In the previous articles we explained the application and the precautions to be taken when implementing an All Volatile program in boilers; we now turn to the importance of pH control.
In an internal boiler treatment program for corrosion control, any of them, the goal is to form an oxide dense enough to act as a protective barrier, isolating the water from contact with the iron. This film is normally associated with magnetite.
For this film to form, conditions for its formation must exist, and pH is one of the main factors. It influences the formation and growth of the oxide layer, the solubility of iron and the transport of corrosion products. Thus, keeping pH within the range specified for each variant of the AVT program, AVT(R) and AVT(O), allows the formation and maintenance of this film to be optimized.
In high-pressure boilers, as pH decreases, under certain conditions of temperature and electrochemical potential, there is an increased tendency for iron to dissolve. However, it is important to ask what the stability of the oxide layer is and what the solubility of the iron species is under the specific temperature, pressure, pH and redox potential conditions inside the boiler.
At higher pH values, excess alkalinity can increase the risk of caustic/alkaline corrosion, even though the solubility of some iron oxides may remain low. In addition, in systems containing copper metallurgy, the presence of ammonia can increase the transport capacity of dissolved copper.
The iron solubility × pH curve for a given temperature and redox condition will frequently show an approximately "U"-shaped behavior, that is, there is a range of minimum solubility, and this is the region of interest.
There is also the effect on FAC, Flow-Accelerated Corrosion. Under certain conditions, increasing pH can decrease FAC by reducing the solubility of magnetite. Under reducing conditions, AVT(R), magnetite is the main protective layer of iron. Under conditions of ORP close to zero, AVT(O), the presence of oxygen can provide a more protective oxide layer.
The important thing is to work under the optimal conditions for each type of system. Those containing copper should operate at a lower pH (8.8–9.2) to avoid attack on the copper, while at the same time minimizing the potential for FAC, whereas those containing only iron can work at a higher pH (EPRI 2013 for combined cycle: 9.6-10.0) and more oxidizing conditions.
For iron, increasing pH within the appropriate range generally helps to decrease oxide solubility, stabilize the magnetite layer, reduce iron transport and reduce FAC.
In the case of copper, the situation is more complex because there are different copper oxides, ammonia can complex copper, and redox potential has a strong influence. Thus, a chemical condition that is excellent for steel is not necessarily the optimal condition for copper.
Ammonia is the standard alkalizing agent of AVT programs because of its low cost, because it does not raise the cation conductivity of the water, and because it is easy to dose. Changes in cation conductivity could be mistaken for contamination.
The point to watch is its very high distribution ratio (the ratio between the concentration in the steam and the concentration in the liquid phase). This means that in a two-phase system, ammonia migrates preferentially to the steam and leaves the condensate in the initial sections with a lower pH than expected, favoring corrosion and FAC.
To manage this problem, neutralizing amines are used, where each amine has its own distribution ratio, which allows the best arrangement to be chosen. For example, morpholine has a low distribution ratio, which protects the initial sections. In practice, commercial formulations use a blend, combining a low-volatility amine to protect the beginning of the system with a high-volatility one to protect remote stretches of piping.
Another important characteristic is basicity. In boiler chemical treatment, this property is important because the amine reacts with water and with acidic species, helping to raise and maintain pH in an appropriate range, reducing corrosion. The higher the basicity, the greater the tendency of the amine to raise pH.
At high temperature, some neutralizing amines decompose into organic acids, and the effect is a decrease in the pH of the liquid phase. This makes amine selection also a matter of thermal stability analysis, in addition to volatility.
Ammonia does not change cation conductivity, which preserves this indicator as a clean reference for contamination. Neutralizing amines tend to raise cation conductivity, forcing the monitoring program to distinguish real contamination from the expected presence of amine. Replacing ammonia with an amine without adjusting the alarm criteria of continuous monitoring is a risk.
The decision between ammonia and a neutralizing amine, or a blend, depends on the design of the system. Systems with short steam and condensate lines, unlike long systems, may dispense with the amine blend. The use of amines requires periodic reassessment of the thermal stability of the chosen product and adjustment of the cation conductivity limits in analytical monitoring.
In the next article we will discuss OT - Oxygenated Treatment.
Has your plant assessed the pH control of its steam generation system? Talk to us for a technical assessment.
References
- ELECTRIC POWER RESEARCH INSTITUTE. Cycle Chemistry Guidelines for Fossil Plants: All-Volatile Treatment. Palo Alto, CA: EPRI, 1996. TR-105041.
- ELECTRIC POWER RESEARCH INSTITUTE. Cycle Chemistry Guidelines for Fossil Plants: All-Volatile Treatment, Revision 1. Palo Alto, CA: EPRI, 2002. 1004187.
- ELECTRIC POWER RESEARCH INSTITUTE. Comprehensive Cycle Chemistry Guidelines for Combined Cycle/Heat Recovery Steam Generators (HRSGs). Palo Alto, CA: EPRI, 2013. 3002001381.
- ScienceDirect Topics. All-Volatile Treatment - an overview. Available at: sciencedirect.com.
- Water Tech Online. Experiences Using Neutralizing Amines to Control pH, Minimize FAC in a Combined-Cycle Power Plant. Available at: watertechonline.com.