Diğer metallerle karşılaştırmalı olarak pirit'in (FeS²) korozyon davranışı üzerinde bazı maddelerin etkisi

Abstract

Bu çalışmada alüminyum alaşımı bakır alaşımları ve karbon çeliğinin nötral ortamlardaki korozyon davranışı üzerinde sodyum molibdat sodyum metgsilikat, çinko klorür ve toliltriazolein inhibitor etkisi incelenmiştir. İyi bir elektronik iletken olan ve son yıllarda göze çarpan piritin korozyon davranışının araştırılmasındaki amaç ise metallerle bir karşılaştırma yaparak yüzey özellikleri hakkında bilgi sahibi olmaktı. Bu amaçla asidik, bazik ve nötral ortamlardaki davranışı incelenmiştir. Daha önce yapılan çalışmalara göre KI/I_ elektrolitin de pirit iyi fotoaktif özellik göstermektedir. Fotoaktif özelliğinin geliştirilmesini sağlayıcı yönde fikir sahibi olabilmek için bu elektrolitteki anodik ve katodik davra nışı incelenmiştir. İnhibitor etkileri potansiyostatik olarak elde edilen polarizasyon eğrileri yardımıyla belirlenmiştir. Korozyon hızları Stern-Geary eşitliği yardımıyla ya da Tafel ekstrapolasyonu yöntemiyle hesaplanmıştır.

