Charged Interfaces - Corrosion

Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING

ME 575

(Advanced Corrosion Engineering)

Chapter # 3

Charged Interfaces


Types Interfaces

Electrolytes

The solution/air interface

Metal/solution interface

Metal Ions in Two Different Chemical Environments

The Electrical Double Layer & Models

Significance of the Electrical Double Layer to Corrosion

Measurement of Electrode Potentials

Reference Electrodes


Charged interfaces

Interfaces form at the physical boundary between two phases, such as:solid

and a liquid (S/L), a liquid and its vapor (L/V), or a solid and a vapor (S/V).

There can also be interfaces between two different solids (S1/S2) or between

two immiscible liquids (L1/L2).

Two special interfaces, the solution/air interface and the metal/solution


Electrolytes: The Interior of an Electrolyte

The interior of an electrolyte mayconsist of a variety of charged and

uncharged species.

(1) H2O molecules

(2) Na+ ions

(3) Cl ions

(4) Organic molecules (which may be

present as impurities, biological entities,

or may be intentionally added as a

corrosion inhibitor).

Fig. 3.1 (a) The water dipole. (b) In the bulk, liquid water

consists of an array of randomly oriented dipoles, so the

net charge is zero

No net electrical field in the interior of liquid water.



Fig. 3.2 A volume element of sodium chloride solution showing

the distribution of ions

no net charge within anyvolume element of

solution due to the

existence of these

dissolved ions of NaCl



Fig. 3.3 Primary waters of hydration for (a) Na+ ion, (b) Clanion. Primary hydration numbers are from Bockris and

Reddy [1]

Central ion is surrounded by watermolecules, which are called primary

waters of hydration.

Secondary region of partially orderedwater molecules (outside the primary

sheath) is called secondary waters

of hydration, which balance the

localized oriented charge which has

developed in the primary water sheath.

No net charge due to ionic hydration.


Interfaces: Solution/Air Interface

Fig. 3.4 Water molecules at the water/air interface

and the origin of surface tension

An imbalance of forces for molecules located in the surface regionresults in a net force inward into the liquid, and this net inward force

is the origin of the surface tension of the liquid.

What about NaCl solution


Interfaces: Metal/Solution Interface

Fig. 3.7 The orientation of water molecules at a metal/s

olution interface. Top: the flop-down orientation of the water dipole. Bottom: the flip-up orientation [3]

More complicated than the solution/airb/c of:

First, being a good conductor, metal side of theinterface can be charged negatively or positively

, respectively.

Second, chloride ions are adsorbed at metal/solution interfaces.

Third, the water molecule itself is adsorbed atmetal/solution interfaces

Fourth, the metal/solution interface is notalways a stable one(neither chemically nor

geometrically)



Array of positive ions in a Fermi sea of electrons.



Corrosion process transfer of a positive ion from the metal lattice into solution Inthe metal lattice, the positive ion is stabilized by the Fermi sea of electrons Insolution, the positive ion is stabilized by its water of hydration


Metal having a positive charge, ispartially balanced in solution by a diffuse

layer of negative ions.

In diffuse layer, ions are in thermalmotion.

An overall increase in the concentrationof negative ions within diffuse layer (par

tly balance the positive charge on the

metal side of the interface)

Net charge within the diffuse part of theelectrical double layer (no net charge in

bulk/interior of soln)

Electrical Double Layer- GouyChapman Model

Fig. 3.10 The GouyChapman model of the electrical doublelayer at a metal/solution interface


Electrical Double Layer- The Electrostatic Potential and Potential Difference

The electrostatic potential (at some point)is the work required to move a smallpositive unit charge from infinity to the

point in question. Assumption is

(1) the positive test charge is smallenough not to perturb the existing

electrical field;

(2) the work involved is independent ofthe path taken.

The potential difference (between two points) is the work required to move a small unit

positive charge between the two points [PD

= B A ( joules per coulomb, or volts)


Electrical Double Layer- Stern Model Model

Fig. 3.12 The Stern model of the electrical double

layer at a metal/solution interface

Adsorption of anions or cations at the metalsurface.

Plane through the center of these adsorbedions is called the Helmholtz plane.

