Tutorial Corrosion II. Electrochemical characterization with EC-Lab techniques

Transcription

1 Tutorial Corrosion II Electrochemical characterization with EC-Lab techniques 1

2 OUTLINE 1. Introduction 2. Types of corrosion a) Uniform corrosion b) Localized corrosion 3. Corrosion experiment 4. EC-Lab corrosion techniques a) Uniform corrosion b) Localized corrosion 5. Conclusions 2

3 INTRODUCTION Corrosion is an electrochemical interaction between a material and its environment which result in changes in the properties of the material Two types of corrosion Corrosion is commonly classified based on the appearance of the corroded material: uniform and localized corrosion 1. Uniform Corrosion Corrosion over the whole surface of the material 2. Localized Corrosion Corrosion at small areas or local sites of the material surface in contact with the corrosive solution 3

4 INTRODUCTION Corrosion involves two electrochemical processes: Oxidation and reduction 1. Oxidation The oxidation process takes place at the anodic sites according to the reaction M M n+ + ne - (1) 2. Reduction Reduction reaction takes place at the cathodic sites The main reduction reactions encountered during the corrosion of a material is the oxygen reduction (2,3) or the hydrogen ion reduction (4) In acidic media O 2 + 4H e - 2H 2 O (2) In alkaline/neutral media O 2 + 2H 2 O + 4 e - 4OH - (3) Hydrogen ion reduction 2H e - H 2 (4) 4

5 OUTLINE 1. Introduction 2. Types of corrosion a) Uniform corrosion b) Localized corrosion 3. Corrosion experiment 4. EC-Lab corrosion techniques a) Uniform corrosion b) Localized corrosion 5. Conclusions 5

6 UNIFORM CORROSION Uniform Corrosion Also known as general corrosion, this kind of corrosion corresponds to an uniform attack over the entire surface of the material. It is the most classical form of corrosion characterized by a uniform reduction of thickness of the surface of material. It is relatively easy to measure, predict and control this form of corrosion 6

7 UNIFORM CORROSION Cathodic domain Anodic domain E corr Corrosion potential Reduction reactions Hydrogen reduction 2 H e - H 2 Oxygen reduction O H 2 O + 2 e - 4 OH - Oxidation/dissolution reaction M M n+ + n e - Polarization curve 7

8 LOCALIZED CORROSION 1. Introduction 2. Types of corrosion a) Uniform corrosion b) Localized corrosion 3. Corrosion experiment 4. EC-Lab corrosion techniques a) Uniform corrosion b) Localized corrosion 5. Conclusions 8

9 LOCALIZED CORROSION Localized corrosion types In this section, six types of localized corrosion are described 1. Pitting corrosion It is the most insidious form of corrosion. It is resulting in small cavities or holes extending from the surface into the material usually occurs in halide containing solutions (Cl -, Br -,..) 2. Crevice corrosion Usually associated with small volumes of stagnant solution leading to a limitation in the oxygen diffusion to a confined space (under gasket, insulation material,..) 9

10 LOCALIZED CORROSION 3. Selective corrosion Preferential attack of a particular component of an alloy in presence of electrolyte. The most well-known examples are: dezincification of brass (Cu-Zn) destannification of tin bronze (Sn-Cu) 4. Intergranular corrosion As the name suggests, this form of corrosion occurs along the grain boundaries of metal and is usually results from local differences in composition due to chemical segregation effects or to specific phases precipitated on the grain boundaries 10

11 LOCALIZED CORROSION 5. Galvanic corrosion Corrosion resulting from contact between two different metals or alloys in a conducting corrosive environment. This corrosion occurs when three conditions are present: a) Metals must be far apart on the galvanic series b) These metals must be in electrical contact, c) The metals must be exposed to a conductive electrolyte 6. Stress corrosion cracking (SCC) the cracking is induced from the combined action of tensile stress and a corrosive environment. 11

12 LOCALIZED CORROSION Activity Passivity Transpassivity Cathodic domain Anodic domain Passive film M M x O y Sharp increase of anodic current Etp Transpassive potential Dissolution Passivation (without Halide species) Polarization curve of a passivable material 12

13 LOCALIZED CORROSION Activity Passivity Pitting corrosion Pitting Cathodic domain Anodic domain Passivity breakdown with Halide species ( Cl -, Br -, I -, F - ) E pit Pitting potential Polarization curve of a passivable material 13