In simplest terms, the corrosion of a metal in aqueous solution may be thought of as a two-step process, The anodic (oxidative) reaction: k1 M ; * Mn+ + ne" (M = Cu,Fe,Zn etc.) (1) k2 This reaction results in the dissolution of the metal and can occur over the whole metal surface or be restricted to specific areas. The cathodic (reductive) reaction: e.g. 2H20 + 02 + 4e~ a 4DH~ (2) 2H+ +2e" » H2 (3) e" + 2H+ + ND" ? N02 + H2G U) 3e~+^H+ +NO3 ? N0+ 2H20 (5) These reactions utilise the electrons produced by the anodic reaction. If the corroding metal is made the test electrode of an electrochemical cell it is found to have a characteristic potential (E ) relative to reference corr electrode when it is corroding freely in the absence of any applied potential. The variable voltage source can be used to polarize the test electrode positive (anodic) and negative (cathodic) with respect to E cor The current flowing in the circuit in the anodik direc tion is a measure of the rate of dissolution of metal (i.e corrosion rate) from the test electrode. As the electrode is made more positive the current will increase up to a maximum value determined by both kinetic and diffusional factors at test and counter electrodes. Polarisation in the negative direction will suppress the anodic reaction until at a low enough potential the direction of current flow will reverse and the test electrode will become the cathode in the cell. Polarisation diagrams are a useful tool in under standing and interpreting the complex electro chemistry of corroding systems. Additional utility and insight may be gained from a consideration of these diagrams in terms at modern electrochemical kinetics. There corrosion method" or Geary have of the sha electrodes metal is d tion of an exchange corrosion i. and net.,. compined ı is sevaral methods for determination of rate. One of these "linear polarisation "polarization resistance method" Stern and presented in 1957 a theoretical analysis pe Df polarisation curves for both reversible and corroding electrodes. If a corroding isplaced slightly from equilibrium by applica- external current it may be shown that current,. io is replaced by!, the current, a linear relationship exists Between AE near equilibrium. These two facts may be n the resulting form". A E A I 4. E " net corr Ba. Br^ a c 2.3 i (B +B ) corr a c (6') i = i - i net anodic cathodic VI Uhere B and B is Tafel slopes of anodic and cathodic reactions, respectively. If B values is known for the oxidation and reduction portion of the corrosion reactions the İ can be calculated from measurement of AE/Ai. cor Since B values can be determined by the polarisation measurement at high current, equation 6 represents per haps the most practical and direct application of electrode kinetics to corrosion processes and presents a unique nondestructive method for determining corrosion rates. There are some instances where B is grossly different from Be. Here it must be deduced that oxidation and reduction proceed by different routes. In this study corros alloy (no=?D75,compositi % 0.3 Cr, % 0.2 Mn), copp CDA110 and CDA443) carbon position is % C=G.DB-0.13 % Smax= D.Ü5) and pyrite bitors which are used for silicate for aluminium al carbon stell, and tolyltr are produced Turk-Henkel n-propanol, aniline, sodi cerium(III) nitrate were Na2504 ion behaviour of aluminium on is % 5.5 Zn %2, 5 Mg,% 1.5Cu er alloys ( Metal sample no= steel (SAE no=1 01 0 and com-, % Mn = 0.3-0.6, %Pmax = G.0ff) (FeS") mere investigated. Xnhi- corrosion uıere sodium meta- loy, sodium molibdate for iazole for copper alloys, which Company. The effect of phenol, urn molibdate, borax and carried out on pyrite in 0.1M There are not enough knowledge about pyrite corro sion in literature. So, pyrite which is an alternative solar cell substance was examined in different medie, i.e. acidic, neutral and basic. Hlectrodes were prepared from 75x10x1mm3 coupons of copper alloys and carbon steel. The area which is immersed in solution was about 2,5cmz. Pyrite electrode that was used in this work prepared from pyrite mineral which was obtained from the area of Karadeniz Bakır İşletmeleri Murgul mine. Minerals were cut and shaped into circular plates, mounted onto a copper rot, using silver paste (ScDtchcast 3M) and carefully centered. The insulation between disk vil and copper rot was made uiith epoxy qlue (scotchcast 3MXR 5241) Aluminium electrode was prepared in a similar way. Prior tD each experiment carbon stell and copper alloy were wet polished with detergent and then rinsed withdistilled water. Aluminium electrode also was wet polished with 600-grit £ic paper. It was degreased with acetone for about one hour, and then rinsed with distilled water. Pyrite electrode was etched in HF/CH3CDDH/HN03 (1:1:2 by volume) which is known to liberate the surface from oxides for two minutes. A standart three-electrode cell was used in electrochemical measurements. Reference electrode was satureted calomel electrode (SCE) and counter electrode was platinum. The polarisation measurements were carried out with corrovit (Taccusel) and potentiostat (Uenking Pos 73) by potentiostatic method. The open circuit specimen potential that is the corrosion potential was recorded after one hour immersion. For the determination of polarization resistance Rp, this time was longer (about 2-3 hours) to reach a stable potential. Corrosion rates, the anodic and the cathodic Tafel slopes (Ba and Be) were evaluated from polarisation curves. Ba and Be were used to calculate B, which was introduced in the corrosion rate calculation by Stern- Geary equation İ nT, = B/Rp B = cor Ba * Be 2.3 (Ba + Be) All experiments were carried out in oxygen containing conditions and at room temparature. Inhibotor concent ration was 1gr/lt. The results were shown in Table 1 and 2. Vlll M VP I I CO -P m tn 3 D.H U co u D cn CD 3 co.H.p c m.p D D. TD c ro -p c tu u u CM E D et a. u u D U in in U3 I CO.H.D 01 CJ CD CM O ? j h in in o till co o m *- *- aınjnmmuım -d-j-nın^-UDinın + I + I I I I I c D.H CD O u u D CJ LıJ m et rol m xi IX a)- Measured by patentionstat (Uenking Pos 73)' b)- Measured by corrovit (Taccussel) c)- Synhetic cooling water CaCl2= 332.7 mg/lt Na25D4=35B mg/lt MgSD^. 7H20 = 246 mg/lt d)- These values obtanied from Tafel extrapolation of polarization diagrams which obtained potenti ostati- cally. TABLE 2. Corrosion Potential andCurrent Values far Murgul-Pyrite in Various Media. Media D.1M Na"5D, 2 k pH = 2 pH = 12 0.1 Na2SD4(2h) 0.1 Na2S0^(1 day) D.1 MNa25D^(2 day) 45 icor ( fj A/cm2 ) 3.5 1.7 1.0 10.0 5.5 5.0 (d) a"50, media Inhibition effect of sodium metasilicate in is better than Na"S0,. Which might be due tOpSÎronger pitting and general corrosion of CI" than SO,. It is known that triazole inhibitors prevent corrosion by forming a Cu( I) -inhibitor film on the copper surface. But on the other hand the polymeric film which is formed on zinc were not stable like on copper. This might be explained by the different valency of zinc (2+ instead of 1 ? for Cu/inhibitör complexes) and the coordination to 3 inhibitor molecules or by the higher solubility of the Zn/inhibitör complexes. Alloy II has higher content of Zn, than Alloy I. Because- of the reason that mentioned above the inhibitor efficiency of alloy II isgmaller than that of alloy I. as shown Table I. Sodium molibdat has been known acts as a passivator, which shifts the corrosion potential to a more positive value. This result has been observed for carbon steel which is given in Table 1. pH changes also effects the corrosion current in some extent. At pH=2 higher hydrogen ion content and dissolved oxygen increases the corrosion rate. Although the corrosion current of pyrite in neutral, acidic or basic media does not change very much, the corrosion potential changes in basic media by comparision with carbon stell. Lower corrosion currents in pyrite shows that pyrite is more stable than carbon steel. Because of this reason there are efforts on protective surface coating of carbon steel by farming surface. FeS, on After one or two days corrosion potential of pyrite does not change much, and corrosion current also becomes more stable. In the case of organic inhibitor higher open circuit potentials are obtained than inorganic inhibitors. These results might also be helpfull for the improvement of FeS? surface by showing a small value of icorr, which was proposed as an alternative material for solar cells.