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The object of the present invention is to provide alloys combining low material costs and a high corrosion resistance. In particular, the alloys should have a high wet corrosion resistance in water with high salinity, especially at temperatures above 100°C. The alloys should have a good resistance against pitting and crevice corrosion attack. Preferred alloys have a good resistance to reducing conditions (as measured e.g. by ASTM G 28 A) and at the same time to pitting corrosion and chloride ion attack (as measured e.g. by ASTM G 28 B). Advantageous alloys combine a high corrosion resistance with good mechanical properties, e.g. a high strength. Alloys with such properties are particularly suitable for, but not limited to, down-hole-headers in geothermal power plants operating using hot geothermal fluids containing high chloride concentrations.

 

This object is solved by a nickel-chromium-iron-molybdenum alloy, comprising 40 to 48 wt% (percent-by-weight) nickel, 30 to 38 wt% chromium, 4 to 12 wt% molybdenum, and iron, wherein the alloy optionally further comprises up to 5 wt% manganese, up to 2 wt% copper up to 0.6 wt% nitrogen, up to 0.5 wt% aluminium and up to 0.5 wt% vanadium. According to a second aspect of the present invention, the object of the invention is solved by a nickel-chromium-iron-molybdenum alloy, consisting of 40 to 48 wt% nickel (Ni), 30 to 38 wt% chromium (Cr), 4 to 12 wt% molybdenum (Mo), optionally manganese (Mn), optionally copper (Cu), optionally nitrogen (N), optionally tungsten (W), optionally niobium (Nb), optionally cobalt (Co), optionally carbon (C), optionally tantalum (Ta), optionally titanium (Ti), optionally silicon (Si), optionally aluminium (Al) and optionally vanadium (V) and balance iron (Fe) plus impurities.

 

Surprisingly, it was found that it is possible to achieve a high corrosion resistance in spite of reducing the amounts of molybdenum and nickel, which in practice due to their high price are the two primary constituents that determine the overall final cost of a corrosion resistant alloy. In order to obtain a single phase alloy, the inventors have found that it is possible to increase the chromium content at the expense of the molybdenum and nickel instead of following the conventional way of increasing the iron content. It has been found that it is possible to dissolve very high amounts of chromium in an (austenitic) single phase matrix together with an optimal amount of molybdenum and very low amounts of nickel and iron. The alloys according to the invention have the desired good corrosion resistance and allow for achieving favourable mechanical properties. At the same time the amounts of costly materials may be reduced. This way the alloys according to the invention provide an economically viable and robust alternative for demanding applications such as contact with hot fluids having a high salinity (e.g. above 100°C, above 100 g/l chloride ions).

 

The indicated ranges of nickel, chromium and molybdenum allow balancing these three main alloying elements in order to achieve the desired favourable properties. Outside the ranges of 40 to 48 wt% nickel, 30 to 38 wt% chromium and 4 to 12 wt% molybdenum, at least one of the following properties cannot be expected to be favourable: corrosion resistance, structural properties (e.g. number of phases) and mechanical properties. Additionally, higher amounts of nickel and/or molybdenum would render the alloy uneconomical. Other elements may be additionally added to the alloy according to the invention.

 

 

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