Immersion pitting temperature test

In an effort to confirm the performance of Hypalon liners in concentrated brine solutions at high temperatures, immersion tests were set up. Potable grade Hypalon, which was used in the brine pits as well as in the New Mexico salt gradient pond was compared to a new grade of Hypalon - Industrial grade, which shows less water swell at elevated temperatures. Both materials were tested in distilled water as a control and in 20%... 

Determination of the critical pitting or critical crevice temperature is often used as an alternative to comparing the severity of attack on several samples [2f-23], Procedures for this testing are described in Methods C and D of ASTM G 48. Specimens are immersed for a given period (e.g., 72 h) at a temperature that is not expected to cause attack. Fresh specimens are then exposed at 5°C increments for the same duration until the temperature at which attack occurs (critical temperature) is determined. For many materials, the critical temperatures of replicate specimens fall within a 5°C variation. Retesting of the same specimen can affect the critical temperature [24]. Testing may be initiated just below the critical temperature, if it can be estimated. If not, this technique can require many specimens and a few weeks to find the critical temperature. 

The decision to test is usually driven by unusual chemistry such as a new catalyst, reaction components, or reaction conditions. Immersion/mass loss method is primarily used in conjunction with microscopic examination. Process fluids from production or pilot runs are primarily used to best simulate potential corrosion. The test methods are custom designed based, in part, on cost and the ability to obtain sufficient quantities of test fluids, and handle the process conditions. Process conditions are tested outside the process control limits (e.g., temperatures, pH) to better accentuate the corrosion potential. These extremes have to be tempered by the stability of the products in the stream. For wall thicknesses greater than 0.250 in. (6.35 mm), a uniform loss of less than 10 mil/year (0.26 mm/year) is considered structurally acceptable. Signiflcandy lower levels of uniform loss are of concern for product or process contamination issues. Microscopic examination is used to determine potential localized corrosion concerns, such as pitting or stress cracking. Indications of pitting or stress corrosion in stamped areas of the coupon are of particular concern. U-bend tests are rarely used because of insufflcient test fluid quantities and availability. 

The tests are typically carried out in a thermostatically controlled electrochemical cell at a temperature between 18 and 25°C with a test solution containing 0.3% sodium chloride. A potentiostat is required to polarize the test electrode at controlled scan rates. The auxiliary electrode can be any inert electrode, and the reference electrode can be either calomel or silver/silver chloride. The anodic polarization curve is plotted for one specimen by scanning the potential from a cathodic value of. B = -1.16 V vs. SHE at a scan rate of 0.15 mV-s to the pitting potential Bpf, i.e., the potential at which the density of current is increased by at least one order of magnitude in the anodic polarization process. Another specimen made of the same alloy and temper is then immersed in the cell and allowed to rest for 5 min, and the potential is then moved to... 

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