Water Treatment

HRSG issues: Reemphasizing the importance of FAC corrosion control – Part 4

This is Brad Buecker’s final installment of a four-part series on flow-accelerated corrosion control.
HRSG issues: Reemphasizing the importance of FAC corrosion control – Part 4

This is the final installment of a four-part series by Brad Buecker, Buecker & Associates. Catch up with parts 1, 2 and 3 here.

An issue that became readily apparent when researchers began investigating flow-accelerated corrosion (FAC) is that a small amount of chromium in carbon steel will greatly inhibit the corrosion mechanism. The common 1¼ (percent) and 2¼ chrome-alloy carbon steels are highly resistant to FAC. Fabrication of economizer and HRSG evaporator elbows and other FAC-susceptible components with these alloys is a straightforward solution. 

However, the chrome-steel alloys are more expensive than plain carbon steel, and thus this option is often not considered during project design. Also, the frequent load swings that most power units now experience expose more locations to FAC than exist in base-loaded units. Materials cost would be greater if this factor was included in equipment design.

The last 15 years or so have seen a gradual re-emergence of film-forming chemistry to protect steam generator metal surfaces from corrosion, including FAC. The concept goes back decades, but results then and even now have been mixed. 

This section briefly highlights some of the most important pros and cons for adopting film-forming chemistry in steam generators, with the understanding that many details are omitted due to space constraints. Readers are encouraged to download the Reference 1 document for additional information on film-forming amines.

Figure 1.  Topotactic and epitactic oxide layers on carbon steel. (2)

Film-forming chemistry basics

As the name implies, the key function of film-forming products (FFP) is to protect metal surfaces. The general chemical structure of these products is a rather long organic chain with a hydrophilic and hydrophobic end. Most common are amine-based compounds, which we will refer to as AFPs. But also available are non-amine compounds, or NFPs. 

To obtain a better understanding of important FFP action mechanisms, consider the common oxide layers that form on steam generator carbon steel.

Topotactic is the tight layer that initially forms from oxygen reaction with the base metal. The epitactic layer consists of loose iron oxides, usually produced elsewhere in the steam generating network, which precipitate on metal surfaces.

Table 1 compares some of the most important properties of AFPs and NFPs.

Figure 2 outlines the general chemical structure of one AFP.

Figure 2.  AFP bonding with iron oxide. This compound is a di-amine. Other configurations are possible. Illustration courtesy of ChemTreat, Inc.

As the figure depicts, the compound bonds with the topotactic layer and the long, hydrophobic molecular chains extend outward from the oxide surface, providing a shield from the fluid.

Figure 3 outlines the general structure of an NFP.

Figure 3.  Basic structure of ethylenebis stearamide.

The active groups in this compound are amides, HN-C=O.

An issue that arises with FFP use is the inclusion of other chemical products in a treatment program. The FFP formulation may contain a surfactant to enhance bonding with the metal surface. Beyond that is the inclusion of ammonia or an alkalizing amine to maintain the moderately basic pH that is recommended for the AVT programs outlined in the previous installments to this series. 

In this author’s opinion, such pH control remains important, because if the filming product did not provide a uniform layer throughout the feedwater network, the ammonia alkalinity would help protect exposed metal surfaces. A question that arises is whether the alkalinity booster should be included in the FFP formulation or be fed separately. The latter arrangement allows more chemical feed flexibility.

With respect to boiler water chemistry, many conventional drum units and the intermediate-pressure and high-pressure circuits of many HRSGs still rely on tri-sodium phosphate or perhaps caustic treatment. The literature suggests that these programs can be continued with FFP treatment. From my perspective, and having directly observed serious boiler tube corrosion caused by impurity in-leakage from steam surface condenser tube failures, I would feel more comfortable continuing the phosphate chemistry. Others might have a different opinion.

