June 15, 2008

Choice of Metallic Coating (2)

The metals above hydrogen in the electrochemical series, which have a more positive potential than hydrogen, are less pores to progressive corrosion in aqueous solutions. Those metals with the most positive potentials, such as gold and platinum metals, indeed have practically no tendency to pass into the combined state, accordingly they are designated noble metals.

The position of iron in the electrochemical series should particularly be noted; it is more negative or base than hydrogen, and considerable more negative than copper, tin, and nickel, almost identical in position to cadmium, and considerable more positive than zinc.

Where two different metals are in electrical contact with each other an also with an aqueous solution, they form a galvanic cell or battery, because of their difference of potential. The metal with the more negative potential is stimulated to corrode faster, and the metal with the more positive potential is discouraged by the difference of potential. A current flows around the circuit made up from the two metals and the solution, it passes out of the corroding metal (the anode) into the solution and thence back into the protected metal (the cathode). The anode and the cathode may be some distance apart in this action, since both the metals and the solutions are electrically conducting to the electron transfer.

This situation of two different metals in contact being exposed to a corrosive liquid, arises when a protective metallic coating is discontinuous for some reasons. It may be inherently porous, or it may be locally deficient, or it may have become cut or damaged in service, or even by former corrosion. The corrosion of the basis or undermetal thus exposed will be stimulated or protected, according to the relative potentials in the electrochemical series of the coating and the basis metal. The more positive metal will always stimulate the corrosion of the more negative metal. If it is the corrosion of the basis metal which is stimulated in this way the effect can be serous, because the area exposed will be small compared with that of the undamaged coating metal, and therefore the current generated by the difference of potential will be concentrated on a small area of reactive basis metal. On the other hand, if the corrosion of the basis metal is inhibited by the adjacent presence of a more negative coating quite large gaps in the coating may not be very detrimental.

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