Page 61 - MODERN ELECTROCHEMISTRY

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field underlying all those phenomena in chemistry, biology, and engineering that
involve real (and therefore moist or wet) surfaces.

1.4. PROPERTIES OF MATERIALS AND SURFACES
It is now well recognized that the properties of most materials are controlled by
the properties of their surfaces. Furthermore, most surfaces are “wet.” The quotation
marks are used because “wet” does not here mean “placed in a solution.” It can also
mean “covered by a film of moisture.” How pervasive are—in this sense—wet
surfaces! Indeed, most outdoor and some indoor surfaces are in contact with invisible
films of moisture, and the presence of these films is a required element in the corrosion
and decay of metals. Thoroughly dry surfaces, removed from contact with air, do not
corrode.
Wet surfaces have interfacial regions. One is in the solid, the other is in the
solution. Across these regions are super-intense electric fields that accelerate or
decelerate the passage of electrons from solid to solution and vice versa.
Corrosion—the gradual decay of materials—occurs in many ways, all involving
electrochemical surface reactions. The essence of it is the electrochemical dissolution
of atoms in the surface into the ion-containing film that is in contact with the corroding
metal. However, such dissolution has to be accompanied by a counter-reaction and
this is often the electrochemical decomposition of water to form hydrogen on the metal
surface. If that occurs, the H in the form of minute protons, may enter the metal,
diffuse about, and cause a weakening of metal–metal bonds and hence stress-corrosion
cracking.
Local mechanical stress also plays an important part in determining the behavior
of materials: combined with electrochemical corrosion, it may lead to bridges collaps-
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ing and ships splitting in half.
Is friction electrochemical also? At least on moist surfaces, the distance between
surface promontories—the protrusions of the metal–metal contacts—is controlled by
the repulsion of like charges from ions adsorbed from electrolyte-containing moisture
films onto surfaces. Indeed, if a pendulum swings on a fulcrum containing a metal-
metal contact, its rate of decay (which is increased by the friction of the contact)
maximizes when the interfacial excess electrical charge is a minimum; the friction
therefore is a maximum (because the metal contacts, unrepelled by charges, are in
closer contact).

1.4.1. Interfaces in Contact with Solutions Are Always Charged
Even when a solid–solution interface is at equilibrium (i.e., nothing net happen-
ing), electron transfer occurs at the same speed in each direction, for there are excess

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This can happen when the front and back of a ship are momentarily suspended on the peaks of waves in
rough weather.