Contents Fundamental of AC theory Review of EIS Equivalent Circuit - PowerPoint PPT Presentation

Contents  Fundamental of AC theory  Review of EIS  Equivalent Circuit Models for Coatings  Change in EIS during Deterioration of a Coating  Combination of assessment of coating  Few coating examples and case studies

What is EIS and what can do?  A metal is held at its OCP, E corr , in an electrolyte  A small sinusoidal AC voltage is applied to the metal, e.g., 10-100 mV  The response, a small sinusoidal current, is measured  The ratio of the two is not a resistance, but an impedance  The frequency of the AC voltage is varied from 10 5 to 10 – 2 Hz (typically)  The Impedance value is plotted vs. the frequency  The curve is modeled by an Equivalent Circuit that gives the same frequency dependence  The EC consists of resistors, capacitors and other elements, whose values can be calculated if the model fits the data  These elements represent the Polarization Resistance R p and Double Layer Capacitance C dl of the metal in the electrolyte  As it is based on AC, EIS can also be applied to metals with a dielectric coating, such as a paint, immersed in an electrolyte

What is EIS can do? EIS is not only a corrosion measurement technique  In that case we measure the properties of the coating, such as thickness, dielectric constant, porosity, water uptake, etc.  The equipment we use in EIS (potentiostat, corrosion cell, reference electrode, counter electrode), are the same as in the DC methods  In addition, a frequency response analyzer and a lock-in amplifier are used  EIS has become very popular in recent years and is now a routine tool  An ASTM standard has been written  The areas that are considered appropriate for using EIS are: 1.Rapid estimation of corrosion rates, within 30 min. 2.Estimation of very low corrosion rates (<0.01 mpy) 3.Estimation of corrosion rates in low-conductivity media 4.Rapid assessment of corrosion inhibitor performance 5.Rapid evaluation of coatings 6.Evaluation of metal pretreatments, e.g., chromates, phosphates

EIS – Theory - Simple Linear Input/output Systems SYSTEM Input Output Light Current Temperature Voltage Sound Voltage Ohm’s law: R = V / I Impedance (Z) = across function (V O ) / through function (I O )

EIS – Frequency response analysis Input and output as function of time Sinusoidal response in a linear system Magnitude and Phase shift are frequency dependant, Impedance is vector quantity IZI = Z’(real) – j Z’’(imaginary)

How does EIS work? Sinusoidal response in a linear system

EIS

How is shown on a oscilloscope? E (t) plotted vs. I (t)  Frequency response analyzers and lock-in amplifiers are required to convert these figures to EIS spectra.  During the recording of an EIS spectrum this figure is displayed

DC/AC-Experimental set up Reference electrode Counter electrode sample in contact with the electrolyte through a hole of 1cm 2 Electrochemical flat cell

Electrochemical Impedance Data Acquisition • Measure E ocp and allow it to stabilize • Apply DC voltage equal to the measured value of E ocp • In addition to the DC voltage, apply a small sinusoidal voltage (10 mV) perturbation of fixed frequency and measure the current response • Calculate the impedance and the phase shift. • Repeat the measurement at a wide range of frequencies. Data Analysis • Model the electrochemical process with electrical circuit elements such as resistors, capacitors, and inductors. Adjust the values of the circuit elements to fit the model to the EIS data.

EIS applications • Coatings performance prediction (e.g pre-treatment/primer or any polymer system) • Film evaluation (free standing or CCVD) • Corrosion prevention/control • Inhibitor studies • Electroplating, Electro deposition • Conducting polymers • Battery and fuel cells, Membrane/separators • Metal processing/recovery • Corrosion (Pitting / SCC etc.) • Metal oxide formation

A Bode Plot: Magnitude & Phase Shift

EIS data can be presented as a Bode Plot or a Nyquist Plot

Modelling • Electrochemical cells can be modeled as a network of passive electrical circuit elements. • The network is called an “equivalent circuit”. • The EIS response of an equivalent circuit can be calculated and compared to the actual EIS response of the electrochemical cell. • The values of the circuit elements in the model are calculated by varying them in an iterative fashion until acceptable agreement is reached (non-linear least squares) with the actual EIS curve of the sample.

