Knowledge have about material choices for resisting erosion damage from fluids that are also corrosive? Since erosion resistance typically has an inverse relationship to corrosion resistance, would a Rc 55 low alloy steel (e.g. Nitralloy 135) suffer from corrosion to the point of wearing out faster than 17-4PH H900 at Rc 45 when subjected to,
You may also try a coating to help improve the erosion resistance. TMT Research in Smith River, CA offers some diffusion coatings for erosion protection on pumps and valves. Contact them if you are interested in that option.
Since erosion resistance typically has an inverse relationship to corrosion resistance, <
I will concede that NACE stipulates "Do not exceed" hardness numbers, but that is the limit to which I agree with your premise.  Stellite is more corrosion resistant to almost every process fluid than carbon steel, and harder, and more erosion resistant.  Still you must primarily select an alloy that is corrosion resistant to the process fluid, or the corrosion products will likely be washed away.  Back when I went to "sandbox 1 school" as a young pup control valve engineer, they called this process KE-rosion-or combination corrosion/erosion. 
If you address corrosion, you can control erosion by limiting velocity. 

MORE NEWS