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CORROSION COUPON MANUALCORROSION COUPON MANUALCONFIDENTIALCORROSION COUPONSOverviewCorrosion coupons have long been used as a monitoring tool in cooling water systems. The information they provide has been used to evaluate current treatment programs as well as to compare current results with those seen in thepast. By comparing the final coupon weight with the initial weight, it is possible to calculate a projected annual mils per year (mpy) corrosion rate. Historically, thishas been the primary way in which coupons were used.There is, however, far more information that can be gathered from a corrosioncoupon than corrosion rate. Some indication of scaling tendencies as well as the deposition resulting from corrosion can be observed upon visual examination ofthe coupon surface. Microbiologically Influenced Corrosion (MIC) can also be observed on these test strips.This manual of photographs has been assembled to assist in the evaluation ofcooling water treatment performance. An attempt was made to secure across-section of coupon conditions for inclusion in this manual. Conditions suchas dezincification, microbiologically influenced corrosion, cathodic protection,and fouling are presented. Included with each set of photographs is an explanation of the probable cause of the condition observed and appropriate recommendations. It is hoped that the field person, by comparing actual couponsto these photographs, will be able to give a reasonably accurate evaluation of thecause of the condition. Remember, however, that coupons are only one tool in evaluating program performance. Coupon corrosion performance must be assessed in relation to water chemistry, treatment residuals and control of operating parameters (pH, halogen, cycles, etc.).It is important to remember how to properly use corrosion coupons. They are most valuable when used to compare with one another and to evaluate such things as program changes or system upsets. By maintaining a constant size, metallurgy, location,water flow and replacement time, the only variable becomes the water condition.Corrosion CouponsPre-weighed metal coupons are still widely used as a reliable method for monitoring corrosion in cooling systems. Coupon weight loss provides a quantitative measure of the corrosion rate, and the visual appearance of the coupon provides an assessment of the type of corrosion and the amount of deposition in the system. In addition, measurement of pit depths on the coupon can indicate the severity of the pitting. The drawbacks to metal coupons are they do not have an applied heat flux (they are not heated), and the flow velocity of a specific heat exchanger may be difficult to simulate in the coupon bypass rack.Coupon RackTest coupon racks are made of various materials, with the selection determined by the application. GE Betz distributes and recommends using a PVC SCH 80 1-inch arrangement of pipes, nipples, elbows, and tees to be assembled into a coupon rack. The coupon racks are made of other materials, such as carbon steel or stainless steel. These metal racks are selected over the standard GE Betz rack for a number of reasons, including pressure, temperature, or ASTM standard restrictions. The PVC rack can be used with any coupons in a cooling system where pressure and temperaturelimitations are approximately 70 psig (4.8 bar) of pressure and 140°F (60°C), respectively.Coupon Rack InstallationThe coupon rack should be installed on the return header because this water is warmer than the supply water header from the tower and will simulate a more severe duty. It is important to have the coupon rack line installed off the top of the header to minimize debris and solids plugging the rack. To avoid air binding, pipe the coupon rack so that water will flow upward through it, and in such a way that it will remain full of