Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment – Metal or metal alloy
Reexamination Certificate
2000-03-24
2002-02-12
Bell, Bruce F. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
Metal or metal alloy
C205S740000, C204S196010, C204S196100, C204S196190, C204S196210, C204S196250, C204S196270, C204S196360, C204S196370
Reexamination Certificate
active
06346188
ABSTRACT:
FIELD OF THE INVENTION
The present disclosure relates to a cathodic protection system. More particularly, the present disclosure relates to a method and apparatus for providing power to impressed current cathodic protection systems at relatively low voltage to inhibit chloride-induced corrosion of the reinforcing steel embedded in concrete structures.
BACKGROUND OF THE INVENTION
Chloride-induced corrosion occurs in salt water and brackish water areas, causing substantial structural damage to steel-reinforced bridge pilings, marine substructures, concrete balconies, and other steel reinforced concrete structures. In northern climates where road de-icing salts are employed, similar reinforcing steel corrosion is observed. Corrosion of the reinforcing steel, which is embedded into concrete structures to impart strength to the concrete, is a well-known problem in the art. The concrete comprising many bridge pilings, substructures, piers, wharves and the like is generally porous and permits the ingress of salt water. In tropical and subtropical environments, warm temperatures accelerate the diffusion rate of the chloride (which comprises salt) toward the steel reinforcing members. Warm temperatures can also cause a partial drying-out of the structures resulting in evaporative drying which increases the concentration of the chloride within the steel-reinforced concrete structure. Also, warm temperatures accelerate the diffusion of atmospheric oxygen into the porous concrete. When the chloride concentration reaches approximately 0.6 to 0.8 kg per cubic meter, sufficient chloride is present to initiate the corrosion process, specifically, corrosion of the iron contained within the steel reinforcing bars.
The familiar iron oxide (commonly known as rust) compounds which form as a result of the reaction of iron with atmospheric oxygen in the presence of water, occupy considerably more volume than the parent iron contained in the steel. Due to the limited amount of space available for volume increases, as little as 0.002 in. (2 mils) of corrosion on the surfaces of the reinforcing members is sufficient to induce tremendous tensile stresses on the neighboring concrete. When the exerted stress exceeds the tensile strength of the concrete, severe cracking and/or spalling of the concrete can occur. Eventually, the mechanical integrity of the structure becomes sufficiently compromised that costly repair or complete replacement is necessary.
Several techniques are currently in use to inhibit or completely halt the deleterious corrosion process described above. As the corrosion process is oxidative in nature, rendering the steel cathodic (i.e., placing the steel in a reducing state) effectively prohibits the oxidation process from occurring. This is accomplished by supplying electrons to the steel reinforcing members so that oxidation of the steel does not occur. Instead, the oxygen at the surface of the steel is electrochemically reduced. In other words, if electrons are provided to replace those that would normally be lost during the corrosion reaction (in that iron loses electrons when it oxidizes), the iron will not oxidize.
One known technique used to protect reinforcement steel is to place it in electrical contact with a sacrificial material such as zinc or aluminum. For instance, a zinc anode can be used to protect the steel from oxidizing. In such an arrangement, as the iron in the reinforcing steel attempts to rust, the zinc anode preferentially sacrifices its electrons to the iron. In this process, the zinc sacrifices itself and corrodes instead of the steel. This technique, known as galvanic protection, while relatively simple, suffers from several drawbacks. First, because the zinc is sacrificial it is eventually consumed. Where the zinc anode is disposed in a jacket of concrete as in pile applications, this corrosion of the zinc will result in zinc corrosion products building up adjacent to the zinc base metal surface. Such reaction products can increase the electrical impedance on the pile jacket, thereby affecting the amount of protecting current that can be supplied to the steel. If the resistance of the zinc pile jacket increases over time, and if sufficient current (i.e., electron flow) cannot be provided to the steel, the steel will then rust. Due to this phenomenon, pile jacket replacement or costly repair may eventually be required since zinc exhaustion eventually occurs.
A second technique has been developed which affords corrosion protection for the life of the structure to which it is applied. In this technique, a dimensionally stable titanium anode coated with a catalytic coating is used which promotes electrochemical activity. As in the method described above, the anode can be, for example, disposed in a pile jacket about a bridge pile to be protected. However, since titanium-based dimensionally stable anodes are not capable of providing a source of electrons (i.e., they do not induce a current flow to the steel reinforcing members), an external power supply must be provided to supply an impressed current between the anode and the reinforcing steel (cathode). Normally, this current is supplied by connecting the negative terminal of the power supply (i.e., electron source) to the reinforcing steel, and the positive terminal of the power supply to the catalyzed titanium anode. In the pile application, the electrical circuit is completed with the salt water which penetrates the concrete existing between the anode and the reinforcing steel to be protected. Once configured in this manner, energizing the power supply results in current flow to the reinforcing steel. In previous impressed current systems, controllable direct current (DC) power supplies that take commercially available alternating current (AC) line power and rectify it into DC power have been used. This rectification of the AC power is accomplished through use of one or more rectifiers.
Although conventional impressed current techniques present advantages over galvanic methods, these techniques also present several important drawbacks. First, AC power typically must be available at the site. In many rural or remote locations, power is not available and the cost of running power lines to the site can be considerable. Since one, or at most only a few, power supplies is usually used, extensive wiring conduits normally must be installed between the power supply and the individual pilings, substructures, or road deck areas to be protected. Depending on the particulars of the application, wiring and conduit installation can be extremely expensive. Therefore, externally powered cathodic protection systems are not cost-effective on structures where only a part of the structure requires immediate protection. In addition, power supply vandalism and damage due to lightning strikes often requires greater-than-desirable maintenance. Furthermore, in order to be economically viable, AC to DC power supplies must have sufficient voltage to power a large structure. Such voltage is needed to overcome the resistance of the wiring running between the power supply and the structure. In the event of a malfunction or drift in the control section of the power supply, an abnormally high voltage output can give rise to excessive voltage at the surface of the reinforcing steel. The resulting impressed current therefore can be greater than that needed to electrochemically reduce the available oxygen and prevent corrosion. Under these conditions, the steel can be forced to negative potentials that are sufficient to support water electrolysis with concomitant nascent hydrogen evolution. Nascent hydrogen, generated at the surface of the steel, represents a potentially serious hazard in that hydrogen embrittlement of the steel reinforcing members can occur.
To address these limitations, large zinc-air batteries have been used on bridges to protect groups of four to six pilings (commonly referred to as a bent). Although solving several of the problems associated with the aforementioned AC/DC systems, the batteries proved to be too large to be
Gabert Gregory J.
Shuster Nicholas
Bell Bruce F.
Enser Corporation
Thomas Kayden Horstemeyer & Risley LLP
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