14 OUTLINE 1. Introduction 2. Types of corrosion a) Uniform corrosion b) Localized corrosion 3. Corrosion experiment 4. EC-Lab corrosion techniques a) Uniform corrosion b) Localized corrosion 5. Conclusions 14

15 CORROSION EXPERIMENT Experimental Set up WE= Working electrode RE= Reference electrode Potentiostat Corrosion Cell CE= Counter electrode (Pt, Graphite,..) 15

16 CORROSION EXPERIMENT Role of the potentiostat steady-state current-voltage curves I E A. Controlled voltage techniques (Except ZRA) 1- Apply a voltage between RE and WE 2- Record the current flowing between the WE and the CE B. Controlled current techniques 1- Apply a current between CE and WE 2- Record the voltage between the WE and the RE 16

17 CORROSION EXPERIMENT Connection to the cell Each potentiostat cell cable has 6 terminal leads: Cell connection modes Standard mode VMP3 series SP300 series E WE = Ref1-Ref2 E WE = S1-S2 CE to Ground mode (used for multielectrode inverstigation) E WE = Ref3-Ref2 E WE = S1-S2 17

18 OUTLINE 1. Introduction 2. Types of corrosion a) Uniform corrosion b) Localized corrosion 3. Corrosion experiment 4. EC-Lab corrosion techniques a) Uniform corrosion b) Localized corrosion 5. Conclusions 18

19 EC-LAB CORROSION TECHNIQUES Corrosion techniques Uniform Corrosion E corr vs time Linear Polarization (LP) Corrosimetry (CM) Generalized Corrosion(GC) VASP CASP ZRA Localized corrosion E corr vs time Cyclic Potentiodynamic Polarization (CPP) Multielectrode Potentiodynamic pitting (MPP) Multielectrode Potentiostatic pitting (MPSP) CPT Depassivation Potential: DP Corrosimetry (CM) ZRA 19

20 OUTLINE 1. Introduction 2. Types of corrosion a) Uniform corrosion b) Localized corrosion 3. Corrosion experiment 4. EC-Lab corrosion techniques a) Uniform corrosion b) Localized corrosion 5. Conclusions 20

21 EC-LAB CORROSION TECHNIQUES E corr vs time It is a basic technique used to monitor the corrosion potential of the working electrode versus time Passivation / Inhibition E corr Activation/dissolution E corr vs time set up Time 21

22 EC-LAB CORROSION TECHNIQUES Linear Polarization The LP technique consists of a potential scan around the E corr. It is used to determine polarization resistance, corrosion current and Tafel parameters. E corr vs time E L -E i 2 kinds of information Potential scan LP description Linear polarization set up 22

23 EC-LAB CORROSION TECHNIQUES Linear Polarization LP E corr ± 250 mv Polarization in a wide ΔE I corr, β a and β c Tafel Fit LP E corr ± 25 mv Polarization in a narrow ΔE R p R p Fit LP stainless steel in NaCl media (30 gl -1 ) 23

24 Corrosimetry: CM EC-LAB CORROSION TECHNIQUES The CM technique is used in corrosion for the determination of R p versus time by a repetition of the linear polarization around the corrosion potential at fixed time interval. Polarization resistance Corrosion rate Corrosimetry description Corrosimetry set up 24

25 EC-LAB CORROSION TECHNIQUES Corrosimetry (CM) CM technique is advisable for long-time corrosion study corrosion rate vs time 25

26 EC-LAB CORROSION TECHNIQUES Generalized corrosion Technique dedicated to study the general corrosion. It consists of half a cycle or a cycle of usual cyclic voltammetry with a digital potential sweep E corr vs time E corr GC parameter I corr Courbe à ajouter Reverse scan Generalized corrosion set up 26

27 EC-LAB CORROSION TECHNIQUES VASP VASP (Variable Amplitude Sinusoïdal micro-polarization) is based on the EIS measurements. It consists in the determination of the change of the polarization resistance in function of the potential amplitude variation V(t) Electrochemical system I(t) I a V a V( t) Ecorr V sin(2 f t) with Va min Va Va max a VASP set up For each amplitude V a, R p is determined and the R p =f(v a ) is plotted 27

