Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For corrosion
Reexamination Certificate
2000-05-18
2001-09-25
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For corrosion
C205S777000, C204S280000, C204S404000, C204S412000
Reexamination Certificate
active
06294074
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to corrosion monitoring systems, and more particularly to, an improved electrode design for corrosion monitoring utilizing electrochemical noise measurements.
DESCRIPTION OF THE PRIOR ART
Metal corrosion is a major and costly problem for many industries. Two basic kinds of corrosion can be identified. General corrosion occurs uniformly over the entire surface of a metal structure and the rate of this form of corrosion can be easily monitored and predicted. Localized corrosion, however, is a more serious form of corrosion where rapid and sustained localized metal pitting occurs. This localized pitting corrosion can lead to the premature and catastrophic failure of metal pipes and storage vessels. The localized pitting corrosion can shorten the material life by orders of magnitude as compared to general corrosion or uniform corrosion. Early detection of localized pitting corrosion would result in cost savings because metal parts could be treated, repaired or replaced only when necessary thus avoiding unscheduled failures.
For years, engineers have been trying to develop effective monitoring methods to detect localized pitting corrosion. Among the innovative methods that were evaluated, electrochemical noise analysis (ENA) is recognized as one of the potential monitoring techniques. For example, U.S. Pat. No. 4,575,678 issued Mar. 11, 1986 discloses corrosion monitoring apparatus and corrosion monitoring method utilizing electrochemical noise analysis.
Electrochemical noise analysis is a non-destructive, in-situ monitoring method of the natural corrosion process that measures the electrochemical corrosion current and potential fluctuations. However, due to the chaotic nature of the corrosion process, signal processing of the recorded current and potential noise becomes very critical in determining the meaning of the recorded data.
Researchers have been interpreting the electrochemical noise analysis data by using different signal processing algorithms to quantitatively or qualitatively characterize the corrosion process. In an effort to specify the corrosion mechanisms and distinguish between uniform and localized pitting corrosion, they have monitored the signals for potential and current noise levels, noise resistance and pitting index (i.e., standard deviation of current noise divided by average current noise). Attempts have also been made to increase the sensitivity of the electrodes by applying an anodic bias voltage to the electrodes to accentuate localized corrosion as in U.S. Pat. No. 6,015,484. However, it was found that these results alone do not effectively identify the different corrosion mechanisms.
U.S. Pat. No. 5,888,374 issued to Daniel H. Pope, YuPo J. Lin, Edward J. St. Martin, and James R. Frank, on Mar. 30, 1999 and assigned to the present assignee disclosed an improved method and apparatus for monitoring localized pitting corrosion in metal pipes or storage vessels. The electrochemical probes include a pair of working electrodes formed of the same material as the monitored metal pipes or storage vessels and a reference electrode formed of a corrosion resistant material. Electrochemical probes are used for sensing electrochemical noise voltage and current values. The root-mean-square of the electrochemical voltage values are calculated and stored as the sensed electrochemical noise voltage level. The stored electrochemical noise voltage level values are processed by transforming the stored electrochemical noise voltage level values into power spectral density data utilizing a fast Fourier transform. A slope of the power spectral density data relative to frequency is calculated. A linear slope of a low-frequency portion of the power spectral density data is calculated by using a least-square method.
A principal object of the present invention is to provide an improved electrode design for corrosion monitoring utilizing electrochemical noise measurements.
It is another object of the present invention to provide such an improved electrode design for monitoring corrosion in metal pipes or storage vessels that utilizes electrochemical noise analysis.
It is another object of the present invention to provide such an improved electrode design for monitoring corrosion in metal pipes or storage vessels that utilizes electrochemical noise data obtained with a plurality of electrochemical probes including a generally non-corroding reference electrode and a pair of working electrodes formed of the same material as the monitored metal pipes or storage vessels.
It is another object of the present invention to provide such an improved electrode design for monitoring corrosion in metal pipes or storage vessels that utilizes electrochemical noise data obtained with a plurality of electrochemical probes including a reference electrode and a pair of working electrodes, each having a defined surface roughness.
It is another object of the present invention to provide a method to increase the probability that sustained localized pitting corrosion will occur on the more polished surface of the electrodes used for monitoring corrosion in metal pipes or storage vessels and be detected by analyzing voltage noise data obtained from electrochemical noise measurements.
It is another object of the present invention to provide a reproducible and accurate measure of the general corrosion rate on the unpolished electrode by analyzing the current noise data obtained from the electrochemical noise measurements.
It is another object of the present invention to provide such an electrode design that overcomes many of the disadvantages of prior art arrangements. For instance, the sensitivity of an unpolished electrode to develop localized pitting corrosion is low and it is hard to correlate the general corrosion rate on a pair of working electrodes with the ECN measurements.
SUMMARY OF THE INVENTION
In brief, these and other objects and advantages of the invention are provided by an electrode design for corrosion monitoring using electrochemical noise measurements. Electrochemical probes are used for sensing electrochemical noise voltage values and electrochemical noise current values. The electrochemical probes include a pair of working electrodes and a reference electrode. Each of the pair of working electrodes has a defined surface roughness. One of the pair of working electrodes has reduced roughness, whereby sensitivity to sustained localized pitting corrosion is increased in the working electrode with the reduced roughness.
In accordance with features of the invention, one of the pair of working electrodes has a defined surface roughness prepared by surface polishing the surfaces by using different grits of polishing cloth and/or different particle sizes of polishing slurries. The electrode surfaces were polished by using either a 600 grit polishing cloth or polishing slurry with particle sizes in the range of 1.0 micrometer to 0.05 micrometer. Thus, by reducing the surface roughness of one of the working electrodes, increased sensitivity to sustained localized pitting corrosion is provided. Meanwhile, the unpolished electrode with a rougher surface has higher general corrosion attack than the smooth surface. Therefore, we have found that, by electrically connecting these electrodes, the unpolished electrode will serve as the anodic site (i.e., corrosion) and the polished electrode will serve as the cathodic site for the corrosion process. Thus, the net corrosion current measured by a zero-resistance amperometer can be accurately used to measure the general corrosion rate of the unpolished electrode.
REFERENCES:
patent: 5139627 (1992-08-01), Eden et al.
patent: 5275704 (1994-01-01), Yang
patent: 5286357 (1994-02-01), Smart et al.
patent: 5425867 (1995-06-01), Dawson et al.
patent: 5888374 (1999-03-01), Pope et al.
patent: 6010889 (2000-01-01), Geary et al.
patent: 6015484 (2000-01-01), Martinchek et al.
Frank James R.
Lin YuPo J.
Pope Daniel H.
St. Martin Edward J.
Bell Bruce F.
Pennington Joan
The University of Chicago
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