Chapter 10 - High-Temperature & High Pressure
Dr. Russell D. Kane, InterCorr International, Inc. Houston, Texas
USA
Autoclave corrosion tests are a convenient means for laboratory simulation of many service environments for the purpose of evaluating the corrosion resistance of materials and for determining the effects of metallurgical, processing, and environmental variables on corrosion processes. The reason for such tests is to more closely recreate the high temperatures and pressures commonly occurring in commercial or industrial processes.
In many of these cases, the factors affecting corrosion behavior are 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, chemicals, aerospace and transportation where reliability, serviceability and concerns for corrosion are paramount. In these situations, greater effort must be taken to carefully reproduce conditions of high temperature and pressure in laboratory test vessels in which corrosion coupon, electrochemical, or environmentally assisted cracking (EAC) experiments can be conducted in a credible manner.
Basic Principles
Equipment
The simplest type of HT/HP corrosion test is that which is conducted in a sealed, static, pressurized test vessel. The test vessel contains a solution and a vapor space above the solution. The liquid and gas phases will be determined by the amount and vapor pressures of the constituents in the test vessel as well as by the test temperature. 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 (i.e., linear polarization resistance (LPR), polarization curve generation, and cyclic polarization). In these cases, it is often necessary to modify the simple static test vessels with additional electrical feed-throughs to accommodate the electrodes required for electrochemical testing. If multiple liquid phases are present (e.g., aqueous and liquid hydrocarbon phases) it is often 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.
Refreshed and Recirculating Tests
The limitation of volume of corrosive environment in most HT/ HP tests makes issues such as solution volume to specimen surface area ratio a critical factor. In most cases, it is advantageous to limit this ratio to no less than 30 mL/cm'. In any event, care should be taken to prevent depletion of' critical corrosive species or contamination of the test solution with unacceptably high levels of metal ions produced by corrosion. Such conditions may require periodic changes in the test constituents after a certain period of testing time (e.g., 3 days, 7 days, 1 month, etc.) depending on their rate of consumption or contamination by corroding specimens. In particularly critical situations, it is possible to minimize such concerns by using constant or periodic replenishment of either the gaseous or liquid phases in the autoclave while under pressurized conditions.
Factors Affecting HT/HP Test Environments
For simple HT/HP exposure tests involving either aqueous or nonaqueous phases, the total pressure is usually determined by the sum of the pressures of the constituents of the test environment which will vary with temperature. Where liquid constituents are being used for the test environment, the partial pressure is usually taken to be the vapor pressure of the liquid at the intended test temperature. Vapor pressures for several other volatile compounds used in HT/HP corrosion testing can be found in the technical literature. In some cases, higher test pressures can be obtained by pumping additional gas into the test vessel using a special gas pump. Alternately, hydrostatic pressurization may be employed whereby there is no gas phase in the test vessel and the pressure is increased by pumping additional liquid into the test vessel in a controlled manner.
The importance of partial pressure in HT/HP corrosion testing is that the solubility of' the gaseous constituents in the liquid phase is usually determined by its partial pressure. For a particular mol fraction of gas in the vapor phase, its concentration in the liquid will usually increase with increasing total pressure. This is why the effects of low-level (ppm) corrosive impurities such as oxygen, hydrogen, H2S, and many other species on corrosion and cracking are often found to be magnified at high pressure and exhibit corrosive effects not commonly found in conventional low-pressure corrosion tests.
Special HT/HP Corrosion Test Conditions
Example: Hydrogen
A chemical species whose chemical behavior impacts corrosion resistance and materials performance is hydrogen. It has been known for decades that atomic hydrogen can produce embrittlement in many metallic materials to varying degrees, particularly under conditions of high-pressure hydrogen. Under high hydrogen environment pressure or electrochemical reaction, or both, hydrogen atoms can enter the material where it can react by one of' the following mechanisms:
There has been much interest in conducting hydrogen embrittlement cracking (HEC) tests in aqueous media which can produce atomic hydrogen on the surface of materials as a result of corrosion or cathodic charging. In most cases, these tests can be conducted at ambient pressure and al. temperatures from ambient to elevated depending on the application. When aqueous hydrogen charging is involved, pressure is usually not a major factor. However, as in the case of steels exposed to aqueous hydrogen sulfide containing environments, the atomic hydrogen is produced as a result of sulfide corrosion. The severity of the mass-loss corrosion and hydrogen charging is directly dependent on the amount of hydrogen sulfide dissolved in the aqueous solution. In applications involving petroleum production and refining, compressed natural gas storage, chemical, processing and heavy water production such effects are compounded by exposure to HT and/or HP conditions. Additionally, variations in pH which control the type and amount of dissolved sulfide species and the severity of corrosion and hydrogen charging can be affected by hydrogen sulfide pressure.
Test Variations
The HT/HP corrosion test methods described previously require many specialized procedures, evaluation techniques and test equipment. In this section, 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. These include:
Typically, the major differences between these types of tests conducted at high pressure and in conventional low-pressure glassware are the special requirements and limitations imposed by the test vessel, vessel feed-throughs, and the high-pressure environment. However, these types of evaluations can be accomplished through careful planning and test vessel design.
Example: Electrochemical Measurements
Electrochemical corrosion measurements can be conducted in HT/HP environments with modification of conventional test vessels and by using few special techniques. In general, most all of the conventional electrochemical techniques performed in glassware have also been conducted inside of HP test equipment. In general, the most critical portion of any electrochemical system is the reference electrode. The design and construction of the reference electrode for HT/HP corrosion testing is particularly important as it must provide a stable reference potential (relatable to other common reference electrodes) to which the working electrode (i.e., test specimen) can be compared. It must also give a reasonable service life to remain functional throughout the intended test period.
In many applications, a test vessel modified with an external reference electrode cell has become the most common system to minimize effects of temperature, pressure, or contamination, or a combination thereof, from the corrosive environment. This approach helps to prolong the usable life of the reference electrode. One of the most common reference electrode systems used in HT/HP corrosion testing is Ag/AgCl. However, other reference systems have been developed and are used for application in specific environments [S]. The external reference electrode is usually contained in a separate pressurized compartment. The reference electrode is then connected to the test vessel using a bridge or conductive solution conduit.
To minimize penetrations into the lest vessel for HT/HP electrochemical tests, it is possible to use the test vessel directly as the counter-electrode. This requires special selection of materials of test vessel construction so that it will be effectively inert to the test environment. In systems that contain impurities which will contaminate common reference electrode systems and degrade their performance, it is possible to pressure balance the electrode and to even provide a reverse flow into the test vessel to minimize ingress of the contaminant species into the reference electrode.
Another type of reference system that can be employed effectively in HT/HP systems where conventional stable reference electrodes are not usable is the use of an inert (i.e., graphite or platinum) or a corroding metal to produce a stable potential. Such electrodes are considered "pseudo-reference" systems since they may tend to exhibit a drift in potential with time. However, in environments where no other convenient reference electrodes are available, they can provide at least a measurement of relative potential between materials in the environment even if their reference potentials cannot be considered truly constant.
Standards, Significance and Use
The number of standards involving HT/HP corrosion testing is somewhat limited. In general, the procedures used for HT/HP corrosion tests have been highly specialized to the particular nature of the corrosion test and the test environment. Those which have been identified are described in this section.
ASTM Standards
There are several ASTM standards that describe testing techniques
for the evaluation of corrosion and materials compatibility which
involve HT/HP conditions. These are: