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.
FGD designers, fabricators and operators learned several significant lessons when stainless steel 904L is used in quencher and sump of a double loop scrubber at Laramite river unit damaged due to pitting corrosion. Several damaged taken place in stainless steel 904L alloy matching composition welds in weld heat influenced zones, in the heat tint resulting by welding at scratches produced while fabrication and in the paint markings. Inconel alloy 625 welding materials were utilized to repair the steel 904L apparatus. It was evident that enhanced fabrication processes were important in obtaining increased FGD operation life. It was resulted that molybdenum based stainless steel alloys needed overalloyed welding products comprising of 9 to 16% molybdenum for adequate weld metal corrosion resistance. The importance of eradicating heat tint, embedded iron and paint marks from stainless steels was also stated.
FGD material technique followed another major step during the same period when a glass flake polyester lining could not succeed in combining chamber of the direct bypass reheat outlet duct at the Mississippi. A substitution lining of Hastelloy G failed during six months of its installation. Although, it was noticed that Hastelloy C-276 welds are not corroded. The attacked alloy G was shielded with thin sheet of Hastelloy C276. This repair showed that Hastelloy C-276 was identified as the solution for FGD troubles. The technology of overlaying a steel structure with thin alloy sheet found to be an effective and affordable method for repair of corroded FGD parts and for manufacturing new alloy linings. This lining technology is also popular in the modern time and normally called as wallpapering.
The Nickel-Chromium alloys and iron-nickel-chromium alloys have been determined to offer the necessary corrosion resistance for prolonged time. The content of molybdenum is found responsible for offering corrosion resistance to alloys. Niobium and tungsten also increase the corrosion resistance.
For normal evaluation, magnitude of molybdenum can be utilized as a resistance measure of nickel-chromium alloys to corrosion in the FGD system conditions. This evaluation and instructions are very normal.
The austenitic stainless steels containing molybdenum such as steel 316 and steel 317 comprise of 3 and 4-1/2% molybdenum respectively, and are utilized in the nominal aggressive region of the system. These alloys are normally observed to be unimportant to unapproved in any portion of the vessel or duct. The conditions for these materials are usually 5,000 ppm chlorides and pH 5 or more. The latest grades of steel containing 6% molybdenum and nitrogen are called as super austenitic stainless steels are utilized in the nominally more rigorous conditions. The standard limit for application of these alloys is 10,000 ppm to 20,000 ppm chlorides however some applications report 40,000 ppm. Acidic limits for these materials is pH 4 or more. These alloys are utilized basically in the same applications as that for Hastelloy G containing 6% molybdenum, but are economical due to smaller magnitude of nickel. The inclusion of nitrogen in these steels retains a completely austenitic structure while improving strength and corrosion resistance.
There are four commonly used types of applications for nitinol.
Free Recovery: nitinol is deformed at a low temperature, and heated to recover its original shape.
Constrained Recovery: The same, except that recovery is rigidly prevented, and thus a stress is generated.
Work Production: Here the alloy is allowed to recover, but to do so it must act against a force (thus doing work).
Superelasticity: As discussed above, here the nitinol acts as a super spring.
Nitinol is also popular in extremely resilient glasses frames. It is also used in some mechanical watch springs.
It can be used as a temperature control system; as it changes shape, it can activate a switch or a variable resistor to control the temperature.
It is used in cell-phone technology as a retractable antenna, or microphone boom, due to its highly flexible and mechanical memory nature.
It is used in some novelty products, such as self-bending spoons which can be used by amateur and stage magicians to demonstrate "psychic" powers or as a practical joke, as the spoon will bend itself when used to stir tea, coffee, or any other warm liquid.
It can also be used as wires which are used to locate and mark breast tumours so that following surgery can be more exact.
Due to the fact it can change shapes it is also used as a golf club insert.
Nickel titanium can be used to make the underwires for underwire bras.
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