Charged Interfaces

Interfaces form at the physical boundary between two phases :

Introduction

a solid and a liquid (S/L) a liquid and its vapor (L/V), a solid and a vapor (S/V). two different solids (S1/S2) two immiscible liquids (L1/L2).

Electrolytes

The Interior of an Electrolyte

An electrolyte is a solution which contains dissolved ions capable of conducting a current.

Interfaces

The Solution/Air Interface

Water molecules at the water/air interface and the origin of surface tension

Reduction in the surface tension of a solution by a dissolved surface-active agent

Orientation of CH3 (CH2)10COOH molecules at the

solution/air interface and the formation of an electrical double layer

The Metal/Solution Interface

The orientation of water molecules at a metal/solution interface.

Metal Ions in Two Different Chemical Environments

Schematic illustration of valence electrons for 1 Mg atom, 4 Mg atoms, and the Fermi sea of delocalizedelectrons for solid magnesium metal

Stability of a Mg2+ ion in two different environments

The Electrical Double LayerThe electrical double layer is an array of charged species which exist at the metal/solution interface. The metal side of the interface can be charged positively or negatively by withdrawing or providing electrons.

The Electrostatic Potential and Potential Difference

The electrostatic potential (at some point) is the work required to move a small positive unit charge from infinity to the point in question.

The potential difference (between two points) is the work required to move a small unit positive charge between the two points, as shown in Fig

Significance of the Electrical Double Layer to Corrosion

The significance of the electrical double layer (edl) to corrosion is that the edl is the origin of the potential difference across an interface and accordingly of the electrode potential.

Simple equivalent circuit model of the electrical double layer. Cdl is the double layer capacitance, RP is theresistance to charge transfer across the edl, and RS is the ohmic resistance of the solution

As shown earlier, there is no net charge in the interior of solution, so that φS = φS’

(φM − φs) = PDMS = PDM/S, where the notation M/S refers to the interface formed between metal M and solution S.

Then, Eq. (2) can be rewritten as

PDS/M + PDM/M1 + V + PDM1/ref + PDref/S =0 (3)

Thus

V = −PDS/M − PDM/M1 − PDM1/ref − PDref/S (4)

The terms PDM/M1 and PDref/M1 are small and can be neglected.

In addition, PDS/M =−PDM/S.

Thus, Eq. (4) becomes

V = PDM/S−PDref/S

Relative Electrode Potentials

The potential difference across a metal/solution interface is commonly referred to as an electrode potential.

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

In the special case

the half-cell potential is arbitrarily defined as E◦ = 0.000 V.

Experimental determination of a standard electrode potential for some metal M using a standard hydrogen reference electrode

The following limitations must be recognized:

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

(2) The relative ranking of metals in the emf series is not necessarily the same (and is usually not the same) in other media (such as seawater, groundwater, sulfuric acid, artificial perspiration).

(3) The emf series applies to pure metals only and not to metallic alloys.

(4) The relative ranking of metals in the emf series gives corrosion tendencies (subject to the restrictions immediately above) but provides no information on corrosion rates.

Reference Electrodes for the Laboratory and the Field

The copper/copper sulfate reference electrode for use in soils

Measurement of the electrode potential of a buried pipe using a copper/copper sulfate reference