50 permalloy alloyed with Cr for increased corrosion resistance

The soft magnetic alloy 50 Permalloy (∼50Fe-50Ni), widely used in e.g. electro-mechanical relays in telephone exchanges, has been alloyed with 0-12 wt% chromium in order to increase the corrosion resistance. The saturation magnetization decreases linearly with the chromium addition and is reduced to half its value for 9 wt% chromium. However, chromium also substantially lowers the crystal anisotropy and saturation magnetostriction and the net effect of chromium is higher initial permeabilities, roughly constant maximum permeabilities and lower coercive field strengths. Chromium also stabilizes a favourable cubic texture obtained at the primary recrystallization during the final heat treatment. The electrical resistivity is increased and is doubled at a chromium content of roughly 5 wt%. The corrosion properties have been investigated by measurement of current-voltage curves in a sulphuric acid solution and by a salt spray test. It is found that the corrosion resistance can be substantially improved while the magnetic properties are moderately degraded or even improved.