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Duplex stainless steels (DSS) are an attractive alternative to conventional austenitic 300 series. They are becoming more and more present in industrial applications requiring high mechanical properties combined with good corrosion resistance. UNS S32202 is a lean duplex grade designed to guarantee corrosion resistance superior to that of 304L in most environments and even equivalent to 316L in NaCl environment at room temperature. Its yield strength is twice as high as 304L and 316L allowing thickness and weight reduction in structural components. With low nickel content (2.5%) and no molybdenum addition, the impact of raw material price fluctuation is reduced. It makes UNS S32202 suitable for a high number of applications including public transportation, building & construction, watersystems, liquid storage and pulp&paper industry.
Duplex stainless steel (DSS) provides a good alternative to austenitic stainless steel to build engineering designs thanks to excellent mechanical properties and corrosion resistance associated to their biphasic microstructure. Their microstructure changes during the welding. The influence of microstructure and elemental distribution on pitting corrosion resistance of different zones of the lean DSS UNS S32202 welded by Gas Tungsten Arc Welding (GTAW) was analyzed. The welded transversal cross section surface after polishing and the welded surface after pickling were studied. Comparing both surfaces, the welded surface is more resistant to pitting corrosion than welded transversal cross section surface, according to the electrical charge and the total mass loss. The High Temperature Heat Affected Zone (HTHAZ) surface after pickling has no pitting thanks to the presence of the superficial layer (DSS with a specific microstructure). However, the HTHAZ transversal cross section has the lowest pitting resistance. Its pit initiation sites could be associated to Cr2N. Pit initiation sites in the molten zone (MZ) could be linked to oxide particles and Cr2N for both surfaces. In all pits (in MZ and HTHAZ, for both surfaces) ferrite dissolves preferentially (austenite remains intact). Austenite is more resistant to pitting due to its higher nitrogen content than in ferrite.
Silicon is an important element of our Planet’s crust, which is transferred into water streams through dissolution.1 Hence, it is usually found as water-soluble silicate or colloidal silica in natural surface waters (sea, rivers, lakes), or underground waters. When such water is used for industrial purposes (eg. industrial cooling), silicate can enter the operating system and can pose a threat to its proper operation. The main reason is the solubility of amorphous silica, a product of the silicate polycondensation process.
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Inorganic scaling and fouling are undesirable processes occurring in process waters that are supersaturated with respect to scaling cations and anions. Mineral scaling is the result of nucleation and crystal growth phenomena that follow predictable pathways. This is because the mineral scales are composed of well-defined crystalline salts, the most common being calcium carbonate, calcium sulfate dehydrate (gypsum), metal sulfides, depending on the particular water chemistry and operational parameters (eg. temperature).
Fouling of equipment surface by unwanted materials is a well-documented problem inindustrial water systems.1 The foulants are generally classified into four categories: a)mineral scales i.e., calcium carbonate, calcium sulfate, calcium phosphate, magnesiumhydroxide, etc., b) corrosion products i.e., iron oxide, zinc oxide, copper oxide, etc., c)microbiological mass i.e., bacteria, algae, fungi, and other organisms, and d) depositsi.e., clay, silt, calcium-inhibitor salts, corrosion products, etc.