Coupon

Most users require extensive composite qualification testing programs to assure acceptable end use product properties. The magnitude of the qualification depends entirely on the end use requirements and may range from extensive testing of laminates and final parts made from several production lots of fiber, to a single lot laminate evaluation or test coupon verification of properties. 

Sample tests in the field. These include coupons, stressed samples, electrical-resistance probes exposed to the plant corroding medium, or samples exposed to the atmosphere, to soils, or to fresh, brackish, or sahne waters. 

The primaiy use of this laboratoiy technique today is as a quick check to determine the order of magnitude of a corrosion reaction. Sometimes the calculated rate from an immersion test does not look correct when compared to the visual appearance of the metal coupon. 

There is no single standard size or shape for corrosion-test coupons. They usually weigh from 10 to 50 g and preferably have a large sur-face-to-mass ratio. Disks 40 mm (IV2 in) in diameter by 3.2 mm Vh in) thick and similarly dimensioned square and rectangular coupons are... 

Weight lo.s.s probe.s. Coupons for measuring weight loss are still the primai y type of probe in use. These may be as simple as samples of the process plant materials which have been fitted with electrical connections and readouts to determine intervals for retrieval and weighing, to commercially available coupons of specified material,... 

Electrical re.slstance probe.s. These probes are the next most common type of corrosion probes after coupons. This type of probe measures changes in the electrical resistance as a thin strip of metal gets thinner with ongoing corrosion. As the metal gets thinner, its resistance increases. This technique was developed in the 1950s by Dravinieks and Cataldi and has undergone many improvements since then. 

Factors may throw off these rates—these are outlined in ASTM G3I, Standard Practice for Laboratory Immersion Corrosion Testing of Metals. Coupon-type tests cannot be correlated with changing plant conditions that may dramaticahy affect process equipment lifetimes. Other methods must be used if more frequent measurements are desired or correlation with plant conditions are necessary. 

Choices of alternative materials. Corrosion probes are carefully chosen to be as close as possible to the alloy composition, heat treatment, and stress condition of the material that is being monitored. Care must be taken to ensure that the environment at the probe matches the service environment. Choices of other alloys or heat treatments and other conditions must be made by comparison. Laboratory testing or coupon testing in the process stream can be used to examine alternatives to the current material, but the probes and the monitors can only provide information about the conditions which are present during the test exposure and cannot extrapolate beyond those conditions. 

Frequently, so-called crevice washers are used in coupon studies to test the environment for its ability to produce crevice corrosion (Fig. 2.22). There are several designs most consist of a small Teflon washer with radially oriented, wedge-shaped teeth. The washer is held to the coupon by a mounting bolt that passes through a central hole. The spaces between teeth form small crevice-shaped areas in which attack may occur (Fig. 2.23). The test is somewhat subjective and is not easily quantified. Using this test, attack in crevices either occurs or does not. 

Figure 2.23 Shallow pitting in crevice areas on a 304 stainless steel coupon exposed to a misting atmosphere. Note the relatively clean areas where the washer teeth contacted the coupon surface. (Magnification 7.5x.)...

Specimen Location Corrosion coupon from mill coolant tank... 

After increasing coolant corrosion-inhibitor concentration, coupon corrosion rates decreased by almost 70%. 

Figure 4.27 Mild steel coupon removed from a rolling mill cooling tank. Note the thick greasy deposits.

When a clean steel coupon is placed in oxygenated water, a rust layer will form quickly. Corrosion rates are initially high and decrease rapidly while the rust layer is forming. Once the oxide forms, rusting slows and the accumulated oxide retards diffusion. Thus, Reaction 5.2 slows. Eventually, nearly steady-state corrosion is achieved (Fig. 5.2). Hence, a minimum exposure period, empirically determined by the following equation, must be satisfied to obtain consistent corrosion-rate data for coupons exposed in cooling water systems (Figs. 5.2 and 5.3) ... 

Austenitic stainless steel coupons were placed in a large electrostatic precipitator. Each coupon rapidly developed pits. Attack was caused by chlorides dissolved in acidic aqueous solutions. 

Coupon tests involved a number of metallurgies and were done to evaluate precipitator-plate alloys. Test stainless steel plates failed, not only because of pitting but also because stress-corrosion cracks developed. 

Figure 7.22 Pitting on a 304 stainless steel coupon caused by acidic, chloride-containing water condensating and evaporating. High chloride concentrations were produced locally.

Inappropriate Select compatible materials of construction for materials of con- the specific process struction lead to, Change process parameters (e.g., different acid, corrosion and reduce temperature). Evaluate changes with test potential failure. coupons off-line Dillon 1992... 

Incorrect/incom- Review material of construction requirements vs. patible materials existing equipment before changing service of construction, corrosion coupons during pilot/develop-used in transfer- ment/scale-up ring/charging line or equipment. CCPS G-23... 

Fig. I. Comparison of unprimed and eleetroprimed single lap-shear adhesive joint strengths for steel coupons bonded with imidazole-cured epoxy [43].

Fig. 9. Oil absorption vs. oil MW. 1x1x0.050 inch coupons of SBS (Stereon 841A) were immersed in paraffinic process oils of varying molecular weight (kinematic viscosity) for 300 days.

Three different commercial formulations of silicone sealants from Dow Corning was used in the NSF sponsored studies. They were DC-790, DC-995, and DC-983, in the order of increasing modulus. Dumbbell test coupons (samples) were prepared as per the ASTM standards. Some test coupons were maintained at ambient conditions as control and the rest were subjected to simulated weathering. The weathered coupons were removed from the test layout at regular intervals of time and were tested for any changes in crosslink density due to exposure. 

In our study, the effect of moisture over the nonneutral pH range of 3-11, direct sunlight, ozone at a concentration level of 6000 ppm, and the effects of loading stresses, were investigated for the three commercial sealants. A characteristic variation of crosslink density for the typical silicone sealants is shown in Fig. 29. This figure depicts the results for the coupons exposed to moisture and sunlight. Initially upon exposure, the crosslink density of the sealants exhibit an increase due to the availability of residual uncurred crosslink sites... 

This transitory behavior was observed to arise from all the weathering agents considered in this study except ozone. Instead, test coupons exposed to ozone exhibited an initial decline in the crosslink density of the silicone with the formation of surface cracks, which were difficult to distinguish with the naked eye. With continued exposure to ozone, however, the material would begin to crosslink. We proposed that ozone s greatest affinity... 

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