Chemical Cleaning is a pre-commissioning procedure that generally refers to the cleaning of a boiler or HRSG with chemistry formulated to remove mill scale from the water tubes. Chemical Cleaning is also used on piping and vessels that need to have mill scale removed as well. This mill scale removal minimizes tube failures during start up and improves heat transfer making the boiler/HRSG more efficient during operation. A typical chemical cleaning is a single volume cleaning and consists of first filling the equipment with high quality water, raising the temperature of the water, then adding an emulsifier to remove the oils and light greases that will place these in suspension. This is followed by adding an inhibitor to prevent good steel from being attacked by the acid chosen, then a weak chelating acid is added to dissolve and also place in suspension any mill scale and iron that is to be removed. The acid stage is then followed by a neutralizing phase, and finally the cleaned metal is passivated with a passivating agent. The boiler/HRSG is then drained and rinsed ready to place into operation after restoration. The passivating agent places a vapor barrier on the steel to help prevent flash rust during restoration.