Flow-Accelerated Corrosion
By Marvin D. Silbert, Marvin Silbert and Associates
From The Analyst, Fall 2002

Abstract
Most cooling and boiler systems are constructed of carbon steel, a material that is very prone to corrosion when it comes into contact with water. While the water-treatment industry has developed many products to retard that corrosion, it is important to recognize that boilers and cooling systems are very dynamic. The success or failure of any chemical treatment program is highly dependent upon the balance between two opposing processes: a chemical treatment program that forms a stable protective surface, and a flow that is continuously trying to remove it. This article shows some examples that demonstrate the role of flow with respect to corrosion in cooling and boiler systems.

Introduction
For years, the metals that form the pipe work, tubes, shells and other components of cooling systems and boilers were chosen on the basis of cost-effectiveness, mechanical strength and heat-transfer characteristics. Water chemistry was rarely considered until it suddenly came into play when that first drop of water was added to the system. It must be remembered that these metals started as oxides or sulfides dug from the ground. A large input of energy was needed to convert them into the metallic form. Once they come into contact with water, they are going to find ways to revert back to their natural state. Depending upon the type of system, they may be attacked by dissolved oxygen, galvanic action or microbiological activity either individually or in combination.

There are two practical and workable routes.

A. Anodic inhibition - Prevents metallic ions from entering the water under a very specific set of conditions by forming a highly protective oxide surface.

B. Cathodic inhibition - Blocks the corrosion process by depositing a protective film that isolates the metal surface from its surroundings.

In neither case is the protective layer either fixed or permanent. The flowing water is constantly trying to dissolve or erode that surface and the higher the velocity, the more effective the removal of the protective layer. Thermal stresses form cracks that allow water to seep under the deposit where impurities can concentrate. Multiple cracks grow together and cause large chunks to break away and be transported with the flow. Over time, the damage may be sufficient to expose bare metal. Wherever it does, corrosion is likely to occur. The ability of any chemical treatment program to protect the base metal becomes a chemical balancing act between the flow trying to remove the coating and the treatment trying to reform or repair it. If it can be repaired as fast as it is being removed, the surface will be protected. If it cannot, the protection will be lost and the metal will corrode. As the corrosion process requires that flow, it is known as Flow-Accelerated Corrosion or Flow-Assisted Corrosion. The acronym FAC is appropriate for either. As the loss can also be considered a combination of mechanical wear or erosion, followed by chemical attack or corrosion of the freshly exposed surface, it is also known as Erosion-Corrosion.

Perhaps the best-known example of FAC occurs in domestic hot-water systems for large buildings. Water is distributed using copper pipes and tubing to the individual users from a hot-water heater in the basement or sub-basement. The individuals who use that water do not care that their office or apartment on the 50th floor may be a few hundred metres further from that boiler than one on the 1st floor. They want hot water whenever they turn on the tap. Rather than supply the taps directly from the boiler, these buildings employ a recirculating hot-water loop with roughly the same distance between the loop and the hot water taps in any office or apartment on any floor. While various jurisdictions may prohibit chemical treatment in potable water systems, this does not present any problem as the dissolved oxygen in the water reacts with the copper metal to form an oxide layer that protects the metal as long as the flow velocity in the loop is below three feet per second. What happens when either the design or a valving error allows it to go higher? The oxide gets worn away to expose bare copper metal. As oxygenated water continues to pass over that active surface, a new layer of oxide forms on the bare spots, only to be worn away again. This cycle repeats itself until there is essentially no wall thickness left and the pipe fails. As most of the piping for the recirculation line is hidden behind walls, the failure first appears as water damage to walls or carpets. By this point, it is too late to do anything but replace major pipe sections. The repairs can be quite expensive, especially when walls must be knocked down to allow access to the pipes.

Cross Section of Copper Riser from Recirculating Hot-water System. This pipe was removed when the system failed due to FAC. The photographs show the extent of the wall thinning. Magnification is 10x for the upper view and 60x for lower view.

Boiler Water Treatment
FAC is well known in boiler feedwater systems. Most boiler treatment programs are based upon a high-quality treated feedwater with an oxygen scavenger added to keep the oxygen below 10 mg/L and a volatile amine to raise the pH. When the feed train is made from ferrous metals, a pH of 9.5-9.8 will effectively minimize corrosion in carbon steel. In a well controlled system, the low oxygen and high pH result in the formation of a stable and protective magnetite (Fe₃O₄) layer on the carbon steel surfaces and a circulating iron concentration below 10 mg/L. While magnetite is considered as insoluble, it does have a very small solubility that is dependent upon both temperature and pH. This means there will be regions where the magnetite is more soluble than others. As the boiler feedwater moves up the cycle, magnetite will move from those regions where it is most soluble to those where it is least soluble. When copper alloys are present, that pH range must be reduced to 8.8-9.2 to prevent attack on the copper. Unfortunately, this makes those differences in solubility much greater. For many years, it was common to design the feedwater trains for the high-pressure boilers used for electrical generation with Admiralty Brass condenser tubes, Admiralty Brass or carbon steel low pressure (LP) feed heater tubes, and carbon steel high pressure (HP) feed heater tubes. This combination may have had good thermal-transfer properties and kept the price down, but it was also a guarantee that the HP feed heater tubes would suffer thinning from FAC as the magnetite moved from the HP feed heater and deposited on the boiler tubes. Most tube failures tend to occur when the full output of the unit is badly needed. The quickest action is to take the unit down, locate the failed tubes, plug them and get the unit back online as quickly as possible. To continue operating with the same output, that same flow will now have to go through fewer tubes. This means that the flow velocity will be higher to compensate and that will accelerate the time to the next failure. Eventually, it will become necessary to retube the heat exchanger. The choice should be a more resistant alloy such as a stainless steel for the tubes and, if at all possible, eliminate all the copper alloys in the system.

