483 Budi_agung Corrosion Measurement Techniques

Corrosion Measurement Techniques A copy of this presentation is available in the CAL group in the computers in the Teaching Lab, or via the WWW at http://www.cp.umist.ac.uk/CPC/L_Notes (c) Bob Cottis 1995

Corrosion Measurement Techniques ❖Polarization curves ❖Linear Polarization Resistance ❖Open Circuit Potential Decay ❖AC Impedance Measurement ❖Electrochemical Noise Measurement ❖Weight Loss Measurement

(c) Bob Cottis 1995

Polarization Curves ❖Measurement ❖Cell

design

❖Plotting

data

❖Interpretation

(c) Bob Cottis 1995

methods

Measurement Methods ❖

Objective – determine current density under steady-state conditions as a function of potential – not really practical, as this would strictly require one sample for each potential – therefore compromise on ‘closeness’ to true steady-state

(c) Bob Cottis 1995

Measurement Methods Connect electrodes to corresponding terminals ❖ Potential control on potentiostat Reference Electrode Luggin Probe - connection allows reference for Potentiostat AE potential potential to be detected measurement close to metal surface RE WE Counter Electrode (or Auxilliary Electrode or Working Electrode - Secondary Electrode) Potentiostat controls metal being studied provides current path potential into solution (c) Bob Cottis 1995

Measurement Methods ❖

Current control Potentiostat

Current path

AE RE WE

Counter Electrode R

Reference Electrode Current controlled by Luggin Probeacross still only used monitor potential, control oftovoltage Working Electrode to (I=V/R) limit IR error notneeded connected to potentiostat resistor (c) Bob Cottis 1995

V

Measurement Methods ❖

Swept potential or current – Use sweep generator to produce slowly changing potential – Sweep generator output controls potentiostat – Record response on chart recorder (or use computer monitoring) – Swept current not often used, as it moves through corrosion potential very quickly

(c) Bob Cottis 1995

Measurement Methods ❖

Potential or current step – Step potential or current from one value to the next, allowing time to stabilise at each new value – Record current or potential – May be manually controlled, or use computer to step potential/current and take readings

(c) Bob Cottis 1995

Measurement Methods ❖

Sweep direction – Aim to perform experiment in such an order that the initial polarization affects subsequent results as little as possible – Options ◆ ◆

◆

(c) Bob Cottis 1995

new specimen for each potential one specimen for cathodic polarization, and one for anodic, both start at corrosion potential one specimen, sweep from cathodic to anodic

Measurement Methods ❖

Sweep rate (or step rate) – Ideal, all measurements made at steady-state – Time-dependent effects include: ◆ ◆ ◆

Charging of double layer capacitance (I = C dV/dt) Mass transport effects (t ∝ L2/D) Adsorbed species and surface films (Faraday’s Law)

– Typical sweep rates are of the order of 1 mV/s or less

(c) Bob Cottis 1995

Questions ❖

Consider the corrosion of iron in aerated neutral solution, with the following parameters: DO2 = 1.2 x 10-5 cm2 /s – Cdl = 35 µF / cm2 – Boundary layer thickness, δ = 100 µm – Number of iron atoms on surface ≈ 2×1019/cm2 – Charge on the electron = 1.6 x 10-19C

❖

Calculate – Capacitive current at 1 mV/s – Characteristic diffusion time – Limiting current density for O2 reduction (8 ppm O2) – Time to oxidise Fe surface to FeOH (Fe+) at ilim

(c) Bob Cottis 1995

Cell Design ❖ ❖ ❖ ❖ ❖

Working electrode Reference electrode Counter electrode Solution Mass transport

(c) Bob Cottis 1995

Working Electrode ❖

Requirements – – – – – –

(c) Bob Cottis 1995

reproducible representative free of crevices free of edge effects free of galvanic effects free of water-line effects

Working Electrode ❖

Epoxy embedded electrode:

Apply thin layer of epoxy Weld or solder connecting to minimise stress and risk Apply thick layer of epoxy wire to specimen of crevice Pretreatformation specimen for to seal connecting tube and good adhesion Carefully grind surface for strength to expose metal

(c) Bob Cottis 1995

Clean surface - don’t use acetone

Working Electrode ❖

Stern-Makrides electrodes:

