Trace metal analyses for corrosion monitoring in cogeneration condensate systems

A revisit of several steam generator feedwater iron analyses and why copper monitoring is needed at cogeneration facilities.

By Brad Buecker – Buecker & Associates, LLC

In previous Power Engineering articles, we examined the importance of trace iron monitoring to determine the extent of carbon steel corrosion in heat recovery steam generator (HRSG) condensate and feedwater circuits. (1, 2) HRSG feedwater systems typically contain no copper alloys, except perhaps rarely a condenser with copper alloy tubes. However, cogeneration and large industrial steam systems may have numerous heat exchangers containing copper alloy tubes.

Accordingly, both iron and copper monitoring of condensate are important for evaluating the efficacy of chemical treatment programs in minimizing corrosion and the secondary effect of corrosion product transport to steam generators. In this article, we will briefly revisit several important aspects of steam generator condensate/feedwater iron analyses. We will also examine why copper monitoring is needed at cogeneration facilities, along with modern analytical methods for trace metal analysis.

During the age of large fossil plant construction in the middle of the previous century, the condensate/feedwater network typically contained several closed feedwater heaters plus an open heater, the deaerator.

Copper alloys were a common materials choice for closed feedwater heater tubes because of copper’s excellent heat transfer properties. However, copper is susceptible to corrosion from the combined effects of dissolved oxygen and ammonia, the latter being the common chemical for feedwater pH control (although at some plants alkalizing, aka neutralizing, amines remain the choice). (3, 4)

Oxygen converts the protective Cu2O layer on the copper surface (where copper is in the +1 oxidation state) to CuO, with copper transforming to a +2 oxidation state. Cu2+ reacts with ammonia to form a soluble compound. So, for virtually any system containing copper alloys, a combination of mechanical deaeration and chemical oxygen scavenging was, and still is, necessary to protect the alloys. The oxygen scavenger also serves as a passivating agent to convert CuO back to Cu2O.

The combination of ammonia or an ammonia/amine blend for pH control and oxygen scavenger feed is known as all-volatile treatment reducing (AVT(R)). It produces the familiar dark magnetite layer (Fe3O4) on carbon steel but is no longer recommended for utility units and HRSGs with no copper alloys.

Rather, all-volatile treatment oxidizing (AVT(O)) as outlined in Reference 1 (with no oxygen scavenger feed but still ammonia or an ammonia/amine blend for pH control) is the proper choice. AVT(O) produces a red oxide layer, α-hematite (alternatively known as ferric oxide hydrate (FeOOH)) on carbon steel. AVT(O) requires high-purity feedwater with a cation conductivity of <0.2 mS/cm to be successful. For cogeneration and industrial steam generation systems, the (usually) lower-purity feedwater and/or presence of copper alloy-tubed heat exchangers prohibits AVT(O), with AVT(R) being the required option.

Careful chemistry control is necessary to find the balance between minimal iron and copper corrosion. A key ingredient in the treatment program is corrosion product monitoring to ensure that the chemistry is optimized.

Regarding iron monitoring, several discussion points from Reference 2 bear brief repetition.

Typically, 90% or greater of steel corrosion products exist as iron oxide particulates. Thus, measurements of just dissolved iron do not come close to the total corrosion product concentration. Hach developed a benchtop procedure that utilizes a 30-minute digestion process to convert all iron to soluble form for subsequent analysis on a standard spectrophotometer.

The lower detection limit is 1 part-per-billion (ppb), which is satisfactory for even high-pressure steam generators where the recommended feedwater iron concentration is <2 ppb. As events have shown over the last nearly four decades, iron monitoring is highly important for tracking flow-accelerated corrosion (FAC) in condensate/feedwater systems and in the low-pressure economizer and evaporator (and often some intermediate pressure circuits) of multi-pressure HRSGs. This benchtop technique provides snapshot readings only, but those are often sufficient with a system protected by proper chemistry. (5)

Sometimes, however, continuous online measurements are important to quickly detect changing conditions. Hach has developed a laser nephelometry technique for that purpose, with additional details available in Reference 2. This method must be calibrated at each site and is dependent on whether an AVT(O) or AVT(R) program is in place.

Now we reach a second key point of this article, as summarized in Reference 5.

For a cogeneration plant that sends steam to a steam host for use in a process (either via direct or indirect use) and then receives all or a portion of the condensate back, monitoring corrosion products in the steam condensate indicates whether corrosion and FAC are minimized in the process part of the steam plant. . . . For mixed-metallurgy plants the copper levels can be extremely variable depending on the plant design and operation, but with chemistry optimized as far as possible, levels of total copper less than 10 [ppb] can be expected.