water at all times and not backdrain when the main recirculating system shuts down. To maintain a constant flow velocity, a Dole~ or other suitable flow control valve isrecommended to ensure constant velocity. For additional installation and/or general information on the coupon rack, refer to Equipment Fact Sheet EF14-08 0010.For even more representative corrosion and deposition data, coupon racks and coupons can also be installed throughout a unit. The outlet of an exchanger simulates temperature and other conditions closer to that exchanger than the return water header. The hotter temperatures better simulate scaling tendencies on unheated coupons. There are no electrical parts to a coupon rack, therefore they can be located in any environment, including an explosion proof rated area.Coupon HoldersTest coupon holders are made of various materials with the selection determined by the application. The material of the coupon holders are typically the same as the coupon rack. GE Betz distributes and recommends using the PVC SCH 80 1-inch NPT coupon holders.The material of choice for screws and nuts is nylon. The conditions in a cooling water loop typically involve low temperatures and pressures, where PVC coupon holders and nylon screws are applicable. If nylon screws are unavailable, the metallurgy of the screws and nuts must match the metallurgy of the coupons to prevent galvanic interference. GE Betz does not recommend attempting to simulate galvanic coupling within the unit by mix-matching metals with monitoring coupons. This could lead to a large misrepresentation of system conditions and treatment performance.Retractable coupon holders can be used to place coupons near, or in, key exchangers. The theory behind installing the coupons close to, or into, an exchanger is to attempt to most closely simulate system conditions. Closer to actual temperatures can usually be obtained in the outlet of an exchanger, rather than the return water header. Therefore, the result of these coupons may be different than the ones in the return header, because of the differences in conditions.Coupon MetallurgyThe coupon metallurgy is selected to match the metallurgy of the cooling system. The type of metallurgy is extremely important in order to assess the treatment program performance on the correct metallurgy. The use of the coupons is necessary to measure a representative corrosion rate. Carbon steel and Admiralty coupons are the most commonly used coupons although others are available. Carbon steel coupons are available in a pretreated or unpretreated finish. The pretreated coating on the carbon steel coupons is a zinc phosphate coating. The pretreated coupons are used to represent the metallurgy in a prepassivated system. Generally, non-pretreated coupons provide a better indication of treatment performance.Coupon InstallationCoupons should be installed properly in a corrosion coupon bypass rack with continuous, controlled water flow past the coupons. The normal flow rate through the bypass is 3 to 5 gpm (1.35 to 2.25 ft/sec or 0.40 to 0.67 m/sec velocity with 1-inch pipe) to simulate velocities in the actual equipment. The coupons should be installed in the direction of the water flow (i.e., water should flow from the coupon holder to the tip of the coupon).There are a number of locations within the coupon holder where a number of metallurgies can be monitored with one holder or rack. The positions and locations that the coupons are installed into the rack could affect the results when using certain metallurgies. The order and the orientation of the coupons in the rack are important and will cause the results to be misleading. The copper, 90/10 copper/nickel or Admiralty coupons should always be installed in the last positions of the coupon rack (closer to the discharge of the rack) to avoid galvanic attack of the carbon steel coupons. The