Neutralizing amines, filming products show promise for protecting critical equipment against corrosion

Amines—both neutralizing and filming, as well as proprietary filming products—have the potential to reduce corrosion in heat-recovery steam generators (HRSGs), condensers, and steam turbines at combined-cycle plants, and provide offline protection, EPRI’s Mike Caravaggio and Steve Shulder told the editors at the first HRSG Forum with Bob Anderson. Caravaggio is the research organization’s senior program manager for major component reliability, Shulder the program manager for boiler and turbine steam and cycle chemistry.

Until recently, little independent information was available on these chemicals for powerplant applications. To understand how amines and filming products work (sidebar) and to effectively apply them, the power industry needs independent research and proven treatment guidelines, say Caravaggio and Shulder.

Neutralizing, organic-based amines have advantages over ammonia treatment for corrosion control in two-phase environments because of their better dissociation and distribution properties, resulting in a higher at-temperature liquid-phase pH. These advantages have been demonstrated in the nuclear industry for improving corrosion control in reheat moisture separators.

Similar improvements have been noted, in some cases, in corrosion control of water- and air-cooled condensers in fossil applications. Nonetheless, while neutralizing amines may have benefits in these environments, they can also have a negative impact if the product breaks down too quickly.

Filming (or film-forming) amines and products also have advantages in corrosion control over traditional treatments. Through the formation of hydrophobic films on internal steam/water surfaces, they can disrupt corrosion cells. However, questions remain regarding their powerplant applications.

For example, while filming has been demonstrated in field applications on boiler waterwalls and in superheat and reheat tubing, it has not been demonstrated for steam turbines, although a side-stream low-pressure (LP) phase transition zone test loop has indicated significant protection when subjected to contaminant addition.

Also, although filming amines and products have been shown to form hydrophobic films that are anticipated to inhibit corrosion, the level of this corrosion inhibition has not been quantified for known offline and online tube corrosion mechanisms in conventional boilers and HRSGs.

Moreover, the minimum concentration of filming amines or products and the application duration for various filming agents to achieve a protective film under various powerplant conditions are not well established. Variables such as makeup rate, unit configuration, unit cleanliness, and feedwater component metallurgy have all been demonstrated to have a significant effect.

Conventional powerplant chemistry monitoring techniques are incapable of evaluating whether film formation has occurred, significantly complicating continuous control and optimization of filming-amine- or filming-product-based treatments. Additionally, variations in filming-amine and filming-product formulations mean no single analytical technique is available for evaluating the presence of the chemicals in water and steam samples.

In the future, the requirements for corrosion protection in power cycles are expected to increase as plants cope with the demands of increasing cycling operation, low-load operation, and extended layup protection. Exclusive use of ammonia might be inadequate to meet these future challenges. Judicious investigation is needed of alternative treatments—such as amines and filming products.

In recent years, Caravaggio and Shulder said, the Electric Power Research Institute has tried to fill the gaps in information on amines and filming products in powerplant applications by conducting an ongoing series of research studies. They are aimed at building a body of knowledge on a broad range of aspects of these chemicals and compiling that information into comprehensive treatment guidelines.

The sections below offer some background information and highlights of that research:

State of knowledge on filming amines

This 2015 report compiles current EPRI and publicly available information on film-forming amines in conventional fossil and combined-cycle plant steam- and water-cycle applications—specifically:

      • Establishes the state of knowledge with respect to filming amines for steam- and water-cycle treatments in powerplants.

      • Contains information on filming products in addition to amines. Focus is on filming amines because more independent research has been done on them.

      • Provides detailed information on the chemistry of filming amines, (1) their mechanism of action, (2) volatility in steam and water cycles (distribution), (3) thermal stability in steam and water cycles (decomposition), (4) dissociation and impact on pH and conductivity, (5) requirements for chemical addition, (6) beneficial corrosion prevention/reduction effects throughout the cycle (by component/damage mechanism), and (7) negative effects throughout the cycle (impact on cation conductivity, ion-exchange resin, and instrumentation).

      • Recommends where research and application knowledge are needed to facilitate the adoption of filming amines as a power-cycle treatment.

How amines and filming products work

Amines are a large class of organic compounds. Those that may be applied for chemistry- and corrosion-control purposes come under two broad categories: neutralizing and filming. In addition, some filming products are available that are not necessarily based on amines.

Neutralizing amines (such as dimethylamine and ethanolamine) behave in much the same way as ammonia, reducing the solubility of iron oxides by increasing the pH of the condensate, feedwater, or evaporator/drum water. Neutralizing amines have been commonly used as alkalizing agents for feedwater treatment since the 1950s. Their role in the steam/water cycle is understood from the standpoint of their physical and chemical properties (dissociation, distribution, and decomposition). Dissociation and distribution are equilibrium related; decomposition is permanent.

