Like galvanizing, films of ZRC afford protection to steel substrates electrically. Applied to a steel substrate the zinc film completes an electrochemical cell with the substrate in which the zinc metal of the ZRC film assumes the anode of the couple, and thereby ensures that, in water or a corrosive environment, the steel will behave entirely cathodically (Fig. 1). As such, while the zinc primer maintains contact with the steel, the steel cannot oxidize, and all corrosion is confined to the film of ZRC.
To complete the cell, in addition to the electrical connection between the zinc and the steel (the adhesion of the ZRC film to the substrate), a common electrolyte across both metals is also necessary (an aqueous solution which will conduct electricity, and will serve as a medium in which the corrosion reaction can occur). This electrolyte may be rainwater, seawater or any other electrically conductive solution.
The electrical current (I) that is naturally set up as the zinc begins to corrode and galvanically protect the steel can be measured electrically, as can the potential difference or voltage set up between the zinc and the steel substrate. The extent of corrosion is proportional to the magnitude of the corrosion current, which is, in turn, related to the voltage of the cell (V) and its resistance (R). The relationship is that known as Ohm's Law.
I = V/R
As the potential difference between the zinc and the steel is fixed (more or less), the overall resistance of the cell (R) most significantly determines the amount of corrosion.
This resistance is made up of the individual resistances of the various components of the cell. These include the metallic resistances within the zinc film and within the steel substrate, the electrolytic resistance of the electrolyte, and the resistance of any corrosion product that may build up on either the steel or the zinc.
Hot dipped galvanized films are made up of solid zinc metal, melted onto the steel. In these films the resistances are, like those of the steel itself, virtually zero. The electrical resistivity of zinc metal is 5.5 microhms/cm-1. In pure zinc/steel couples, such as these, current flows readily (so that zinc corrosion takes place rapidly). It is for this reason that galvanizing films corrode rapidly in seawater, until the insulative effects of corrosion product add resistance to the cell.
In films of ZRC the zinc is present in the form of discrete spheres of zinc dust which are packed close enough together to ensure a virtual tangential contact between the zinc particles and the steel substrate (Fig. 2). True contact in ZRC is compromised by the presence of an ultra thin sheath or monomolecular layer of polymeric binder or glue which is necessary to sustain the cohesive integrity of the film and ensure that the film sticks to the steel. The electrical corrosion current must conduct through this insulative sheath.
The presence of this binder sheath, therefore, decreases the conductivity of the ZRC film significantly compared to films of hot dipped galvanizing. Films of dry ZRC have much higher resistivity (the reciprocal of conductivity). Laboratory determined values have approached 2.5 X 106 ohm/cm. This reduced conductivity does not impede the protective properties of the film, for as long as enough current can continue to flow through the couple toward the zinc (i.e. as long as current is not discharged from the steel), the steel will remain galvanically protected. In fact, in aggressive environments such as seawater the ZRC film itself corrodes more slowly (compared to pure zinc) and will, therefore maintain protection for a longer duration. Only when the resistance of the cell becomes so high from corrosion bi-product that current ceases to flow between cathode and anode will cathodic protection be nullified.
Because of the inevitable structure of the ZRC film that is imposed by the design requirements necessary for good corrosion resistance, the films are relatively porous. Thus, under wet service conditions, water may actually enter the film, taking up the space in the film porosities. The presence of water (rather than air) within these porosities, increases the electrical continuity of the film, and the electrical resistivity drops. Depending on the electrolytic conductivity of the electrolyte which wets the film, the change in electrical resistivity may be slight or appreciable. If the film is wet with distilled water (of low electrical conductivity) the depression of resistivity will be small; where the more conductive salt water wets the film the resistivity change may be appreciable with resistivity values 6 x 105 ohm/cm and lower. As the resistance is lowered corrosion current may flow more readily, and so in sea water the corrosion of zinc (the consumption of the anode) will occur more rapidly, and ZRC will not protect steel for as long as it will in fresh water.
As the ZRC film naturally weathers and/or the zinc corrodes in service, the surface of the film and to a lesser extent the interior spaces of the film are coated and filled with zinc corrosion product. This corrosion product serves to insulate the zinc metal from the corrosive environment, and as a consequence causes the electrical conductivity of the film to fall progressively. Eventually, as the film becomes nonconductive, cathodic protection ceases entirely, and the protection offered by the primer is thought to be then of a barrier nature. Protection in the latter stages of the life of the ZRC film therefore is possibly a result of deprivation of oxygen access to the metal and/or by reduction in the electrical conductivity of any water that can actually reach the steel beneath the film (resistance inhibition). At this stage in the life of the primer, the film's resistance will be so high that ZRC becomes virtually non-conductive, although it will remain protective wherever the steel is coated.
Electrical resistivity of a substance is defined as the electrical resistance offered by the material to the flow of current multiplied by the cross-sectional area of the current flow and per unit length of current path. In actuality, the term defines the property of bulk materials such as a cube of zinc. Its application to thin films is not entirely appropriate. The resistivity of ZRC films may be approximated by measuring the resistance of the film over a non-conductive substrate such as glass, as long as the film will adhere to that substrate (Fig. 3). Conventionally, the resistivity is measured (using an ohmeter) between two sharp linear electrodes which cut through the film across a square (wherein the distance between the two electrodes is equal to the length of the electrodes). The size of the square does not affect the resistivity value, as long as the distance between the electrodes "d" is equal to their length. The data is conveniently expressed in ohm/cm or ohm/m (SI units) which can be calculated from the measured resistance, the value "d" and the film thickness of the film, "t".
What will markedly affect resistivity values of a newly applied film of ZRC is the substrate over which the film being measured is applied. To measure the true resistivity of the film the substrate must not be steel (or other conductive material). When non-conductive substrates are used, the current can only flow across the film and the ohmeter value is that of the film alone (Fig. 4a). Where the substrate is conductive, such as steel, an alternative (and less resistant route of current passage) will be across the low resistivity substrate (Fig. 4b). The ohmeter value is in consequence misleadingly low. Also, the electrodes must cut through the film, so that the entire film cross-section abuts the electrode faces (Fig. 4a). The electrodes should not merely sit on top of the film (Fig. 4c). Unless the resistance is measured through the entire thickness of the ZRC film, the resistivity values will be misleadingly high.
A final point! ZRC is formulated so that the amount of polymeric binder employed is just sufficient to hold the film together and onto the steel while still allowing sufficient conductivity to ensure the steel substrate is cathodically protected. The relationship of binder volume to zinc volume is precise and cannot be varied. In providing the wet product we can, however, only ensure that what goes into the can is so properly balanced. Zinc is a heavy pigment and will in storage tend to settle in the can. In order to re-establish the engineered ratio of pigment to binder, it is essential that the product be stirred and totally homogenized before application. ZRC applied from an incompletely stirred can will be laid down with some areas having inappropriately high zinc loading and, therefore, unnecessarily high electrical conductivity but reduced physical strength. Other areas will be vehicle rich and strong but highly resistant to the passage of electricity, and, therefore, poorly protective. Only by stirring up the can contents properly before application, and frequently during long applications, will the applicator ensure that he gets the outstanding protection against corrosion we put into every gallon of ZRC.