last position is in reference to the flow direction. Coupon ExposureCoupons should be exposed for 30, 60 or 90 day intervals. The length of time a coupon is exposed depends on the objective of the study. If fast results are desired, test specimens are usually removed in pairs after exposure for 7 - 21 days. For routine monitoring, where a stable, well established program is in progress, single coupons may be removed at monthly intervals, or even once every two or three months. For purposes of comparison, it is suggested that the coupon replacement period remain fixed. If it is necessary to remove coupons at times different from the scheduled interval, install additional coupons for this purpose. Coupon ResultsInterpretation of corrosion coupon results is conducted in a number of ways, including both physical appearance, weight loss, and surface analyses. The appearance and weight loss for corrosion rates should both be considered for a complete description of analyses. A surface analysis can be conducted to determine the components of the corrosion product or deposition on the coupon surface. The weight loss per known surface area leads to a known quantity of metal loss, with units of mils per year (mpy) where there are 1,000 mils per inch. Copper is one thing that is typically looked for by doing a surface analysis.During the time the coupon is installed, routine inspections can be performed to assess program results (e.g., once a week). If a change in coupon appearance is noted, changes in the treatment active level(s) or system operating conditions can be made, if warranted, to optimize program performance. During these inspections it is recommended that the coupon be air dried to determine if there are any trace amounts of deposit on the coupon surface. Normally a trace amount of deposit on the coupon surface will not be detected when the coupon is first removed and will appear only as the coupon dries. If a trace amount of deposit is observed, the necessary corrective action should be taken to ensure that similar deposits are not occurring on exchanger heat transfer surfaces. If pits or tubercles are observed during a routine inspection, the coupon should be rechecked the following week. During the next inspection, if the pit has not increased insize, the treatment program is providing effective corrosion control and no furtheraction is required. On the other hand, if the pit or tubercle has increased in size,this is an indication that the pit is an active site and that the program is not providing sufficient cathodic protection. Changes in the treatment program and/or operating conditions need to be made to stop the documented pitting.Coupon Monitoring Tipsaluminum coupons. Aluminum coupons should be installed upstream of any copper or copper alloy coupons.∙Ni/Cr mesh coupons are good for monitoring microbiological sessile populations.∙If the outlet of the coupon rack does not open to the atmosphere, an appropriate pressure difference needs to be maintained to develop an acceptable flow velocity.∙Install coupon racks away from chlorination and chemical feed points that could bias results.IDENTIFICATION OF COUPONSADMIRALTYA-1 BLACK DISCOLORATION - Neutral pH program with Zinc A-2 DEZINCIFICATION: - Neutral pH programA-3 DEZINCIFICATION: Recycle water, High Cl2 - Neutral pH program with TTA A-4 DEZINCIFICATION: High Cl2, H2S - Neutral pH program with TTACARBON STEELS-1 GAMMA IRON OXIDE: Neutral pH programS-2 CALCIUM PHOSPHATE BARRIER FILM: Phosphate based program for Once Through CoolingS-3 ETCHED APPEARANCE: Possible Process Leak - Neutral pH programS-4 GENERAL CORROSION - UNIFORM: Low calcium, Well water iron - Neutral pH program with zincS-5 GENERAL CORROSION - NON-UNIFORM: High Suspended Solids, Oil Contamination - Polymer/Lignin programS-6 GENERAL CORROSION - NON-UNIFORM: High Suspended Solids, Oil Contamination - Polymer/Lignin programS-7 LOW CALCIUM, PITTING: Phosphate program for Once Through CoolingS-8 LOW CALCIUM, BLACK STREAKING CORROSION: High pH, high Alkalinity program with HEDPS-9 LOW CALCIUM, PITTING: Neutral pH programS-10 MB CORROSION: Neutral pH programS-11 MB CORROSION: Sulfate Reducers - Neutral pH program S-12MB CORROSION: Sulfate Reducers - Neutral pH program S-13 MBCORROSION: Neutral pH programS-14 CORROSION AND DEPOSITION: Neutral pH programS-15 CALCIUM PHOSPHATE DEPOSITION: Neutral pH programS-16 CALCIUM/IRON/PHOSPHATE DEPOSITION: Well water iron - Neutral pH programS-17 PITTTING / GALVANIC CORROSION: Neutral pH program with TTACORROSION COUPON MANUALCONFIDENTIAL A-1CORROSION COUPON MANUALCONFIDENTIAL ADMIRALTYA-1Metallurgy: Days Exposed: Admiralty 42Corrosion Rate: Treatment Program: Controls: 0.2 mpyNeutral pH program with zinc pH = 7.0 - 7.2Water Analysis (Typical): Cycles = 6 - 7pH =6.8Cu= 0.15 Conductivity =3,600 Fe =0.29M-Alkalinity =10 Na =292SO4 =1,030SiO2 =40Cl =503Zn=2 Total Hardness = 1,110Ca =692Mg =418Probable Cause of The black tarnish is indicative of a copper oxide layerCoupon Condition: on the Admiralty coupon. Although the couponappearance does not look good, the corrosion rateof 0.2 mpy is within the acceptable limits forAdmiralty.Recommendations: Continue to monitor corrosion results closely. Monitor azoleresiduals. If azole residual <1.0 and/or corrosion rateincreases, increase the azole concentration. Note: the0.15 ppm copper concentration detected in the tower wateris high and can lead to potential galvanic corrosionproblems if sufficient azole levels are not maintained tocomplex the copper. Examine carbon steel coupon forcopper plating.CORROSION COUPON MANUALCONFIDENTIAL A-2A-3CORROSION COUPON MANUALCONFIDENTIALADMIRALTYA-2Metallurgy: Days Exposed: Admiralty 49Corrosion Rate: Treatment Program: Controls: 0.2 mpyNeutral pH programpH = 7.0 - 7.2 Conductivity = 3,500- 4,000Chlorination:Water Analysis (Typical):ContinuouspH = 5.8Cu <0.05 Conductivity =3,200Fe =0.3M-Alkalinity =10 Na =383SO4 =1,350SiO2=54 Cl = 291 T (Total PO4) =12.9Total Hardness = 1,100TIP (Inorg. PO4) =10,9Ca =676 OP(o-PO4) =8.2Mg =424Top of coupon contains some noticeable dezincification. Thereis also a dark discoloration/tarnish to the typical Admiralty yellow.Both phenomenons might be the result of low pH at high chlorine levels. The acceptable corrosion rate and low copper levels in the bulkwater do not reflect a problem, but the noticed dezincification indicates a localized corrosion mechanism.Control system pH within the target range and limit free chlorine residuals to <0.5 ppm. The water analysis indicates low orthophosphate levels, which infers low azole residuals sincethe inhibitors are all in one product. Make sure the appropriateazole residual is maintained to minimize Admiralty corrosion.Probable Cause of Coupon Condition: Recommendations:CONFIDENTIALADMIRALTYA-3Metallurgy: Days Exposed: Admiralty 41Corrosion Rate: Pitting Rate: 1.6 mpy 14.4 mpyTreatment Program: Controls: Neutral pH program with TTA pH = 7.2 - 7.4Chlorination:Water Analysis (Typical):NaOCl is added to maintain a free residual of 0.5 - 0.8 ppmand a total residual of 1.0 - 1.5 ppm.pH = 6.8 Cu =0.06 Conductivity =6,410 Fe =2.0M-Alkalinity =44 Na =1,050SO4=1,398 SiO2 =28.7Cl =1,205 T (Total PO4) =18.6Total Hardness = 709 TIP (Inorg. PO4) =14.5Ca =402 OP(o-PO4) =11.9Mg =306The tower is subjected to recycle water that increases thechlorine demand dramatically. The high conductivity, chlorideand sulfate concentrations make the water very aggressive tothe system metallurgy and in particular to yellow metal. Included in this recycle water is a very high level of sulfide.During these times the chlorination rate is extremely high. Thevery high Admiralty coupon corrosion rate appears to be typicalof low pH and/or high chlorine environments. Some dezincification is also