Boiler Primer 2: Treatment chemistry (low pressure)

Chemical reactions happen pretty quickly in a boiler because the temperature is so high, and reaction rate is generally dependent on temperature.  Some reactions that happen in a boiler are good, and some are bad.

Bad reactions:
Calcium scale formation on boiler tubes (solid deposits reduce heating efficiency)
Oxygen corrosion of steel (pitting and metal loss)

Good reactions:
Steel passivation (forms protective layer)
Calcium salt formation that remains soluble or in the bulk water (we can remove it with blowdown)

We combat bad reactions like calcium scale formation by adding chemicals that the calcium can react with to form small crystals in solution, instead of on the steel tubes in the boiler.  Combating oxygen corrosion involves the use of oxygen scavengers which react with the oxygen to form other compounds, and remove it from the water--what's not there can't do damage.  Nearly all of the following are formulated as sodium salts.  Sodium ions play nice, in general calcium do not.  Calcium often comes into the boiler through makeup water (replacement water for leaky valves or water lost to steam)--it is preferred to be removed with a water softener if possible, before being added to the boiler.

Corrosive Water	Scale-forming Water
low pH soft or with primarily noncarbonate hardness low alkalinity	high pH hard with primarily carbonate hardness high alkalinity

The central dogma of boiler treatment currently is: mitigate corrosion almost completely through the use of an alkaline pH, and use chemistry to stymie unfavorable reactions.  There are some acidic treatments, but today most work is done at elevated pH (table source).

Hot loop chemistry:

Boilers that circulate hot water through a building are typically not blown down (removing water with a high level of dissolved salts).  Once properly treated, they can remain stable for months.  These systems are typically treated with nitrite (above), which passivates the steel by reacting to form a layer of magnetite (Fe3O4).  This is a less reactive oxide of iron that, unlike rust (Fe2O3), will not contribute to loss of boiler steel.  We like to see boiler water a little black (magnetite), rather than red (rust), because it means the steel has a protective layer on it.

Steam boiler chemistry:
Steam boilers present several problems that hot water boilers do not have.

The first difference is that these boilers are constantly losing water as steam, and need more water added as a result.  The steam that condenses has a relatively low pH (7 as opposed to boiler water, which is kept from 9-11).  This makes the water rather acidic and corrosive to the steel.  Combine that with carbon dioxide, which is dissolved in this water creating carbonic acid, and you get a nasty bit of acidic water.  To combat this, amines (cyclohexylamine, above) are added to the boiler water.  Amines vaporize within boiler operating temperatures and leave with the steam.  They also condense with the water and raise the pH (pKa similar to sodium hydroxide which is strongly basic).  This reduces the corrosion in the steel pipes that return the condensed water to the boiler.

Oxygen scavengers such as sulfite (above) and erythrobate are used to react with the oxygen that is dissolved in the feed water make up to the boiler.

Phosphates (above) are added to bind the calcium in small insoluble crystals that are unlikely to cause scale on the boiler tubes.  These eventually leave with the water that is blown out of the boiler periodically.

The above classes of chemical are the most common course of treatment for boilers, though there are others.
These are important considerations for low pressure boilers.  Boiler drums found in higher pressure systems such as power plants have different chemistries and considerations.