Salt bath composition based on alkali nitrates for oxidizing ferrous metal to improve its corrosion resistance

Abstract

A salt bath composition is adapted to form a layer of magnetite Fe.sub.3 O.sub.4 on the surface of ferrous metal parts, including nitrided ferrous metal parts, to protect the underlying iron against corrosion. This layer is impermeable and of good crystalline order, as indicated by a deep black color and a corrosion potential greater than 1 000 mV. The composition includes at least nitrate anions and sodium and lithium cations, the latter in a proportion by weight relative to the bath between 0.1% and 5%, preferably between 0.5% and 1.75%. Preferred compositions contain sodium, potassium and lithium cations and nitrate, carbonate and hydroxyl anions, within the following percentage ranges: 8.5 .ltoreq.CO.sub.3.sup.2- .ltoreq.26 15 .ltoreq.NO.sub.3.sup.- .ltoreq.41.5 4.7.ltoreq.OH.sup.- .ltoreq.21.5 Apart from the exceptional reliability of the oxidation treatment, the presence of lithium associated with the sodium and the potassium and with the nitrate or carbonate anions reduces the formation of carbonate-containing sludge in the bath, probably because the composition forms ternary mixtures of nitrate or carbonate with a low melting point.

Dependencies are inferred from the claim text.