The erosion-corrosion behavior of two stainless steels (UNS S42000 and UNS N08028) has been assessed under gaseous-liquid-solid impingement conditions. Pure corrosion and erosion-corrosion impingement tests were conducted at three different impact angles and at three different impact velocities up to 60 m/s. CO2 at a pressure of 1,500 kPa was used as the gas phase. The sand content, with grain size below 150 m, was 2.7 g/L brine. Artificial brine with a NaCl content of 2.7 % was used as liquid phase. The damaged surfaces of samples exposed to the high velocity multiphase flow were investigated by scanning electron microscopy (SEM) and an optical device for 3D surface measurements to assess the depth of attack. Electrochemical investigations according to ASTM G 61 were performed to determine electrochemical behavior of tested materials including critical pitting potentials Epit and repassivation potentials Erepass. Furthermore, the surfaces near regions of the samples tested were investigated by applying atomic force microscopy (AFM), magnetic force microscopy (MFM) and nano-indentation measurements. From the investigations, impact velocity shows the greatest effect on the degradation rate of both stainless steels UNS S42000 and N08028 under three-phase impact conditions with sand. In contrast, pure corrosion impingement tests without sand did not show an increase of mass loss rate with rising impact velocity. The steel grades investigated show mainly erosive attack and no corrosion under all tested conditions. Only some samples of the lower alloyed grade UNS S42000 exhibit scattered pits which do not seem to contribute to the degradation within the focal point. Therefore it can be concluded that degradation of these highly alloyed steels mainly happens by erosion due to impacting sand particles. Grade UNS N08028 clearly shows better erosion-corrosion resistance than alloy UNS S42000 at impact velocities up to 20 m/s. This is expected to be caused by its higher tensile strength and fracture elongation. Moreover, the superaustenitic grade UNS N08028 appears to have a higher capacity for absorbing and dissipating the impact energy. This behavior is due to its face centered cubic lattice in comparison to the body centered (slightly tetragonal distorted) lattice of alloy S42000.