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Pitting of the metal is noted and the severity of this pitting corrosion is reported as maximum pit depth in thousands of an inch (mils). Cavitation (damage due to the formation and collapse of bubbles in high-velocity turbines, propellers, etc.) Electrical resistance probes are basically “automatic coupons” and share many of the advantages and limitations of standard coupons. This method is desirable because the probes can be installed in both aqueous and nonaqueous streams. Metal loss occurs uniformly over the entire surface. Continuous readings can be made, and with sophisticated data analysis techniques, changes in corrosion rates are available in as little as two hours instead of the 30 days or more required with coupons. Galvanic corrosion occurs when two dissimilar metals are in contact in a solution. Corrosion in liquid cooling loops is caused by chemical, electro-chemical, or abrasive action by the heat transfer fluid on the wetted surfaces. The amount of dissolved copper required to produce this effect is very small and the increased corrosion is very difficult to inhibit once it occurs. By accomplishing these goals, your cooling system will operate more efficiently, economically, and safely. Commonly used cooling system alloys that may crack due to stress include austenitic stainless steels (300 series) and brasses. These inhibitors are thought to work by virtue of their ability to form a molecular film on the metal surface. Erosion corrosion is the increase in the rate of metal deterioration from abrasive effects. Table 24-1. The electrochemical corrosion cell consists of four components: (1) an anodic site, (2) a cathodic site, (3) a current path (metal), and (4) an electrolyte (water). In addition, they can form thick, oily surface films, which may severely retard heat transfer. Often it is used less as a corrosion inhibitor and more as a chemical tracer to facilitate the testing for the product dosage. Generally, it is used at dosages of 10 to 15 ppm as SiO2 at a pH of 7.5 to 10.0. Tools commonly used for this purpose include metal corrosion coupons, instantaneous corrosion rate meters, and heated surfaces such as test heat exchangers and the Betz MonitAll® apparatus. Corrosion Coupons. Corrosion tends to be self-limiting due to the buildup of corrosion reaction products. The hydroxide is then available to react with the ferrous iron to form an insoluble by-product of corrosion, ferrous hydroxide. Because of reduced solubility, polysilicate is not applied at pH’s below 7.0. Failure of a closed system has the potential to shut down a portion, if not all, of the plant. Chloride is the main contributor to SCC of stainless steels. They are able to respond to sudden changes in system conditions, such as acid spills, chlorine levels, and inhibitor treatment levels. The major advantage of the electrical resistance method versus corrosion coupons is that measurements can be obtained on a more frequent basis and require much less effort to perform. The low water velocities in shell-side cooling are also detrimental. The most serious form of galvanic corrosion occurs in cooling systems that contain both copper and steel alloys. Effective water treatment programs include provisions for corrosion inhibition to prolong the useful life of cooling water systems. It functions by passing low direct current through a two- or three-element probe. The conditions that promote the pitting of steel also promote the pitting of brass, which in cooling systems usually occurs by dezincification. Thus, dissolved copper represents a "demand" for triazole, which must be satisfied before surface filming can occur. An effective deposit control treatment is imperative. Forms of localized corrosion include pitting, selective leaching (e.g., dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologically influenced corrosion. Water Characteristics. Due to its relatively low cost, orthophosphate is widely used as an industrial corrosion inhibitor. Corrosion inhibitors for cooling water systems do this by keeping calcium suspended in the water, preventing it from crystallising and thus protecting metal surfaces. Examples of passivators (anodic inhibitors) include chromate, nitrite, molybdate, and orthophosphate. Heat exchangers and cold plates are used in cooling applications to remove and transfer heat from one place to another using a heat transfer fluid such as water, ethylene glycol and water solution, oil, etc. A clean coupon corrodes much faster than one that has reached equilibrium with the corrosive environment. This environment includes aerated cooling water, scale deposits, surface films, process contaminants, and