Metal treatment – Process of modifying or maintaining internal physical... – Processes of coating utilizing a reactive composition which...
Reexamination Certificate
2000-05-18
2002-08-06
Sheehan, John (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Processes of coating utilizing a reactive composition which...
C148S265000, C148S273000, C148S279000, C427S305000, C427S328000, C427S405000
Reexamination Certificate
active
06428630
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of protective coatings, specifically, to the method of depositing a multilayer coating having improved properties, which can be easily applied and repaired.
BACKGROUND OF THE INVENTION
Modern waste incinerators burn waste to avoid the need to store waste in landfills, and to produce energy. The walls of incinerator combustion chambers (combustion zones or incinerator areas) are subject to extremely harsh conditions, including exposure to extreme heat, noxious gases, and corrosive chemical compounds which are the by-products of combustion.
Incinerators burn, for example, garbage, biomass and biological waste. Combustion of these materials produces CO
2
and H
2
O vapor, plus acid gases, HCl, NO
x
(where NO
x
is any of a series of oxides or nitrogen) and SO
x
(where SO
x
is SO
2
or SO
3
). Combustion of plastics, such as polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), polyvinylidene chloride (PV
2
C), etc., can produces free halides and hydrogen halide gases. All of these are highly corrosive.
In addition to the combustion of incinerator waste, impure fuels are used as the combustion means in incinerators of all types. Such fuels include, for example, high sulfur coal, lignite, plant matter and asphalt and tar products such as orimulsion. These fuels also produce acidic gases such as NO
x
and SO
x
, which can cause corrosion and oxidation of the incinerator surface in the combustion zone.
Molten ash is produced by complex low melting point salts. This molten ash rapidly attacks steel.
All of these products of waste combustion create an extremely harsh environment, causing extensive corrosion and oxidation of exposed incinerator surfaces. It has been found that in some cases an uncoated carbon steel surface lasts only approximately 12 months in this environment.
Accordingly, there exists a significant need to protect surfaces, exposed to such a corrosive and oxidizing environment, such as those found in an incinerator.
The need to protect the exposed surfaces of incinerators from corrosion and oxidation in modern waste management plants must be balanced with the goal of extracting energy from the combustion process. The steam generated from the utilization of the heat of combustion can be used for home heating, industrial use, or to produce power via turbine driven generators. Thus, modern waste management plants are referred to as “waste-to-energy” plants.
Corrosion can be controlled by decreasing the heat acting on the incinerator walls by using, for example, refractory coatings, or by cooling the incinerator walls with water. However, this loss of heat results in a proportional loss of energy. If cooling is used as the means of corrosion prevention, the water-to-energy transformation is detrimentally compromised.
The combustion zone of an incinerator is typically formed from steel. Some incinerator combustion zones are formed from steel tubes. The life of an incinerator is measured in wall thickness, expressed in millimeters (mm) or mils (thousandths of an inch), lost per year as the steel is corroded and oxidized. Absent any protective coating on the steel surface, corrosion and oxidation occur rapidly.
Various solutions have been proposed to address the corrosion and oxidation problems faced by incinerators operators, each having its drawbacks:
(a) Nickel Based Superalloy Tubes.
One proposed solution to the problem of corrosion and oxidation in incinerators is to fabricate an entire incinerator from specially formulated nickel based superalloys. The proposed nickel based superalloys usually contain significant percentages of Cr and Mo, and are extremely expensive.
In addition to the prohibitive cost of using such superalloys, this solution does not address the need to provide corrosion and oxidation protection to preexisting facilities, short of rebuilding the entire facility.
While nickel based superalloys display effective corrosion resistance, they may still need to be replaced, in time. Moreover, if there is wall failure in a nickel based superalloy structure, there exists no simple, effective and cost effective method of repairing the wall.
(b) Composite or Coextruded Tubes.
Composite or coextruded tubes are produced by coextrusion of two component tubes metallurgically bonded together during the coextrusion process.
Composite tubes are high in cost. In addition, where the two components of a composite tube differ in their coefficient of thermal expansion, cracking can occur due to temperature changes.
Repair of composite tubes is difficult, and may require total replacement of the tubes.
(c) Weld Overlay of Steel Tubes with Nickel Alloy.
One proposed solution to the problems of corrosion and oxidation of incinerator walls is welding nickel alloy overlays over damaged portions of steel or steel tubes. This process is expensive, and requires welding equipment.
Weld overlays cannot conform to irregularly shaped surfaces, such as where two adjacent incinerator tubes are joined. Weld overlays leave gaps where tube surfaces can still be attacked.
In addition, the process of welding is highly stressful on the underlying structure. The welding process can thus further damage or crack previously damaged or weakened tubes.
(d) Thermal Spraying Techniques.
Thermal spray coating processes are often employed in order to protect surfaces which are exposed to extremely hostile conditions, such as environments exposed to high temperatures or noxious substances. Thermal spray coating processes are used in a variety of industries. A benefit of thermal spray coating is that an inexpensive base metal, such as carbon steel, can be treated on its surface, creating new surface properties that are far superior to that of the base metal. In this way, thermal spray coatings save costs associated with fabrication of entire structures from prohibitively expensive superalloys.
Currently, thermal spray coating processes exist which must be performed under tightly controlled conditions, such as at a manufacturing plant or laboratory, and must invariably be applied under ideal conditions. Accordingly, these known thermal spray coating processes are often applied to surfaces in controlled booths or cabins using robotic techniques. In some cases inert shrouding is used or even vacuum chambers. These coating processes are performed on discrete, pre-assembled component parts. The use of controlled conditions reduces the number of variables that effect the quality of the coating. Because these spray coating processes must be performed under ideal conditions, they are not practical where a thermal spray coating must be applied to an existing surface in situ.
Thermal spray coating processes fall under the following general categories, listed from lowest cost, least dense, and lowest perceived quality coating, to highest cost, most dense and highest quality coating:
(1) Combustion powder/wire (“Flame spraying”). Combustion flame spraying employs compressed air or oxygen, mixed with one of a variety of fuels (e.g., acetylene, propylene, propane, hydrogen), to both melt and propel the molten metal particles. Generally, the process yields low density coatings. Combustion flame spraying uses either powder, wire or rod as the feedstock material and has found widespread usage around the world for its relative simplicity and cost effectiveness. Combustion flame spraying provides for a thermal coating having a density range of approximately 85-90%.
(2) Arc wire. Arc wire spraying involves two current-carrying electrically conductive wires fed into a common arc point at which melting occurs. A high-velocity air jet blowing from behind the moving wires strips away the molten metal which continuously forms as the wires are melted by the electric arc. Arc wire spraying provides for a thermal coating having a density range of approximately 80-95%.
(3) Plasma spray. A plasma gun operates on direct current, which sustains a stable non-transferred electric arc between a cathode and an annular anode. A plasma gas is introduced at the back of t
Mor GianPaolo
Mosser Mark F.
Drinker Biddle & Reath LLP
Oltmans Andrew L.
Sermatech International Inc.
Sheehan John
LandOfFree
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