noted on this particular specimen.Limit NaOCl feed to 4 gal/day/1,000 gpm recirculation rate. Ifeffective MB control cannot be maintained at this upper NaOCllimit, shock feed a non-oxidizing biocide with continuous feed ofa biosurfactant. Increase azole concentration (TTA) or switch to aproduct containing HRA, the azole of choice for these conditions.Probable Cause of Coupon Condition: Recommendations:CONFIDENTIAL A-4CORROSION COUPON MANUALCONFIDENTIAL ADMIRALTYA-4Metallurgy: Days Exposed Admiralty 25Corrosion Rate: Treatment Program Controls: 0.9 mpyNeutral pH program with TTA pH = 7.0 - 7.5Chlorination:Water Analysis (Typical): Conductivity = 5,000 - 6,000Orthophosphate = 10 - 15 ppm0.6 Continuous to maintain a 0.2 ppm free residualpH =6.8 Cu =Conductivity =6,250 Fe =0.36M-Alkalinity =44 Na =980SO4 =620 SiO2 =147 Cl = 1,610 TP (Total PO4) = 11.3Total Hardness = 861 TIP (Inorg PO4) =11Ca =583 OP(o-PO4) =10.7Mg =278Probable cause of This is an excellent example of dezincification. It iscoupon condition: generally associated with low pH and excessivechlorination. H2S is sometimes present in this tower. Recommendations: Limit Cl2 feed to 4#/day/1,000 gpm recirculation rate. If effective MBcontrol cannot be maintained at this upper Cl2 limit, shock feeda non-oxidizing biocide with continuous feed of a biosurfactant.Increase azole concentration (TTA). Switch to a productcontaining HRA, the azole of choice for these conditions. Note:the 0.6 ppm copper concentration detected in the tower water isvery high and can lead to potential galvanic corrosion problemsif sufficient azole levels are not maintained to complex thecopper. Examine carbon steel coupon for copper plating.CORROSION COUPON MANUALCONFIDENTIALS-1S-2CARBON STEEL S-1Metallurgy:Days Exposed: Corrosion Rate: Treatment Program Controls: Carbon Steel300.2 mpyNeutral pH programo-PO4= 12 -14 ppm Calcium = 600 (max) ppm pH = 7.0 - 7.2 Cycles =4-5Water Analysis (Typical): pH= 7.1Cu =<0.05Conductivity =2092 Fe =0.5M-Alkalinity =52 Na =99SO4 =342 SiO2 =26Cl = 144 TP (Total PO4) =17.3Total Hardness = 785 TIP (Inorg PO4) =15Ca = 555 OP (o-PO4) =11.7Mg =230Probable Cause of This is a good example of a gamma iron oxide film formedCoupon Condition: by high orthophosphate concentrations. The passivatedcoupon often is dark gun metal blue in color.Recommendations: Keep the soluble orthophosphate residuals within the required controlrange to maintain low carbon steel corrosion rates.CARBON STEELS-2 Metallurgy: Carbon SteelDays Exposed: 183Corrosion Rate: 0.7 mpyDeposit Analysis: P2O5 (Major) Ca (Major)Fe2O3 (Major) Cu(Trace)Treatment Program: Phosphate based program for once through coolingProbable Cause of The film appears to be calcium phosphate. Where breaks Coupon Condition: occurred in the film, corrosion began. Pitting then led to thesubsequent deposition at the pit sites. In this case, however, thecorrosion rate is acceptable since the deposit provided a barrier typefilm which minimized corrosion.Recommendations: Monitor exchangers to ensure that calcium phosphate is notdepositing out on the heat transfer surfaces.CORROSION COUPON MANUALCONFIDENTIAL S-3S-4CORROSION COUPON MANUALCONFIDENTIAL CARBON STEELS-3Metallurgy: Days Exposed: Pretreated Carbon Steel 49Corrosion Rate: Treatment Program: Controls: 0.3 mpyNeutral pH programpH = 7.0 - 7.2Chlorination:Water Analysis (Typical): Conductivity = 3,500 - 4,000Phosphate = 10 - 15 ppmContinuouspH =7.0 Cu <0.05 Conductivity =3,200 Fe =0.3M-Alkalinity =10 Na =383SO4 =1,350 SiO2=54Cl = 291 TP (Total PO4) = 16.9Total Hardness = 1,100 TIP (Inorg PO4) =12.9Ca =676 OP(o-PO4) =12.2Mg =424Probable Cause of There appeared to be some etched areas but they have noCoupon Corrosion: real depth to them. The low corrosion rate indicates that no seriousproblem exists. The appearance could be indicative of poorpretreatment of the specimen or possibly the result