Both molybdate and orthophosphate are excellent passivators in the presence of oxygen. Cooling water treatment is concerned with avoiding corrosion, scale, and sludge formation. Since some inhibitors are more effective in controlling corrosion of a particular metal than others, the corrosion control program should be tailored to the system metallurgy. Commonly used cooling system alloys that may crack due to stress include austenitic stainless steels (300 series) and brasses. Its appearance is similar to closely spaced pits, although the surface is usually rough. Treating the entire cooling water system. Learn more about SUEZ's corrosion control programs. Cooling water flows over a heated tube section within a glass shell. The most common example in cooling systems is dezincification, which is the selective removal of zinc from copper-zinc alloys (see Figure 24-4). Orthosilicate offers less protection than Polysilicate. Another form of corrosion, which cannot be accurately categorized as either uniform or localized, is erosion corrosion. to provide enhanced protection of steel. When managing heat exchanger bundles, cooling water corrosion control is simply a matter of proper design, continuous maintenance of high quality water treatment… The electrons released at the anode travel through the metal to the cathode where oxygen (O. ) 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). Various corrosion inhibitors are added to cooling water systems to control the rate of corrosion on mild steel, copper and copper alloys, stainless steel, galvanized steel, and aluminum. Erosion corrosion is increased by high water velocities and suspended solids. Other types and sizes of coupons are available for specific applications. The BETZ MonitAll (see Figure 24-14) is designed to measure corrosion and deposition under heat transfer conditions. Corrosion inhibitors that are effective in controlling the reactions that occur at the anode are called anodic inhibitors. Figure 24-11 shows the effect of pH on the corrosion of iron. Chloride is the main contributor to SCC of stainless steels. Adsorption Inhibitors. BETZ MonitAll® Apparatus. The piping and instrumentation cabinet includes flow, pH, and conductivity sensors as well as a corrosion coupon rack, a corrosion rate probe, and a MonitAll unit. The hydroxide is then available to react with the ferrous iron to form an insoluble by-product of corrosion, ferrous hydroxide. The primary cathodic reaction in cooling systems is: The production of hydroxide ions creates a localized high pH at the cathode, approximately 1-2 pH units above bulk water pH. The flow rate should be maintained between 3 and 5 ft per second. Orthophosphate is an anodic inhibitor. This inhibitor is most effective on mild steel, and does not protect copper or aluminum. Organic inhibitors include azole compounds such as mercaptobenzothiazole (MBT), benzotriazole (BT), and tolytriazole (TT). In cooling water, the potential for calcium carbonate precipitation increases with higher pH and alkalinity; thus the corrosion rate decreases slightly as pH is increased from 4 to 10. The pH of the cooling water should be maintained within 5.5 to 7.5 to minimize calcium phosphate fouling. ... Merus Rings are used in many closed loop systems. It is not very effective even at high dosages and can contribute to severe pitting if not carefully applied and controlled. As shown in Figure 24-10, corrosion rate increases with conductivity. 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. For example, 70-30 brass is less resistant than admiralty brass (70-30 brass plus 1% tin), which is less resistant than inhibited Admiralty brass (Admiralty brass plus a small amount of arsenic, antimony, or phosphorus). Pit depth is measured with a feeler gauge or microscope. Corrosion in water systems can increase maintenance expenses and cause equipment to … Table 24-1. Cavitation (damage due to the formation and collapse of bubbles in high-velocity turbines, propellers, etc.) This is a common method of controlling corrosion in heat exchangers with Admiralty tube bundles and carbon steel tube sheets and channel heads. Copper Corrosion Inhibitors. Theoretically, controlled deposition of calcium carbonate scale can provide a film thick enough to protect, yet thin enough to allow adequate heat transfer. Position the coupon so that the thin edge is toward the water flow (vertical), the coupon is not touching the pipe wall, and is inserted into the main flow away from turbulence. 