RP0775-2005residual stress (such as introduced by stamping) andimposed stress. 2.1.2.3 Instances of SCC of carbon steel couponsunder oilfield conditions have rarely been reported. Nevertheless,broken pieces of coupons or holders can lodge downstream in valvesand intetfere with their normal operation. 2.1.3 Machine or polishthe edges of the coupon to remove cold-worked metal if thecold-worked edges adversely affect the data. Coupons formed bystamping are less expensive than machined coupons. Stamped couponsare satisfactory without additional machining for most oilfieldmonitoring. 2.1.4 Ideally, match the sutface finish of the couponswith the finish of the metal being investigated, .e., the pipe orvessel wall. Because this is seldom practical, other sutfacefinishes are applied. No specific sutface finish is absolutelyessential but uniformity is very important when data from differentsets of coupons are being compared. Coupons may be prepared bygrinding smooth with 120 grit paper, by tumbling with loose grit,or blasting with abrasive blasting material. A consistent finishmay be obtained by blasting with glass beads, but glass beads maynot remove mill scale or rust. All abrasives should be free ofmetallic particles. 2.1.5 After the coupons have been cleaned,handle them by suitable means to prevent contamination of thesutface with oils, body salts, and other foreign materials. Clean,lint-free cotton gloves or cloths, disposable plastic gloves,coated tongs, or coated tweezers should normally be used. 2.1.6Under a ventilated hood, remove any residual oils with ahydrocarbon solvent such as xylene, toluene, or 1 , l , Itrichloroethane and rinse with anhydrous isopropyl alcohol. If oilsare not present, cleaning with alcohol or acetone should besufficient. 2.1.7 Dry, measure, and weigh the coupons to withinkO.l mg. Record the mass, serial number, and exposed dimensions.Calculate the sutface area (including the edges) and record. Theareas covered by the coupon holder and shielded areas offlushmounted coupons must be excluded. (For test nipples or otherlarge corrosion test pieces, see Paragraph 3.6.) 2.1.8 Prior toshipment, store the individually packaged coupons in a closedcontainer with indicating silica gel.") Coupons may be wrapped inpaper or placed in envelopes impregnated with a vapor-phasecorrosion inhibitor. 2.2 Procedure for Field Handling of CouponsBefore and After Exposure 2.2.1 Prior to coupon installation,record the following information: coupon serial number,installation date, name of system, location of the coupon in thesystem (including fluid or vapor phase), and orientation of thecoupon and holder. A typical corrosion coupon report is shown inAppendix A. 2.2.2 During installation, handle the coupon carefullyto prevent contamination of the coupon sutface. (See Paragraph 2.15.) 2.2.3 When the coupon is removed, record the coupon serialnumber, removal date, observations of any erosion or mechanicaldamage, and appearance of scale or corrosion product. Any otherpertinent data such as shut-in time and changes in velocity andinhibitor treatment should also be recorded. The coupon should bephotographed immediately after removal, particularly if appearanceof the corrosion product or scale is important. 2.2.4 Protect thecoupon from contamination by oxidation and handling. Place thecoupon in a moisture-proof or special envelope impregnated withvolatile corrosion inhibitor and ship immediately to a laboratoryfor analysis. Do not coat the coupon with grease or otherwise alterit. Gentle blotting with tissue paper or a clean soft cloth may bedesirable to remove moisture prior to shipment. Corrosion productsor scale deposits should not be removed in the field. 2.3Laboratory Procedure for Cleaning and Weighing Coupons AfterExposure 2.3.1 Record the coupon serial number. If the coupon wasnot photographed in the field, it should be photographed in thelaboratory before and after cleaning. Prior to any cleaning, weighthe coupon to within kO.l mg. 2.3.2 Visually examine the coupon andrecord observations. Qualitative analysis of adherent scale orforeign material may be petformed. 2.3.3 Immerse the coupon in asuitable hydrocarbon solvent, such as clean xylene or toluene, longenough to remove the oil, oil-wet materials, and paraffin. Rinsewith isopropyl alcohol or acetone. Handle solvent under aventilated hood. Dry in a gentle dry air stream and weigh thecoupon to within kO.l mg if quantitative analysis of acid-solubledeposits is desired.
