CORROSION THERMODYNAMICS

PRESENTED BY:GAUTAM AHUJAM.TECH-POLYMER TECHNOLOGY2K11/PTE/02DTU

INTRODUCTION

Corrosion the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. Corrosion also refers to other materials than metals, such as ceramics and polymers, although in this context the term degradation is more common. In the most common use of the word, this means electrochemical oxidation of metals in reaction with an oxidant such as oxygen.

UTILITY OF THERMODYNAMICS

• 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

Four components are essential to set up an electrochemical corrosion cell:

1. ANODE – The corroding electrode

2. CATHODE- The passive, non-corroding electrode

3. THE CONDUCTING MEDIUM – The electrolyte or the corroding fluid

4. COMPLETION OF THE ELECTRICAL CIRCUIT- Through the material

CORROSION MECHANISM

FREE ENERGY OF A REACTION

The tendency for any chemical reaction, including corrosion, is measured by the G.

Mg + H2O +1/2O2 Mg(OH)2 G° = -142,600 cal

Cu + H2O +1/2O2 Cu(OH)2 G° = -28,600 cal

Au + 3/2H2O +3/4O2 Au(OH)3 G° = +15,700 cal

Tendency to corrosion : Mg > Cu Au≫

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

STANDARD ELECTRODE POTENTIALS

The potential difference across an electrochemical cell is the potential difference measured between two electronic conductors connected to the electrodes.

A voltmeter may be used to measure the potential differences across electrochemical cells but cannot measure directly the actual potential of any single electrode.

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. )

Hydrogen Electrode

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

• So

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

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.

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.

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

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