Protective coating system for high temperature stainless steel

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Reexamination Certificate

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C219S076100, C219S076120, C219S076160, C427S405000, C427S419200, C427S456000, C428S679000, C428S685000, C428S629000, C428S652000, C428S653000, C428S938000, C428S939000, C428S941000, C585S403000, C585S920000

Reexamination Certificate

active

06475647

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a coating system for the generation of protective surface alloys for high temperature metal alloy products and, more particularly, relates to the provision of a metal alloy coating on the internal wall surfaces of high-temperature stainless steel tubes to produce a coating that provides corrosion resistance and reduces the formation of catalytic coking in hydrocarbon processing such as in olefin production and in direct reduction of ores.
2. Description of the Related Art
Stainless steels are a group of alloys based on iron, nickel and chromium as the major constituents, with additives that can include carbon, tungsten, niobium, titanium, molybdenum, manganese, and silicon to achieve specific structures and properties. The major types are known as martensitic, ferritic, duplex and austenitic steels. Austenitic stainless steel generally is used where both high strength and high corrosion resistance is required. One group of such steels is known collectively as high temperature alloys (HTAs) and is used in industrial processes that operate at elevated temperatures generally above 650° C. and extending to the temperature limits of ferrous metallurgy at about 1150° C. The major austenitic alloys used have a composition of iron, nickel or chromium in the range of 18 to 41 wt % chromium, 18 to 48 wt % nickel, balance iron and other alloying additives. Typically, high chromium stainless steels have about 31 to 38 wt % chromium and low chromium stainless steels have about 20 to 25 wt % chromium.
The bulk composition of HTAs is engineered towards physical properties such as creep resistance and strength, and chemical properties of the surface such as corrosion resistance. Corrosion takes many forms depending on the operating environment and includes carburization, oxidation and sulfidation. Protection of the bulk alloy is often provided by the surface being enriched in chromium oxide (chromia) and aluminum oxide (alumina).
These two metal oxides, or a mixture thereof, are mainly used to protect alloys at high temperatures. The compositions of stainless steels for high temperature use are tailored to provide a balance between good mechanical properties and good resistance to oxidation and corrosion. Alloy compositions which can provide an alumina scale are favoured when good high temperature oxidation resistance is required, whereas compositions capable of forming a chromia scale are selected for resistance to hot corrosive conditions. Unfortunately, the addition of high levels of aluminum to the bulk alloy is not compatible with retaining good mechanical properties. Therefore applying a coating containing aluminum onto the bulk alloy is a good way to provide the desired alumina surface oxide while maintaining desired mechanical properties.
One of the most severe industrial processes from a materials perspective is the manufacture of olefins such as ethylene by hydrocarbon steam pyrolysis (cracking). Hydrocarbon feedstock such as ethane, propane, butane or naphtha is mixed with steam and passed through a furnace coil made from welded tubes and fittings. The coil is heated on the outerwall and the heat is conducted to the innerwall surface leading to the pyrolysis of the hydrocarbon feed to produce the desired product mix at temperatures in the range of 850 to 1150° C. An undesirable side effect of the process is the buildup of coke (carbon) on the innerwall surface of the coil. There are two major types of coke: catalytic coke (or filamentous coke) that grows in long threads when promoted by a catalyst such as nickel or iron, and amorphous coke that forms in the gas phase and plated out from the gas stream. In light feedstock cracking, catalytic coke can account for 80 to 90% of the deposit and provides a large surface area for collecting amorphous coke.
The coke builds up and constricts flow in the tubes and acts as a thermal insulator, requiring a continuous increase in the tube outer wall temperature to maintain throughput. A point is reached when the coke buildup is so severe that either the pressure drop reaches unacceptable levels or the tube skin temperature cannot be raised any further and the furnace coil is then taken offline to remove the coke by burning it off (decoking). The decoking operation typically lasts for 24 to 96 hours and is necessary once every 10 to 180 days. During a decoke period, there is no marketable production which represents a major economic loss. Additionally, the decoke process degrades tubes at an accelerated rate, leading to a shortened lifetime. In addition to inefficiencies introduced to the operation, the formation of coke also leads to accelerated carburization, other forms of corrosion, and erosion of the tube inner wall. The carburization results from the diffusion of carbon into the steel forming brittle carbide phases. This process leads to volume expansion and the embrittlement results in loss of strength and possible crack initiation. With increasing carburization, the alloy's ability of providing some coking resistance through the formation of a chromium based scale deteriorates. At normal operating temperatures, half of the wall thickness of some steel tube alloys can be carburized in as little as two years of service. Typical tube lifetimes range from 3 to 6 years.
It has been demonstrated that aluminized steels, silica coated steels, and steel surfaces enriched in manganese oxides or chromium oxides are beneficial in reducing catalytic coke formation. Alonizing™, or aluminizing, involves the diffusion of aluminum into the alloy surface by pack cementation, a chemical vapour deposition technique. The coating is functional to form a NiAl type compound and provides an alumina scale which is effective in reducing catalytic coke formation and protecting from oxidation and other forms of corrosion. The coating is not stable at temperatures such as those used in ethylene furnaces, and also is brittle, exhibiting a tendency to spall or diffuse into the base alloy matrix. Generally, pack cementation is limited to the deposition of one or two elements, the co-deposition of multiple elements being extremely difficult. Commercially , it is generally limited to the deposition of only a few elements, mainly aluminum. Some work has been carried out on the codeposition of two elements, for example chromium and silicon. Another approach to the application of aluminum diffusion coatings to an alloy substrate is disclosed in U.S. Pat. No. 5,403,629 issued to P. Adam et al. This patent details a process for the vapour deposition of a metallic interlayer on the surface of a metal component, for example by sputtering. An aluminum diffusion coating is thereafter deposited on the interlayer.
Alternative diffusion coatings have also been explored. In an article in “Processing and Properties” entitled “The Effect of Time at Temperature on Silicon-Titanium Diffusion Coating on IN738 Base Alloy” by M. C. Meelu and M. H. Lorretto, there is disclosed the evaluation of a Si-Ti coating, which had been applied by pack cementation at high temperatures over prolonged time periods.
The benefits of aluminizing an MCrAlX coating on superalloys for improved oxidation and corrosion resistance have been previously well documented. European Patent EP 897996, for example, describes the improvement of high temperature oxidation resistance of an MCrAlY on a superalloy by the application of an aluminide top coat using chemical vapour deposition techniques. Similarly, U.S. Pat. No. 3,874,901 describes a coating system for superalloys including the deposition of an aluminum overlay onto an MCrAlY using electron beam-physical vapour deposition to improve the hot corrosion and oxidation resistance of the coating by both enriching the near-surface of the MCrAlY with aluminum and by sealing structural defects in the overlay. Both of these systems relate to improvement of oxidation and/or hot corrosion resistance imparted to superalloys by the deposition of an MCrAlY thereon. These references do n

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