Molybdenum corrosion resistance

ASTM A 890/A 890M-99(2003) Standard Specification for Castings, Iron-Chromium-Nickel-Molybdenum Corrosion-Resistant, Duplex (Austenitic/Ferritic) for General Application (contains the major duplex grades)... 

ASTM A 890, Standard specification for castings, iron-chromium-nickel-molybdenum corrosion-resistant, duplex (austeniticiferritic) for general application. 

It is used in certain nickel-based alloys, such as the "Hastelloys(R)" which are heat-resistant and corrosion-resistant to chemical solutions. Molybdenum oxidizes at elevated temperatures. The metal has found recent application as electrodes for electrically heated glass furnaces and foreheaths. The metal is also used in nuclear energy applications and for missile and aircraft parts. Molybdenum is valuable as a catalyst in the refining of petroleum. It has found applications as a filament material in electronic and electrical applications. Molybdenum is an... 

Tantalum and 2kconium exhibit the highest corrosion resistance to HCl. However, the corrosion resistance of 2ironium is severely impaHed by the presence of ferric or cupric chlorides. Tantalum—molybdenum alloys containing more than 50% tantalum are reported to have exceUent corrosion resistance (see Molybdenumand molybdenum alloys) (69). Pure molybdenum and tungsten are corrosion resistant in hydrochloric acid at room temperature and also in 10% acid at 100°C but not in boiling 20% acid. 

Molybdenum improves the corrosion resistance of stainless steels that are alloyed with 17—29% chromium. The addition of 1—4% molybdenum results in high resistance to pitting in corrosive environments, such as those found in pulp (qv) and paper (qv) processing (33), as weU as in food preparation, petrochemical, and poUution control systems. 

Vitahium FHS ahoy is a cobalt—chromium—molybdenum ahoy having a high modulus of elasticity. This ahoy is also a preferred material. When combiaed with a properly designed stem, the properties of this ahoy provide protection for the cement mantle by decreasing proximal cement stress. This ahoy also exhibits high yields and tensile strength, is corrosion resistant, and biocompatible. Composites used ia orthopedics include carbon—carbon, carbon—epoxy, hydroxyapatite, ceramics, etc. 

Stainless Steels. Stainless steels are more resistant to msting and staining than plain carbon and low ahoy steels (47—50). This superior corrosion resistance results from the presence of chromium. Although other elements, such as copper, aluminum, shicon, nickel, and molybdenum, also increase corrosion resistance these are limited in their usefiilness. 

The Tribaloy aUoy T-800, is from an aUoy family developed by DuPont in the eady 1970s, in the search for resistance to abrasion and corrosion. Excessive amounts of molybdenum and sUicon were aUoyed to induce the formation during solidifica tion of hard and corrosion-resistant intermetaUic compounds, known as Laves phase. The Laves precipitates confer outstanding resistance to abrasion, but limit ductUity. As a result of this limited ductUity the aUoy is not generaUy used in the form of plasma-sprayed coatings. 

The two corrosion-resistant alloys presented ia Table 5 rely on chromium and molybdenum for their corrosion resistance. The corrosion properties of IJ1 timet are also enhanced by tungsten. Both alloys are available ia a variety of wrought product forms plates, sheets, bars, tubes, etc. They are also available ia the form of welding (qv) consumables for joining purposes. 

Nickel increases toughness and improves low-temperature properties and corrosion resistance. Chromium and silicon improve hardness, abrasion resistance, corrosion resistance, and resistance to oxidation. Molybdenum provides strength at elevated temperatures. 

Stainless Steel There are more than 70 standard types of stainless steel and many special alloys. These steels are produced in the wrought form (AISI types) and as cast alloys [Alloy Casting Institute (ACI) types]. Gener y, all are iron-based, with 12 to 30 percent chromium, 0 to 22 percent nickel, and minor amounts of carbon, niobium (columbium), copper, molybdenum, selenium, tantalum, and titanium. These alloys are veiy popular in the process industries. They are heat- and corrosion-resistant, noncontaminating, and easily fabricated into complex shapes. 

The addition of molybdenum to the austenitic alloy (types 316, 316L, 317, and 317L) provides generally better corrosion resistance and improved resistance to pitting. 

These alloys have extensive applications in sulfuric acid systems. Because of their increased nickefand molybdenum contents they are more tolerant of chloride-ion contamination than standard stainless steels. The nickel content decreases the risk of stress-corrosion cracking molybdenum improves resistance to crevice corrosion and pitting. 

The resistance of a metal to erosion-corrosion is based principally on the tenacity of the coating of corrosion products it forms in the environment to which it is exposed. Zinc (brasses), aluminum (aluminum brass), and nickel (cupronickel) alloyed with copper increase the coating s tenacity. An addition of V2 to 1)4% iron to cupronickel can greatly increase its erosion-corrosion resistance for the same reason. Similarly, chromium added to iron-base alloys and molybdenum added to austenitic stainless steels will increase resistance to erosion-corrosion. 

For corrosion resistance, these steels (18% nickel, 9% cobalt, 3% molybdenum, 0.2% titanium and 0.02% carbon) are similar to the 13% chromium steels and, therefore, are suitable for mildly corrosive situations. Because of their very high strength after heat treatment (yield strength—1390 N/mm, elongation—15%, impact strength) maraging steels find some use in a very high-pressure equipment. 

Steel is essentially iron with a small amount of carbon. Additional elements are present in small quantities. Contaminants such as sulfur and phosphorus are tolerated at varying levels, depending on the use to which the steel is to be put. Since they are present in the raw material from which the steel is made it is not economic to remove them. Alloying elements such as manganese, silicon, nickel, chromium, molybdenum and vanadium are present at specified levels to improve physical properties such as toughness or corrosion resistance. 

Cast irons are iron with high levels of carbon. Heat treatments and alloying element additions produce gray cast iron, malleable iron, ductile iron, spheroidal cast iron and other grades. The mechanical properties vary significantly. Nickel-containing cast irons have improved hardness and corrosion resistance. Copper or molybdenum additions improve strength. 

Nickel is usually alloyed with elements including copper, chromium, molybdenum and then for strengthening and to improve corrosion resistance for specific applications. Nickel-copper alloys (and copper-nickel alloys see Section 53.5.4) are widely used for handling water. Pumps and valve bodies for fresh water, seawater and mildly acidic alkaline conditions are made from cast Ni-30% Cu type alloys. The wrought material is used for shafts and stems. In seawater contaminated with sulfide, these alloys are subject to pitting and corrosion fatigue. Ammonia contamination creates corrosion problems as for commercially pure nickel. 

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