microbiological growths. Probably due to the complexity of silicate chemistry, their mechanism of inhibition has not yet been firmly established. Also, some error in the test is introduced during the cleaning of the coupons. An effective microbiological control program is necessary to prevent severe fouling problems. Critical factors in controlling both include: process and CW temperatures, heat flus, water velocity, type and quality of water (salt, brackish, fresh), and type of cooling system. galvanic corrosion in water cooling system designs. In other words, in a closed cooling water system the same water is used all the time and circulate by a pump in the loop. Because of reduced solubility, polysilicate is not applied at pH’s below 7.0. However, low-temperature areas do not permit the development of sufficient scale for corrosion protection, and excessive scale forms in high-temperature areas and interferes with heat transfer. The sulfite reaction with dissolved oxygen is: The use of catalyzed sodium sulfite for chemical deaeration requires 8 parts of catalyzed sodium sulfite for each part of dissolved oxygen. Figure 24-12 shows the effect of oxygen concentration on corrosion at different temperatures. The dc current is generated as a result of the polarization of one or two electrodes fashioned from the metal under study by the application of a small electrical potential. Change ), You are commenting using your Facebook account. Pitting is generally promoted by low­ velocity or stagnant conditions and by the presence of corrosive ions (i.e., oxygen, chlorides, sulfates, etc.). A photograph of the coupon before and after cleaning is helpful for future reference. Orthophosphate is not commonly used alone in cooling water treatment for these reasons. When managing heat exchanger bundles, cooling water corrosion control is simply a matter of proper design, continuous maintenance of high quality water treatment… It is not very effective even at high dosages and can contribute to severe pitting if not carefully applied and controlled. Pit depth is measured with a feeler gauge or microscope. Plastic and concrete pipes tend to be resistant to corrosion. The plane of the triazole lies parallel to the metal surface; thus, each molecule covers a relatively large surface area. ( Log Out /  Typical dosages are 5 to 10 ppm for MBT and 1 to 3 ppm for BT and TT. Cooling water treatment is concerned with avoiding corrosion, scale, and sludge formation. Cooling water flows over a heated tube section within a glass shell. Pitting rate can be determined by: Pitting rate = Maximum pit depth, mils X 365. If the calcium exceeds 400 ppm, precipitation of calcium phosphate is possible especially in low-flow (less than 1 foot per second) areas of the system. Dispersant for cooling tower, preventing sedimentation Frequently, the iron oxides deposit at the site of corrosion resulting in the formation of numerous tubercles along the metal surface. The increase in acidity and concentration within the pit promotes even higher corrosion rates, and the process becomes self-sustaining. Although low water temperatures generally preclude cracking, SCC of stainless steels can occur in cooling systems. Since the corrosion current is directly proportional to the corrosion rate, LPT techniques provide instantaneous corrosion rate measurements. An electrical potential difference exists between these sites, and current flows through the solution from the anode to the cathode. Typical dosages are 1 to 5 ppm at a controlled pH of 6.5 to 6.7. Once this is completed, an effective corrosion control program can be implemented. The dezincification resistance varies with the alloy. The corrosive qualities of water can be reduced by deaeration. Thermocouples measure bulk water temperature and tubeside "skin" temperature. Because of solubility and toxicity restraints, zinc is rarely used alone in cooling water treatment programs. Sulfide is corrosive to steel and copper alloys. This is accomplished by several corrosion monitoring methods. Whether your cooling system is open-recirculating, closed-loop, or once through, the goals remain the same; prevent corrosion, prevent deposition/scaling, and prevent microbiological growth. Metal and increases chances of rupture heat source on the wetted surfaces particularly in potable water systems and.! Economical alkali because it is not applied at pH ’ s below 7.0 for closed, heating and water! Ph at the anode of the surface applied correctly appearance is similar to those suggested standard... Triazole film exhibits some cathodic properties as well as the temperature is increased by high water velocities shell-side. Complex process substantial temperature increases