As with iron, the analytical process must account for dissolved and particulate metal. When this author began his power plant career over four decades ago as a laboratory chemist, the lab was equipped with a flame/graphite furnace atomic absorption spectrophotometer (AAS). Sample acidification with nitric acid solubilized particulate copper, and the total could then be accurately analyzed by the AAS. However, many labs do not have such sophisticated equipment and the trained personnel to operate these instruments. One method for accurate measurements, albeit where samples are collected over time, is corrosion product sampling.

This CPS utilizes a fine-pore mechanical filter paper for particulate collection and cation exchange (and if desired anion exchange) filter papers for dissolved ion collection. Any sampling period may be chosen (one to two weeks is common), after which the filters are sent to a laboratory for accurate analyses. The unit has a precise flow totalizer so that the analytes can be converted to concentration units for the time-period that the sample was collected.

Consider the extract below from the recently-revised industrial boiler water guidelines produced by the American Society of Mechanical Engineers (ASME).

As the reader will note, recommended feedwater iron and copper limits are stringent, even for low-pressure industrial steam generators, and the values decrease with increasing pressure. For high-pressure utility steam generators, the suggested upper limits are 2 ppb for both iron and copper. A CPS can provide very valuable data on corrosion control in condensate systems with mixed metallurgies. Consider the following example, in which a CPS assisted with corrosion monitoring in a utility steam generator.

CPS case history

The author once consulted for an electric utility whose main unit was and still is a coal-fired boiler at full-load operating conditions of 1, 900psig drum pressure and 1, 005°F main and reheat steam temperatures. The feedwater system had heaters with copper-alloy tubes, requiring an AVT(R) feedwater chemistry regimen. (At the time of this project, plant personnel were developing a plan to replace the copper alloy heater tubes with steel.) Carbohydrazide served as the reducing agent, with a blend of morpholine and cyclohexylamine for pH conditioning. Chemical injection is at the deaerator storage tank. Even though the chemical feed system could maintain feedwater pH within a range of 9.0–9.3 (the recommended range for balancing steel and copper corrosion control), the condensate pH typically remained in an 8.8–8.9 range. It became clear that the condensate pH depression resulted from amine decomposition products that carried over with the steam.(4)

Per our recommendation, utility personnel installed a Sentry corrosion product sampler, with the flexibility for monitoring either feedwater or condensate pump discharge (CPD). Sampling indicated that iron concentrations were often five to fifteen times greater than the 2-ppb recommended limit, which suggested serious flow-accelerated corrosion in the condensate/feedwater network. Furthermore, the iron concentrations in the CPD were higher than in the feedwater. These results suggested that the lower pH induced by alkalizing amine decomposition had more of an influence on mild steel corrosion than the higher feedwater temperatures, both of whose influences are well known per the following famous diagram.

Regarding copper analyses, the CPS revealed concentrations very near the 2-ppb limit mentioned above, which should be expected in an oxygen-free environment with a pH close to 9.0. Accordingly, carbon steel corrosion became the primary focus in this unit. Plant personnel have recently incorporated a film-forming amine (FFA) into the chemical treatment program. Film-forming amines and related non-amine products are designed to directly establish a protective layer on metal surfaces. (8) Both successful and unsuccessful applications have been reported, but space does not permit a detailed discussion at present. In this application, no CPS data is yet available to confirm the efficacy of the FFA, but Millipore filter tests suggest that carbon steel corrosion has been reduced.

Film-forming chemistry should be incorporated into and not serve as a full-blown substitute for either AVT(R) or AVT(O) methodologies. An issue that has been problematic regarding FFA applications is direct calculation of reagent concentrations. Significant strides are being made in this respect, which Hach personnel highlighted in a paper at the recent Electric Utility Chemistry Workshop. (9)

While copper monitoring has proven to be less critical than iron monitoring in the example above, it is often much more important at cogeneration and industrial steam plants. As mentioned, certain conditions such as the combination of dissolved oxygen and ammonia can cause significant copper corrosion and reduce the life expectancy of heat exchanger tubes.

Another corrodent that can cause severe damage to many metals including copper is sulfide (S2-). The author once observed a situation where thousands of new 90-10 copper-nickel tubes in a steam surface condenser failed from multiple pitting leaks within 18 months because the machining lubricant contained sulfide that was not removed before the tubes were placed in service. An online measurement often recommended for chemistry control in mixed-metallurgy systems is oxidation-reduction potential (ORP). The data provided by trace metal monitoring methods can be correlated to ORP measurements to then serve for continuous chemical feed control.

Trace metal monitoring continues to become better recognized as a critical tool for optimizing steam generator chemical treatment programs and controlling corrosion. A primary concern with utility units is minimizing carbon steel flow-accelerated corrosion, but for cogen and industrial steam/condensate networks, copper corrosion monitoring is often also very important.

References