Nace Rp 0775 Pdf Freel
RP0775-20052.3.4 Immerse steel coupons in 15% inhibitedhydrochloric acid to remove mineral scale and corrosion products.Ultrasonic agitation may be used to accelerate the cleaningprocess. Numerous commercial inhibitors are available to protectthe steel during acid cleaning. The following inhibitor solutionhas been successful: A stock solution is made of 37.5% HCI to which1O g/L of 1,3-di-n-butyl-2 thiourea (DBT) has been added.3Immediately prior to use, the stock solution is diluted by slowlyadding a measured volume of stock solution to an equal volume ofdistilled water with stirring. Additional information on cleaningmetals other than steel should be consulted.4a5a6a7 2.3.4.1 Couponsthat are not coated with hard scale or tightly adhering corrosionproducts may be cleaned by blasting with glass beads. Mass lossduring blast cleaning should be determined by cleaning unexposedcoupons in accordance with Paragraph 2.3.7. 2.3.5 After cleaning,immerse the coupon in a saturated solution of sodium bicarbonatefor one minute to neutralize the acid. Rinse with distilled waterto remove the neutralizer. 2.3.6 Rinse the coupon immediately inisopropyl alcohol or acetone and dry in a stream of dry air. Airlines should be equipped with traps and filters to remove all oiland water. Coupons with tenacious films should be scrubbed with ahousehold cleanser and O00 steel wool prior to drying with alcoholor acetone. Visually examine the coupon and record observations.2.3.7 Subject a preweighed blank that was not exposed to thecorrodent to the cleaning process to ensure that mass loss fromcleaning is not significant. 2.4 Calculation of the AverageCorrosion Rate (CR). The following procedures should be used tocalculate the average corrosion rate. 2.4.1 Determine the mass lossof the corrosion coupon and divide the mass loss by the product ofthe metal density (Table I ) , the total exposed surface area, andthe exposure time to obtain the average rate of corrosion. Thefollowing equations may be used to determine the average corrosionrate depending on the units desired.6 2.4.1.1 A calculation ofaverage corrosion rate, expressed as a uniform rate of thicknessloss per unit time in millimeters per year or millimeters per annum(mm/y or mm/a), is shown in Equation (1):
RP0775-2005depth of deepest pit (mm) x 365 PR (mmly) = exposuretime (days)bottom of the pit. Metallographic cross sections throughpits provide an accurate measurement of pit depth if a high degreeof accuracy is deemed necessary. The same measurement techniqueshould be used on all coupons from a given system. Pit density perunit area should be reported. Additional information on themeasurement of pits can be found in ASTM) G 46. 2.5.3 Pittingcharacterization by calculation of pitting rate may be misleadingif pitting onset occurs after an incubation period. Time to pittingonset varies and pit growth may not be uniform. Therefore, careshould be exercised in applying calculated pitting rates to projecttime-to-fa iIure.
RP0775-2005with conventional valves is shown in Figure 5. Aninstallation assembly that requires a special fitting on a line orvessel is shown in Figure 6. When installation and removal ofcoupons from a pressurized system is contemplated, the systemdesign must accommodate the tool length. Overall length depends onthe distance from the access valve to the final insertion depth inthe pipe or vessel. 3.3.5 Coupon holders to secure a disk-typecoupon flush with the pipe wall are available. Coupons flush withthe pipe wall are subject to less turbulence than flat or roundcoupons that protrude into the flowing stream. Therefore, theflush-mounted coupons should provide information that is morerepresentative of corrosion on the pipe wall. The disk-type couponsshould be held in place with either plastic or coated steel screws.In some systems, iron sulfide may bridge between the coupon andpipe wall. The resulting short circuit can increase or decrease therate of corrosion on the coupon. 3.3.6 Coupon holders for placingcoupons in well tubing are also available. Coupons can be attachedto a tubing stop (see Figure 7),14 which may be available from somesubsurface pump suppliers and wire-line service companies. Anothercoupon holder that can be set by wire line in a side-pocket mandrelis available from gas lift equipment suppliers and wire-lineservice companies. 3.4 Location in theSystem--`,,```,,,,````-`-`,,`,,`,`,,`---
RP0775-20053.5.2 When coupons are used to evaluate and monitorcorrosion-inhibitor treatment, new coupons should be installed justprior to treatment. This is particularly important when there is along period between treatments (as in inhibitor squeeze, tubingdisplacement, and infrequent batch treatment of gas we IIs). 3.6Other Monitoring Devices 3.6.1 Test NippleslSpools. These arenormally short (300- to 900-mm [ I - to 341) lengths of tubulargoods of the same size and metal composition as the material usedin the system. If test nipples are made from the same material asadjacent piping, galvanic corrosion of the test nipple is not aproblem and insulating the nipples from the pipe should not benecessary. If the compositions of test nipples and piping aredifferent, electrical isolation should be used to prevent galvaniccorrosion. Electrical isolation of test nipples in lines operatingabove 14 MPa (2,000 psi) and 93C (200F) is practical only ifflanged spools are used for test nipples. nipples can be splitlongitudinally after mass loss is determined. 3.6.1.4 The externalsutface of the test nipple should be protected from atmospheric orsoil corrosion if the mass loss is to reflect only internalcorrosion. The addition of heavy flanges to a corrosion nipple mayprevent accurate direct mass-loss measurements. However, flangednipples can provide useful data on pitting rates. 3.6.1.5 Testnipples/spools should be cleaned, and volume, mass, or wallthickness measurements accurately determined prior to and afterexposure to allow calculations of corrosion rate during theexposure period.
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