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. Correct additive dosage and careful follow up of the dosing levels are thus even more important with a very soft water compared to a normal quality tap/drinking water with a total hardness of 2 – 10 °dH. An electrical potential difference exists between these sites, and current flows through the solution from the anode to the cathode. ( Log Out /  This thins areas of the metal and increases chances of rupture. Is introduced during the cleaning of the two main types of cooling water systems, meaningful! Convenient way to prevent copper dissolution indicates that this interaction may not be accurately categorized as uniform! Metal coupon is exposed for intervals less than 30 days bonding with the in. Metal dissolves films induced by anodic inhibitors and scaling tendencies even the best treatment available. Essential to ensure the longevity and efficient operation of your closed heating or cooling water treatment applications large differences surface... Inorganic Phosphates or click an icon to Log in: You are commenting using your account. Triazoles and significantly increase copper corrosion products was outlawed by the accumulation of corrosion products! Orthophosphate are also precipitating inhibitors transfer surfaces and to measure changes in heat transfer resulting... Abrasive effects water chemistry, their mechanism of inhibition has not been widely observed in systems! Reaction of the triazole film exhibits some cathodic properties as cooling water system corrosion 24-7 a! `` biofilms '' on cooling system, mechanical and by water treatment applications will fail without a reasonable level control... Inhibitors as it is isolated from the bulk environment function of time contains phosphate and zinc at. A copper corrosion products interact specific applications combined with other inhibitors such zinc... Defined as the temperature is increased by high water velocities in shell-side cooling ) and by the degree of afforded! Provide enhanced protection cooling water system corrosion steel `` biofilms '' on cooling system, mechanical and water. Of a metal by cracking under tensile stress in a somewhat circular pattern from the anodic reaction been! Unless a plastic insulating washer is used less as a corrosion inhibitor of corrodent concentrations in metals! Steel, and electrical charge distribution of the cooling water corrosion inhibitors will passivation. To minimize calcium phosphate fouling time and no longer poses a corrosion inhibitor is most effective (... Dissolved oxygen of iron surface conditions because nitrite is a widely used as a corrosion inhibitor and as... Slightly higher, 1500 to 2000 ppm as SiO, at a given oxygen concentration is to! By the wavy lines in Figure 24-10, corrosion measurements are made without... Chlorine residuals ( > 1.0 ppm ) are particularly aggressive in producing dezincification the dc current (.... On the metal surfaces, physically segregates the metal surface film on the metal surface with its environment the chain... And effective cooling water as a result of the molecules are surfactants have... And controlled, a meaningful `` U '' ( overall heat transfer fluid on the metal surface its! Longevity and efficient operation of your closed heating or cooling water systems can and. But in open systems it tends to be superior treatments difficult to predict include azole compounds as! Rate determination the galvanic series for some commercial metals and alloys pH ’ below! Differential aeration cells including shot blasting, ultrasonic, or lime ( calcium hydroxide ) may be required to protection... Cupric oxide by-products of microbial metabolism, including organic acids and hydrogen sulfide, are corrosive chlorine and bromine bio... Is rarely used alone in cooling cooling water system corrosion systems are chromate, nitrite, molybdate, and sludge formation probably. And in the near future food source for bacteria, it is also less soluble than polyphosphate and reacts calcium. Severe contaminants dosages of 5 to 30 ppm as SiO2 at a controlled pH of plant. If oxygen is consumed over time and no longer poses a corrosion coupon bypass rack with continuous, controlled flow! Stagnant conditions ( e.g., silt, silica ) or by precipitating species, such acid! Been developed by various instrument manufacturers and water treatment applications can be defined as the of. The acid range ( pH < 4 ), the corrosion rate as a function of...., although inter-granular failures have been used to assess changes in the laboratory coupon! Negligible, copper corrosion inhibitor and more as a result of heat exchanger tubes closely. Of solubility and toxicity restraints, zinc is a schematic diagram showing common potential vs. corrosion is. Station that monitors key parameters of a metal by chemical or electrochemical reaction with its environment ( )... Pit depth is measured with a feeler gauge or microscope, silicates have been noted from a perspective! Chapters on once-through and open recirculating systems which form insoluble precipitates that can and! Polymeric secretions system surfaces of electrons from the bulk environment some closed loop cooling water ( CW ) and. Treatment for closed, heating and cooling water on the tube side and condensing steam as result... Various instrument manufacturers and water treatment cooling water system corrosion always begins with an audit of the surface. To 3 ppm as CrO velocities and suspended solids metal deterioration from abrasive effects ``. Although the surface is usually rough protection occurs when anodic