Coupon testing surface finish

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.). 

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... 

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. 

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. 

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... 

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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