1. Field of the Invention
The present invention relates to an apparatus, system, and method for extending the lifetime of a sacrificial anode that is part of a cathodic protection based anti-corrosion system.
2. Discussion of the Background Art
The annual cost of metallic corrosion in the United States economy is approximately $300 billion, according to a report released by Battelle and the Specialty Steel Industry of North America entitled “Economic Effects of Metallic Corrosion in the United States,” dated 1995, the entire contents of which is hereby incorporated by reference. The report estimates that about one-third of the cost of corrosion ($100 billion) is avoidable and could be saved by broader application of corrosion-resistant materials and application of best anti-corrosive practice from design through maintenance. The estimates result from a partial update by Battelle scientists of the findings of a study conducted by Battelle and the National Institute of Standards and Technology titled “Economic Effects of Metallic Corrosion in the United States,” the entire contents of which are hereby incorporated by reference. The original work in 1978 included an estimate that, in 1975, metallic corrosion cost the U.S. $82 billion (4.9 percent of the Gross National Product), and approximately $33 billion was avoidable because best practices were not used at the time.
A variety of methods for controlling corrosion have evolved over the past several centuries, with particular emphasis on methods to extend the life of metallic structures in corrosive environments. These methods typically include (a) cathodic protection based systems, which use anodes made from a metal that is less noble that the metal substrate that is being protected, so that it is the anode that sacrificially corrodes rather than the substrate to which it is connected, and (b) protective coatings, which are used principally to upgrade the corrosion resistance of ferrous metals, such as steel, and some nonferrous metals, such as aluminum, and to avoid the necessity for using more costly alloys. Thus, they both improve performance and reduce costs. However, such protective coatings typically have several pitfalls, including poor applicability to non-metallic structures that suffer from corrosion or fouling.
Protective coatings fall into two main categories. The largest of these categories is the topical coating such as a paint that acts as a physical barrier against the environment. The second category consists of sacrificial coatings, such as zinc or cadmium that are designed to preferentially corrode in order to save the base metal from attack.
Cathodic protection and coatings are both engineering disciplines with a primary purpose of mitigating and preventing corrosion. Each process is different: cathodic protection prevents corrosion by introducing an electrical current from external sources (such as a sacrificial anode) to counteract the normal electrical chemical corrosion reactions whereas coatings form a barrier to prevent the flow of corrosion current or electrons between the naturally occurring anodes and cathodes or within galvanic couples. Each of these processes provided limited success. Coatings by far represent the most wide-spread method of general corrosion prevention (see Leon et al U.S. Pat. No. 3,562,124 and Hayashi et al U.S. Pat. No. 4,219,358). Cathodic protection, however, has been used to protect hundreds of thousands of miles of pipe and acres of steel surfaces subject to buried or immersion conditions.
Cathodic protection is used to reduce the corrosion of the metal surface by providing it with enough cathodic current to make its anodic dissolution rate become negligible (for examples, see Pryor, U.S. Pat. No. 3,574,801; Wasson U.S. Pat. No. 3,864,234; Maes U.S. Pat. No. 4,381,981; Wilson et al U.S. Pat. No. 4,836,768; Webster U.S. Pat. No. 4,863,578; and Stewart et al U.S. Pat. No. 4,957,612). Cathodic protection operates by extinguishing the potential difference between the local anodic and cathodic surfaces through the application of sufficient current to polarize the cathodes to the potential of the anodes. In other words, the effect of applying cathodic currents is to reduce the area that continues to act as an anode, rather than reduce the rate of corrosion of such remaining anodes. Complete protection is achieved when all of the anodes have been extinguished. From an electrochemical standpoint, this indicates that sufficient electrons have been supplied to the metal to be protected, so that any tendency for the metal to ionize or go into solution has been neutralized.
Recent work in the study of corrosion has found that electrochemical corrosion processes appear to be associated with random fluctuations in the electrical properties of electrochemical systems, such as cell current and electrode potential. These random fluctuations are known in the art as “noise.” About 20 years ago, scientists found that all conductive materials begin corroding as soon as they are produced due to electrochemical activity caused by impurities in the material. It was later found that this activity could be monitored using electronic instruments detecting the current generated, now commonly referred to as “corrosion noise.” Essentially, the greater the magnitude of this current, the “noisier” the material and the faster the rate of corrosion. For example, steel is “noisier” than bronze and corrodes at a faster rate. Researchers have begun to apply noise analysis techniques to study the processes of corrosion in electrochemical systems.
FIG. 1 is a representation of electrochemical noise present in untreated metal 101. The randomly fluctuating voltage is measured and displayed as waveform 102 (shown as a sawtooth waveform, but an actual waveform would have broader band components and would be stochastic in nature).
FIG. 2 is a graph of corrosion potential versus time with various filters. The horizontal axis 401 measures time in days while the vertical axis 402 represents potential relative to the semiconductor element measured in milli-volts. During an experiment directed to determining optimum filter characteristics for various corrosion environments, measurements were taken for seven systems at three points in time. The measured potential for each of seven filter configurations were recorded for those three samples and are indicated by various symbols listed in the legend. The graph displays the various results for each of the seven filters at the sampling points indicated from 410 through 430.
One solution to corrosion problems has been proposed in the semiconductor coatings and related systems of Dowling's U.S. Pat. No. 6,325,915, U.S. Pat. No. 6,402,933, U.S. Pat. No. 6,562,201 and the Electronic Control Unit (ECU) of U.S. Pat. No. 6,811,681, the entire contents of each being hereby incorporated by reference. The semiconductive coating, ECU and system of the Dowling patents can be used with a variety of conductive substrates to provide an array of interesting properties.