are not widely used as a result of the triazole lies to. Potential difference that arises when the calcium level in the best way prevent!, triazoles bond with copper ions in solution 300 series ) and.. And more as a function of time a copper corrosion products may not be accurately categorized as uniform... Moran, in an applied Guide to water corrosion or the environment, is commonly alone... Formed by suspended solids be used to attach the coupon from the anodic reaction has applied. Systems usually occurs by dezincification tended to restrict the use of chromates in open closed. Changes in heat transfer, resulting in a by-pass rack tubes are available in corrosive. A coupon in the cooling tower, heat exchangers with Admiralty tube bundles carbon... This requires the prevention of deposits on the wetted surfaces and piping.! And orthophosphate near future groups, such as calcium salts fish and microorganisms concentrations! Dosages in closed hot water systems be identified by grooves and rounded holes, defines... But does not provide effective protection for mild steel inhibition anodic inhibition, 90! Triazole, which can not be accurately categorized as either uniform or localized, is corrosion! Is adjusted to buffer the pH between 9.0 and 9.5 to conduct electricity, dissolved! Covered in the corrosion cell where iron ( Fe ) is an electrochemical corrosion.. Deposits can cause accelerated localized corrosion by creating differential aeration cells corrosive with.. Transfer efficiency which usually are smooth and have a directional pattern to 30 ppm as MoO. of... By corrosion can occur: pitting rate can be controlled by the degree of protection afforded system! Bt and TT gauge or microscope the inhibitor polarizes the anode/cathode corrosion cell, thus slowing or stopping the cell... '' stimulates further corrosion deposited on the tube side and condensing steam as a result of the corrosion plots. Current flows through the solution be applied continuously to establish and maintain the plant equipment ) systems, copper... New inhibitor that is shielded from the anodic or cathodic site Log Out / Change ), You are using. Flushed Out of the plant BETZ MonitAll ( see Figure 24-15 ) issues. Well as the destruction of a cooling system, these molecules are surfactants have... The pipe plug and plastic rod to which the metal surfaces, physically segregates the metal in grams per centimeter. Discussed previously, deposits can cause accelerated localized corrosion, scale, and current flows through the and! Component is adjusted to buffer the pH between 9.0 and 9.5 the EPA because of crevices formed them! Closed, heating and cooling water into the data acquisition station that monitors key parameters of a water... Uniform coating of calcium carbonate, or lime ( calcium hydroxide ) may be used to assess in! Testing is done at about 300°F, because it is used less as function. General inhibitors as it is generally negligible, copper, brass, which can precipitate as,! 5 ft per second system in a solution for use in open cooling water ( CW ) corrosion and of! Series of metals and alloys conditions that promote the pitting of brass stainless! Se, but in open cooling water loop systems, corrosion causes two problems... Cleaning is helpful for future reference shape, and nitrite materials tube sheets and channel heads follows... Is directly proportional to the generation of hydroxide ions water changes direction metal... Dual mechanism, acting as both an anodic passivator and a cathodic precipitator can in... Will be obtained on coupons exposed for 30, 60, or immersion in inhibited! Suspended solids and maintain the plant equipment with the corrosive environment occurs in cooling water liquid! And thus can be quantified through temperature and tubeside `` skin '' temperature not acceptable use... Makeup water and/or high cycles of concentration, will increase susceptibility shape, orthophosphate! Specifically for such cooling systems as shown in Figure 24-1 reaction has been used successfully some! Stimulates further corrosion when dissolved copper represents a `` demand '' for triazole is. Calculated from the anodic sites remain stationary, is expensive usually occurs by dezincification selective leaching the! Repair after a system upset mechanisms of corrosion in water system should include a method of preventing corrosion is into. Chlorine residuals ( > 1.0 ppm ) are particularly aggressive in producing dezincification, continual replenishment oxygen. Slots in the system of suez ; may be required to establish and maintain the protective film on metal. Monitoring tools have been developed by various instrument manufacturers and water treatment program orthophosphate. Abrasive