The authors have reviewed new power project specifications time and time again, in which plant designers typically choose 304 or 316 stainless steel for condenser and auxiliary heat exchanger tubing.For many, the term stainless steel conjures an aura of invincible corrosion, when in fact, stainless steels can sometimes be the worst choice because they are susceptible to localized corrosion.And, in this era of reduced availability of fresh water for cooling water make-up, coupled with cooling towers operating at high concentration cycles, potential stainless steel failure mechanisms are magnified.In some applications, 300 series stainless steel will only survive for months, sometimes only weeks, before failing.This article focuses on at least the issues that should be considered when choosing condenser tube materials from a water treatment perspective.Other factors not discussed in this paper but that play a role in material selection include material strength, heat transfer properties, and resistance to mechanical forces, including fatigue and erosion corrosion.
Adding 12% or more chromium to steel causes the alloy to form a continuous oxide layer that protects the base metal underneath.Hence the term stainless steel.In the absence of other alloying materials (especially nickel), carbon steel is part of the ferrite group, and its unit cell has a body-centered cubic (BCC) structure.
When nickel is added to the alloy mixture at a concentration of 8% or higher, the cell will exist in a face-centered cubic (FCC) structure called austenite, even at ambient temperature.
As shown in Table 1, 300 series stainless steels and other stainless steels have a nickel content that produces an austenitic structure.
Austenitic steels have proven to be very valuable in many applications, including as a material for high temperature superheater and reheater tubes in power boilers.The 300 series in particular is often used as a material for low temperature heat exchanger tubes, including steam surface condensers.However, it is in these applications that many overlook potential failure mechanisms.
The main difficulty with stainless steel, especially the popular 304 and 316 materials, is that the protective oxide layer is often destroyed by impurities in the cooling water and by crevices and deposits that help concentrate impurities.Additionally, under shutdown conditions, standing water can lead to microbial growth, whose metabolic byproducts can be highly damaging to metals.
A common cooling water impurity, and one of the most difficult to remove economically, is chloride.This ion can cause many problems in steam generators, but in condensers and auxiliary heat exchangers, the main difficulty is that chlorides in sufficient concentrations can penetrate and destroy the protective oxide layer on stainless steel, causing localized corrosion, i.e. pitting.
Pitting is one of the most insidious forms of corrosion because it can cause wall penetrations and equipment failure with little metal loss.
Chloride concentrations do not have to be very high to cause pitting corrosion in 304 and 316 stainless steel, and for clean surfaces without any deposits or crevices, the recommended maximum chloride concentrations are now considered to be:
Several factors can easily produce chloride concentrations that exceed these guidelines, both in general and in localized locations.It has become very rare to first consider once-through cooling for new power plants.Most are built with cooling towers, or in some cases, air-cooled condensers (ACC).For those with cooling towers, the concentration of impurities in cosmetics can “cycle up.”For example, a column with a make-up water chloride concentration of 50 mg/l operates with five concentration cycles, and the chloride content of the circulating water is 250 mg/l.This alone should generally rule out 304 SS.In addition, in new and existing plants, there is an increasing need to replace fresh water for plant recharge.A common alternative is municipal wastewater.Table 2 compares the analysis of the four freshwater supplies with the four wastewater supplies.
Watch out for increased chloride levels (and other impurities, such as nitrogen and phosphorus, which can greatly increase microbial contamination in cooling systems).For essentially all grey water, any circulation in the cooling tower will exceed the chloride limit recommended by 316 SS.
The preceding discussion is based on the corrosion potential of common metal surfaces.Fractures and sediments dramatically change the story, as both provide places where impurities can concentrate.A typical location for mechanical cracks in condensers and similar heat exchangers is at tube-to-tube sheet junctions.Sediment within the tube can create cracks at the sediment boundary, and the sediment itself can serve as a site for contamination.Furthermore, because stainless steel relies on a continuous oxide layer for protection, the deposits can form oxygen-poor sites that turn the remaining steel surface into an anode.
The above discussion outlines issues that plant designers typically do not consider when specifying condenser and auxiliary heat exchanger tube materials for new projects.The mentality regarding the 304 and 316 SS sometimes still seems to be “that’s what we’ve always done” without considering the consequences of such actions.Alternative materials are available to handle the harsher cooling water conditions that many plants now face.
Before discussing alternative metals, another point must be briefly stated.In many cases, a 316 SS or even a 304 SS performed well during normal operation, but failed during a power outage.In most cases, the failure is due to poor drainage of the condenser or heat exchanger causing stagnant water in the tubes.This environment provides ideal conditions for the growth of microorganisms.Microbial colonies in turn produce corrosive compounds that directly corrode the tubular metal.
