Pipeline coupons

The importance of the hydrogenase enzyme was also demonstrated in experiments in which mixed cultures of SRB recently isolated from the production water of two pipeline systems in Alberta were circulated through two Robbins Devices (McCoy et al. 1981) at a flow rate of 4Lmin Both loops showed detectable SRB attached to corrosion coupons. One population of the SRB had a high level of hydrogenase activity, which correlated well with the subsequent high corrosion of the metal coupons the other population of SRB had low levels of hydrogenase and low levels of iron loss detected (Bryant et al. 1991). 

The ASTM Corrosion Test procedure by Total Immersion Method requires that all specimens in a test series should have the same dimensions when comparisons are to be made. In these experiments, carbon steel was used representing the pipeline material. The coupons were cleaned, polished and weighed. Coal-water slurry, 10 to 40 weight percent, was used in the corrosion tests. For the coal-water slurry the intitial pH of the medium varied from 2.3 to 2.6 and the test series containing the nutrient media, the microorganisms and the inhibitors the pH varied from 2 to 2.5. Specimens were immersed in the reaction vessel maintained at a constant temperature of 86°F for 72 hours or the specified time. The coupons were removed, washed with deionized water, dried and weighed. The loss in weight of the specimen, before and after the test was attributed to corrosion. 

The effectiveness of cathodic protection can be accomphshed by monitoring the pipeline potential using the close interval potential survey (CIPS) method or by using direct current voltage gradient (DCVG) or IR coupon techniques. Also, physical and electrochemical methods can be used to estimate the corrosion rates of a cathodicaUy protected system. 

The coupons have been used to monitor cathodic protection systems in Europe since 1960 [70]. Most of these coupons were installed since 1975. Coupons are used in the form of steel electrodes of a strictly determined shape and surface area, protected by cathodic protection together with the structure. The schematic of the coupon installation on a pipeline is shown in Fig. 15.10. 

FIGURE 4.27 SEM of a coupon made of pipeline steel exposed to a mixed culture containing SRB as well as Clostridium after 2 h (a) and 1 month (b). (From Fickert R. et al. Biotic pit initiation on pipeline steel in the presence of sulfate reducing bacteria, Paper No. 04590, Corrosion 2004, NACE, USA, 2004.)... 

Coupon Tests. A weighed metal coupon shaped to conform to the outside surface of a buried pipe is attached by a brazed connecting cable, and both the cable and surface between coupon and pipe are overlaid with coal tar. After exposure to the soil for a period of weeks or months, the weight loss, if any, of the cleaned coupon is a measure of whether cathodic protection of the pipeline is complete. 

Test coupons of steel of a specified size are buried near the pipeline and connected by cable at the test point with the cathodically protected pipeline. They simulate artificial defects in the coating. The protection current taken from the test coupon can be measured via the cable connection and the true potential determined from a reference electrode in front of the test coupon by momentarily interrupting the cable connection [28]. Ohmic potential drops between the reference electrode and the test coupon are obtained from a measuring test probe that has a built-in reference electrode on the back of it to measure the 7/ -free potential directly [see Eq. (2-34) with 5 0] without having to switch off the protection... 

One method of getting around these problems is to use a small working electrode coupon connected to the pipe. The size of the electrode is based on a reasonable estimate of the holiday sizes on the pipe. The polarization curve is obtained by observing the potential on the known electrode. Such data must be evaluated carefully, since it represents only one point. On a long pipeline, several test locations should be evaluated. For more details on electrochemical polarization, the reader is advised to consult Refs 39 to 41 and 42. 

Procedures for monitoring corrosion of pipelines and related surface facilities using retrievable specimens are described in NACE Standard Practice for Preparation and Installation of Corrosion Coupons and Interpretation of Test Data in Oilfield Operations (RP0775). One of the main concerns in the use of such specimens is the problem of location, since corrosion is generally associated with a separate water phase. In some cases, special dropout pots are used to trap a water phase to provide a severe location for specimens. Electric resistance probes can be used in place of specimens. In locations where there is a continuous water phase and fouUng is not a problem, polarization type probes can be used [74,75]. 

Samples for analysis are usually obtained by scraping accessible surfaces. In open systems or on external surface of pipelines or other underground facilities this can be done directly. For low pressure water systems, bull plugs, coupons or inspection ports can provide a way to expose specimens representative of internal surfaces [15], However, more sophisticated devices are required for pressurized systems to allow mounting an assembly on a standard pressure fitting [16]. 

Once the site has been exposed, corrosion monitoring devices (e.g., coupons, electrical resistance probes) can be installed to identify and monitor corrosivity in the pipeline. Inline inspection results may also provide information to assess the downstream conditions of the pipe. Once regions most susceptible to corrosion are free of damage, it is then considered that pipeline integrity to a large extent is assured. 

Perhaps the most important consideration in the installation of corrosion coupons is that a coupon must be representative of the actual pipeline surface/defect. The exact metallurgical detail and surface finish as found on the actual pipeline are therefore required... 

Corrosion coupons. Corrosion coupons connected to cathodically protected structures are finding increasing application for performance monitoring of the CP system. Essentially these coupons, installed uncoated, represent a defect simulation on the pipeline under controlled conditions. These coupons can be connected to the pipeline via a test post outlet, facilitating a number of measurements such as potential and current flow. 

Methodology. Perhaps the most important consideration in the installation of corrosion coupons is that a coupon must be representative of the actual pipeline surface and defect. The exact metallurgical detail and surface finish as foimd on the actual pipeline are therefore required on the coupon. The influence of corrosion product buildup may also be important. Furthermore the environmental conditions of the coupon and the pipe should also be matched (temperature, soil conditions, soil compaction, oxygen concentration, etc.). Current shielding effects on the bonded coupon should be avoided. 

Several measurements can be made after a coupon-type corrosion sensor has been attached to a cathodically protected pipeline. on potentials measured on the coupon are in principle more accurate than those measured on a buried pipe, if a suitable reference electrode is installed in close proximity to the coupon. The potentials recorded with a coupon sensor may still contain a significant IR drop error, but this error is lower than that of surface on potential measurements. Instant-OFF potentials can be measured conveniently by interrupting the coupon bond wire at a test post. Similarly, longer-term depolarization measurements can be performed on the coupon without depolarizing the entire buried structure. Measurement of current flow to or from the coupon and its direction can also be determined, for example, by using a shunt resistor in the bond wire. Importantly, it is also possible to determine corrosion rates from the coupon. Electrical resistance sensors provide an option for in situ corrosion rate measurements as an alternative to weight loss coupons. 

Properly installed coupons are excellent for monitoring corrosion. They are not very successful in pipeline programs because they need to be installed in places that are generally not easily accessible. 

Offshore pipelines often have coupons installed both at the bank and on the platform, at the inlet of the system. The coupons on the bank are of httle benefit because most of the corrosion has occurred before the fluids reach the bank thus, the corrosive gases are no longer present. The coupons at the inlet are located upstream of water fallout so there is no stagnant liquid area that is constantly replenished with corrosive gases. 

Proper field monitoring and inspection techniques can be used to measure the real corrosion rate, to find safety risks on time and to avoid corrosion accidents. The field monitoring and inspection techniques normally include weight-loss coupons, electrical resistance probes, ultrasonic wall thickness measurements, infrared thermal imaging systems, etc. Ultrasonic wall thickness measurement is widely used to monitor the general and erosion corrosion rates of the equipment and pipelines in crude oil processing plants. 

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