A proposed extension of the impressed-current method of cathodic protection of metal against corrosion in wet environments would reduce the current and power needed. The proposal involves (1) the application of a special multilayer coat to the metal part to be protected and (2) the use of a small dc power supply that operates at a precisely chosen low output potential. The effectiveness of protection should justify the costs of power sources and coatings in many situations: metal structures that could be protected by the extended method include ships, automobiles, bridges, metal buildings, radio towers, pipelines, and storage tanks.
The proposed multilayer protective coat would include an electrically insulating layer in contact with the metal surface to be protected. The next layer would be electrically conductive; it could be made of conductive plastic or carbon-filled paint, for example. If the conductive layer were not sufficiently durable to withstand prolonged exposure to the anticipated environment, a third protective layer could be added. The conductive layer would be connected to the positive terminal of the dc power supply, while the metal to be protected would be connected to the negative terminal (see figure).
If there were no cracks or pinholes in the multilayer coat, then when the power supply was first turned on, current would flow initially by virtue of the capacitance between the metal substrate and the conductive layer. Once the capacitor was charged to the output potential of the power supply, the current would fall to a negligible value, and only a static electric field would be maintained. As cracks or pinholes formed during deterioration of the outer layers, water and oxygen would come in contact with the metal substrate. By keeping the metal substrate negative (cathodic) relative to the conducting layer, a protective shield of hydrogen ions would be made to form on the exposed parts of the metal substrate, thereby inhibiting electrochemical reactions in the pinholes and cracks.
To protect effectively against corrosion, in the presence of water, the output potential of the power supply must be greater than the corrosion-induced potential of the metal substrate. For example, the potential for iron to corrode to common rust is 0.44 V. At the same time, the supply voltage should be less than the minimum voltage to electrolyze water (1.229 V), so that the impressed current could be kept much smaller than it would be if electrolysis were occurring. On the basis of previous experience with sacrificial anodes of zinc (which has an oxidation potential of 0.76 V), it appears that a suitable potential to protect steel without consuming excessive current in electrolysis would lie between 0.7 and 1.1 V.
This work was done by Leonard M. Weinstein of Langley Research Center. LAR-15069