producing formation, however, information gainedf roms uch chemical
analysis will most likely be meaningless.
11.1.6C oupons
Coupons are the oldest and simplest device used in the monitoring of
corrosion. Couponsare smallpieces of metal, usually of arectangular shape
that are inserted in the process stream and removed after aperiod of time for
study.The most commonand basic use of couponsistodetermine average
corrosion rate over the period of exposure. This is accomplished by weighing
the couponb efore and after exposure (coupons must first be cleaned
following exposure to remove corrosion products and any other deposits)
and determiningt he weight loss. The average corrosionr ate can easily be
calculated from the weight loss, the initialsurfacearea of the coupon,and the
time exposed. It is advisable to leave acoupon exposed for at least 30 daysto
obtain valid corrosion rate information. There are two reasons for this
recommended practice. First, aclean coupon generallycorrodes much faster
than one that has reached equilibrium with its environment. This willcause
ah igher corrosion rate to be reported in as hort test than is actually being
experienced on the pipe or vessel. Second, there is an unavoidable potential
for error as aresult of the cleaning operation. Coupon cleaning procedures are
designed to remove all of the deposits without disturbing the remaining
uncorroded metal of the coupon. Asmall amount of the underlying metal is
oftenremoved with the deposits, however,and if the actual metal loss from
corrosion is small (as would be the case in as hort test), the effect of metal
removed during cleaning would create asignificant error.Care must be taken
to correct for this effect. It should be recognized that ac oupon can only
provide corrosion rate data based on the total weight loss dividedbythe total
time of exposure. Am ajor shortcoming of coupon monitoring is that high
corrosion rates for short periods of time may be undetectable and cannot
be correlated to process upset conditions. If more frequent information
on weight loss is desired, the use of electrical resistance monitoringsystems is
recommended. In many cases, it is also recommended that couponsa nd
electrical resistance (ER) probes be used in conjunction.
One of the most important roles of couponsistoprovide informationabout
the type of corrosion present. Unlike ER probes that only detectthe amount of
metal removed, couponscan be examined for evidence of pitting and other
localized formso fa ttack. It is also important to remember that coupons or
monitoring probes indicate the attack of the environment only at the point of
exposure. It is important, therefore, that the coupon or probe be installed at
representative locations as close as possiblet oc ritical pointsw here corrosion
measurements are desired (e.g., vessel or pipe walls, tube sheets, trays, lateral
lines upstream to major tie-ins). Conditions of flow,t emperature, concentration, etc., may change considerably onlyafew inches away from any given
location with resulting differences in corrosion rates.Coupons suspended in
the center of apipelineorvessel may corrode at different rates than coupons
Corrosion of Linings and Coatings
536
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547
analysis will most likely be meaningless.
11.1.6C oupons
Coupons are the oldest and simplest device used in the monitoring of
corrosion. Couponsare smallpieces of metal, usually of arectangular shape
that are inserted in the process stream and removed after aperiod of time for
study.The most commonand basic use of couponsistodetermine average
corrosion rate over the period of exposure. This is accomplished by weighing
the couponb efore and after exposure (coupons must first be cleaned
following exposure to remove corrosion products and any other deposits)
and determiningt he weight loss. The average corrosionr ate can easily be
calculated from the weight loss, the initialsurfacearea of the coupon,and the
time exposed. It is advisable to leave acoupon exposed for at least 30 daysto
obtain valid corrosion rate information. There are two reasons for this
recommended practice. First, aclean coupon generallycorrodes much faster
than one that has reached equilibrium with its environment. This willcause
ah igher corrosion rate to be reported in as hort test than is actually being
experienced on the pipe or vessel. Second, there is an unavoidable potential
for error as aresult of the cleaning operation. Coupon cleaning procedures are
designed to remove all of the deposits without disturbing the remaining
uncorroded metal of the coupon. Asmall amount of the underlying metal is
oftenremoved with the deposits, however,and if the actual metal loss from
corrosion is small (as would be the case in as hort test), the effect of metal
removed during cleaning would create asignificant error.Care must be taken
to correct for this effect. It should be recognized that ac oupon can only
provide corrosion rate data based on the total weight loss dividedbythe total
time of exposure. Am ajor shortcoming of coupon monitoring is that high
corrosion rates for short periods of time may be undetectable and cannot
be correlated to process upset conditions. If more frequent information
on weight loss is desired, the use of electrical resistance monitoringsystems is
recommended. In many cases, it is also recommended that couponsa nd
electrical resistance (ER) probes be used in conjunction.
One of the most important roles of couponsistoprovide informationabout
the type of corrosion present. Unlike ER probes that only detectthe amount of
metal removed, couponscan be examined for evidence of pitting and other
localized formso fa ttack. It is also important to remember that coupons or
monitoring probes indicate the attack of the environment only at the point of
exposure. It is important, therefore, that the coupon or probe be installed at
representative locations as close as possiblet oc ritical pointsw here corrosion
measurements are desired (e.g., vessel or pipe walls, tube sheets, trays, lateral
lines upstream to major tie-ins). Conditions of flow,t emperature, concentration, etc., may change considerably onlyafew inches away from any given
location with resulting differences in corrosion rates.Coupons suspended in
the center of apipelineorvessel may corrode at different rates than coupons
Corrosion of Linings and Coatings
536
CAT8247—CHAPTER 11—6/10/2006—12:56—SRIDHAR—XML MODEL B–pp. 533–547