The properties of some neutralizing amines have the potential to improve the pH conditions in HRSG LP and HP evaporators and economizers, the phase transition zone (PTZ) of the LP steam turbine, the condensing steam in wet- and air-cooled condensers, and the pH conditions at any other two-phase flow locations.

The potential to improve pH in these environments arises from lower volatility to improve distribution of the amine to the liquid phase in low-pressure applications and higher basicity caused by higher dissociation of the amine, resulting in higher pH at operating temperatures compared to ammonia.

A potential limitation of neutralizing amines is that they may break down into organic acids at high temperature in fossil-unit steam and actually give rise to a lower pH than with ammonia alone.

Filming amines and products provide corrosion protection by forming a physicochemical barrier between the metallic surfaces and the working fluid (water) to prevent corrosion from occurring. Filming amines and products can also provide a film on the steam-touched surfaces and offer protection against oxygen pitting when units are offline and exposed to humidified air or water formed via condensation.

The term “filming amines” denotes not just one chemical but a family of chemicals that is relatively broad and diverse. A filming amine can have a primary, a secondary, or a tertiary amine structure or a combination of these functionalities attached to any number of hydrocarbons greater than 10.

In many cases, the vendors of modern filming-amine formulations do not provide a detailed definition of the specific chemical structures for this class of materials. Filming amines generally are formulated with one or more neutralizing amines to maintain their stability. Such combination amines normally increase pH in addition to establishing a protective film.

Some filming products currently on the market may not be based on an amine chemistry. “Filming products” typically do not include a neutralizing amine in the product and just consist of a proprietary filming chemical so they do not influence pH.

Filming amines and products can be used in continuous feeding mode to reduce iron corrosion, or short-term for preservation of idle equipment during layup. Continuous feed and layup protection are the primary applications in fossil plants.  

Filming amines and products have to be applied with sufficient time and concentration to provide a protective film on all metal surfaces. The larger the unit or surface area the longer it will take to provide protection. Because this is an equilibrium reaction, a sufficient “active” residual must be maintained throughout the steam/water cycle.

Currently, there are no independent methods available to measure the active residual of commercially available products. Overfeeding of filming amines should be avoided to minimize the potential for fouling online instrumentation and the formation of “gunk balls” (gelatinous spheroids). Careful attention also should be paid during the initial application for the possible release of anions trapped within deposits, often referred to as “cycle clearance.”

Monitoring of amines, filming products

Ongoing EPRI field studies are investigating the monitoring and control requirements to achieve corrosion control when applying commercially available neutralizing amines, filming amines, and filming products on a continuous basis in conventional fossil and combined-cycle plant steam/water cycles.

Corrosion control in the PTZ

For turbines, offline periods can result in pitting corrosion, which might lead to corrosion-fatigue and stress-corrosion-cracking failures during operation. With increased power generation from renewable sources and more stringent environmental legislation, both conventional and combined-cycle plants are running in a more flexible mode, with more frequent cyclic/low-load operation, seasonal operation, and shutdowns.

The chemistry of the steam prior to a unit shutdown can have a significant impact on the formation of pits and the pitting rate. Adding a filming amine or product could be effective in mitigating or preventing these damage mechanisms.

An ongoing EPRI project aims to determine whether filming amines and products can be used to significantly reduce the risk of corrosion and damage in the phase transition zone (PTZ) of LP turbines during and after a steam-chemistry excursion with elevated levels of chloride and sulfate.

The resulting data are being used to quantify the risk or benefit compared to conventional steam chemistry from a previous study where neutralizing amines were used and ammonia was added for feedwater pH control.

To date, the project has established the ability of the four commercially available filming amines/products to substantially reduce corrosion when subjected to non-optimal steam chemistry conditions and elevated anionic constituent levels compared to the use of ammonia for pH control. Additional testing is being performed to determine if filming products may reduce corrosion on previously pitted specimens. Current testing with the commercially available filming amines/products also has indicated a substantial reduction in corrosion.

Quantifying corrosion improvement

The power industry has had difficulty quantifying the corrosion improvement of filming amines and products by materials of construction, because of the confluence of uncontrollable variables present in field applications. As a result, plant operators need tools to assist in quantifying—across the range of commonly used materials—the corrosion inhibition of different filming amines and products under powerplant conditions, and in quantifying the impact of water chemistry on the ability of filming amines and products to inhibit corrosion.

For powerplant feedwater, an EPRI study plans to quantify the effectiveness of filming amines and products in reducing the general corrosion rate of carbon steel and admiralty brass.

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