitivanim rastvorima. Inhibitorski efekat povećanih koncentracija hloridnih jona, uočen je za koncentracije od 5•10-2M i delimično 1•10-1M. Na osnovu izgleda površine ispitivanih uzoraka mesinga, doneti su i određeni zaključci u pogledu njihove sklonosti prema procesu decinkacije. U rastvorima bakar(II)-jona, pri njegovoj najvećoj koncentraciji od 5•10-2M, prisutne su i najveće vrednosti gustina korozionih struja, za sve uzorke mesinga. Inhibitori korozije, korišćeni u ovim ispitivanjima, poznati su kao inhibitori korozije bakra, legura bakra i čelika. Ispitivanja su vršena u kiselom rastvoru natrijum-sulfata. Sulfatni rastvor i kisela sredina su izabrani nakon detaljno proučenih literaturnih podataka, kao i na osnovu uslova proizvodnje i eksploatacije bakra i legura bakra. U rastvorima bakar(II)-jona, svi ispitivani inhibitori su pokazali određeni stepen inhibiranja, pri čemu, najbolji stepen zaštite pruža inhibitor BTA. U rastvorima hloridnih jona, pri koncentraciji inhibitora od 1•10-2%, jedino inhibitor tiourea pokazuje inhibitorski efekat. Ukoliko se poveća koncentracija inhibitora korozije na 1•10-1%, inhibitorski efekat pokazuju inhibitori BTA i tiourea. Stepen inhibiranja inhibitora BTA kreće se u granicama 94-99% i tiouree 40-63%. Na osnovu predočenog, može se zaključiti da koroziono ponašanje ispitivanih vrsta mesinga, CuZn-42 i CuZn-28, sa pet stepeni deformacije, zavisi od: koncentracije bakra u mesingu, stepenu deformacije mesinga, koncentracijama Cu(II)-jona i Cl--jona, pH-vrednosti rastvora i vrsti i koncentraciji korišćenih inhibitora korozije. Podaci dobijeni tokom izrade ove doktorske disertacije objavljeni su u obliku naučnih radova u časopisima sa SCI liste.