WATER TREATMENT: Protect equipment against corrosion with neutralizing amines, filming products

By Mike Caravaggio and Steve Shulder, Electric Power Research Institute

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 in combined-cycle plants, and provide offline protection.

However, until recently, little independent information has been available on these chemicals for powerplant applications. To understand how amines and filming products work and to effectively apply them, the power industry needs independent research and proven treatment guidelines for proper application.

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 (Fig 1) 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, the Electric Power Research Institute (EPRI) 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.

This article provides some background information and highlights of that research.

Amines and filming products

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.

Unfortunately, there are currently no independent methods 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.”

State of knowledge on filming amines

In 2015, EPRI’s “State of Knowledge on Film-Forming Amines” (Product ID: 3002003678) compiled current EPRI and publicly available information on film-forming amines in conventional fossil and combined-cycle plant steam- and water-cycle applications. This report does the following:

  • 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.

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.

The studies aim to determine what monitoring is required to achieve optimal corrosion results in plants applying amines or filming products. For plants using neutralizing or filming amines or filming products, pH control based on measurements at 25C (77F) may be insufficient to determine the optimal corrosion-control conditions. The studies are attempting to assess this situation and outline a path toward corrosion optimization for plants applying these chemicals either in isolation or in combination.

For neutralizing amines, field studies on corrosion optimization are being conducted at several combined cycles. Work includes sample collection from available monitoring locations in the steam/water cycle. Analysis of grab samples includes cations (sodium, ammonia, and neutralizing amines), anions (chloride, sulfate, acetate, and formate), total organic carbon, and suspended iron.

The participating powerplants are applying treatments that include neutralizing amines. The application of these treatments is being optimized through field-work analyses. Optimized corrosion product transport results for iron have been less than 1 ppb in the condensate/feedwater and evaporator circuits.

Results to date indicate that, for neutralizing-amine treatments, the conventional corrosion-control parameter of pH measured at 25C does not correlate well with iron corrosion product transport. Analysis of the calculated at-temperature pH using EPRI’s MULTEQ (MULTiple EQuilibrium) program shows better correlation and is being used in corrosion-control optimization activities for the neutralizing amines. EPRI also has identified a control process using conventional online chemistry monitoring.

The study and its latest findings are described in the report “Neutralizing Amine Application for Continuous Service in Fossil Plants and Heat Recovery Steam Generators” (3002006356).

An ongoing project complementing this work has designed and installed a pH monitoring system that splits a fully condensed sample into a two-phase mixture for controlling a neutralizing amine treatment.

The project is evaluating whether the split-sample pH can be used to determine if the combined impact of a neutralizing amine treatment and its breakdown products are improving or having a detrimental impact on the at-temperature two-phase pH under operating conditions and how this relates to iron corrosion-product release rates.

For filming amines and products, EPRI research is seeking to develop electrochemical techniques used for evaluating corrosion inhibition on a continuous online basis. In the future, it is anticipated that a prototype field probe will be available for installation at a powerplant applying a filming amine or product treatment, and that a field trial will allow for evaluation of the probe as an online continuous monitor of film integrity that can be used for treatment optimization.

Ultimately, this work will allow for independent monitoring of film formation of different filming amines/products and provide a tool for owner/operators to begin optimizing these treatments.

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 steam turbine-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.

The project and its findings are described in the report “Control of Corrosion in the Steam Turbine Phase Transition Zone (PTZ) Using Filming Amines” (3002006103).

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. First phase of the study was a proof-of-concept trial in a laboratory setting to evaluate the feasibility of using an in-situ electrochemical corrosion probe to measure corrosion rates in simulated feedwater. The research will examine the effect at various temperatures typically experienced in the feedwater systems for a variety of test solutions that would be expected in plants applying a filming amine or product treatment.

Test coupons of carbon steel and various copper alloys will be immersed in flowing solutions with a fixed residual concentration of the filming amine or product and fixed chemistry parameters (hydrazine, oxygen, and pH levels, for example) at elevated temperatures. Corrosion rates will be determined under these defined conditions. Success in the laboratory may lead to implementation of a corrosion probe for these systems at power stations.

The research and its findings are described in “Effect of Filming Amines in Steel and Copper Alloy Corrosion” (3002006101).

For powerplant boilers, EPRI studies will take a dual approach to assess the corrosion protection provided by filming amines and products in field applications. The first approach has been to develop laboratory tests for evaluating corrosion protection afforded by films on boiler (waterwall and superheater/reheater) tubing under simulated offline conditions and online conditions. These tests are being used to evaluate tube samples collected from plants applying filming amines and products for the level of corrosion protection achieved.

An additional laboratory test is being constructed to evaluate the effect of filming amines and products on corrosion fatigue. The second approach: Continue to collect case-study information with regard to the rate of corrosion-fatigue and stress-corrosion-cracking failures in fossil plants susceptible to those problems—both before and after the application of filming amines or products.

Case study

Limited documented cases exist about the effectiveness of filming amines for corrosion control. A recent EPRI report, “Filming and Neutralizing Amine Application for Continuous Service in Fossil Plants and Heat Recovery Steam Generators” (3002007937), provides a case study documenting the experience of a 3 × 1 combined-cycle plant that implemented use of filming and neutralizing amines for improved corrosion control in its steam and water cycles.

The plant featured in this case study is PSEG’s Bethlehem Energy Center, in Bethlehem, NY. It is equipped with three 160-MW gas turbines and a 300-MW steam turbine. All three HRSGs are triple pressure with feed-forward LP drums.

Initially the plant operated on an all-volatile treatment with ammonia in the condensate/feedwater as well as in the three evaporator sections of the HRSGs. However, the plant was incapable of optimizing chemistry control with ammonia alone. In 2012, it began applying an ammonia and neutralizing amine blend (Fig 2). In 2015, Bethlehem continued to refine the chemistry treatment with the addition of filming amine.

The case study documents both chemistry and iron corrosion-product analysis results. It analyzes these data versus the known iron corrosion mechanisms in a combined-cycle steam and water cycle. These numbers are both less than those for the ammonia/neutralizing amine blend and the ammonia/neutralizing amine/filming amine blend.

Additional ongoing research

To further the understanding of filming amines/products, EPRI is conducting research to determine the influence of the products on deep-bed condensate polishers and online instrumentation used to monitor cycle chemistry. A recently released report, “Filming Product/Amine Impact on Condensate Deep-Bed Resin Polishers” (3002008140) reviews the influence of these products on polisher resins.

Laboratory testing is being conducted to assess the ability of filming amines/products to minimize single- and two-phase flow-accelerated corrosion and to mitigate corrosion damage in ACCs. Testing also is being conducted to evaluate the ability of ion chromatography to be provide an independent method to measure the active residual of these products in the steam/ water cycle. CCJ

Mike Caravaggio, EPRI’s senior program manager for major component reliability, can be reached at 704-595-2589 and mcaravaggio@epri.com; Steve Shulder, program manager for boiler and turbine steam and cycle chemistry, at 704-595-2953 and sshulder@epri.com.