Self-passivating tungsten alloys

To optimise the safety properties of a future power plant IPP is developing tungsten alloys which safeguard themselves against oxidation when they come into contact with oxygen.

In a future fusion power plant the plasma-facing surfaces will presumably be protected with tungsten. Material erosion by high-energy hydrogen is kept low by this robust metal. This, however, constitutes a disadvantage to the safety properties of the power plant. Like other wall materials, tungsten is activated by the fast fusion neutrons. In the event of a loss of cooling the surfaces can heat up to more than 1100° C due to afterheat. Air ingress causes formation of tungsten oxide, which is volatile at high temperatures.

The positive safety features of a power plant are maintained in spite of this effect. Nevertheless, it is desirable to suppress oxidation and evaporation of the radioactive material. For this purpose IPP is developing tungsten alloys which are self-passivating.

Electron-microscopic cross-sectional image of a tungsten alloy after oxidation at 900 °C: The protective chromium oxide layer appears in the upper part of the image as a dark belt.

To achieve this, tungsten is mixed with chemical elements which form a protective oxide layer on the surface of the alloy when they come into contact with oxygen. This layer then prevents further oxidation of the material. Samples of such alloys are being prepared at IPP by magnetron sputtering in the form of thin films for analysis and investigation. An alloy of tungsten, chromium, and titanium proved to be particularly efficient: Upon addition of 15 weight per cent chromium and 2 per cent titanium the alloy oxidises ten thousand times more slowly than pure tungsten.

The self-passivating tungsten alloys can also be used outside of fusion research – in any application where susceptibility (of tungsten) to oxidation is a problem.

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