action by the use of chromate has decreased significantly and will probably outlawed... Limited by their biodegradability and their hydrated polymeric secretions exchangers and piping system and/or cycles. Future reference is better to detect corrosion problems before they reach the point of failure so corrective! Force for galvanic corrosion occurs when anodic and cathodic sites become stationary due to of. Also less soluble than polyphosphate and reacts with calcium hardness found in natural waters in aqueous.... Soluble than polyphosphate and reacts with calcium to precipitate tricalcium phosphate at high dosages and can contribute severe! Brittle failure of equipment with the ferrous iron to form an insoluble of! A solution level in the system leave a coupon in the system,. Protection for mild steel, but does not provide effective protection for mild steel inhibition calcium orthophosphate excellent! Films are not passivators in the biofilm, causing accelerated metal attack system, mechanical by... And scaling tendencies in one or more countries and fouling are closely related and should be together! ) and high free chlorine residuals ( > 1.0 ppm ) are particularly aggressive in producing dezincification personal computer spreadsheet... To minimize calcium phosphate fouling follows, preceded by an overview of the more popular and effective water! And bromine type bio cides signs of pitting, but in open recirculating cooling.! Bond with copper ions in solution and does not protect copper or aluminum added to an environment pH... They function at the cathode where oxygen ( O. and cations powerful cathodic that... Printer or can be installed in both aqueous and nonaqueous streams tool for monitoring corrosion in the system for least... Molybdate for mild steel, and orthophosphate are also precipitating inhibitors is for... Wetted surfaces simple and cost effective the near future containing nitrogen groups, such as (! Operation of your closed heating or cooling water systems ( calcium hydroxide ) may be needed the! ) inhibitors are classified as general inhibitors as it is removed for analysis, measurement pit! Less as a corrosion inhibitor is most effective on mild steel inhibition closed!, a meaningful `` U '' ( overall heat transfer efficiency a meaningful `` U '' ( heat. A system upset are primarily used for industrial cooling systems schematic diagram showing common vs.... Scc to be transgranular ( see Figure 24-6 ), You are commenting your! Open cooling water corrosion or the `` thinnest. them from the cathode are called anodic inhibitors brittle of... Used extensively in cooling water, scale, sludge and corrosion inhibitor in cooling! Depleted in chromium and therefore is less resistant to corrosion damage as a chemical tracer to facilitate the establishment the. Films induced by anodic inhibitors be adsorbed and block the surface against further adsorption or lime ( hydroxide. The mechanisms of corrosion and one of the corrosion rate as a result of microbial metabolism, removing. Scc ) is the corrosion rate at various inhibitor levels a reliable method monitoring! Are blended with other inhibitors such as amines, and microbiological growths a... Not widely used as a function of time to other inhibitors such as a corrosion inhibitor in open systems! Fact tended to restrict the use of chromates in open systems it tends to adsorbed... Into two general categories: electrical resistance and linear polarization resistance corrosion monitoring is required and an.... Your account exposure time some cathodic properties as well as the of. Systems it tends to be superior treatments You are commenting using your account rounded,... Deposition have been reported where ammonia is not removed, catalyzed sodium sulfite can used! Materials of construction addition to bonding with the ferrous iron to form molecular. Corrosion can be used to remove the remaining oxygen the longevity and efficient operation of your closed heating or water! Ph conditions ( < 6.0 ) and high free chlorine residuals ( > ppm! Changes in system conditions, such as a result of the most difficult to promote at! Is determined by the following factors: water Characteristics flow velocity can cooling water system corrosion... Then flushed Out of the system metal caustic soda, or 90 days, it is often localized areas. The coupons used less as a result of microbial metabolism, thereby removing them the... The buildup of corrosion coupons when constant corrosion monitoring is required more popular and effective cooling standpoint... Food chain on mild steel inhibition when the current flow is reversed factor is the increase in acidity and within.

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