Recommended Practice andProcedure for the Use of

Corrosion Coupons

In practice, the corrosion coupon is one of themost useful tools for monitoring corrosion. Couponsprovide accurate results at a reasonable cost, areeasy to use and can provide general information thatis both quantitative and visual on many types ofcorrosion. The information obtained is reliable,providing adequate care has been taken in handlingthe coupons and the relevant exposure parametershave been accounted for.

Some knowledge of the type of corrosionproblems that may occur in a line or system (and thetype of monitoring which is best suited to theseconditions) can be predicted at the design stage.While most other monitoring devices are designed togive information on one specific type of corrosionproblem, the corrosion coupon can record details ofseveral types of corrosion and should, therefore, beused from the outset of any corrosion monitoringprogram.

COUPON INSERTION, REMOVAL AND CLEANING

When working with corrosion coupons, followingprocedures for insertion, removal and cleaning iscritical in order to obtain accurate results.

Caproco coupons are supplied in hermeticallysealed bags, and each coupon is individually wrappedin vapour phase inhibited paper to give it a shelf life ofapproximately one year. The seal on the bag shouldnot be broken until the coupons are to be used, toensure that atmospheric corrosion of the coupons iskept to a minimum.

When attaching coupons to the coupon holder, itis recommended that clean laboratory gloves (P/N11239) are worn to prevent moisture and body oilspresent on the skin from coming in contact with theunexposed coupon. Gloves should also be wornwhen removing exposed coupons from the couponholder.

Exposed coupons should be cleaned andweighed as soon as possible after their removal.There are several methods for cleaning coupons aftertheir exposure. These fall into two basic categories:mechanical cleaning and chemical cleaning. Bothapproaches have advantages and disadvantages.

Chemical cleaning methods are sometimesunable to remove scale and dirt that has bonded itselfonto the coupon surface. Mechanical cleaning, onthe other hand, may remove metal from the surface ofthe coupon, although this can easily be accounted forby including a metal loss factor when calculating thecorrosion rate of the coupon.

TYPE OF CORROSION COUPONS

1. Strip Coupons

Due to its large surface area, the strip couponprovides the most area of all coupon types on whichcorrosion can develop and be readily observed. Thestrip coupon also gives more accurate results ofweight loss because it has a larger mass than othercoupons.

2. Disc Coupons

The disc coupon can most closely simulate theconditions that are occurring on the pipe wall. Unlikeother coupons, they can be flush mounted. This givesthem the advantage of not interfering with any piggingthat may be done on the line. The disc coupon ismounted on the coupon holder in a plane at rightangles to the holder stem, thereby avoiding anynecessity to orientate them with the system flow.Multiple disc coupon holders are very popular in lineswhich carry more than one phase, since a disccoupon can be positioned in each of the phases.

3. Rod Coupons

Rod coupons are generally used where a longerexposure time is required. Since six coupons can bemounted on a single holder, the insertion and removalof the coupons is staggered, with only two couponsbeing removed at any one time. This allows theoperator to collect data from the coupons regularly,but at shorter intervals than would otherwise bepossible.

4. Coupons with Applied Stress

Coupons with applied stress are proven to yielduseful results in environments where stress corrosioncracking is suspected to occur. A strip coupon isclamped to a holder so that a stress simulating that ofthe pressure vessel or pipeline is present in the

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coupon. The coupon is inserted and removed in thesame way as other coupons so that the resultsproduced with the coupon can be readily inspectedand tested.

5. Coupons with Residual Stress

The coupon with residual stress has beenpurposely cold worked with a definite force.Consequently, this type of coupon contains elasticstrain prior to exposure. While cold-workingincreases the hardness of the metal, it reduces itsductility. In an embrittling environment, the metal issusceptible to catastrophic failure. With the use ofthis type of coupon, the effects of the environment canbe detected and when necessary, corrective actiontaken before any resulting failure occurs.

6. Scale Coupons

This type of coupon is useful in determiningwhether scaling in the pipeline or vessel is asignificant problem by artificially encouraging scale todevelop upon it. The coupon has several holes ofvarious sizes to allow scale to accumulate, and ismounted similarly to strip coupons on a strip couponholder.

RECOMMENDED PROCEDURE FOR THE APPLICATION OFCORROSION COUPONS

The procedure is similar for the various types ofcoupons and their appropriate holders. Couponholders are mainly designed to be attached to solidplugs, but Caproco offers coupon holders that may beused with a hollow plug.

The instructions for insertion and removal of acoupon holder attached to a solid or hollow plug insystems under pressure are presented in detail in the�Caproco Retriever and Service Valve Operation�Manual.

1. The solid plug should be removed from theaccess fitting using the retriever and servicevalve as described in the operations manual.

2. The solid plug should be removed from theretriever, taking care not to touch the coupons onthe coupon holder.

3. Inspect the coupons and coupon holder formechanical damage. Note the condition of thecoupon(s) (e.g. heavily coated with dirt, coveredwith scale, covered with greasy/non-greasy film,no deposits, etc.).

4. Remove the coupons from the coupon holder,taking care not to touch the coupons with barehands or tools.

5. Wrap each coupon separately after making anote of the serial number on the coupon and itslocation in the line (e.g. top of line, bottom ofline, 3 o�clock, etc.)

