In this paper, a low activation of ferritic-austentic steel was studied when it was exposed with plasma. The experiment was carried out on a plasma focus device with energy of 1.9 kJ, with a pressure of 2.5 torr of deuterium. AFM analysis shows that cracks up to 2 microns and holes from 81 to 281 nm are found on steel surfaces. The mechanism of the occurrence of erosion of low-activation ferritic-austentic steel is also indicated. It is established that the mechanism of erosion depends on the momentum.