Reduced activation ferritic-martensitic steels are considered for application in future fusion technology as structural materials. In the past, corrosion testing was performed at moderate temperatures of about 480 °C and showed acceptable homogeneous corrosion behavior. However, in the mean time the envisaged temperature limits increased to around 550 °C. Due to large uncertainties in extrapolations of the corrosion behavior from 480 to 550 °C, long-term tests were initiated which include also the development of modeling tools and analyzing for the first time in more detail transport and precipitation phenomenas. The tests performed at 550 °C and flow rate of 0.22 m/s revealed a corrosion rate of about 400 μm/year with emphasis to the long term exposed samples. The high amount of dissolved components formed precipitations at cooler loop positions which led to line blockages after short operation of about 3000 h. A comparison with global Pb–17Li data and 480 °C testing will be shown.