1. Introduction. Figure 1: Open Recirculation Cooling Water Tower (image courtesy of Solenis) Page 3

Transcription

1

2 Table of Contents 1. Introduction Description of Root Causes of Corrosion Electrochemical Corrosion Under-Deposit Corrosion Microbial-Induced Corrosion Corrosion Monitoring Conclusions... 11

3 1. Introduction Modern industrial facilities in light and heavy industries are often equipped with cooling systems to safely and reliably remove waste heat. Cooling may be achieved with air-cooled systems such as condensers or more complex systems with pumps and piping that use water as the working fluid. Water-based cooling systems are very common since the fluid has a high heat capacity and is relatively inexpensive, safe to handle, and readily available. Cooling water systems may be open recirculation, closed-loop or once-through systems. Open recirculation systems are very common and utilize cooling towers and evaporative cooling principles to indirectly exchange heat from the process to the atmosphere, (Figure 1). Cooling water systems are increasingly automated, complex and may be subject to upset conditions including process changes, makeup water variability, heat exchanger leaks, and upset conditions such as interruption in chemical treatment. Prolonged operation outside of appropriate conditions can lead to mechanical degradation of system materials and decreases in system performance due to corrosion, deposition, and microbiological activity. Figure 1: Open Recirculation Cooling Water Tower (image courtesy of Solenis) Corrosion s effects are wide ranging across industries, adversely affecting infrastructure like oil pipelines and steel bridges used in transportation. Corrosion products can lead to fouling, reduced heat exchanger efficiency and increased operating costs. Costs related to corrosion vary across industries, with some global estimates as high as $2.2 trillion annually. 1 NACE conducted a study in 2002 which found estimated annual costs of corrosion are $276 billion in the US. 2 The Electric Power Research Institute (EPRI) study in 2001 attributed $17 billion of corrosion-related costs in the electric power generation industry Page 3

4 each year. 3 In cooling water systems, uncontrolled corrosion can lead to high operating and maintenance costs and reliability issues including equipment and system failure. To minimize corrosion-related costs, an understanding of the corrosion mechanisms and potential solutions is critical to preserve assets, ensure reliability and cost-effectively manage risks. Robust cooling treatment programs are required to effectively deliver corrosion, deposition and microbial control to improve reliability, safely comply with health regulations and cost-effectively manage risk. This paper is intended to discuss root causes of corrosion in industrial cooling water systems and provide deeper insight into best practices to prevent and minimize corrosion. 2. Description of Root Causes of Corrosion The leading cause of failure in cooling water systems is corrosion of metallic systems such as heat exchangers. Non-metallic corrosion resistant materials, such as fiberglass reinforced pipe (FRP) may be very good at corrosion resistance; however, these materials are very poor at heat transfer. Common metals in cooling water systems include mild steel, stainless steel (304 and 316), admiralty brass, copper, and copper nickel blends (90/10 and 70/30). In the presence of oxygen, these refined metals revert back to their oxidized states. Since it is not practical to remove oxygen from open recirculation cooling systems, other approaches must be utilized to prevent or minimize corrosion of process metallurgy. The most cost-effective solution often involves chemical solutions in the form of a variety of corrosion inhibitors. One common corrosion mechanism in cooling systems is an electrochemical reaction, due to a break in the passivated film barrier or under-deposit corrosion (due to fouling or microbial biofilms). When under-deposit corrosion is enhanced by microbiological activity, it is referred to as microbially-induced corrosion (or microbially-influenced corrosion ) and abbreviated as MIC. (There is an emerging trend to recognize this phenomenon as microbial-enhanced corrosion or MEC.) Physical and chemical factors can impact corrosion rates including temperature, ph, water composition (including aggressive anions like chlorides and sulfates, for example), and water velocity. Corrosion inhibitors and operational management are required to mitigate corrosion risks from a variety of process conditions. 3 Page 4

5 Metal loss is commonly expressed in mils (thousands of an inch) per year (mpy), and the type of attack (general or local) is important to determine causes and potential solutions. Mils per year correspond to thousandths of an inch (0.001 ) of metal loss within a given system. Figure 2 summarizes an expected lifespan of Schedule 40 mild steel pipe (1/8 wall thickness, or 125 mils) at various general corrosion rates. Figure 2: Schedule 40 Mild Steel Pipe Service Life vs Corrosion Rate (Sources: Solenis and industry guidelines) Corrosion rates can be measured through destructive testing of process metallurgy. However, a more common and more economical approach uses preweighed metallic tags called coupons, immersed in process conditions according to ASTM guidelines. After predetermined exposure times, the coupons are extracted, re-weighed and examined to gain insight into corrosion potential of the system. General corrosion indicates that the predominant corrosion is occurring over 50% of the surface of the coupon. Pitting type corrosion indicates a localized corrosion mechanism, (100 mil per year of material loss). Although Figure 2 may appear as if low corrosion rates still allow long life, this false sense of security can be misleading. General corrosion is often accompanied by a more aggressive localized form of corrosion, called pitting. Page 5

6 Figure 3: Localized pitting on Coupon and Micrograph of Stress Corrosion Cracking with pit initiation site in red circle (images courtesy of Solenis) Pitting is a more severe form of corrosion considered more dangerous than general corrosion due to the potential to cause mechanical failure of the system. Pits can be shallow or deep, narrow or wide, and open to bulk water or partially or completely concealed by corrosion products. Pitting can also lead to microscopic fractures called stress corrosion cracking (Figure 3). Expert metallurgical analysis must be utilized to properly identify and plan for the remediation of systems experiencing pitting. 3. Electrochemical Corrosion The mechanisms of electrochemical corrosion are important to understand when considering cooling corrosion inhibition programs. The root cause of electrochemical corrosion is unchecked movement of electrons from the anode to the cathode in an electrolyte solution. There are various ways to stop the anodic/cathodic reaction of electrons through metal or ion flow in solution including using corrosion-resistant materials and coatings, employing methods of galvanic protection in the form of sacrificial anodes (such as zinc, magnesium and aluminum), and improving process control and monitoring (such as limiting chlorides). After these options have been evaluated and implemented, a corrosion inhibitor chemical protection program is applied for additional protection. When evaluating chemical treatment programs to inhibit corrosion, there are primarily three categories of corrosion control based on mechanisms: Page 6

7 1. Anodic inhibitors reduce the rate of metal dissolution by modifying passivity and producing a protective oxide film layer via chemisorption. 2. Cathodic inhibitors prevent the reduction of oxygen with a physically- and electrically-impermeable barrier film. 3. Film-forming azoles, which create a protective passive film to prevent copper re-deposition. Calcium is a unique species known for inverse solubility in aqueous environments: at high temperature and concentration the dissolved species precipitates out of solution as calcium salts. While deposit control is outside the scope of this paper, it is important to note that calcium can play a beneficial role in corrosion remediation. In the presence of certain phosphorus-bearing compounds (such as phosphates and phosphonates), calcium phosphate (CaPO4) will act as a cathodic corrosion inhibitor for metals. Cathodic inhibition can be supplied with chemical species like zinc or phosphate salts. Anodic inhibition can be provided by silicate, orthophosphate, molybdate or nitrites. Industry providers can offer combined anodic and cathodic inhibitors in the form of phosphonates, a molecule with carbon-phosphorus-oxygen bonds, and proprietary formulations like Stress Reducing Inhibitor (SRI). Azoles are a unique class of ring-shaped molecules used to prevent electrochemical corrosion in copper or copper alloy yellow metal systems. Corrosion in these systems liberate copper atoms, which can re-deposit on other metals like low-carbon steel and cause severe localized corrosion in the form of pitting. Chemicals in this category include benzotriazole (BZT), tolyltriazole (TTA) and a chlorinated tolytriazole called halogen-resistant azole (HRA). When considering treatment with any of these chemicals, a thorough understanding of makeup water chemistry, system operation, and process metallurgy is crucial to accurately select products and set dosages to be cost effective and technically effective. Corrosion cannot be avoided entirely; however, a well-implemented cooling water treatment program will reduce corrosion rates to acceptable levels, (Figure 4). Page 7

8 Material Generally Acceptable Corrosion Level (mpy) Mild Steel 1 3 Aluminum Copper Alloys Stainless Steel Figure 4: Generally Acceptable Corrosion Levels, Open Circulating Cooling Systems (Source: Solenis) 4. Under-Deposit Corrosion Under-deposit corrosion can be challenging to predict since bulk water analyses may not indicate corrosion risks and examination of corrosion coupons may not "adequately identify the phenomenon. The deposit may be due to corrosion products (such as iron) accumulating in a low-flow area such as the back of a weld (Figure 5). Figure 5: Effects of Under-Deposit Corrosion, after cleaning (image courtesy of Solenis) Deposits may also be due to uncontrolled precipitation of calcium phosphate or calcium carbonate. Under-deposit corrosion also may occur beneath bacterial slime deposits, which are discussed in the next section. Solutions for underdeposit corrosion control include adhering to industry best practices as mentioned in this section to reduce risks of localized corrosion, as well as maintaining an effective deposit control program. Page 8