Metal rod Retaining nut Washers Heavy-walled glass tube

Lip seal between PTFE case and electrode

PTFE Washer Electrode

(c) Bob Cottis 1995

Working Electrode ❖

Avesta cell: Pure H2O feed

NaCl Solution

Specimen

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Filter paper

Reference Electrode ❖

❖

Commonly use Saturated Calomel Electrode (SCE) Properties may degrade with time (and misuse) – check one against another (should not be more than 1 to 2 mV difference) – do not pass current through the reference electrode (e.g. do not connect to working or counter electrode) – do not allow to dry out

(c) Bob Cottis 1995

Reference Electrode ❖

Solution in SCE (or Ag/AgCl electrode) is saturated KCl – beware of chloride contamination of test solution by Cl- leaking from reference electrode – make sure solution remains saturated

(c) Bob Cottis 1995

Luggin Probe A Luggin probe should be used whenever there is a significant current applied to the electrode

Electrode

❖

(c) Bob Cottis 1995

Luggin probe allows point at which potential is measured to be close to electrode surface (around 3 times tip diameter is best)

Counter electrode ❖

❖

❖

Counter electrode should allow current to pass with tolerable polarization Often claimed that counter electrode should have much larger area than working electrode, but this is not often necessary for corrosion studies Usually use platinum or graphite, although stainless steel can be used in some situations (e.g. where only anodic polarization of specimen is used)

(c) Bob Cottis 1995

Solution ❖

Requirements: – as high a conductivity as possible (add supporting electrolyte, such as sodium perchlorate?) – remain the same (pH, composition) throughout the experiment - ensure that volume is adequate – oxygen concentration often critical - aerate by bubbling air or O2 or deaerate with N2 or Ar – most reactions temperature sensitive, so control, or at least record, temperature

(c) Bob Cottis 1995

Mass transport ❖

Methods of controlling mass transport – – – –

(c) Bob Cottis 1995

rotating disk or cylinder flow channel jet impingement gas bubbling

Plotting of Polarization Curves ❖ ❖

❖

Comparison of log-i and linear-i plots Identification of anodic and cathodic regions on log-i plots Orientation of plots

(c) Bob Cottis 1995

E-i Plot 1.000E-01 8.000E-02

Fe anodic

H cathodic

6.000E-02

O2 cathodic

Net

Current Density

4.000E-02 2.000E-02 0.000E+00 -2.000E-02 -4.000E-02 -6.000E-02 -8.000E-02 -1.000E-01 -1.500

-1.300

-1.100

-0.900

-0.700

-0.500 Potential

(c) Bob Cottis 1995

-0.300

-0.100

0.100

0.300

0.500

E log |i| Plot 1.000E-01

Current Density

1.000E-02

Fe anodic

H cathodic

O2 cathodic

Net anodic

Net cathodic

1.000E-03

1.000E-04

1.000E-05

1.000E-06 -1.500

-1.300

-1.100

-0.900

-0.700

-0.500 Potential

(c) Bob Cottis 1995

-0.300

-0.100

0.100

0.300

0.500

E log |I| - old plotting method

E

log |i| (c) Bob Cottis 1995

Interpretation of Polarization Curves ❖ ❖ ❖ ❖ ❖ ❖

Addition of reactions on log-I graphs Tafel regions Mass transport control Active-passive transition Transpassive corrosion Pitting Corrosion

(c) Bob Cottis 1995

Tafel regions ❖

❖

A Tafel region is a straight line in the E-log|i| plot For a reliable Tafel slope: – the line should be straight for at least one decade (in this context a decade implies a change of current density by a factor of ten, i.e a difference of 1 in log i ) – the region should be next to Ecorr

(c) Bob Cottis 1995

E log |i| Plot 1.000E-01

Current Density

1.000E-02

Fe anodic

H cathodic

O2 cathodic

Net anodic

Net cathodic

1.000E-03

1.000E-04

1.000E-05

1.000E-06 -1.500

-1.300

-1.100

-0.900

-0.700

-0.500 Potential

(c) Bob Cottis 1995

-0.300

-0.100

0.100

0.300

0.500

Tafel Extrapolation ❖

Extrapolate anodic or cathodic Tafel region, or both, back to Ecorr, when the current density is icorr

❖

In aerated neutral solutions, where mass transport limited oxygen reduction is the main cathodic reaction, the cathodic reaction does not have a valid Tafel slope, but the anodic slope can sometimes be used

(c) Bob Cottis 1995

Question ❖

❖

How can we estimate the rate of hydrogen evolution during free corrosion? Estimate the value for the graph shown.