28 EC-LAB CORROSION TECHNIQUES VASP The frequency f is determined by an EIS technique and corresponds to the R p parameter where Im(Z) =0 and Z(f)= R p f= 0,23 Hz The impedance response of a non-linear electrochemical system depends on the potential amplitude v a Impedance evolution of a non linear system (Test box-3-2) 28

29 EC-LAB CORROSION TECHNIQUES VASP VASP Fit I corr Tafel parameters VASP on a Ni electrode in HCl media 29

30 EC-LAB CORROSION TECHNIQUES CASP CASP (Constant Amplitude Sinusoidal micropolarization) technique is used to determine the corrosion current and the Tafel parameters using Discrete Fourier Transform (DFT) A voltage V(t) with a frequency f is applied around E corr CASP set up Three harmonics:,, Corrosion rate i corr Tafel parameter b a and b c 30

31 EC-LAB CORROSION TECHNIQUES CASP harmonics CASP Fit I corr Tafel parameters 31

32 EC-LAB CORROSION TECHNIQUES Zero Resistance Ammeter (ZRA) ZRA is corrosion technique used to measure the galvanic coupling current between two dissimilar electrodes. It consists of applying zero volts between the WE and the CE and then measuring the current and the potentials (Ewe, Ece) versus the reference electrode. OCV ZRA Coupling current Noise resistance OCV ZRA Set up 32

33 EC-LAB CORROSION TECHNIQUES Three methods are available in EC-Lab to calculate the noise resistance R n 1. Standard deviation (SD). 2. Power spectral density (PSD ) with DFT. 3. PSD with MEM (Maximum Entropy Method) ENA Fit ZRA curves PSD with DFT method PSD with MEM method SD method 33

34 OUTLINE 1. Introduction 2. Types of corrosion a) Uniform corrosion b) Localized corrosion 3. Corrosion experiment 4. EC-Lab corrosion techniques a) Uniform corrosion b) Localized corrosion 5. Conclusions 34

35 EC-LAB CORROSION TECHNIQUES Cyclic Potentiodynamic Polarization (CPP) CPP is a corrosion technique used to evaluate pitting susceptibility and repassivation potential of a material. It is based on a potentiodynamic scan Pitting potential Corrosion potential Repassivation potential Corrosion rate CPP Set up 35

36 EC-LAB CORROSION TECHNIQUES Cyclic Potentiodynamic Polarization (CPP) The figure below give an example of the curve obtained by CPP technique Reverse scan i p E pit i p E corr E repa The pitting potential is easily determined in I-E presentation Pitting potential E pit 36

37 EC-LAB CORROSION TECHNIQUES Multiple Potentiodynamic Pitting (MPP) This technique is designed to study pitting corrosion on one or several electrodes together in the electrochemical cell. CE to ground connection is required in the case of MPP experiment Pitting potential Corrosion potential MPP description Corrosion rate Corrosion resistance MPP set up 37

38 EC-LAB CORROSION TECHNIQUES Multielectrode Potentiodynamic Pitting (MPP) MPP technique can be performed on up to 16 working electrodes simultaneously (with VMP3 in Ce to ground mode) MultiPitting Statistics allows the user to compute data simultaneously from several data files. MP statistics E corr MPP curves of the four steel electrodes MPP investigations and Statistics 38

39 EC-LAB CORROSION TECHNIQUES Multielectrode Potentiostatic Pitting (MPSP) This technique is similar to the MPP technique with a potentiostatic control instead of a potentiodynamic scan. It is designed to study pitting corrosion on one or several electrodes together in the electrochemical cell MPSP description Pitting potential Metastable pits MPSP set up 39

40 EC-LAB CORROSION TECHNIQUES Multielectrode Potentiostatic Pitting MPSP MPSP technique allows the determination of the latency and growth phases for each applied potential E i. Latency phase i p Growth phase MPSP curve for a stainless steel in 30g.L -1 NaCl. 40

41 CONCLUSIONS In the first part of this tutorial: an overview of different corrosion forms was presented. the main characteristics of each form of corrosion was described with photo illustrations. A corrosion experiment with a three electrodes system was presented. The connection modes to Bio-logic potentiostats were also detailed. In the second part of this tutorial: the corrosion techniques available in EC-lab software were detailed. Experimental results were given here to illustrate the use of the corrosion techniques. Numerous corrosion analysis tools, available in EC-Lab softawre were used. 41