Caustic Wash/Cleaning of Pipes

I am designing a pipe system which is category D fluid service (as per B31.3). This piping will be cleaned/washed using caustic (sodium hydroxide, 50%). Because the fluid service is cat. D, hydro static test can be exempted. But as the fluid service will change during caustic wash, can I still consider exempting hydro test OR should I change the fluid category to Cat. "M".