Surface finish, coupons

Coupons These are usually strip, flush discs or cylindrical rods mounted in suitable racks (inserted and retrieved at shutdown), or installed in the plant using high-pressure access systems. Coupons are available from several manufacturers in a variety of materials and surface finishes and are supplied pre-weighed. 

Shapes of typical (commercially available) test coupons are shown in Fig. 25-3 circular in Fig. 25-3a, rectangular in Fig. 25-3b, welded rectangular in Fig. 25-3c, and horseshoe stressed in Fig. 25-3d. Many desired shapes ofcoupons are also available in various sizes and can be obtained in any size, shape, material of construction, and surface finish... 

For wall friction measurements, a wall coupon is inserted between the rings, and powder in the upper ring alone is sheared against a coupon of interest. Wall friction and adhesion, both static and dynamic, may be assessed against different materials of construction or surface finish. 

The latter two materials were employed for comparative studies. Coupons (10 X 10 X 1 mm or 20 x 10 X 1 mm) were cut from the sheets which were thermomcchanically processed in order to obtain a fine-grained near-gamma microslructure of the material [9-11 ].Thc near-surface depletion zones formed during heat treatment of the specimens were mechanically removed.The specimens were subsequently ground to a 600-grit surface-finish, ultrasonically cleaned in acetone and freon and transferred into a preparation chamber for ion implantation. 

The method consists of taking cylindrical test coupons of specified dimensions and surface finish, and suspending them in a specified volume of a specified test solution representative of a moderately aggressive supply water for a period of 24 hours. The immersions are repeated ten times using fresh test solution. Metals are determined using the extract from the final 24-hom period. 

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. 

Good control of surface finish of coupon Surface finish may not reflect reality Authentic surface condition Poor control of surface finish may introduce variability in the answer... 

The coupons were disassembled and photographed during the intermediate inspection. Unlike the coupons received at the Budapest RCM, which had only a machined surface finish, these highly polished coupons showed no deep pits or crevice corrosion. Basin water conductivity was maintained at 3.5-16 pS/cm, with a chloride ion content of less than 2 ppm. The excellent corrosion resistance of these coupons was attributed primarily to the surface finish and the improved purity of the water. 

Opening a crevice couple destroys or drastically changes the crevice features. Therefore a crevice couple should be opened and evaluated only once. If you reuse the couple, the surfaces should be machined and/or polished to reproduce the surface finish of the coupons. 

The material surface should be representative of the finish expected in-service. If the exact service finish is not known, the test surface finish should be noted. For stainless steel or aluminum alloys, where corrosion phenomena can attack specific elements of the metallic structure preferentially, the coupon forming process should also be noted (e.g., annealed, heat-treated, cold-rolled, cast, etc.). 

Rusting, pitting, and SCC tests frequently provide results with poor reproducibility. The percent surface rusted after a salt spray test can also be quite variable. The amount of rust covering a specimen after the salt spray test varies if the rust initiates at the top of the coupon and runs down over the test surface or initiates at the bottom and covers only a small portion of the specimen (Fig. 9). Rusting is also dependent upon the surface finish smoother finishes are usually more resistant. Because of these factors, it is difficult to use the salt spray test for quality control. 

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... 

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. 

Studies of the effectiveness of electropolishing and mechanical polishing in reducing activity uptake have been carried out on the Ringhals 4 reactor in Sweden. Coupons with various surface finishes were attached to the manway insert in the channel head bowl during 1987/88 and 1989/90. The results from the first experiment are summarised in Ref. 4. These experiments have confirmed that... 

ME Pick and R Roofthooft, Examination of Type 304 Stainless Steel and Inconel 600 Coupons with Various Surface Finishes Exposed in Doel 2, Water Chemistry of Nuclear Reactor Systems 5, BNES, London, Vol. l, 23-29, 1989. 

The standard erosion sample size adopted had dimensions 35 x 18 x 10 mm. The sample surface to nozzle-tube distance was fixed at 6 mm. Erosion coupons machined from the reference steel (uncoated substrate) were 35 x 18x8 mm. A shim was used to compensate for differing sample thicknesses in order to maintain constant sample surface to nozzle distance during testing. All erosion samples had a ground flat surface finish of Ra 0.8 pm. Erosion... 

The procedure adopted for preparing the coupons of this batch was slightly different. The surfaces of the coupons were mechanically polished, as opposed to the plain machine finishing that had been adopted for the coupons of Batch I. The racks of Batch II (Fig. 4.4) did not contain the glass ampoules with the tiny tensile test type coupons, and the coupons in each rack of Batch II... 

Coupons are normally 120 grit finished, which is reasonably close to the mill finish found on process equipment. A double disc ground surface is closer to mill finish, and improves reproducibility of results, because it creates a cleaner surfoce than 120 grit. 

The surface condition of the coupon is a critical variable in all hydrogen uptake tests. Studies have shown that abraded or sandblasted surfaces absorb hydrogen more readily than as-pickled surfiices. Thickening of the stirf oxide film by anodizing or thermal oxidation further retards absorption. The actual sur ce finish anticipated in service should be evaluated. 

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