Chemistry: electrical and wave energy – Apparatus – Electrolytic
Patent
1997-01-29
1999-09-07
Tung, T.
Chemistry: electrical and wave energy
Apparatus
Electrolytic
204282, 204291, 204294, C23F 1300
Patent
active
059482184
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a long-line electrode, preferably a long line anode, for an impressed current corrosion protection system, for example for the corrosion protection of buried pipelines or tanks or other substrates.
2. Introduction to the Invention
It is well known to provide impressed current corrosion protection systems in which an electrically conductive substrate is protected from corrosion by establishing a potential difference between the substrate and a spaced-apart electrode. The substrate and the electrode are connected to each other through a power supply of constant sign (DC or rectified AC) and the circuit is completed when electrolyte is present in the space between the substrate and the electrode. In most such impressed current systems, the substrate is the cathode (i.e. receives electrons). However, with substrates which can be passivated, e.g. Ni, Fe, Cr and Ti and their alloys, it is sometimes also possible to use impressed current systems in which the substrate is the anode. In both cathodic and anodic systems, the substrate is often provided with a protective insulating coating; in this case the impressed current flows only through accidentally exposed portions of the substrate. If the system is to have an adequate life, the electrode must not itself be corroded at a rate which necessitates its replacement; this is in contrast to the "sacrificial anodes" which are used in galvanic protection systems. The electrode must also have a surface which is not rendered ineffective by the current passing through it or by the electrochemical reactions taking place at its surface, such as the evolution of chlorine gas.
The electrode and the power supply must be such that the current density at all points on the substrate is high enough to prevent corrosion but not so high as to cause problems such as damage to the substrate (e.g. embrittlement) or disbonding of a protective coating on it. The power consumption of the system depends inter alia on the distance between the various parts of the substrate and electrode. In view of these factors, the theoretically best type of electrode is one which can be positioned so that it is relatively close to all points on the substrate. To this end it may have a shape corresponding generally to the shape of the substrate. Such an electrode is referred to herein as a "distributed electrode".
European Patent Publication No. 0067679 describes a distributed electrode, usually a distributed anode comprising a metal, e.g. copper, conductive core and a conductive polymeric jacket. European Patent Publication No. 0067679 describes a distributed electrode whose electrically active outer surface is provided by an element which is composed of a conductive polymer which is at least 500 .mu.m, preferably at least 1000 .mu.m, thick. The term "conductive polymer" is used herein to denote a composition which comprises a polymer component, and dispersed in a polymer component, a particulate conductive filler which has good resistance to corrosion, especially carbon black or graphite. In particular the electrode comprises a low resistance core electrically surrounded by a conductive polymer composition, wherein the anode is an electrode spaced apart from the substrate, the electrode being in the form of an elongate flexible strip which can be bent through an angle of 90.degree. over a 10 cm radius, the electrode comprising resistivity at 23.degree. C. of less than 5.times.10.sup.-4 ohm.cm and a resistance at 23.degree. C. of less than 0.03 ohm/meter; and of at least 10%, according to ASTM D1708, of the electrode, and is in electrical contact with the core, and which is at least 500 .mu.m thick.
The entire disclosure of European Patent Publication No. 0067679 is incorporated herein by reference.
Where a conductive polymer based anode as described in European Patent Publication No. 0067679 is used alone for cathodic protection, after many years, in extreme environments, some of the carbon of the conductive polymer jacket
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Cordia Johannes Maria
Kheder Joseph
Burkard Herbert G.
Gerstner Marguerite E.
N.V. Raychem S.A.
Richardson Timothy H.P.
Tung T.
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