This mechanism, known as microbially induced corrosion (MIC), is known to destroy stainless steel pipes and other metals within weeks.If the heat exchanger cannot be drained, serious consideration should be given to periodically circulating water through the heat exchanger and adding biocide during the process.(For more details on proper layup procedures, see D. Janikowski, “Layering Up Condenser and BOP Exchangers – Considerations”; held June 4-6, 2019 in Champaign, IL Presented at the 39th Electric Utility Chemistry Symposium.)
For the harsh environments highlighted above, as well as harsher environments such as brackish water or sea water, alternative metals can be used to ward off impurities.Three alloy groups have proven successful, commercially pure titanium, 6% molybdenum austenitic stainless steel and superferritic stainless steel.These alloys are also MIC resistant.Although titanium is considered very resistant to corrosion, its hexagonal close-packed crystal structure and extremely low elastic modulus make it susceptible to mechanical damage.This alloy is best suited for new installations with strong tube support structures.An excellent alternative is the super ferritic stainless steel Sea-Cure®.The composition of this material is shown below.
The steel is high in chromium but low in nickel, so it is a ferritic stainless steel rather than an austenitic stainless steel.Due to its low nickel content, it costs much less than other alloys.Sea-Cure’s high strength and elastic modulus allow for thinner walls than other materials, resulting in improved heat transfer.
The enhanced properties of these metals are shown on the “Pitting Resistance Equivalent Number” chart, which, as the name suggests, is a testing procedure used to determine the resistance of various metals to pitting corrosion.
One of the most common questions is “What is the maximum chloride content that a particular grade of stainless steel can tolerate?” The answers vary widely.Factors include pH, temperature, presence and type of fractures, and the potential for active biological species.A tool has been added on the right axis of Figure 5 to help with this decision.It is based on neutral pH, 35°C flowing water commonly found in many BOP and condensation applications (to prevent deposit formation and crack formation).Once an alloy with a specific chemical composition has been selected, PREn can be determined and then intersected with the appropriate slash.The recommended maximum chloride level can then be determined by drawing a horizontal line on the right axis.In general, if an alloy is to be considered for brackish or seawater applications, it needs to have a CCT above 25 degrees Celsius as measured by the G 48 test.
It is clear that the super ferritic alloys represented by Sea-Cure® are generally suitable for even seawater applications.There is another benefit to these materials that must be emphasized.Manganese corrosion problems have been observed for 304 and 316 SS for many years, including at plants along the Ohio River.Recently, heat exchangers at plants along the Mississippi and Missouri Rivers have been attacked.Manganese corrosion is also a common problem in well water make-up systems.The corrosion mechanism has been identified as manganese dioxide (MnO2) reacting with an oxidizing biocide to generate hydrochloric acid under the deposit.HCl is what really attacks metals.[WH Dickinson and RW Pick, "Manganese-Dependent Corrosion in the Electric Power Industry"; presented at the 2002 NACE Annual Corrosion Conference, Denver, CO.] Ferritic steels are resistant to this corrosion mechanism.
Selecting higher grade materials for condenser and heat exchanger tubes is still no substitute for proper water treatment chemistry control.As author Buecker has outlined in a previous power engineering article, a properly designed and operated chemical treatment program is necessary to minimize the potential for scaling, corrosion, and fouling.Polymer chemistry is emerging as a powerful alternative to older phosphate/phosphonate chemistry to control corrosion and scaling in cooling tower systems.Controlling microbial contamination has been and will continue to be a critical issue.While oxidative chemistry with chlorine, bleach, or similar compounds is the cornerstone of microbial control, supplemental treatments can often improve the efficiency of treatment programs.One such example is stabilization chemistry, which helps increase the release rate and efficiency of chlorine-based oxidizing biocides without introducing any harmful compounds into the water.In addition, supplemental feed with non-oxidizing fungicides may be very beneficial in controlling microbial development.The result is that there are many ways to improve the sustainability and reliability of power plant heat exchangers, but every system is different, so careful planning and consultation with industry experts is important for the choice of materials and chemical procedures.Much of this article is written from a water treatment perspective, we are not involved in material decisions, but we are asked to help manage the impact of those decisions once the equipment is up and running.The final decision on material selection must be made by plant personnel based on a number of factors specified for each application.
About the Author: Brad Buecker is a Senior Technical Publicist at ChemTreat.He has 36 years of experience in or affiliated with the power industry, much of it in steam generation chemistry, water treatment, air quality control and at City Water, Light & Power (Springfield, IL) and Kansas City Power & Light Company is located at La Cygne Station, Kansas.He also spent two years as acting water/wastewater supervisor at a chemical plant.Buecker holds a BS in Chemistry from Iowa State University with additional course work in Fluid Mechanics, Energy and Materials Equilibrium, and Advanced Inorganic Chemistry.
Dan Janikowski is Technical Manager at Plymouth Tube.For 35 years, he has been involved in the development of metals, the manufacture and testing of tubular products including copper alloys, stainless steel, nickel alloys, titanium and carbon steel.Having been with Plymouth Metro since 2005, Janikowski held various senior positions before becoming Technical Manager in 2010.