CS is suitbale only if the acid is normal to room temperature i e say around 30-35 Deg C. If the temperature reaches above 50 Deg.C you can expect severe corrosion and CS is not suitable for high temperature in that case you have to select high Chrome and high Ni steel.

Carbon steel is normally used for Sulphuric Acid service for concentration 70% above and temperature upto 40Deg.C. With this temperature soft,non-adherent,non soluble iron sulfate layer is formed on the pipe inner wall.Due to high temperature(in your case the local contact area due to steam flashing)the acid will reach the bubbling temperature causing more vapor generation over liquid thus by damaging the above protective layer.Once the layer is damaged then the corrosion rate will be very severe .Further Hydrogen grooving is another phenomenon with damaged protective layer under high temperature especially for carbon steel piping wherever the velocity is increased beyond 3ft/sec. So it is better to avoid contact of high temperature steam on the acid pipeline.