Boiler

Steam boilers are mainly made of carbon steels, and the internal surface of these metals that are in contact with water must be free of unwanted deposits. Deposits have a lower thermal conductivity than metals, which causes the temperature of the metal to rise on the combustion wall side. Therefore, more fuel is required to produce steam. (The heat transfer coefficient decreases.)

Formation of magnetite layer in a steam boiler

Steam at high temperatures reacts with iron ions and forms a uniform layer of iron oxide Fe3O4, known as magnetite. In a reducing atmosphere at temperatures above 100 degrees Celsius, water acts as an oxidizing agent and an electrochemical reaction occurs, causing the formation of a layer of magnetite.

Advantages and disadvantages of magnetite layer

The magnetite layer formed is dark, thin, and highly adhesive, which acts as a passive layer on the steel surface to prevent further surface corrosion. The formation of this magnetite layer in steam boilers is considered an advantage, as under ideal conditions the water-side surfaces of these metals are covered with this thin film. However, over time the magnetite layer grows and becomes porous or even breaks and separates. So this deposit layer becomes extremely thick locally. In addition, it has a negative effect on heat transfer and the non-uniformity of the deposit thickness causes the adhesion of other unwanted salt deposits such as calcium carbonate.

Reduced efficiency and the decision to clean the boiler

When the boiler efficiency decreases significantly or blockages or perforations occur in the pipes, it is usually decided to clean the boiler. This is usually done by cutting several pipes from the parts where the accumulation of deposits is expected to be higher in those areas. After extensive metallurgical examinations, the pipe sample along with the relevant deposits is tested for cleaning or the so-called acid washing process.

Pickling Steps

In order to carry out the appropriate process for pickling, knowing the amount of deposits per unit area is a prerequisite for the process. The steps are as follows:

1. Degreasing
2. Pickling with a mixture of HF and HCl acids
3. Passivation using ammonium citrate and an oxidizing agent

Use of a preservative during pickling

During the pickling process, which is carried out by rotating hot hydrochloric and hydrofluoric acids, the magnetite layer reacts with the acid. Therefore, the concentration of iron in the pickling solution will increase, because it dissolves the magnetite. To protect the bare steel against the effects of acid, an acidic preservative is added to it. This acidic preservative reduces the corrosion rate to mpy50. The amount of iron in the pickling solution is actually a parameter of the pickling result and how the pickling was carried out. When the amount of iron in the solution remains constant, all the magnetite has dissolved and the pickling step is over.

Removing Iron Rust After Pickling

After the pickling step, the steel surface will be reactive and wet, and oxygen will form oxide. In fact, the last step in boiler cleaning is to remove this iron rust and create a passive iron oxide film. To remove this iron rust, a citric acid solution is first used, then the citric acid is neutralized with ammonia to form ammonium citrate with an alkaline pH. Usually, the dissolved iron will precipitate as hydroxide. However, due to the strong tendency of ammonium citrate to form complexes, the iron remains in solution.

A strong oxygenating agent is added to the ammonium citrate to activate the ferrous ions to oxidize, thus converting the metallic iron to a uniform ferric state.

This action causes the appearance of a thin, passive, and adherent layer of iron oxide Fe2O3.