Corrosion prevention in steam generators is of critical importance.
Corrosion-induced events can lead to equipment failure, unit shutdowns, and in some cases injury or death. The most notable mechanism at present in this regard is flow-accelerated corrosion (FAC), which afflicts many units around the world. Corrosion issues are being exacerbated by the change from base-loaded to cycling operation for most steam generators, particularly in North America.
For decades, chemistry control in steam generators, and particularly the condensate/feedwater system of these units, has been based on feed of ammonia or a neutralizing amine for pH control. Operation at moderately basic pH (typical range of 9 to 10) helps to minimize general corrosion of carbon steel. A reducing agent, aka oxygen scavenger, was also once a common feature of treatment programs, but this chemistry is no longer recommended unless the feedwater system contains copper alloys, which is almost unknown for heat recovery steam generators (HRSGs). Even so, both single-phase and two-phase FAC continue to be problematic in many steam generators. Furthermore, the high cycling frequency nowadays of most units increases the potential for other corrosion mechanisms, including fatigue, corrosion fatigue, stress corrosion cracking (SCC), and oxygen pitting during shutdowns. A holy grail of boiler water treatment is chemistry that can protect all water- and steam-touched surfaces from corrosion during all conditions. Gaining in popularity is the use of film-forming substances (FFS) for universal corrosion protection. However, application of this technology is not without concerns, as we will explore.
Film Forming Substances
The concept behind film-forming substances, as the name implies, is to protect metal surfaces by establishing a thin, uniform barrier on the metal. The point of attachment can be different depending upon the compound employed and the metallurgy of the surface to be treated. This article focuses in large measure upon carbon steel, which is the common metal for feedwater systems, boiler tubes, and other steam generator components. When carbon steel is first exposed to water in a steam generator, the surface forms a layer of grayish-black magnetite (Fe3O4) via a several-step process. For those units that are on all-volatile treatment oxidizing (refer to “The Integral Benefits of Iron Monitoring for Steam Generation Chemistry Control, Power Engineering, January 2019 for a brief explanation of AVT(O))—which should be virtually all heat recovery steam generators (HRSGs)—the magnetite may become overlaid and interspersed with a coating of red hematite. Regardless, the structure of the oxide layer is represented in the following diagram.
Figure 1. General schematic of the protective oxide layer that forms on carbon steel surfaces.
As illustrated, an inner, dense topotactic layer of oxide forms, which is overlaid by a looser, epitactic layer composed of iron oxide deposition products. Ions can move in and out of these layers, and in fact it is movement of ferrous ions (Fe2+) out of the metal substrate at flow disturbances that leads to FAC. Temperature, pH, and feedwater chemistry also influence FAC. (The previously referenced Power Engineering article, “The Integral Benefits of Iron Monitoring for Steam Generation Chemistry Control,” outlines these additional factors.) Much valuable information regarding FAC can be found on the websites of the Electric Power Research Institute and the International Association for the Properties of Water and Steam.
With this information as a background, we will consider the basic chemistry of some FFS molecules, how they interact with either the oxide or base metal of carbon steel, and why application criteria varies between compounds.
It should be noted that the exact composition of the various FFS molecules are closely guarded secrets, and thus the following discussion outlines general principles.
Film-Forming Amines
A particular type of FFS is the film-forming amines (FFAs), with one such structure shown below.
Figure 2. Attachment of an FFA to the iron oxide surface of carbon steel.
As the diagram illustrates, the active end of the molecule containing the amine group (NH2) attaches to an iron atom on the substrate of iron oxide. The non-polar carbon chain of each molecule extends into the water and provides the barrier. Figure 3 shows a protected metal surface (during off-line conditions), where the barrier prevents penetration of water.
Figure 3. A metal surface protected by an FFA. Note how the water does not spread out and penetrate the surface, but beads.
Decades ago, filming amine chemistry was attempted in steam generators, with a common compound being octadecylamine (C18H39N). However poor control and lack of detailed knowledge often led to problems, including formation of “gelatinous spheroids,” or “gunk balls” in the common vernacular, which fouled steam generators. Now, specialized compounds with modified chemistry are proving to be more effective corrosion inhibitors. For example, the compound shown in Figure 2 has a second amine group as part of the organic chain.
Research with advanced spectroscopy and other methods indicates that some of the FFAs, when applied to a steam generator, will remove about 50 percent of the epitactic iron oxide from the surface. Thus, during initial application a significant amount of iron may appear in the boiler water. This iron needs to be blown down. Once the chemistry stabilizes, iron concentrations will decrease to a low parts-per-billion (ppb) level.