FFP positives and negatives

For any well-formulated FFP, “ideally, the product establishes a complete hydrophobic barrier on metal surfaces to provide protection during operation and down times.” (2)

Figure 4.  Water beads on a metal surface protected with a FFP. (2)

However, not all FFP applications have been successful, and there are still a number of “gray areas” with regard to feed control, accurate chemistry monitoring, and side effects on chemistry and equipment. These include:

  • Some vendors closely guard the chemical formula(s) of their product, which makes it difficult to determine the precise chemistry mechanisms within the steam generator.
  • Direct measurement of FFP concentration is often problematic. This makes feed control and monitoring difficult. Researchers continue to improve monitoring methods.
  • Common is to set an initial dosage per manufacturer’s guidelines and then make adjustments as the film becomes established. Overfeed can result in the formation of “gunk balls” that may cause fouling. Also, feed rates may need adjustment per unit load swings, which in turn may require a somewhat sophisticated control system.
  • By their very nature, FFPs may also form a layer on other equipment, including instrument probes and condensate polisher resin for units so equipped. 
  • FFPs that carry over into superheaters and reheaters, and particularly additions to the formulations such as surfactants and/or alkalizing amines, will break down to small-chain organic acids that lower condensate pH and increase condensate/feedwater cation conductivity.  Chemistry and corrosion control problems may result.

Corrosion product monitoring is one method to evaluate the efficacy of a program. After a program has been well established, iron and other metal concentrations in the feedwater and boiler water should be at a low parts-per-billion level. However, during program initiation, AFPs in particular will fracture much of the epitactic iron oxide, which then enters the boiler water. This must be understood at the onset of a program. 

If metals concentrations do not stabilize after a sufficient treatment period, investigation is necessary. Visual inspections of boiler and feedwater systems, when possible, can be valuable. Important questions during these inspections include, “Do internal components exhibit hydrophobicity when water droplets are applied? Is more or less sludge visible in boiler drums and headers than in the past?” (2)

FFPs for air-cooled condensers

Air-cooled condensers (ACCs) are becoming increasingly common for combined cycle power plants. Text from Reference 1 sums up the potential advantages of film-forming amine products (FFAP) to protect these units from corrosion.

Unlike water-cooled condensers made of corrosion-resistant alloys like red or yellow metal, stainless steels, or titanium grades, ACC tubes are normally fabricated in carbon steel. They add a vast steel surface area to a water/steam cycle, typically of the order of thousands of square meters, and hence constitute a major potential source of total iron corrosion products. Conditions both in the [steam] transport lines [to the ACC] and at the tube entries of the ACC make them susceptible to FAC, with the tube entries in the upper ducting operating under the most severe two-phase FAC conditions.     

The text goes on to say, “In this case, the dosage of the FFAP into the steam line from the turbine to the condenser can be considered in order to directly provide the FFA to the huge iron surfaces of the ACC.” (1) Regardless of the possible success of a program, typical guidelines recommend a condensate iron filter to remove the usually large amount of iron oxides that could otherwise precipitate in boiler tubes and cause overheating and under-deposit corrosion difficulties.

This final part to this FAC series outlines additional methods for potentially mitigating FAC (and other corrosion) in steam generators. The text only provides a brief overview of these technologies. 

For anyone considering FFP chemistry, due diligence is of key importance. Any test or full-scale application of any product(s) should be done in full consort with a reputable vendor, and which includes the development and use of detailed test, monitoring, and inspection protocols. Not to be overlooked is regulatory approval of any products that might be discharged from the plant to the environment.

References

  1. International Association for the Properties of Water and Steam, IAPWS TGD 8-16, Technical Guidance Document: Application of Film Forming Amines in Fossil and Combined Cycle Plants.
  2. Buecker, B., and Shulder, S., “Combined Cycle and Co-Generation Water/Steam Chemistry Control”; pre-workshop seminar for the 40th Annual Electric Utility Chemistry Workshop, June 6-8, 2022, Champaign, Illinois.

About the Author: Brad Buecker is president of Buecker & Associates, LLC, consulting and technical writing/marketing.  Most recently he served as Senior Technical Publicist with ChemTreat, Inc.  He has over four decades of experience in or supporting the power and industrial water treatment industries, much of it in steam generation chemistry, water treatment, air quality control, and results engineering positions with City Water, Light & Power (Springfield, Illinois) and Kansas City Power & Light Company’s (now Evergy) La Cygne, Kansas station.  Buecker has a B.S. in chemistry from Iowa State University with additional course work in fluid mechanics, energy and materials balances, and advanced inorganic chemistry.  He has authored or co-authored over 250 articles for various technical trade magazines, and has written three books on power plant chemistry and air pollution control.  He may be reached at beakertoo@aol.com.