Pre-commissional passivation-Before a boiler is put into service, it is customary to follow a "pre-commissional" chemical cleaning procedure.

Pre-commissional passivation

Before a boiler is put into service, it is customary to follow a "pre-commissional" chemical cleaning procedure. A newly constructed boiler will be contaminated by particulate debris, oils and greases, and rust. These are all removed in a sequence of steps, both chemical and mechanical. The resulting surface is chemically passivated to form a semi-conductive iron oxide film or layer of Fe2O3.
 The Fe2O3 is a poor conductor of ions, (e.g. Fe2+ , Fe3+ ) therefore protecting the steel from further corrosion.
The passivating iron III oxide is not a permanent addition to the steel. It is easily removed if water in the boiler is acidic or contains chlorides.
It is also extremely thin ( 40 -100 A). In fact, when viewing the grey colour of a passivated boiler surface, we are really seeing the true colour of the steel itself.
The mechanism of passivation is thought to be as follows:
Following chemical cleaning itself, the surface is that of the steel itself, with no other layers. It can therefore quickly rust in the presence of water and oxygen. The boiler is filled with a dilute citric acid solution, which dissolves this rust. The pH is raised to an alkaline value using ammonia, and the sequestered iron remains in solution. Dissolution of iron on the surface stops and an oxidizing agent is added. This has the effect of impressing a positive surface potential on the steel. In other words, it initiates oxidation of the surface to iron oxide by withdrawing electrons.
As the potential increases, so does the oxidation, shown by the increase in the corrosion current. When the potential reaches about 0.6V for steel, the oxidation takes place as the formation of a semi-conductive layer of iron oxide. This layer can conduct electrons but not ions. Without a flow of ions, the steel cannot corrode and therefore the corrosion current decreases to the so-called passive current.
If the potential is further increased, the corrosion current remains constant until a point when the semi-conductive layer becomes transpassive, and ionic species are conducted through it. The corrosion current will again rise and passivity is lost. For iron the value of this potential is about 1.6V.
This means that by introducing an oxidizing agent to the ammonium citrate solution, which can impose a potential of between 0.6V and 1.6V on the steel surface, passivation will occur. After allowing time for the reaction, rapid draining of the solution removes the electrolyte and the steel is left in a temporarily passive state. A good choice of oxidizing agent is sodium nitrite, although sodium bromate or hydrogen peroxide can be used.
In-service conditions
If filled quickly with correctly treated water, and put into immediate service, the clean boiler will be operating at maximum efficiency and will have a basic passive layer intact.
Assuming good maintenance of the water supply, the boiler will operate for several years without further cleaning.
During operation, the boiler is fed by de-aerated, de-mineralized water containing additives. These basically scavenge for oxygen and control the pH of the feed.
By almost eliminating dissolved oxygen, while controlling pH and not overdosing additives, the boiler is kept in an optimum condition for steam production.
The choice of additives to boilers is based on many years of research. The object is always to minimize non-mobile deposits and corrosion, both of which can lead to failure.
When boilers are fired up after cleaning and adding treatment compounds, a reaction occurs between the surface of the boiler and the water. Another form of iron oxide is formed. This is magnetite, or Fe3O4 , which is black in colour.
Its formation is a complex process and can be summed up as follows:
The temporary iron oxide film, only a few angstrom thick, will break down.
A series of reactions occur between the iron and the water which result in the following two to form magnetite:
3Fe(OH)2 ---> Fe3O4 + H2 + 2H2 O and
3Fe + 4 H2 O ---> Fe3O4 + 4H2
Some intermediate reactions also produce hydrogen ions.
These lower the pH of the water during start up of boilers and have to be adjusted for with additives under monitoring. Care must be taken to monitor boiler conditions. Overdosing to raise pH too much will accelerate magnetite production by removing hydrogen ions too quickly. This film will be less dense and weaker.
However, if the pH is allowed to drop too far, the film is pickled away. The magnetite will continually be formed at an ever decreasing rate. Its formation can be monitored by analysing for free hydrogen. After a period of between 25000 and 40000 hours use, the magnetite film will be too thick and will require removing by chemical cleaning.
It may be that during the wildly fluctuating conditions during start-up that dosage of oxygen scavengers, such as hydrazine, is too high. The excess will dissociate to form ammonia. This will react with copper in condenser components to form the soluble species Cu(NH3)42+.
Cu + 4NH3 + 1/2 O2 + H2O ---> Cu(NH3)42+ + 2OH-
This reacts on return to the boiler as follows:
Cu(NH3)42+ + Fe ---> Cu + Fe2+ + 4NH3
This is undesirable since the ammonia is recycled for further damage, while the copper corrodes the boiler. Tube scale analysis may reveal metallic copper under magnetite, with copper I oxide mixed in the magnetite in small quantities. This copper, and its oxide must be removed during cleaning, together with the magnetite.