03.Thermodynamics in Corrosion Engineering(1)

Thermodynamics in Corrosion Engineering

Lecture#03-04

Utility of Thermodynamics in Electrochemistry

Thermodynamic considerations allow the determination of whether a reaction can occur spontaneously

If metal dissolution is unfavorable thermodynamically in a given set of circumstances – the job of the corrosion engineer is done Example: Copper in pure deoxygenated water

Objectives

To relate your thermodynamic knowledge with the thermodynamics of corrosion-related electrochemistry

To describe the need for and characteristics of reference electrodes

To describe the origin, use, and limitations of electrochemical phase diagrams (such as Pourbaix diagram)

Free Energy: Driving Force of a Chemical Reaction

Spontaneous

Spontaneous

Relation of ΔG and emf

EnFG ΔG is in Joules E is emf in volts n is the number of electrons

involved in the reaction F is the Faraday (96500

C/equivalent)

The larger the value of E for any cell – more is the tendency for the overall cell reaction to proceed

Ecell = Ecathode - Eanode

The Nernst Equation

rRqQmMlL

....

....ln0

mM

lL

rR

qQ

aa

aa

nF

RTEE

General Reaction for a Galvanic Cell

Nernst Equation:

Half Cell Potential

When a metal M is immersed in an aqueous electrolyte, it acquires a certain potential. If the activity of the metal ions M++ in the aqueous environment is unity, then the acquired potential is known as standard potential φ0

Potential of each electrode can be calculated using Nernst equation

Example: Zinc Electrode

2Zn0

Zn

2

Zn

Znln

2

Zn2eZn

F

RT

Hydrogen Electrode

It is assumed arbitrarily that the standard potential for the following reaction is equal to zero at all temperatures

So

Standard Hydrogen Electrode (SHE) The potential of the electrode equals zero if the

hydrogen ion activity and the pressure of hydrogen gas in atmospheres are both unity. This is the standard hydrogen potential

The half - cell potential for any electrode is equal to the emf of a cell with the standard hydrogen electrode as the other electrode.

The half - cell potential for any electrode expressed on this basis is said to be on the normal hydrogen scale or on the standard hydrogen scale , sometimes expressed as φH or φ ( S.H.E. )

Convention of Signs and Calculation of EMF

V 763.0 Zn2e -Zn

V 763.0Zn 2eZn

02

02

It was agreed at the 1953 meeting of the International Union of Pure and Applied Chemistry that the reduction potential for any half - cell electrode reaction would be called the potential

Pt; H2, H+, Zn2+; Zn Cell

Ecell = Ecathode – Eanode= ??

Reference Half Cells

It is not always convenient to have a hydrogen electrode in the laboratory

Other reference half-cells (reference electrodes) have been introduced. Calomel reference electrode Ag-AgCl half cell The Saturated Copper-Copper Sulfate half cell

Calomel Reference Electrode

Ag-AgCl Reference Electrode

Cu-CuSO4 Half Cell

Number Line for Potential Conversion Among Different Reference Electrode Scales

Oxygen Electrode

Oxygen Electrode and Differential Aeration Cell Consider two O2 electrodes:

one in contact with O2 at 1 atm

other in contact with O2 at 0.2 atm

Oxygen Electrode and Differential Aeration Cell

The reaction is not thermodynamically possible as written

Thus, the electrode 1 is cathode electrode 2 the anode.

In a differential aeration cell, the electrode in lower O2 pressure acts as the anode and the one in higher O2 pressure acts as the cathode

EMF Series

All metals have been arranged in a series according to their standard potential (φ0) values.

The more positive value corresponds to noble metals and the more negative value corresponds to more reactive metals (when arranged according to reduction potential)

Of the EMF series – if two metals make up a cell, the more active metal acts as the anode and the more noble metal of the two will act as cathode

EMF Series

Problems with EMF Series

In real situation, the activities of the metal ions in equilibrium with the respective metals usually do not equal unity

The position of a metal in the EMF series with respect to another metal may change because of complex formation as is the case with tin (Sn) and steel (Fe)

Alloys are not included in the EMF series

In oxidizing environment, some metals undergo passivation and are known as active-passive metals. Transition metals usually show passive behaviour in aerated aqueous environment. This dual position of some metals is not reflected in the EMF series.

Galvanic Series

Galvanic series is an arrangement of both metals and alloys according to their actual measured potentials in a particular environment. There would be one Galvanic series for each environment

Metals and alloys showing active-passive behaviour are listed in both active and passive states.

Galv

an

ic S

eri

es

in

Seaw

ate

r

Pourbaix Diagram

Marcel Pourbaix developed potential-pH diagrams to show the thermodynamic state of most metals in dilute aqueous solutions

With pH as abscissa and potential as ordinate, these diagrams have curves representing chemical and electrochemical equilibria between metal and aqueous environment

These diagrams ultimately show the conditions for immunity, corrosion or passivation.

Simplified Pourbaix Diagram for Iron

Pourbaix Diagram for Iron

Pourbaix Diagram for Iron at 25°C

Benefits of Pourbaix Diagram

Pourbaix diagrams offer a large volume of thermodynamic information in a very efficient and compact format.

The information in the diagrams can be beneficially used to control corrosion of pure metals in the aqueous environment By altering the pH and potential to the regions of immunity and

passivation, corrosion can be controlled. For example, on increasing the pH of environment in moving to slightly alkaline regions, the corrosion of iron can be controlled

Changing the potential of iron to more negative values eliminate corrosion, this technique is called cathodic protection.

Raising the potentials to more positive values reduces the corrosion by formation of stable films of oxides on the surface of transition metals

Limitations of Pourbaix Diagrams These diagrams are purely based on

thermodynamic data and do not provide any information on the reaction rates

Consideration is given only to equilibrium conditions in specified environment and factors, such as temperature and velocity are not considered which may seriously affect the corrosion rate

Pourbaix diagrams deal with pure metals which are not of much interest to the engineers