This paper presents the findings of corrosion studies of cast steel 20GL (structural alloyed steel: G—1% of Mn, L—alloyed) with boriding, selected as a promising passive method of protecting hydraulic machine elements from hydro-solid particle erosion and corrosive effects. Earlier studies have shown the boriding efficiency to increase the solid particle erosion resistance of cast steel 20GL by two–eight times depending on the solid particles’ impact angle of 30° and 90°. The boriding was carried out in a melt based on sodium tetraborate, sodium fluoride and sodium chloride salts in a shaft furnace with external heating. Results indicated that the boriding process did not affect the initial roughness of cast steel 20GL but increased the surface microhardness by more than six times, up to 1680–2080 HV0.01. The total layer thickness after two boriding processes was from 80 to 150 µm. The results of corrosion resistance studies by electrochemical methods obtained in this work showed the positive effect of boriding of steel 20GL. Boriding turns steel 20GL from a low-resistant class (score 6) to a resistant class (score 4–5) on a 10-point scale of corrosion resistance of metals. The boriding at a depth of 150 ± 5 µm reduces the corrosion rate in the environment of 3% NaCl solution by 2.8 times and in the environment of 0.7% Na2SO4 solution by 4.1 times, compared with the initial material without modification. It is revealed that an increase in boriding depth leads to an increase in corrosion resistance. The results indicate that the boriding of hydraulic machine parts made of 20GL steel will increase their corrosion resistance, thus prolonging the operating and overhaul period.