7.4 Corrosion Testing Methods:
Corrosion rate can be determined by metal loss method and the metal loss can be determined gravimetrically, volumetrically, or radiometrically. These all are a direct measure of corrosion and of these, gravimetric or weight loss methods are most used for inhibitor testing corrosion.
Volumetric methods are associated with inspection or monitoring techniques such as the ultrasonic inspection and the electric resistance (ER) probe monitoring, although both are sometimes used in long-term inhibitor evaluations of corrosion. The radiometric methods are used as monitoring methods such as in thin layer activation but could be ‑used for the inhibitor evaluation. The corrosion wheel test used to evaluate oilfield inhibitors is an example of the weight loss testing. Coupons from the weight loss experiment should be examined visually for localized corrosion seen as pits or the edge attack. Analysis can be as simple as the "none, some, or lots" or as detailed as the counting and depth measurement. The ASTM G 46 (Practice for Examination and Evaluation of Pitting Corrosion) provides a complete procedure for evaluating the pitting attack.
The electrochemical testing has two major benefits, one major limitation, and one lesser limitation. The benefits are the short measurement time and mechanistic information. The severe limitation of it is requirement for a conductive corrosive environment. Less troublesome from a testing perspective is requirement for a corrosion model for study. The rapidity of measurement makes these techniques useful in characterizing the inhibitor performance. The corrosion rates can be determined electrochemically in minutes while the weight loss methods can take days. With near instantaneousness of the electrochemical methods, the changes of inhibitor performance with time are readily measurable. Questions about the inhibitor persistence and the incubation time are thus experimentally accessible and the experiments concerned with velocity effects become less cumbersome.
Other methods are:
l Potentiodynamic polarization methods
l Linear polarization resistance (LPR)
l Electrochemical impedance spectroscopy (EIS)
l Electrochemical noise (EN)
There exist numerous standardized procedures for measuring the corrosion or chemical durability of glasses in neutral, acidic and basic environments, under the simulated environmental conditions, in the simulated body fluid, at the high temperature and pressure, and under the other conditions.
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