corrosion-mixed potential theory - arizona state university · 2021. 2. 4. · corrosion-mixed...

Corrosion- Mixed Potential Theory

We analyzed this

We about these?

Refe

renc

e El

ectr

ode

Voltmeter

0.1M H2SO4Only acid, no dissolved cations

Corrosion- Mixed Potential Theory

We analyzed this

We about these?

Refe

renc

e El

ectr

ode

Voltmeter ?

Only acid, no dissolved cations0.1M H2SO4

Corrosion- Mixed Potential Theory

We about these?

Refe

renc

e El

ectr

ode

Voltmeter

We analyzed this

Zn

0.1M H2SO4Only acid, no dissolved cations

Corrosion- Mixed Potential Theory

This material is taken from Introduction to Corrosion Science, McCafferty

90° CCW Outof

Planeflip Butler-Volmer Equation

Corrosion- Mixed Potential Theory

Refe

renc

e El

ectr

ode

Voltmeter

Zn

0.1M H2SO4Only acid, no dissolved cations

Zn corrodes in the acid according to

Zn→ Zn+2 + 2e−

What’s reduced; what is the cathodic reaction?

2H + + 2e− → H2( g )

Corrosion- Mixed Potential Theory

Refe

renc

e El

ectr

ode

Voltmeter

Cu

0.1M H2SO4Only acid, no dissolved cations

Cu corrodes in the acid according to

What’s reduced; what is the cathodic reaction?

Cu→Cu+2 + 2e−

�

O2 + 4H + + 4e− → 2H2O

Why is H the cathodic Rx for Zn and O the Rx for Cu?

Corrosion- Mixed Potential Theory

2H+ + 2e

− →H 2 ( g

)

2H ++ 2e −

←H2( g )

io on Zn

EoEZn/Zn2+o

ioZn/Zn2+

Butler-Volmer Equation for both Zn and H

Corrosion- Mixed Potential Theory

Butler-Volmer Equation

ηi0

Analogy with

f = FRT

icorr is not easily measured!

2H+ + 2e

− →H 2 ( g

)

Corrosion- Mixed Potential Theory

Corrosion- Mixed Potential Theory

Evan’s diagram for Fe in non-complexing acid

Corrosion- Mixed Potential Theory

Corrosion- Mixed Potential Theory

The work function of a metal is thework required to remove and electronFrom the metal to vacuum level.

Corrosion- Mixed Potential Theory

Corrosion- Mixed Potential TheoryCathodic protection of Fe by Zinc

Lowering of by almost an order ofmagnitude.

Corrosion- Mixed Potential Theory

1.2 0.80 0.40 0.00 -0.400ESHE

Corrosion of Cu in acid

Corrosion- Mixed Potential Theory

The y-axis is done in terms of theCURRENT rather than the current density