9 5. Microbial-Induced Corrosion Microbial-induced corrosion (MIC) is a special type of corrosion due to the metabolic activity of microscopic organisms including bacteria and related biofilm such as slime often seen in quiescent areas of aqueous systems. In aqueous systems, microbiological (MB) life can form a protective biofilm, consisting of polysaccharides. In addition to loss of heat transfer, the biofilm can be problematic, leading to under-deposit corrosion. Sulfate-reducing bacteria (SRB s) are a particularly problematic class of bacteria, converting aqueous sulfur (such as dissolved sulfates) into an energy source in anaerobic respiration and in the process emitting hydrogen sulfide (H2S). The resulting chemical reaction can aggressively corrode mild steel, leading to localized pitting (Figure 6). Figure 6: Corrosion due to Sulfate-Reducing Bacteria (image courtesy of Solenis) The risk of microbial-induced corrosion can be better managed with an understanding of the concentration and populations of organisms within a cooling water system. Water samples can be obtained and cultured by microbiologists to better understand which lifeforms are present. An alternative method is to measure metabolic activity called ATP (adenosine triphosphate). ATP is considered a reliable indicator because it correlates with biological health and concentration. Bacterial populations are often represented by colony forming units per unit volume, such as cfu/ml. With a better understanding of MB, cooling tower treatment programs can be developed to keep those populations from aggressively depositing in or corroding the process equipment. Page 9

10 Just like in electrochemical corrosion inhibition, there are a variety of chemical treatments available as biocides. General non-specific oxidizing biocides like chlorine or bromine can achieve effective kills across a broad range of organisms. The oxidizing biocides chemically destroy the cellular structure of the organisms, which cannot develop any biological resistance to this approach. Although this is often a lower cost approach, disadvantages of oxidizing biocides include destruction of water treatment chemicals (such as corrosion inhibitors), lack of dispersion effect to facilitate the removal of the dead organisms, and the risks of cooling water process upset due to excess oxidizer. A second approach is a non-oxidizing, targeted solution to interfere with specific classes of organisms by effectively disrupting natural biochemistries. Although this specialty biocide approach is often more expensive on a cost per pound basis, the treatment is often more effective and less corrosive to equipment than the commodity bleach approach. New advances in chemistries including chloramines and biopenetrants are being adopted for improved selectivity and efficacy than traditional oxidizers. Also, just like with electrochemical corrosion inhibition, water treatment professionals can help guide customers to appropriate biocide solutions to ensure public health (e.g. minimize legionella risks), comply with legal guidelines (such as effluent limits into a water body) and can be technically effective and cost effective. 6. Corrosion Monitoring As illustrated in this paper, corrosion can have a dramatic impact on operations including high maintenance costs and failures leading to forced outages. Corrosion monitoring is an important part of any risk management strategy, including chemical additives. There are a number of ways to monitor corrosion in Cooling Water systems. One method, described earlier, utilizes metallic coupons immersed in the process fluid. By monitoring actual corrosion rates of replicated process metallurgy, program changes can be made to respond to changing process conditions and minimize risks of corrosion damage. Care must be taken when handling and setting up coupon racks, including piping arrangement and sequential order of coupons to reduce risks of cross-contamination. A second approach uses corraters, devices which provide real-time assessment of corrosion risk. Corraters use linear polarization resistance between two metal electrodes to measure instantaneous corrosion rate of a metal in a conductive Page 10

11 solution. As with any instrument, the proper setup and operation and interpretation of results is critical when managing corrosion risk. 7. Conclusions Hopefully this paper has helped illustrate the importance of corrosion control in cooling water systems. Many of these principles discussed in open cooling systems can also be applied to closed-loop cooling systems such as chilled water systems and bearing cooling water. Once corrosion is initiated, the effects of an untreated system can be disastrous for an industrial facility including lost revenue from downtime and costs of mechanical system replacement. Many of the factors affecting corrosion can be anticipated, such as water supply quality, materials of construction, and control parameters (such as ph and blowdown). However, upset conditions or other externalities such as changing makeup water qualities can affect operation and risks of corrosion can increase. An improved understanding of root causes and solutions to corrosion will allow owners and operators of cooling systems to proactively manage risks to lower operational and maintenance costs and improve operation and reliability. Page 11