Autoclave corrosion tests are a convenient means for laboratory simulation of many service environments. The reason for such tests is to recreate the high temperatures (HT) and high pressures (HP) commonly occurring in commercial or industrial processes. Factors affecting corrosion behavior are often intimately linked to the conditions of total system pressure, partial pressures of various soluble gaseous constituents and temperature. There are many HT/HP environments of commercial interest, which include those in industries such as petroleum, nuclear, chemicals, aerospace and transportation where reliability, serviceability and concerns for corrosion are paramount (reference).
Corrosion coupons can be placed in the aqueous phase, vapor space, or at phase interfaces depending on the specific interest involved. Additionally, it is also possible to conduct electrochemical tests in HT/HP vessels. If multiple liquid phases are present it can be necessary to stir or agitate the media or test vessel to produce mixing and conditions whereby the corrosion test specimens are contacted by all of the phases present. Special magnetic and mechanical stirrers are available that can be used to produce movement of the fluid to produce a mixing of the phases. In some cases where contact of the specimens with both liquid and gaseous phases is important in the corrosion process, it may be necessary to slowly rotate or rock the test vessel to produce the intended results. HT/HP corrosion tests have special requirements not common to conventional corrosion experiments conducted in laboratory glassware.
Four variations of common HT/HP test methods that have been found to be useful in materials evaluation involving corrosion phenomena will be briefly described. However, these types of evaluations can be accomplished through careful planning and test vessel design. These include:
Windowed test vessels: special transparent windows and other fixtures such as fiber optics have been used to make visual measurements or observations within the confines of test vessels. Besides being able to withstand the pressures, temperatures, and corrosion environments, these windows may have to perform other functions related to the introduction of light or other radiation if these are part of the test variables.
Electrochemical measurements: most conventional electrochemical techniques have been used for experiments conducted inside HT/HP vessels. The most critical electrochemical component in these experiments always has been the reference electrode. The design and construction of the reference electrode is particularly important, as it must provide a stable and standard reference potential. In many applications, test vessels have been modified to accommodate an external reference electrode to minimize the effects of temperature, pressure, or contamination, or a combination thereof.
Hydrogen permeation: hydrogen charging is often a problem that affects materials submitted to HT/HP test conditions. In such cases, it may be necessary to measure hydrogen permeation rates and diffusion constants in order to estimate the potential hazard of hydrogen attack. For hydrogen permeation measurements at high temperatures, it may be imperative to use solid state devices.
Mechanical property testing: HT/HP vessels have been designed to conduct a variety of mechanical tests such as slow strain rate (SSR), fracture or fatigue testing. The main problem is always one of selecting the fixtures that could withstand the corrosive environments generated in HT/HP tests.