It is necessary to clean a boiler and periodically thereafter for its efficient operation, corrosion control and prevention of tube failures. The cleaning of boilers is done by a combination of 

1) Hot alkaline treatment which is used to remove oil and grease & for effective acid cleaning stage , 

2) Solvent cleaning (inhibited acid cleaning) or Chemical Cleaning is used to chemically remove mill scale, mineral deposits from water and corrosion products (Copper & Ferrous Oxide etc.,) 

3) Neutralization and Passivation is used to neutralize acids and to form a strongly adherent protective oxide layer on the wetted surfaces.

Finally, one has to go for Hydro Jetting to remove remaining loose / hard material.