Solubility of Magnetite in Ammoniated Water vs. Temperature and pH By superimposing the approximate temperatures for the various feedwater components, it can be seen that the maximum solubility lies within the HP feedwater heaters. The effect is much more pronounced in the lower pH range required when copper alloys are present within the system. (This graph has been adapted from data supplied to the author by F. Pocock, Babcock & Wilcox.)

FAC can also be a serious problem in a steam system. As the steam gives up energy to heat plant components or turn a turbine, water droplets start to condense and impinge upon the pipe walls. The choice of amine is important. Ammonia tends to stay with the steam. This leaves the liquid phase with insufficient protection to maintain a stable magnetite surface. Morpholine will perform better as it distributes itself into both the steam and liquid phases. This allows the droplets to carry some protection and help maintain the magnetite surfaces.

Comparison of Ammonia and Morpholine as Alkalizing Amines from 0-300°C
A. The upper figure compares their base strengths. As pKb is a negative logarithmic term, a lower value indicates increasing base strength. The curves show ammonia as the stronger base at lower temperatures with a crossover just above 200°C. Above this morpholine becomes the stronger base.
B. The lower curve shows their distribution between the steam and liquid phases. Ammonia favour the steam phase at all temperatures. Morpholine strongly favours the liquid phase below 100°C and keeps a reasonable proportion in the liquid phase.

(This curve has been plotted using the data of J.W. Cobble and P.J. Turner, "Additives for pH Control in PWR Secondary Water", EPRI NP-4209, Interim Report, August 1988. This report details the base strengths and distribution ratios for seventy-nine candidate amines for pH control.)

Cooling Water Treatment
FAC plays an important role in cooling water treatment, but is rarely discussed in this context. All-organic treatment programs protect carbon steel by laying down a protective film. At the same time, the flowing water that passes across that film tries to remove it. If a treatment program is going to succeed, a stable, uniform and adherent film must cover the entire surface. Laboratory testing and field trials during the development of a particular product establish the ideal dosage for a number of standard applications. Whether or not it is said, the reason for that choice is the balance between formation of that protective film and its removal. If the dosage is set lower, the film may be removed. If it is set higher, the application cost ceases to be competitive. Most water treatment programs include a recommendation for a start-up dosage that is double for normal operation. There is a reason for this approach. It can take several weeks to build up a stable and adherent layer and, until it is formed, the balance can quickly tip to let the flow damage or even remove the film. Doubling the dosage accelerates the growth of the film to obtain that balance point. Once the protective film is formed, it becomes imperative that the product dosage be tightly controlled. In one side-by-side comparison, plants "A" and "B" used the same treatment program. Plant A had an automatic feed system. Plant B was manually controlled and lack of attention would allow the dosage to creep down, then up, then drift again. In spite of the fact that both plants averaged the same total dosage over the year, corrosion rates were very low for A and quite high for B. If the addition rates fluctuate, the film can be extensively damaged when the addition rate is too low for extended periods. An extended period of time had to occur to develop the initial film when the program was introduced. A similar length of time will be needed for a major rebuild as compared to performing a simple patch.
    Will the dosage be the same at all plants? If the flow rates are particularly high, it may be necessary to increase the dosage. If they are particularly low, it may be possible to reduce it. Many plant operators complain that water treatment companies always increase their dosage whenever problems arise and hint that this may be a means to increase revenue. That increase is more likely related to the presence of high flow rates and/or turbulence within the system. These flow rates need not be high throughout the entire system. If there is a higher velocity in one section, the overall treatment must be based upon that one section.

Formulations for System Cleanup
Depending upon the conditions within any steel system, circulating iron is either going to form a protective surface or be transported through the system in a particulate form until it settles in a low flow region, bakes unto a particularly hot surface or is trapped by a filter. As too few systems are equipped with filters, it is more likely to be some combination of the first two scenarios. All chemical treatment programs endeavour to minimize corrosion of the carbon steel and maintain a clean system. Even the best of programs can be overwhelmed and fail when corrosion products enter the system from other locations. It may come from a leaking process heat exchanger, cooling sprays or a long length of pipe work. It is common to modify the formulation to cope with this material by adding agents to clean up the iron or at least keep it in suspension until it can be removed by filtration. The most common ingredients to undertake this endeavour are chelants such as EDTA or NTA. The problem is that these materials cannot distinguish between particulate iron laid down as a protective deposit and corrosion products brought into the system by corrosion at the far reaches of a heating or cooling network. If they clean up an unwanted deposit, success has been achieved. If they clean up some of the protective oxide from the surface, that can be detrimental, especially if they reach bare metal with traces of oxygen in the water. A new layer of oxide will form. With higher temperatures and flows involved, the extent of this FAC damage can be particularly damaging in a boiler system. It is very important to realize that the "C" words are fully interchangeable between the terms on-power cleaning and on-power corrosion.

Conclusion
FAC should be considered at the design stage, during operation, and as part of any troubleshooting activity. There is good news on the design end with the newer combined cycle and large electrical generating stations eliminating copper alloys and switching to stainless steel feed heater and condenser tubes. However, there needs to be more emphasis on the potential for FAC in boiler and cooling systems, both at the proposal phase, and during troubleshooting.

About the Author
Marvin Silbert is President of Silbert and Associates in Toronto, Canada. He can be reached at (416) 225-4541.