The excess charge at the metal surface isbalanced in part by ions located in a GouyChapman diffuse double layer, which exists

outside the Helmholtz plane.

A typical potential difference across theHelmholtz plane is of the order of 1 V. The

thickness of the Helmholtz layer is about

10 (1 = 108 cm) and field strength of1 107 V/cm.


Electrical Double Layer- Stern Model Model

Fig. 3.13 The BockrisDevanathanMller model of theelectrical double layer at a metal/solution interface [4]

Two important consideration:

First: adsorption of water molecules at the M/Sinterface.

Water molecule and ions in solution competefor sites on the metal surface (adsorption of Cl

- ion)

The plane through the center of these adsorbedions is called the inner Helmholtz plane.

Second: charge introduced by the adsorption ofanions at the metal surface is balanced in part

by counter ions of the opposite charge. These

counter ions are not adsorbed on the metal surf

ace, but exist in solution, and have associated

with them their waters of hydration.

The plane through the center of these counterions is called the outer Helmholtz plane.


Significance of the Electrical Double Layer to Corrosion

Fig. 3.14 Simple equivalent circuit model of the electrical

double layer. Cdl is the double layer capacitance, RP is

the resistance to charge transfer across the edl, and RS

is the ohmic resistance of the solution

EDL is the origin of the PD across an M/Sinterface, which is responsible for electrode

potential.

Changes in the electrode potential can producechanges in the rate of anodic (or cathodic)

processes.

Emerging (corroding) metal cations must passacross the EDL outward into solution, and

solution species (e.g., anions) which participate

in the corrosion process must enter the EDL

from solution in order to attack the metal. So,

the properties of the EDL control the corrosion

process.


Electrode Potentials: The Potential Difference Across a Metal/Solution Interface

Fig. 3.15 In order to measure the potential difference across the

metal/solution interface of interest (M/S), an additional interface must

be created using a reference metal ref [3]


Electrode Potentials: The Potential Difference Across a Metal/Solution Interface

The sum total of changes in electrostatic potential must be zero by Kirchhoffs law


Fig. 3.16 A standard hydrogen reference electrode (SHE)

Relative Electrode Potentials- Reference electrodes

The hydrogen electrode is universally accepted as the primary standard againstwhich all electrode potentials are compared.

Fig. 3.17 Experimental determination of a standard electrode

potential for some metal M using a standard hydrogen

reference electrode


Table 3.1 Standard electrode potentials at 25C [6]

The Electromotive Force Series

Limitations of EMF series:

The emf series applies to pure metals (notalloys) in their own ions at unit activity.

The relative ranking of metals in the emfseries is not necessarily the same (and is

usually not the same) in other media (such as

seawater, groundwater, sulfuric acid, artificial

perspiration).

The relative ranking of metals in the emfseries gives corrosion tendencies (subject to

the restrictions immediately above) but

provides no information on corrosion rates.


Fig. 3.18 A saturated calomel reference electrode

1) The saturated calomel electrode(SCE) :

most popular ref. electrode for laboratory use.

Hg, Hg2Cl2/Cl-(aq. saturated KCl),

Half cell rx.: Hg2Cl2 + 2e- = 2Hg + 2Cl-

Relative Electrode Potentials- SCE


Fig. 3.19 A silver/silver chloride reference electrode

2) The siver-siver chloride electrode :

preferred for use at high temperature.

Ag, AgCl(s)/Cl-(aq. saturated KCl),

Relative Electrode Potentials- Ag/AgCl


Reference Electrodes for the Laboratory and the Field


Fig. 3.21 The copper/copper sulfate reference electrode for use

in soils

Relative Electrode Potentials- Cu/CuSo4

3) Cu-saturated CuSO4 electrode : commonly

used in field.

Cu/Cu+2(aq. saturated CuSO4) :

Half cell rx.: Cu+2 + 2e- = Cu and ECu+2/Cu =

0.340 - 0.0295 log (Cu+2)


Fig. 3.22 Measurement of the electrode potential of a buried pipe using a

copper/copper sulfate reference [15]

Reference Electrodes for the Laboratory and the Field


Fig. 3.24 A simple cell for measuring electrode potentials in

the laboratory

Measurement of Electrode Potentials

Documents

Charged Interfaces - Corrosion