Stock material or miscellaneous articles – Hollow or container type article – Glass – ceramic – or sintered – fused – fired – or calcined metal...
Reexamination Certificate
2002-08-07
2004-08-10
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Hollow or container type article
Glass, ceramic, or sintered, fused, fired, or calcined metal...
C428S132000, C428S136000, C138S141000, C138S146000, C138SDIG006
Reexamination Certificate
active
06773774
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to the problem of corrosion relating to buried and submerged metallic structures. In even greater particularity the present invention relates to an improved anti-corrosive protective material for use with cathodic protection systems. More particularly, the present invention relates to a protective material having microperforations that is used to encase buried conduits or other metallic structures to enhance cathodic protection and control microbiologically influenced corrosion.
Buried and submerged conduits, as well as other buried and submerged metallic structures such as tanks are ubiquitously used for storing and carrying various materials, such as water, natural gas, oil, and sewage. A major problem with buried and submerged metallic structures, or concrete with metal reinforcements, is corrosion. The severity and rate of corrosion is dependent on the type of material comprising the structure and the environment in which the structure is buried. Buried conduits in particular are an important part of the infrastructure in the United States and the world. Significant costs are involved in design, development, manufacture, and installation of water, gas, and sewage systems to ensure the longevity of buried conduits and other metallic structures. Failure of these systems from conduit corrosion represents appreciable costs. Corrosion of buried conduits and metallic structures is a major economic, environmental, and safety problem.
The control of corrosion of metals has been a quest of producers and consumers for the entire history of ferrous materials. Corrosion is the deterioration of a material, in most cases a metal, because of a reaction to its environment. Corrosion mainly occurs through an electrochemical process. For corrosion to occur, the following basic elements are required: an anode (the corroding metal); a cathode (the non-corroding metal); an electrically-conductive electrolyte in which the anode and cathode are immersed, such as ionized water or soil; and a current path connecting the anode and the cathode. Additionally, there must be an electric potential difference between the anode and cathode. When these elements are present, a complete electrical circuit referred to as a corrosion cell is formed. Through such a corrosion cell, an electric current flows from the anode to the cathode, carrying metallic ions with it and causing the anode to corrode. Corrosion protection systems seek to control corrosion by disrupting the operation of a corrosion cell.
Traditionally, buried and submerged metallic or metal reinforced structures are protected from corrosive environments by means of environmental barrier systems and/or cathodic protection systems. The materials used in barrier systems have high electrical resistance, and disrupt the corrosion cell by electrically isolating the metallic structure from the surrounding electrolyte environment. The types of barrier systems include 1) coatings that are tightly bound to the outer surface of the metallic structures, 2) wraps that are tightly fixed to the outer surface of the metallic structures using adhesives, and 3) loose polyethylene sleeves or wraps that are not bonded to the outer surface. Cathodic protection systems reduce corrosion by disrupting the difference in the electrical potential between the anode and the cathode. The principles of cathodic protection are based on minimizing the electrical potential between the anode and the cathode, as well as producing current flow in the proper direction to protect the metallic structure. This can be achieved by applying a current to the structure to be protected, such as a pipeline, from some outside source. When enough current is applied, the structure becomes a cathode in the corrosion cell, and thus a non-corroding metal. The types of cathodic protection systems are 1) impressed current systems whereby a current is supplied from an external power source or 2) sacrificial anode systems that rely on the corrosive potential of different metals, and include an anode that corrodes sacrificially to protect the metallic structure.
Bonded coatings on the outer surface of metallic structures are utilized extensively to provide barrier-type protection against corrosion. Despite their widespread use, there are significant disadvantages in the use of bonded coatings. One disadvantage is the tremendous expense associated with manufacturing of metallic structures having bonded coating. Placing a bonded coating on metallic structures requires several difficult and costly manufacturing steps, including the surface preparation of the metallic structures required prior to applying the coating, and the application process itself. These steps require the operation and maintenance of costly equipment used in metallic surface preparation and coating application. Further increasing the manufacturing costs of coated metallic structures are the costs of the coating materials, which can be significant. In addition to the initial expenses associated with manufacturing bonded coating structures, additional expenses are incurred by substantial and costly handling techniques for the finished products that are required to minimize damage to the coatings during the transportation and installation of coated metallic structures. Similar limitations are associated with wraps that are tightly fixed to the outer surface of the metallic structures using adhesives. Similar to bonded coatings, such wraps require significant costs associated with metallic surface preparation prior to application of the wraps, costs associated with the application process, and material costs associated with the use of adhesives.
Corrosion-control systems are often relied upon to protect very large surface areas of underground or submerged metallic structures, such as the case with cross-country pipelines. Another significant limitation of bonded coatings, in addition to the high costs associated with manufacturing and use, is that such coatings cannot be applied to large metallic surfaces with 100% coverage. Therefore, unprotected areas of pipe surfaces can occur, leading to disbanded areas of coating. Practice has demonstrated that even well-coated pipeline will have some coating defects, referred to as holidays, that expose bare metal to the corrosive effects of an electrolytic environment. Even if it were possible for a coated pipeline to be free of holidays, bare metal may be exposed by other physical means causing disbonding of the coating, such as pipe movements with temperature variations, soil stresses, and damage from outside sources during handling and burial. In contrast to a bonded coating system, loose polyethylene wrap, referred to as loose wraps, provide an inexpensive passive environmental barrier for corrosion control. Loose polyethylene wrap does not require metallic surface preparation, or the application and consumption of adhesive and coating materials.
To compensate for the coating defects associated with bonded coatings, it is common practice to use cathodic protection systems in conjunction with the use of bonded coating. The use of a barrier system such as bonded coatings with cathodic protection systems has the advantage of greatly reducing the operating costs of the cathodic protection system, because such a cathodic protection system need only protect the areas of bare metal exposed to the electrolytic environment at the holidays or other defects, rather than the whole surface of an uncoated metallic structure. Thus, the ongoing electrical energy consumption required to maintain cathodic protection for a coated structure can be significantly less than required to protect a bare, or uncoated structure. Cathodic protection systems can cause coatings and adhesives used with wraps to disbond from the metallic structures, causing the rate of electrical consumption utilized with a cathodic protection system to increase over time. It can be appreciated that the conventional use of bonded coatings and wraps in conjunction with cathodic
Ash George L.
Crook John A.
Fulton Enterprises
Smith Gambrell & Russell
Tarazano D. Lawrence
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