Electricity: measuring and testing – Impedance – admittance or other quantities representative of... – Lumped type parameters
Patent
1992-03-10
1994-11-29
Regan, Maura K.
Electricity: measuring and testing
Impedance, admittance or other quantities representative of...
Lumped type parameters
324458, 3241581, G01R 3102
Patent
active
053693701
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for the investigation of the condition of a metal surface, which is provided with a coating, especially of a dielectric material, such as a polymer.
An increasingly greater technical significance is being given to polymer coatings of reactive metal surfaces. They serve as protection of useful metals such as iron, zinc or aluminum against corrosion, they serve as insulating cover layer with special dielectric constructive characteristics as in the case of adhesives.
The technical demands of such connecting systems upon coatings, especially polymers on metal substrates, are a high mechanical stability of the phase-boundary as well as a high corrosion-protection of the substrate against aggressive media by means of the applied coating. The failure of the bond between metal and coating is very often based upon the diffusion of components of particles of corrosive media through the coating, the accumulation of water and ions at the metal/coating interface in first monomolecular and then three-dimensional thick films, the possible formation of an electrochemical double layer, the following corrosion reactions, and finally a large scale delamination of the coating caused by the electrochemical reaction. Principally components of a corrosive medium may diffuse through the coating to the substrate/coating interface, but they may also reach the locally unprotected metal directly along local defective places, e.g. pores or local damages, and from there spread out along the phase-boundary. Such undermining and release, which spreads out from local defects and blemishes, is of particular significance. There is no doubt that in this process electrochemical reactions of the phase-boundary between metal and coating are of great significance and that e.g. a cathodic polarization of the phase-boundary accelerates the delamination velocity significantly. It is also known that the rate of the electrochemical reactions is strongly determined by the diffusion of ions through the layer, such as a polymer film. Finally it is known that reactions in inorganic conversion layers in certain cases are the cause of the loss of adhesion of the coating.
The technical significance of the delamination of the coatings, especially polymer films, is reflected in countless industrial corrosion tests, in which very often the delamination starting from a mechanical injury after corrosion at the free corrosion potential (salt spray test, etc.) or during a cathodic polarization (potentiostatic or galvanostatic holding test) is measured. These tests are regarded as one basis for the development of new coating methods or coating materials. It is at any rate extremely difficult to determine the delamination rate directly and in the shortest possible time.
As possible techniques up to now optical measurements are used in order to determine the delaminated distance (counting bubbles, formation of rust) or the delaminated lacquer is removed mechanically be scratching or by adhesive strips. It is disadvantageous that the valuation in both of these known methods is not free from uncertainty; the methods do not permit a continuous measurement of the delmaination rate; the mechanical testing of the delamination is not nondestructive and finally both methods are only useable when a noticeable destruction of the metal/coating phase-boundary has already occurred. Short-term tests with high resolution are hence not possible.
Electrochemical methods for investigating the delamination rate (e.g. impedance spectroscopy) are also known; however, they have up to now not shown unambiguous results.
The formation of delaminated zones under an intact polymer layer, i.e. far distant from an injury, has up to now only been possible to investigate with the help of an ultrasonic microscope. This method is nevertheless very expensive and is not well suitable in cases, in which the substrate is coated with multiple layers, as is frequently the case in modern lacqueres and coatings.
The present invention, accordingly, add
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Stratmann Martin
Streckel Heinz
Max-Planck-Institut fuer Eisenforschung GmbH
Regan Maura K.
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