Measuring and testing – Embrittlement or erosion
Reexamination Certificate
2002-03-08
2004-08-10
Raevis, Robert (Department: 2856)
Measuring and testing
Embrittlement or erosion
Reexamination Certificate
active
06772622
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The present invention relates to a coupon, generally used for monitoring conditions that could lead to corrosion or stress corrosion cracking under a disbanded coating on a metallic surface. More particularly, the present invention relates to a steel coupon, preferably in the form of a pipe segment, on the outer diameter of which various types of coating disbondment geometries anticipated on a pipeline are fabricated, and in which the environment of the coating disbondment geometries can be continuously or periodically monitored and/or measured.
2. Description of Related Art
Metallic surfaces, such as are common in the field of pipelines and related structures, such as buried tanks and distribution systems, are adversely affected by numerous corrosive electrolytic fluids that contact these surfaces. For example, in the natural gas and petroleum industries, corrosion occurs extensively on the outer surface of both buried and above-ground pipelines.
In order to reduce, or preferably entirely eliminate, this undesirable metallic surface corrosion, anti-corrosion protective coatings have been extensively used in the pipeline industry. These ubiquitous anti-corrosion protective coatings frequently take the form of a helically-applied tape-like protective outerwrapping. The tape-like protective component may be applied directly over an unprepared pipeline outer surface, or may, in fact, be overlaid onto a primer-coated, pretreated pipeline outer surface. Other forms of protective coatings also exist, including coal tar epoxy, asphalt, and fusion bonded epoxy coatings.
For example, U.S. Pat. No. 5,391,686 discloses an exemplary pipeline coating material. The patent discloses a polyurethane formulation that is described to be especially effective as a corrosion preventing coating for metallic substrates. The polyurethane is prepared by mixing two components, one being a mixture of high and low molecular weight polyol compounds and the other being a polymeric methylene polyphenyl isocyanate (MDI).
However, despite the use of such pipeline coating compositions and materials, external corrosion of pipelines are a concern, when there is a breach of the coating, called a “holiday.”
To protect against external corrosion, pipelines (and other buried structures) are typically cathodically protected in addition to being coated as described above. The cathodic protection (hereafter alternatively referred to as “CP”) system is designed to protect the pipe where coating defects occur. Cathodic protection, as it is used here, refers to the phenomenon and practice of applying a small potential to a metallic pipeline that is buried in the ground. This imparted cathodic potential of the buried pipeline will tend to limit or protect against corrosion attacking the metal surface.
Cathodic protection provides corrosion protection to any bare metal areas exposed to soil due to coating defects or “holidays” by causing direct current to flow from the soil into the structure, thereby polarizing the structure as a cathode. Protection is ensured by modifying the environment around the steel as well as reducing the dissolution rate of the steel by reducing the anodic overpotential. The required direct current output of the cathodic protection system is reduced to manageable levels by the coating, which substantially reduces the bare metal area of the structure exposed to soil.
Two cathodic protection systems are generally in use for corrosion protection of metal structures. The first, termed an impressed current cathodic protection system, consists of a rectifier, insulated wires connecting the plus terminal of the rectifier to a buried anode (for instance graphite cylinders), insulated wire connecting the negative terminal of the rectifier to the protected structure, and test stations installed at the structure. The test stations typically consist of a pipe or a valve box with one or two insulated wires attached to the structure, typically by brazing, and a terminal board for termination of the wires. The test stations are used for monitoring the corrosion protection levels by measuring potentials between the structure and a reference electrode in an electrical contact with ground above the structure. The reference electrode usually consists of a copper rod fixed in a plastic body filled with saturated copper-sulfate solution, and having a porous plug to facilitate electrical contact with the ground.
The second, termed a sacrificial (galvanic) cathodic protection system, consists of magnesium, zinc or aluminum anodes buried next to the structure and often directly connected by an insulated wire to the structure. The protective current is generated by the potential difference between the structure and the anode. The structure with sacrificial anodes also has test stations for the cathodic protection testing and evaluation of its corrosion protection effectiveness.
Details of different cathodic protection systems and of the pipeline potential measurements can be found in W. von Baeckmann et al.,
Handbook of Cathodic Corrosion Protection, Theory and Practice of Electrochemical Protection Processes
3
rd
Ed.
, Houston, Tex.: Gulf Publishing Co., 1997, the entire disclosure of which is incorporated herein by reference.
Even when cathodic protection is used, corrosion and defects tend to still occur. Coating defects generally take the form of either exposed bare steel or a disbondment where the coating remains intact but a gap is formed between it and the bare steel. CP systems effectively deliver current to defects directly exposed to soil or water and therefore mitigate corrosion. However, defects covered by a disbonded coating are sometimes difficult to protect because the coating shields the CP current from reaching deep inside the crevice (i.e., disbanded region). In addition, stress corrosion cracking and localized corrosion have been observed predominantly under disbanded regions of the pipe.
To determine the level of CP required for protection (in accordance with NACE standard RP0169), voltage measurements are taken of a pipe versus a reference electrode placed on the soil surface. CP coupons are samples of steel that are electrically bonded to the pipeline and CP system and simulate bare steel coating defects exposed to the soil. Briefly disconnecting the coupon from the pipe while measuring the potential of the coupon with respect to the soil, eliminates the ohmic resistance induced potential drop errors associated with coupon potential measurement. The limitation to this design is that only one geometry is represented by the simulated defect, while pipelines have a broad spectrum of defects. The most poorly represented pipeline coating defects are those under disbonded coatings.
Various CP systems are known in the art, as are coupons for assessing and measuring the CP systems. For example, CP system coupons are disclosed in U.S. Pat. Nos. 5,814,982, 6,060,877, and 6,107,811, the entire disclosures of which are incorporated herein by reference.
U.S. Pat. No. 5,814,982 discloses a test station for measuring the effectiveness of cathodic protection. The test station includes a cylindrical plastic reference tube extending downward through the soil to near the protected structure, such as a pipe. Two plastic coupon tubes extend within the chamber of the reference tube and are attached to opposite sides of interior sidewalls of the chamber. First and second circular cylindrical rod-shaped coupons are attached to, and sealed against, the bottom ends of the coupon tubes. A first coupon is electrically connected to the pipe, and the second coupon is used to measure a free-corrosion (native) potential.
U.S. Pat. No. 6,060,877 discloses a flat cathodic protection test probe. The probe comprises a non-metallic probe body with a mounted metal coupon simulating a large coating holiday on a pipeline, an insulated wire attached to the coupon for an electrical connection to the pipeline, a non-metallic tube filled with a conductive backfill, and a narrow and long porou
Dunn Darrell S.
Moghissi Oliver C.
Sridhar Narasi
Raevis Robert
Southwest Research Institute
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