(c) Bob Cottis 1995

E log |i| Plot 1.000E-01

Current Density

1.000E-02

Fe anodic

H cathodic

O2 cathodic

Net anodic

Net cathodic

1.000E-03

1.000E-04

1.000E-05

1.000E-06 -1.500

-1.300

-1.100

-0.900

-0.700

-0.500 Potential

(c) Bob Cottis 1995

-0.300

-0.100

0.100

0.300

0.500

Mass transport control ❖

❖

❖

When the supply of a reactant becomes mass transport controlled, we observe a limiting current density The most common case occurs for oxygen as a cathodic reactant in neutral solutions NOTE - the diffusion of a reaction product away from the electrode will not affect the rate of the forward reaction

(c) Bob Cottis 1995

Solution Resistance Effects ❖

❖ ❖

At high currents the potential drop associated with the solution resistance can be significant It is generally referred to as an IR error Gives a straight line on E-i plots

(c) Bob Cottis 1995

E log |i| Plot 1.000E-01

Current Density

1.000E-02

Fe anodic

H cathodic

Net anodic

Net cathodic

O2 cathodic

1.000E-03

1.000E-04

1.000E-05

1.000E-06 -1.500

-1.300

-1.100

-0.900

-0.700

-0.500 Potential

(c) Bob Cottis 1995

-0.300

-0.100

0.100

0.300

0.500

E-i Plot 2.000E-02

1.500E-02

Fe anodic

H cathodic

O2 cathodic

Net

Current Density

1.000E-02

5.000E-03

0.000E+00

-5.000E-03

-1.000E-02

-1.500E-02

-2.000E-02 -1.500

-1.300

-1.100

-0.900

-0.700

-0.500 Potential

(c) Bob Cottis 1995

-0.300

-0.100

0.100

0.300

0.500

Active-passive transition ❖

As a passive film develops, it covers the surface and shuts off the dissolution reaction, leading to an active-passive transition

E

log |i| (c) Bob Cottis 1995

Active-passive transition ❖

For stainless steel we sometimes see two active-passive transitions, on for Chromium, and one for Iron

E

log |i| (c) Bob Cottis 1995

Transpassive corrosion ❖

❖

❖

A passive metal (notably Cr and Fe) may start to dissolve at a very positive potential when a higher oxidation state (e.g. Cr6+ as chromate) is formed This is known as transpassive corrosion, and will give something like a second activationcontrolled reaction For alloys the behaviour will be complicated by the differing behaviours of the alloy components

(c) Bob Cottis 1995

Anodic Polarization Curve for Stainless Steel

E

Activation-controlled Active-passive Transpassive Oxygen Active peak forcurve iron Overall anodic corrosion dissolution transition reduction of Cr

log |i| (c) Bob Cottis 1995

Pitting Corrosion ❖

❖

Pitting shows up as an increasing anodic current before (at a less positive potential than) transpassive corrosion or oxygen evolution, usually preceded by noise E-log|i| plot does not follow same path if scan direction is reversed, but current is greater (since pit continues to grow)

(c) Bob Cottis 1995

Pitting Corrosion

E

Current continues Noise Pit eventually spikes due re-to to increase after meta-stable passivates pitting reversal of scan

log |i| (c) Bob Cottis 1995

What is going on? Stainless Steel in Aerated Sulphuric Acid

Anodic Cathodic

E

Anodic Cathodic log |i| (c) Bob Cottis 1995

Linear Polarization Resistance Measurement ❖Theoretical basis ❖Measurement ❖Interpretation

(c) Bob Cottis 1995

methods

LPRM Theory ❖

For an activation controlled reaction  E − Eo   i = io exp  β   E − Eo  di io  ∴ = exp dE β  β  i = β

(c) Bob Cottis 1995

Exchange Equilibrium current Tafel slope based on potential density exponential (i.e. mV for a change of 1 in ln(i))

LPRM Theory ❖

Summing for two reactions di ia ic = + dE β a βc  βa − βc  1  = = icorr   β aβc  R p