of a process leak.The exposure time was sufficiently long that if a serious problemexisted it would have been obvious by now. Recommendations: Monitor system closely to ensure that effective corrosion andmicrobiological control are maintained. If an increase inmicrobiological activity is detected, this fact coupled with thecoupon appearance can be indicative of a process leak.CORROSION COUPON MANUALCONFIDENTIALCARBON STEELS-4Metallurgy:Days Exposed: Corrosion Rate: Treatment Program: Controls: Carbon Steel818.3 mpyNeutral pH program + zinco-PO4= 17 - 20 ppm TDS = 6,500 (max.) Cycles= 3 -4pH = 7.6 - 8.0Zn = 0.5 - 1.0 ppmChlorination:Water Analysis (Typical): Continuous to maintain a 0.3 ppm free residual.<0.05pH =7.6Cu =Conductivity =7,630 TotalFe =8.7M-Alkalinity =248 Na =1,720SO4 =2,040SiO2=41 Cl = 1,235 TP (Total PO4) = 13Total Hardness = 47 TIP (Inorg PO4) = 13Ca = 29 OP (o-PO4) =12Mg =18Probable Cause of The make-up water to this system is well water withCoupon Condition: 0.9 ppm of iron. However, this coupon does not look like the classiccalcium-iron-phosphate deposition. It appears to be simplygeneral corrosion due to the low orthophosphate residuals beingmaintained for the tower operating conditions, i.e., low calcium,high conductivity.Recommendations: Maintain soluble orthophosphate residuals within the required controlrange to minimize steel corrosion. If effective steel corrosionprotection still cannot be obtained, increase zinc level.CORROSION COUPON MANUALCONFIDENTIAL S-5CORROSION COUPON MANUALCONFIDENTIALCARBON STEELS-5Metallurgy:Days Exposed: Corrosion Rate: Deposit Analysis: Carbon Steel2521.5 mpyFe3O4 =71% LOI =12% CaO =8% CO2 =5% SO3 =3% SiO2 =1%Treatment Program:Chlorination:Water Analysis (Typical): Polymer / Lignin - Intermittent feedSurfactant (anionic) - Intermittent feed0.05 IntermittentpH =7.6Cu < Conductivity =1,400 Fe =5.5M-Alkalinity =51 Na =98SO4 =408SiO2 =5.5 Cl = 139 TP (Total PO4) < 0.4Total Hardness = 592 TIP (Inorg PO4) < 0.2Ca =406OP(o-PO4) <0.2Mg =186Probable Cause of Severe general corrosion is due to the lack of a corrosion Coupon Condition: inhibitor. The system is subject to high suspended solidsas well as oil contamination.Recommendations: Apply steel corrosion inhibitors (orthophosphate and/or pyrophosphate)at the appropriate concentrations for the tower operatingconditions, i.e., pH, calcium, etc. Polymer must be fedcontinuously and at a level to keep the phosphate soluble sothat it can act as a corrosion inhibitor as well as to keep thesuspended solids dispersed and off the heat transfer surfaces.Although there is no evidence of Microbiologically InfluencedCorrosion (MIC), chlorine should be fed continuously tocontrol microbial activity.CONFIDENTIAL S-6CONFIDENTIAL CARBON STEELS-6Metallurgy: Days Exposed: Carbon Steel 25Corrosion Rate: 27.0 mpyTreatment Program: Chlorination:Water Analysis (Typical):Polymer / Lignin - Intermittent feedsurfactant (anionic) - Intermittent feedIntermittentpH =7.6Cu <0.05 Conductivity =1,400 Fe = 5.5M-Alkalinity =51 Na =98SO4 =408SiO2 =5.5 Cl = 139 TP (Total PO4) < 0.4Total Hardness = 592 TIP (Inorg PO4) < 0.2Ca =406OP(o-PO4) <0.2Mg =186Severe general corrosion is due to the lack of a corrosion inhibitor. The system is subject to high suspended solids aswell as oil contamination.Apply steel corrosion inhibitors (orthophosphate and/or pyrophosphate) at the appropriate concentrations for the tower operating conditions, i.e., pH, calcium, etc. Polymer must be fed continuously and at a level to keep the phosphate soluble so that it can act as a corrosion inhibitor as well as tokeep the suspended solids dispersed and off the heat transfer surfaces. Although there is no evidence of Microbiologically Influenced Corrosion (MIC), chlorine should be fed continuously to control microbial activity.Probable Cause of Coupon Condition: Recommendations:CONFIDENTIAL S-7。