Electrolysis: processes – compositions used therein – and methods – Electrolytic analysis or testing – For corrosion
Reexamination Certificate
1999-12-09
2001-08-28
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic analysis or testing
For corrosion
Reexamination Certificate
active
06280603
ABSTRACT:
TECHNICAL FIELD
The present invention relates to an electrochemical noise technique for determining corrosion rate.
BACKGROUND OF THE INVENTION
Electrochemical noise (ECN) may be defined as the spontaneous fluctuations of current and potential generated by corrosion reactions. Various methods have been used to detect corrosion reactions, including a linear polarization resistance method in which a direct current (DC) signal is applied to a corroding cell consisting of two or three electrodes and the resulting DC polarization is monitored. Provided that the applied current is small so that the potential shift is less than 20 millivolts (mV), the response is linear in most cases and the measured resistance, commonly known as the polarization resistance, may be related inversely to the rate of the uniform corrosion attack. Other techniques include the application of electrochemical impedance in which a sine wave current or potential is applied, in a similar manner to the linear polarization technique, and the sine wave potential or current resulting from the applied current or potential is monitored. Alternatively, a pseudo random noise signal can be applied to a corroding cell, with the electrochemical impedance obtained by time or frequency domain transformations.
Although the above techniques are widely employed, they: (1) possess limitations in that they only provide information on uniform corrosion conditions because they provide an average signal for the surface of the electrode being monitored; and (2) depending upon the environment, metallic material, and corrosion type, the assumption that me corrosion rate is proportional to the measured charge transfer or polarization resistance is invalid because the corrosion is of a localized nature. These problems have been addressed by monitoring localized corrosion via the utilization of electrochemical potential noise analysis. Alternatively, by coupling current analysis with electrochemical potential noise analysis further information can be obtained. For example, two similar electrodes can be coupled together via a zero resistance ammeter with the output of the zero resistance ammeter passed to the input of the electrochemical noise analysis system. In this way, the fluctuation of the coupling current may be analyzed in essentially a similar manner as for the electrochemical potential noise analysis described previously.
U.S. Pat. No. 5,139,627 to Eden et al. discloses a system which employs two working electrodes fabricated with the same material and exposed to the same corrosion conditions as the metallic surface to be tested. This system further employs means for measuring the coupling current between the working electrodes, means for measuring electrochemical potential noise originating from the electrodes, and means for comparing the coupling current with the electrochemical current noise to provide an output indicative of the degree to which corrosion is localized. Eden et al. utilize open circuit potential conditions, employing two working electrodes in an electrolyte environment wherein both electrodes are short circuited with a low resistance amp meter. The current between these two working electrodes is the result of corrosion occurring on them, with the measurement of the net current relating to the corrosion on both of them. Disadvantages of this system, however, range from the fact that the working electrodes need to be identical to obtain accurate readings and obtaining such identical electrodes is difficult, if not impossible, and also that it is unknown which electrode is responding to reveal the corrosion, to the fact that this system requires the use of two working electrodes which limits where this system can be employed. Furthermore, distinguishing between various types of localized corrosion is, at minimal, difficult due to the fact that both electrodes contribute to the system response.
What is needed in the art is a simplified corrosion rate detection system and method.
SUMMARY OF THE INVENTION
The present invention relates to an electrochemical noise method for determining the corrosion rate of a conductive article, comprising: placing a working electrode, reference electrode, and counter electrode in a corrosive environment of interest, wherein said working electrode has substantially the same composition as the article; measuring potential at open circuit between the working electrode and the reference electrode over a first period of time; placing the working electrode under a potentiostatic control; and measuring current between the working electrode and the counter electrode for second period of time.
The present invention further relates to a working electrode which is composed of the material of interest; a counter electrode which is inert in an environment of interest; a reference electrode which is inert in the environment of interest; and a measurement system connected to said working electrode, counter electrode, and said reference electrode, wherein said measurement system is capable of monitoring potential between said working electrode and said reference electrode and monitoring current between said counting electrode and said working electrode.
REFERENCES:
patent: 3716460 (1973-02-01), Weisstuch et al.
patent: 5139627 (1992-08-01), Eden et al.
patent: 2218521A (1989-11-01), None
patent: WO98/50786 (1998-11-01), None
Baker Hughes Incorporated
Bell Bruce F.
Cantor & Colburn LLP
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