❖

Anodic Because partial βpartial is taken current Cathodic Anodic Tafel slope current Cathodic Tafel slope c based Tafel slope onas a density (=i-i )in) (positive) (= (negative) negative corr corr decade change current (i.e. a change Stern-Geary of 1 in log i ) coefficient

Rearrange and convert to b rather than β Rp =

(c) Bob Cottis 1995

B icorr

,

ba bc B= 2.3( ba − bc )

LPRM Measurement Methods ❖ ❖ ❖ ❖

Control variable Waveform Cell configuration Sweep rate

(c) Bob Cottis 1995

LPRM Control Variable ❖

Potential control – potential range can be optimised – problems with drift of Ecorr

❖

Current control – potential range depends on Rp – measurement inherently centred about i = 0

(c) Bob Cottis 1995

LPRM Measurement Waveform ❖

Triangle wave – can measure di/dt at i = 0 – requires relatively complex instruments

❖

Square wave (switch between +i and -i) – simple instruments – simple to automate

❖

Sine wave – simplest theory for frequency effects – complex to perform measurement

(c) Bob Cottis 1995

LPRM Cell Configuration ❖

Two electrode – assume Rp is the same for two similar electrodes and measure cell resistance (= 2Rp + Rsol) – easy, no reference electrode required

❖

Three electrode – use conventional counter, reference and working electrodes – provides lower solution resistance, therefore better for low conductivity solutions – more complex instrumentation

(c) Bob Cottis 1995

LPRM Recommendations ❖

❖

❖

Use three electrode measurement with triangle waveform for laboratory studies Use two electrode measurement with square waveform for simple corrosion monitoring (use three electrodes for high resistance solutions) Use potential control when icorr variation is large

❖

Use current control when Ecorr varies a lot

❖

When both icorr and Ecorr vary use current control, but adapt current to keep potential range reasonable

(c) Bob Cottis 1995

LPRM Interpretation ❖

Determination of B value – calculate from Tafel slopes ba bc B= 2.3( ba − bc ) – correlation with weight loss – arbitrary value ◆ ◆

(c) Bob Cottis 1995

26 mV for activation control 52 mV for one reaction at limiting current

LPRM Sweep Rate ❖

❖

❖

Must be sufficiently slow for current charging double layer capacitance to be much less than total current Characteristic time given by RctCdl - cycle time should be at least 3 times this Need not be slow enough to allow diffusion processes to respond (as the basic theory is not valid for diffusion processes)

(c) Bob Cottis 1995

LPRM Problems ❖

Theoretically, either – both reactions must be activation controlled, or – one reaction must be activation controlled and the other mass-transport limited

❖

❖

In practice it is rare for real systems to meet these constraints, and application of LPRM is not theoretically justified Solution resistance adds to measured Rp, and produces lower apparent corrosion rate

(c) Bob Cottis 1995

Equivalent Circuits ❖

❖

An electrical circuit with the same properties as a metal-solution interface The simplest circuit is a resistor, Rct, corresponding to the polarization resistance, in parallel with a capacitor, Cdl, corresponding to the double layer capacitance Solution

(c) Bob Cottis 1995

Equivalent Circuits ❖

❖

An electrical circuit with the same properties as a metal-solution interface The Randles equivalent circuit adds a series resistor, corresponding to the solution resistance Rct Rsol

(c) Bob Cottis 1995

Analysis of Solution Resistance ❖

If we analyse the full response to the LPRM measurement, we can estimate Rsol, the C Rct The voltage Rsol is dl and Estimate Cdlacross from given by V Cdl) exponential decay. Thesoltime oexp(-t/R for V to fall to When t=eR-1sol(37%) Cdl, of the initial value is R C V=V oexp(-1) sol dl

i Vo=iRsol E

V=iRct Time

(c) Bob Cottis 1995

Open Circuit Potential Decay ❖

Similar to analysis of LPRM measurement – charge double layer capacitance by applying a current or potential – disconnect charging current – monitor decay of potential

(c) Bob Cottis 1995

Open Circuit Potential Decay Charging at current i

Disconnected Initial voltage drop = iRsol

E Delayed voltage drop = iRct

Time = RctCdl 0.37iRct Time

(c) Bob Cottis 1995

(c) Bob Cottis 1995