Chromium precipitation

Addition of niobium to austenitic stainless steels inhibits intergranular corrosion by forming niobium carbide with the carbon that is present in the steel. Without the niobium addition, chromium precipitates as a chromium carbide film at the grain boundaries and thus depletes the adjacent areas of chromium and reduces the corrosion resistance. An amount of niobium equal to 10 times the carbon content is necessary to prevent precipitation of the chromium carbide. 

The reduced chromium precipitate is soluble in saturated sodium bicarbonate solution but no additional aldehyde was obtained on extracting this bicarbonate solution with ether. 

Very often, Celite is added to the Collins solution during the oxidation of alcohols in order to prevent loss of product in chromium precipitates.92 The addition of acetic anhydride to the Collins solution, first reported by Garegg and Samuelsson,93 allows a very mild oxidation of alcohols that is particularly suited for sugars and nucleosides. Acetic anhydride helps preventing a -elimination that may occur during the oxidation of alcohols containing heteroatoms at the (3-position.94... 

Very often, ca. 2-7 g of dry Celite per g of O3 are added—normally before the preparation of the Cr03 2Py complex—in order to avoid loss of product on the chromium precipitates during the work-up. Very frequently, ca. 2-5 equivalents of acetic anhydride (MW = 102.1) are added—normally after the preparation of the CrC>3 2Py complex—in order to facilitate a milder reaction, particularly in sugars and nucleosides. It is not common to add both Celite and acetic anhydride in the same reaction. 

Salts of chromium. Precipitate, soluble in cold acetic acid. Precipitate, insoluble in acetic acid. Precipitate, insoluble in acetic acid. 

Chromium trioxide. CrOj. Red precipitate from [Cr04p plus cone. H2SO4, m.p. 198 C, loses oxygen at 420" C. CrOa is a powerful oxidizing agent and is used as such. Acidic, gives [Cr04] - with water. 

Hence the orange colour of a dichromate is converted to the green colour of the hydrated chromium(III) ion, Cr ", and sulphur is precipitated when hydrogen sulphide is passed through an acid solution.)... 

Chromium(III) hydroxide is obtained as a light green gelatinous precipitate when an alkali or ammonia is added to a chromium(III) salt ... 

Hydrated chromium(III) sulphate exhibits different colours and different forms from which varying amounts of sulphate ion can be precipitated by barium chloride, due to the formation of sulphato-complexes. Chromium(III) sulphate can form alums. 

Addition of alkali gives a green gelatinous precipitate of chromium(III) hydroxide, soluble in a large excess of strong alkali. 

AISI 321 and 347 are stainless steels that contain titanium and niobium iu order to stabilize the carbides (qv). These metals prevent iatergranular precipitation of carbides during service above 480°C, which can otherwise render the stainless steels susceptible to iatergranular corrosion. Grades such as AISI 316 and 317 contain 2—4% of molybdenum, which iacreases their creep—mpture strength appreciably. In the AISI 200 series, chromium—manganese austenitic stainless steels the nickel content is reduced iu comparison to the AISI 300 series. 

In addition to these principal alloying elements, which provide soHd solution strengthening and/or precipitation strengthening, wrought alloys may contain small amounts of titanium and boron [7440-42-8J, B, for control of ingot grain size, and ancillary additions of chromium, manganese, and zirconium to provide dispersoids. AH commercial alloys also contain iron and siUcon. 

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