Corrosion in Chemical Industry - Yildiz Technical University

YILDIZ TECHNICAL UNIVERSITY DEPARTMENT OF CHEMICAL ENGINEERING CORROSION IN CHEMICAL INDUSTRY SECTION 3 PROF. DR. EMEK MÖRÖYDOR DERUN RES. ASSIST. ENİS MUHAMMET GÜL 1 POTENTIAL-pH EQUILIBRIUM DIAGRAMS (POURBAIX DIAGRAMS) Pourbaix devised a compact summary of thermodynamic data in the form of potenti...

YILDIZ TECHNICAL UNIVERSITY DEPARTMENT OF CHEMICAL ENGINEERING CORROSION IN CHEMICAL INDUSTRY SECTION 3 PROF. DR. EMEK MÖRÖYDOR DERUN RES. ASSIST. ENİS MUHAMMET GÜL 1 POTENTIAL-pH EQUILIBRIUM DIAGRAMS (POURBAIX DIAGRAMS) Pourbaix devised a compact summary of thermodynamic data in the form of potential – pH diagrams, which relate to the electrochemical and corrosion behavior of any metal in water. ………………………………………………………………………………… ………………………………………………………………………………… ………………………………………………………………………………… 2 POTENTIAL-pH EQUILIBRIUM DIAGRAMS (POURBAIX DIAGRAMS) Zn° + 2H+ → Zn+2 + H2 ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………….. 3 POTENTIAL-pH EQUILIBRIUM DIAGRAMS (POURBAIX DIAGRAMS) E-pH diagrams are drawn according to the ……………………………..and in case of changing concentration the diagram will change at the same time. E – pH diagrams show ……………………………………………. ……………………………………………………………………………… can be drawn by considering the other ions (Cl-, SO4-, cyanide) in the water. They 4 POTENTIAL-pH EQUILIBRIUM DIAGRAMS (POURBAIX DIAGRAMS) The region where metal is stable shows the ideal conditions for studying because in stable region metal do not corrode. The region in which ions are stable shows corrosion of metal. The region in which solid compounds (reaction products) occur can be protective or not according to properties of compounds. If the compounds drift apart the connection between the metal and environment they can prevent the corrosion of metal. On the contrary, in the event of continuing contact the corrosion of metal will resume. 5 MEASURING CORROSION RATE H =……………….. G =………………….. t =……………………. k.G H= A.t. A =……………………  K =………………........ =………………………… 6 MEASURING CORROSION RATE ☺H < 0,15 mm/year ………………………….. ☺0,15 mm/ year < H < 1,5 mm/ year ……………………………. ☺1,5 mm/ year < H ……………………………. CORROSION RATE UNIT K VALUE mpy mile/year 3,45.106 ipy inch/year 3,45.103 ipm inch/month 2,87.102 mm/year mm/year 8,76.104 m/year m/year 8,76.107 g/m2.hour 1,00.104 mgr/dm2.day 2,40.106 gr/m2.second 2,78.106 g/m2.hour mdd gr/m2.s 7 Conversion Factors for Corrosion Rate Corrosion Rate Conversion Factors to Convert mpy ipy 1000 ipm 12000 mm/year 39,4 m/year 0,039 gr/m2.s 345/ mdd 1,44/ gr/m2.s 1,24/ 8 Question 1 weight loss of 100cm2 of a material is calculated as 400 mg. The density of the material is 2.7 g/cm3. Find the corrosion rate in mpy. The …………………………………………………………………… …………………………………………………………………… 9 I.t.M W= n.F I i= ( A/ cm2) A i: Current density W: ……………………………………… I: ……………………………………….. M: ……………………………………… n: …………………………………….... F: ……………………………………… i.A.t.M W= n.F 10 Question 2 The corrosion rate of a steel tank wall is 54.7 mdd. Calculate after how many days the 0.50 mm of the wall will corrode.  =7,87 g/cm3 ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………………………… ………………………………………………………………………… 11 Iois………………………………………………………........... There is a critic Io value for each electrochemical reaction. The critic Io value depends on metal type, solution type, metal-ion concentration and roughness of the metal surface. Meo = Me+2 + 2e- IA Me+2 + 2e- → Me° IC IA = IC = Io 12 If we apply a potential to the metal and force the metal to behave as anode, the path of oxidized ions will be shorter and corrosion will be accelerated. The same operation can be performed by shortening the path of reduced ions. Eo → Equilibrium voltage (Eo) E – Eo =  (overvoltage) Overvoltage is ……………………..………………………………………………………… ……………………………………………………………………………… ……………………………………………………………………………… …………………………………………………………………… 13 When corrosion process takes place on a surface, the real potential adopts a value somewhere between the equilibrium potential of the cathodic and anodic reactions. Equilibrium potential for cathodic reaction = Eoc Equilibrium potential for anodic reaction = Eoa Real potential = E Cathodic overvoltage = c = E – Eoc < 0 Anodic overvoltage = a = E – Eoa > 0 14 Polarization …………………………………………………………………………………… ………………………………………………………………….as polarization. Polarization is the displacement of each electrode potential from its equilibrium value and polarization reduce the potential difference between anode and cathode. Polarization value affects the slopes of the curves and corrosion rate decreases as polarization values increase. 15 16 If is immersed in the solution which consists Me+2 ions; 1) Me+2 ions reach the metal surface  In case of potential difference → ………………….  In case of concentration difference → ………………. In case of stirring the …………………………………… solution → 2) Me+2 ions adsorbed to the surface and if the surface is not suitable for electron exchange ions sprawl to other places. Me+2 + 2e- → Me° 17 3) When Me+2 ions reach the suitable surface get electrons and turn into their metal form. 4) Ions can be diffused into metal at the surface they are reduced. 5) Ions can be combine with other metal atoms and turn into crystal form Me+2 + 2e- → Me° 18 T: Total energy amount in the system T : D + A + c+ cr +......................+I.R D : Diffiusion overvoltage A : Adsorption overvoltage c : Charge overvoltage cr: Crystal overvoltage The bigger overvoltage type controls the reaction rate. The reaction rate controlling step is the step which needs more overvoltage among overvoltage types. Overvoltage types are 1. 2. 3. ……………………………………………………………………………… …………………………………………………………………………….. …………………………………………………………………………… 19 Polarization Types …………………………….…………..: Diffusion polarization occurs when the ion concentration on the electrode surface diverge from the concentration in the solution. …………………………………………: Activation polarization is the formation of a layer containing absorbed hydrogen atoms that block the metal's surface from the corrosion process …………………………………………: When a conductive oxide film or adsorbed oxide film or adsorbed gas layer occur on the electrode surface the resistance of electrolyte – electrode road increases. The increased resistance cause a decrease in the corrosion rate or an increase in electrolyte resistance. 20 HOMEWORK Tafel Equations For activation polarization the relationship between current density, i, and overvoltage, , is given by the Tafel equations. Tafel constans (aa ,ac ,a , c) change according to the vents that occur at electrod and the enviroment that electrod is in. T = C + D + IR System resistance T = -c.log 21 22 THANK YOU FOR LISTENING… 23