Pipe coupon

More recently, attempts have been made to correlate mathematically the chemical composition of natural waters and their aggressivity to iron by direct measurements on corrosion coupons or pipe samples removed from distribution systemsThis work has been of limited success, either producing a mathematical best fit only for the particular data set examined or very general trends. The particular interest to the water supply industry of the corrosivity of natural waters to cast iron has led to the development of a simple corrosion rig for the direct measurement of corrosion ratesThe results obtained using this rig has suggested an aggressivity classification of waters by source type i.e. 

The weep point or strain-to-first-crack in a wall for filament-wound pipe constructed using isophthalic plastic is currently found to be not less than 0.009 in./in. This has been repeatedly demonstrated by careful coupon testing and burst testing of pipes with strain gauge instrumentation attached. 

Coupon corrosion results are also influenced by the mounting and installation process within a bypass unit. Coupons must be mounted with plastic, or other suitable nonconducting, holders to ensure insulation from stray currents and electrolytic effects. The plane of the coupon should be parallel to the direction of flow and it should not touch the wall of the pipe. 

The simplest form of compatibility test is to immerse standard ASTM coupons in a liquid or vapor in the lab. Coupons can be installed in the field in a stream. However making a coupon assembly for installation in a pipe or a vessel may be difficult and persuading plant operators even more so. Lab testing is easy to carry out but will not replicate real life conditions such as flow, agitation, stress (compression for gaskets) and one-sided exposure. For this reason, lab dunk tests are usually done for screening reasons. Application temperature is easy to achieve by space heaters. 

Table 4.6 Ranking severity of contamination of metals from brass coupons exposed to the local laboratory waters being supplied to the respective pipe rigs...

Surface finish of the test coupon Various manufacturing processes can change the physical form of the metal s initial surface or produce surface films, either of which may influence the eharacteristies of the corrosion deposit that is developed when in contact with water. These effeets may influence or change the mechanism controlling the level of eontamination. The standard sit-and-soak test defines a machined smface that will not be representative of a cast surface, for example. In the first set of eonormative tests, aetual galvanized steel pipe was used to overcome precisely this sort of problem. 

RTR pipe designers also use a S-S curve but instead of a yield point, they use the point of first crack (empirical weep point) as shown in Figure 8.11. Either the ASTM hydrostatic or coupon test determines it. The weep point is the point at which the RTR matrix (resin) becomes excessively strained so that minute fractures begin to appear in the structural wall. At this point it is probable that in time even a more... 

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. 

With these factors in mind, disc type coupons were chosen. These were similar to the American Sodety for Testing and Materials (ASTM) test coupons but smaller in diameter (100 and 70 mm) to facilitate transport. Single sheet coupons were used to evaluate pitting corrosion on exposed surfaces, and couples or sandwich type coupons were used to simulate crevice and/or galvanic corrosion. The coupons were cleaned and numbered, and the alloy type was identified using a laser scriber. The coupons were stacked in a stainless steel rack. This consisted of a steel pipe welded to a stainless steel base plate and threaded at the other end to hold the coupons in place with a stainless steel nut welded to a hook to enable the rack to be suspended with a nylon rope. A... 

Lap shear testing should be conducted to ensure that an adequate bond exists between the pipe and the wrap. For composite repair methods that are not monolithic, these tests should also include composite-composite test samples as well as the composite-steel test coupons. The composite-composite sample is used to assess the bond strength between the layers, while the composite-steel samples are used to determine the lap shear strength at the interface between the pipe material and the composite. 

Corrosion monitoring is done directly on the pipe wall, and therefore gives more direct and representative information than traditional intrusive probes or coupons. 

TLA was developed in the United Kingdom [20,27]. The surface of a sample (which can be a coupon or a section or tube or pipe) is irradiated to make it slightly radioactive. Two samples are irradiated. One sample is protected from corrosion the second is exposed to the corrosive fluid, in this case the water system. The radioactivity of each of two samples is monitored 2is a function of time. There will be a radioactive decay curve characteristic of the protected sample and a second decay curve for the exposed sample. The difference between the two curves indicates metal loss, which can be converted to corrosion rate. This technique has not been broadly used, but some field use has been reported [27,22]. It shows promise of being an automated coupon which provides a continuous record of corrosion. Figure 4 shows a pipe specimen with an activated area, and Fig. 5 shows data generated in an inhibited cooling tower system. 

Because of the extensive use of copper-nickel in shipboard piping, the potential corrosivity of RO ultrapure water on these systems is considered important, ASTM G 4 (Standard Guide for Conducting Corrosion Coupon Tests in Field Application) covers in-plant testing of metals and alloys without regard to alloy t q)e, family, or class and generates test results by exposure of the materials to filtered seawater (RO feed stock), an intermediate-pass brine byproduct, and ultrapure product water. 

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. 

Commercially available coupon systems are available for measurement of cathodic protection data, and some are available for measurement of both polarization and corrosion rate data. Care must be exercised when using these methods to ensure that the coupons are installed in an area representative of the pipe and that the coupon size is representative of the coating holidays on the pipe. NACE International has pubhshed a state-of-the art report on the... 

Too high a fiow rate will keep debris off of the coupons and may initiate erosion attack on copp>er-base alloys. Low fiow rates may result in excessive corrosion and fouling. Normally, fiow rates of 1-2 m/s (3 to 5 ft/s) are adequate. The coupx)n rack should be connected to the return riser to the tower or at the exit of the hottest part of the system using plastic piping or any other material that will not influence coupmn corrosion rates. For example, do not use galvanized pip>e, coppjer pip e, or brass v Jves to connect the... 

The LPR probe should be installed perpendicular to the water flow in a representative water stream which is part of the piping system or at the end of the coupon test rack, as shown in Fig. 2. The probe must be installed such that water flows between the two electrodes in order to avoid fouling of the electrodes. An aligning pin at the male cable connector of the probe indicates the electrode position as shown in Fig. 2. The two electrodes should project into the flowing water, with the probe lead flush with the pipe wall as shown in Fig. 3. 

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