Frequency Response of Electrical Circuit Elements 0 ° Phase Shift  Resistor (ohms) Z = R Z = -1/j  C -90 ° Phase Shift  Capacitor (Farads) Z = j  L +90 ° Phase Shift  Inductor (Henrys)  A real response is in-phase (0 ° ) with the excitation. An imaginary response is 90 ° out-of-phase. j =  -1, ω = 2  f radians/s, f = frequency (Hz or cycles/s)

EIS of a Resistor Applied Voltage Phase Measured Current Shift of 0º Magnitude Emag Imag Zmag = Emag / Imag = R Time

EIS of a Capacitor Measured Current Phase Applied Voltage Shift of 90º Imag Emag Magnitude Zmag = Emag / Imag = 1/(2  fC) Time

EIS- Non destructive technique Interface Metal/electrolyte Equivalent circuit for a Electrolyte Metal Cd l simple corroding system Rs with an electrolyte resistance. Rp Rs + Rp Cdl Rs Nyquist frequency decreases Bode IZI impedance W max =1/RpCdl plot and phase angle Rp plots Rs Rs + Rp

EIS- Non destructive – Organic coated products with pore/defect Electrolyte Coating Metal Rs Cc Z” Rct and Cdl Rpo and Cc Rpo Cdl Rct pore or Z’ defect • Water uptake • Evaluate coating resistance/capacitance • Corrosion underneath the coating • Blistering-delamination • Quantification of substrate area non protected ?

Coating Impedance vs. water permeation

Two “Special Problems” with the Measurement of the EIS of Coatings 1. Because of the barrier nature of coatings, currents will always be small, so use a sensitive potentiostat and a Faraday Cage. 2. The initial Open-Circuit Potential of an intact coating is subject to Capacitive Drift. For the first few measurements, use the E ocp of the bare metal.

EIS Seems More Complex, than It Is! • Discussed the theory of EIS/modeling and it’s not particularly simple. • You will be pleased to know that the real-world applications of EIS for coating evaluation are relatively simple!

EIS of Coatings on Metallic Substrates • For coatings on a metal substrate, EIS acts as a very sensitive quantitative detector of changes in both the coating and the metal substrate during long-term exposure to an electrolyte. • Changes in the coating will be apparent in EIS long before any visible damage occurs. • Apply stress to the sample to cause it to fail. The stress should simulate the service environment, which could be weathering or a specific chemical attack, e.g ., seawater.

EIS of Coatings on Metallic Substrates • Measure an EIS Curve immediately upon exposure and periodically thereafter until the test is complete. Changes in the EIS Curve with time reflect changes in the coating or the metal substrate. These changes are accelerated by the artificial stress. • Fit an equivalent circuit to the data to determine the value of the circuit elements. • Evaluate the data to select an “indicator” of coating deterioration. The indicator may be Z total , capacitance, pore resistance, etc . • In many cases, Z at low frequency is satisfactory .

What Do We Mean by “Stress”? • To test a coating, we must cause it to fail by applying an “artificial” stress • The stress must resemble the service environment and it must not change the failure mechanism • The stress may be a solution under various conditions, a temperature, climate or a voltage.

Electrified interface structure for a corroding coated metal

Equivalent Circuit and Schematic of an Organic Coating on a Metal Substrate

Typical Model of a Coated Metal R u = Uncompensated Resistance C coating = Coating Capacitance R pore = Pore Resistance Rp = Polarization Resistance (Corrosion of the Metal Substrate) Cdl = Double Layer Capacitance at the Metal Substrate

Alternative Model of a Coated Metal to Account for Blistering* Ru = Uncompensated Resistance Ccoating = Coating Capacitance C blister = Coating Capacitance Above the Blister Cdl,blister = Double Layer Capacitance of the Metal Under the Blister Rp,blister = Polarization Resistance (Corrosion of the Metal) Under the Blister Rpore = Coating Pore Resistance Cdl = Double Layer Capacitance of the Metal Under the Pore Rp = Polarization Resistance (Corrosion of the Metal) Under the Pore *Kern et al, Journal of Coatings Technology, 71, 67 (1999)

Six Steps of Coating Degradation* 1. Initial Immersion, Purely Capacitive 2. Absorption of Water 3. Development of a Pore Resistance 4. Diffusion-Controlled Corrosion Through Pores 5. Free Corrosion During Blistering 6. Major Coating Damage *J. N. Murray, Progress in Organic Coatings, 31, 375-391 (1997)

1. Initial Immersion, Purely Capacitive Note: • 90 o Phase Angle • -1 Slope in Impedance • High Impedance at Low Freq • Impedance @ 0.16 Hz ~ 1GW