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, H, may enter the metal, diffuse about, and cause a weakening ofmetal-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 collapsing and ships sphtting 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). 

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