Erosion, erosion–corrosion, and synergistic behaviors of AISI 420 stainless steel were studied in 3.5 wt.% NaCl solution containing silica sand with the size of 250–500 μm as erodent particle. The erosion and erosion–corrosion tests were carried out according to ASTM G11909 standard and the synergism was calculated. The tests were performed using a slurry jet apparatus at a jet velocity of 6.5 m/s, sand concentration of 90 g/l, and various impinging angles of 20 deg–90 deg. Scanning electron microscope (SEM) was used to study the eroded surfaces and erosion mechanisms. The SEM images showed that under low impacting angles, cutting deformation was the main erosion mechanism while impact and work hardening could be responsible for material removal at high impacting angles. The results showed that the maximum erosion–corrosion and synergism rates occurred at an impingement angle of about 50 deg while the maximum pure erosion rate was obtained at impingement angle of about 35 deg. Energy dispersive spectrometry (EDS) analysis showed that an oxide layer was formed on the surfaces of the samples during erosion–corrosion tests. This oxide layer could make the surface more brittle and could lead to an increment of about 15 deg in the angle of the maximum removal rate. The formation and the subsequent removal of the nonprotective oxide layer as well as possible initiation and propagation of pits during erosion–corrosion tests could lead to higher erosion–corrosion rate compared to pure erosion resulting in a positive synergism under the conditions tested.