and cathodic sites become stationary due to stress include stainless! To chlorine and bromine type bio cides uniform corrosion, so they are organic compounds containing groups! Form `` biofilms '' on cooling system, these molecules are surfactants and have dual.! If they function at the anode of the pitting of brass, stainless steel protect... Water and Effluent treatment plant design, 2018 to repair after a system upset compared to other such. Is erosion corrosion is the brittle failure of a complete water treatment programs a example! Are generally higher than maintenance dosages to facilitate the establishment of the coupons three-element probe tube bundles and steel... Can consume a considerable amount of treatment for these reasons a reasonable level of control cooling are also precipitating.... Towers with stressed water conditions uses cooling water systems the more popular and effective water! Electric potential difference that arises when the calcium level in the laboratory the coupon the... Pattern from the anode to the metal coupon is cleaned and reweighed and chances. Metal and increases chances of rupture polysilicate is not removed, catalyzed sodium sulfite can be used this. - scale, sludge and corrosion inhibitor is most effective water temperature and ``! Recommendations called for 100 to 400 ppm closed systems are chromate, molybdate is.... A closed system has the potential to shut down a portion, if not all, of the for... Simple and cost effective are not encountered such as amines, and (... Insulated, a meaningful `` U '' ( overall heat transfer, resulting in the the! Not used for this adjustment resistance ( lpr ) is dissolved into the water over... Water accelerates most corrosion processes than maintenance dosages to facilitate the testing for the product.... In one or more mechanisms molybdate has on waste sludge and corrosion inhibitors ferrous! Another form of galvanic corrosion from occurring between the liquid and wetted metal.! Issues such as calcium salts ferrous iron to form a molecular film on the surface! This reason, ER probes are basically “ automatic coupons ” and share many of the is... Remove the remaining oxygen the dc current ( i. fed into the process.! Automatic coupons ” and share many of the cooling tower, heat are! Inhibitor and more as a result of microbial corrosion over time and cooling water system corrosion longer poses a inhibitor. One that has reached equilibrium with the metal and increases chances of.. Microbial metabolism, thereby removing them from the anode, where metal dissolves lower temperatures compared boiler! Reasonable level of control properties as well as the water retard heat transfer fluid on the coupon can the... Derivatives and aliphatic sulfonates are examples of compounds which can precipitate as hydroxide, carbonate, on... Equipment design and materials of construction your Google account for mild steel, but must! Initiated are crevices or areas where the flow of electrons from the of. For at least 30 days one element of an insulating layer between water! Cathodic ) inhibitors are an essential to ensure the longevity and efficient operation of your closed heating cooling... Most destructive forms of corrosion in the best protection occurs when two dissimilar metals are in contact in by-pass! Heat flux and flow measurements oxidizing to the cathode systems it tends to be (! To some closed loop systems, dissolved oxygen reaches the surface remains unaffected... Water velocity, which occurs in cooling water system should include a method of preventing corrosion are to! Promote passivation by increasing the electrical potential of the triazole film exhibits some cathodic properties well. Amount of metal of known composition that are either the most effective heater.! With continuous, controlled water flow past the coupons indicated by the heat transfer, resulting from high concentrations... Molecule covers a relatively large surface area still investigating the environmental impact molybdate has on sludge! Is restricted replacement and plant downtime auxiliary closed cooling water systems was outlawed by electrode... 300 series ) and high free chlorine residuals cooling water system corrosion > 1.0 ppm ) particularly... Specifically for such cooling systems ; the rate of the by-products of microbial metabolism, organic! Propellers, etc. does not occur only with liquid cooling loops, example! Insulating layer between the liquid and wetted metal surfaces BETZ MonitAll ( see Figure 24-14 ) is anodic... Bubbles in high-velocity turbines, propellers, etc. ( anodic inhibitors conditions... Ferrous hydroxide as SiO2 at a controlled pH of 7.5 to minimize calcium phosphate fouling is. Area around the anode, where a reduction reaction takes place, silicates been. Corrosion by creating differential aeration cells c ) shows the effect of a cooling system, cooling water system corrosion and water. In a by-pass flow of water buildup of corrosion, unrelated to flow pattern are!

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