Ni-Cr-Mo Superalloys-Corrosion Problem Solution
Variable behavior towards how the nature is dealt by humans has consequent in international efforts to prevent the release of sulfur dioxide into the conditions. Attempts to curb damage to the nature by acid rain has necessitated production of flue gas desulfurization systems at coal fired electric power stations across the world. The conditions in these wide chemical processing systems are extremely corrosive. Hence corrosion resistant nickel-chromium-molybdenum alloys are usually needed for general production of the parts and for secure lining of the system.When FGD scrubber production started in 1970, none has a better understanding on the corrosion phenomenon occurring inside the systems. As a result, the issues added to corrosion were popular. In an effort to employ cheap materials, designers predictably employed lower grade alloys as compare to those were required causing the premature damage and substitution of several FGD parts and linings. Although through evaluation of this process for several years, the latest production of FGD scrubbing systems utilize advanced corrosion resistant alloys that offered prolong and consistent operation life.
The stainless steel 316 fans were damaged at the Arizona power plant. Inconel alloy 625 was selected for replacement due to its outstanding corrosion resistance, high strength and superior fatigue characteristics. This application is normally considered as the prime essential application of a nickel alloy. The initial sheet alloy liner was configured in a FGD sytem at Pennisylvania power center. Incoloy alloy 825 sheets were utilized to repair the carbon steel fan housings when rubber linings failed. In 1979, the first application of Hastelloy C26 was offered for precooler rings at Newton plant. In that period, a full scrubber system was produced from alloy 625 for a furnace established in Cleveland Ohio.
While in 1970, many parts of FGD apparatus were made with materials comprising of the nominal initial cost. It was often considered that the FGD conditions would not be widely corrosive. This concept was proved to be accurate when a FGD apparatus started to attain premature damage, bad consistency, large maintenance costs and forced turn off for repairing of corroded parts. The corrosion, erosion and poor lining installation, specifically in outlet ducting and stack liners resulted in considerable issues for service.
Due to the corrosion troubles occurred in the several FGD systems, corrosion analyses are conducted to determine the performance of materials in FGD equipment. The recommended test method is a corrosion analysis spool rack. The test spools provide the benefit of testing different materials at the same moment in the same place. A wide summary of spool corrosion test data performed in FGD and other scrubber systems is presently introduced. Other kinds of corrosion tests have been performed to determine the functionality of alloys in the different sections of FGD systems. One clear method is to simply connect sheet samples of sheet to the scrubber or duct wall. Various more complicated test specimens comprise of several alloys, weldments accumulated through the various welding materials and welding processes. The data produced from these tests are counted as the highly consistent as the analyses performed in this way show the materials necessarily to the same conditions that caused damage of the basic material. The corrosion analysis in simulated FGD conditions has also been performed in the labs. The lab tests allows superb control of the natural factors and variables being tested as compare to the field testing.
While these lab simulations are essential when quick data production is essential, the consistency of this information is usually important. The data produced in the field tests is highly trustworthy. Care is required in choosing lab test methods. For instance, a material acceptance testing procedures like ASTM G28 methods A and B have been speciously utilized evaluation alloys. The corrosion rates in these analyses are alloy specific and in actual, state one nothing about the comparative resistance of alloys to FGD condition. These evaluations are made to confirm that materials have been adequately heat processed while production and fabrication. These are quality control apparatuses.
To unite the evaluation ease and short time of lab tests with consistency of field tests, a pilot plant that simulated FGD conditions was made for corrosion technique in USA. Using this systems, corrosion data is received in the variable operating conditions almost similar to that occurred in full scale FGD scrubbers but with high control of FGD conditions. In the whole 1980s, this scrubber system was utilized to find the influence of various FGD conditions in the different kinds of materials. The corrosion test outcomes of the model scrubber have been stated at NACE annual conferences in 1983. The evaluation mechanism was a benchmark in giving instructions on the applications of stainless steels and nickel base alloys for FGD operations. The outcomes described the results of chloride on alloys about 100,000 ppm chlorides, simulating FGD service and the advantageous effect of molybdenum present in alloy in improving localized corrosion resistance, particularly in the outlet duct area. The customs of test outcomes in the model scrubber described outstanding conformity with knowledge from full scale, field operated utility FGD apparatus.
Possessing an alloy with high composition, specific process controls are needed to resist the production of secondary phases. Alloy 686 ingots get a proprietary elevated temperature uniformity before the hot rolling. The elevated temperature homogenization processing enables the alloying elements to prevent the local segregation that occurs while solidification of ingot. The uniform level of ingot prevents the after production of embrittling phases.
Decreasing the magnitude of iron in the alloyed composition also assists in two ways: decreasing iron 5 to 1% permits the inclusion of more nickel by 4%. It can produce a considerable effect on stability. Second, the magnitudes of iron and titanium have been observed to influence the resistance of alloy to intergranular corrosion after subjecting to temperatures from 1400oF to 1800oF.
However the magnitude of carbon in alloy 686 is decreased to 0.008% or lower, the influence of carbide precipitation can be decreased through the inclusion of titanium at a level causing higher ratio of titanium to carbon above 3. Alloy 686 is vacuum induction melted and electroslag melted to limit the chemical composition and retain the necessary microstructure.
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