6. Unscrew the set screw on the coupon holder nutusing an allen key. The coupon holder shouldthen be removed from the solid plug (note: thecoupon holder has left hand threads). Thecoupon holder should be brushed (whennecessary) and wiped clean.

7. Remove the primary packing and O-ring from thesolid plug. Inspect and replace either or bothparts if they are damaged, worn or deformed.

8. The solid plug should be reassembled,tightening the coupon holder nut until it snuglyholds the primary packing (note: the primarypacking should not be allowed to spread open).

9. Tighten the coupon holder set screw with theallen key.

10. The coupons should now be firmly tightenedonto the coupon holder. A pair of clean glovesmust be worn by the person handling thecoupons.

11. The assembly should be checked to ensure thatit is firmly tightened with no loose parts.

12. If strip coupons are used, the orientation of thecoupons should be marked on the �HEX� of thesolid plug using a metal file.

13. The threads and O-ring on the solid plug shouldbe wiped with grease.

14. The coupon holder assembly is now ready to beplaced in the access fitting body. The retrieverand service valve must be used (in pressurizedsystems) to install the solid plug with the couponholder attached. The instructions in the CaprocoRetriever and Service Valve Operation manualmust be followed.

15. Final tightening of the plug in the access fittingshould ensure that the coupons are alignedparallel to the process flow. (Note: do not loosenthe solid plug in order to achieve orientation, asthis may affect the plug seal in the accessfitting).

RECOMMENDED PRACTICE FOR CLEANING EXPOSEDCOUPONS

1. Each coupon should be visually inspected,including verification of the serial number.

2. The coupons should have the insulationremoved without exerting any force or causingany damage to the coupons themselves. If thecoupons need to be handled, gloves must beworn.

RECOMMENDED PRACTICE AND PROCEDURE ~ CORROSION COUPONS

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3. The coupons should be immersed in a bath witha cleaning solvent such as methelyene chloride.The coupons should then be inserted into anultrasonic bath with fresh methelyene chloride.The ultrasonic bath should be run forapproximately three minutes.

4. The coupons should then be chemically cleanedin a 10% HCl solution or be placed in a blastbooth where they are individually cleaned, usingpreviously unused glass bead, at 40 psi (2.8kg/cm2).

5. The coupons should then be weighed accuratelyto 0.1 of a milligram (mg).

6. Inspect the coupon for a specific form ofcorrosion (e.g. pitting, etching, erosion, etc.).

7. Measure any pits using a pit depth gauge.

CORROSION RATE CALCULATION

The rate of corrosion is expressed in severaldifferent forms. The simplest in dimensionless unitsis the percentage change in weight of a coupon in anexposed time interval. This figure is usuallyextrapolated to give the percentage change perannum. Generally the rate of corrosion is more usefulif the figure reflects the impact on a system�s lifespan.

ABSTRACT

The corrosion rate depends on Faraday�s Law:

Weight of metal dissolving (g) - K x I x t

K = atomic weight of metal (g/mol) No. of electrons transferred X 96,500 Amp/Sec

where: I = current (amps) t = time(sec)

By dividing both sides of the equation by thecoupon surface area, this gives the corrosion rate asequal to a constant multiplied by the current density(amp) / area (cm2). In other words, the rate of weightloss from a given area is directly proportional to thecurrent density.

The rate of weight loss from a given area, whilevaluable, does not give a direct correlation with therate of penetration of the corrosion.

When this figure is translated to the radial depthof pipe wall lost per annum, this information becomesvery useful to the corrosion engineer or operator indetermining the life expectancy of the system.Corrective action can then be taken and re-measuredto give an acceptable time to failure/replacement.

The corrosion rate calculated using corrosioncoupons assumes uniform corrosion across thecoupon (e.g. uniform corrosion of the pipe wall at themonitoring point). This approximation is acceptablefor most circumstances to determine the average rateof corrosion. Of course, where a coupon showsevidence of pitting, the pit depth rate is more useful.The pit depth of the deepest coupon pit is measured,allowing the pit depth rate per annum to becalculated. Again, this figure correlates directly withtime to failure for a given pipe wall thickness.

Corrosion Rate Calculation in Metric Units:

umpa

(micrometersper annum)

{ weight loss of coupon (g) }total exposed area of coupon (cm2)

= X

{ 3.65 x 106

}[ exposure timein days ] x [ density

of metal ( g cm3 ) ]

Corrosion Rate Calculation in U.S. CustomaryUnits: (most commonly used)

mpy

(mils peryear)

{ weight loss of coupon (g) }total exposed area of coupon (in2)

= X

{ 2.23 x 104

}[ exposure time

in days ] x [ densityof metal (

g

cm3 ) ]Pitting Rate Calculation

Pit umpa =pit depth(um) x 365

exposure time in days

Pit mpy =pit depth(mils) x 365

exposure time in days

Corrosion and Pitting Rate ComparisonsCorrosion Rate Pitting Rate

mpy umpa mpy umpa

Low < 1.0 < 25 < 5 < 127

Moderate 1.0 - 4.9 25 - 126 5 - 7.9 127 - 201

Severe 5.0 - 10.0 127 - 254 8 - 15 202 - 381

Very Severe > 10.0 > 254 > 15 > 381