Recent full-scale applications of an FFA developed by author Stuart and colleagues indicate that a feed concentration at or near 40 ppb is sufficient to obtain a 100 ppb concentration in the low-pressure circuit of the most common type of HRSG, the feed-forward, triple-pressure configuration. Research indicates that the amine forms a tight bond with the oxide layer, thus allowing a relatively low dosage rate. Continued applications and testing will help to refine these values.
Alternatives to FFAs
Other compounds have been developed, which rely on alternatives to amine as the functional group(s) on the molecules. The general arrangement of one such alternative is shown below.
Figure 4. Ethylenebis Stearamide (EBS)
These non-amine film-forming products (NFP) rely on the amide functional group (HN-C=O). Rather than attaching to the oxide layer, the molecules penetrate the oxide and form a relatively weak bond with the base metal. The compounds are typically fed at 1 ppm (0.2% active), and require about one month for the actives to concentrate to 200 ppb, where the amide molecules become effective. If the concentration drops, the film elutes from the surface. Thus, they require a certain dosage to maintain a continuous, protective barrier.
Benefits and Potential Drawbacks to FFS
The obvious benefit of film-forming substances is that if the product forms a continuous barrier throughout the steam generator circuit, corrosion will be minimized. And, if this barrier remains intact during unit shutdowns, it will protect the metal from off-line corrosion, particularly from air ingress that allows oxygen to concentrate at metal surfaces in a wet environment. However, even though the number of FFS applications continues to increase, intense investigation continues into a number of issues, including:
· Volatility – Do the compounds exhibit sufficient volatility to carry over with steam and protect steam-side components? Also, how well might they protect air-cooled condensers (ACC)? ACCs have a huge volume of carbon steel that typically releases many iron oxide particulates to the condensate. FFS may be of significant help in controlling ACC corrosion.
· Thermal Stability – Are the compounds thermally stable at high temperatures, or will they decompose into smaller organic compounds that are ineffective for corrosion protection, and that may influence chemistry? This has been an issue with neutralizing amines, which decompose to small-chain organic acids and carbon dioxide in high-temperature steam systems. Apart from other factors, such decomposition artificially elevates the cation conductivity of the water, which may be problematic for turbine warranty issues, where turbine manufacturers judge steam purity per cation conductivity measurements.
· Dosage Levels – How should the system be dosed, both at startup and then when a complete barrier has been established?
· Blended or Unblended Feed – Even though a FFS may provide excellent protection in the steam generator, it is still important to continue feed of ammonia or a neutralizing amine for pH control in the system. However, is it best to feed the FFS and neutralizing chemical in a blend or separately?
· Monitoring – These long-chain organic compounds are usually difficult to monitor directly. Surrogate analyses may be required to keep track of residual concentrations in the various circuits. Intense research continues with regard to FFS monitoring techniques, to better perfect the chemistry of these applications. Of course, with any program iron monitoring is still a necessary and valuable tool to determine if the system is properly passivated.
· Fouling – Will the product coat some system components that require clean surfaces, such as condensate polisher ion exchange resin and instruments either with probes immersed in fluid streams, or that monitor samples extracted from the process?
As this list indicates, several questions have not yet been completely resolved. Physically verifying that a passive layer has been established on all surfaces is an impossible task. A key measurement, as it is now for current treatment practices, is iron monitoring. For years, EPRI guidelines have suggested that with proper control of ammonia or neutralizing amine condensate feedwater chemistry, it is possible to maintain total iron concentrations in the feedwater below 2 ppb. This should certainly be the case for FFS-treated systems. One very important parameter regards iron concentration at unit startup. Usually iron levels are much higher at startup than during normal operation. If the FFS is performing properly, little excess iron should be observed.
Concern, and rightly so, has arisen regarding the effects of film-forming products on system components such as instrument probes and, for those units that have condensate polishers, the polisher resin. Results so far indicate that instrument fouling is largely influenced by the product. Some compounds do not cause significant fouling, others might. In regard to condensate polishers, very few drum units have this equipment, which is unfortunate because a polisher is a wise, upfront investment for steam generator protection. Information so far seems to be mixed on the fouling effects of FFS towards polisher resins.
Another item of discussion is whether an FFS should be fed separately or can be blended with the ammonia or neutralizing amine feed. Chemical manufacturers often like to supply blended products, which lowers the complexity and expense of chemical feed equipment, but blended products limit flexibility of system control. For a bit of extra money, the increased flexibility can be well worth the investment.
Conclusion
Film-forming substances may prove to be a game-changer in the high-pressure steam generation industry, power and otherwise, both for on-line and off-line corrosion protection. A number of successful applications have been reported, but as the above discussion indicates, not all the pieces of the puzzle have been totally fitted together. Intense research of this chemistry continues. For those interested, The Third International Conference on Film Forming Substances is coming up from March 19-21 in Heidelberg, Germany.