Inhibitors evaluation tests

In other words, the quest for a standardized corrosion inhibitor evaluation test is as elusive as it ever was and continues to be so. 

An inhibitor evaluation test often will involve more than merely exposing a sample of metal to a corrosive environment. In this sub-section, four laboratory tests wdl be described to Uluslrate the complex conditions or the specific properties that can be encountered in designing a test to meet certain applications. Table 8.4 summarizes the conditions of each of the four tests and the other requirements peculiar to the application. Tests are also discussed below in detail to elaborate on the reasons that make them different from others. 

HIV-1 protease inhibitors were initially tested to determine dosing and interval administration (Markowitz et al., 1995 Danner et al, 1995). These studies provided key insights into the pharmacologic properties for this class of compounds. A direct result of these studies was the early recognition of resistance (since nonfully suppressive doses and intervals were initially tested) to HIV-1 protease inhibitors. The rapid clinical assessment of these medications began with an evaluation of the addition of HIV-1 protease inhibitors for patients with advanced immune destruction as defined by less than 100 CD4 cells. This was followed by HIV-1 protease inhibitor evaluation for patients with modest immune suppression, defined as less than 200 CD4 cells, and then for patients with minimal immune destruction (200-500 CD4 cells), and even exploratory studies for patients with no immune destruction (more than 500 CD4 cells). 

Quinine also has pharmacological activity as an antimalarial, and some studies have been made in order to find a method to diminish or suppress its bitter taste. This can be achieved by adding sweet compounds such as sucrose or aspartame or nonspecific bitter taste inhibitors such as NaCl or phosphatidic add and tannic add [39]. The effects have been studied by sensory evaluation tests in human volunteers, a binding... 

The current method for the hyaluronidase assay described in the United States Pharmacopeia (USP) [132] is based on the inability of hydrolyzed potassium hyaluronate to form a complex precipitate with proteins from added serum, reflected in a decreased turbidity of the reaction mixture (measured after 30 min). The method is, from the enzymological point of view, not well defined since it does not actually evaluate the kinetics of the hydrolysis of the substrate. An assay with end-point determination is only valid if the reaction rate does not change during this reaction time. We found that only with the two lowest test concentrations (0.15 and 0.3 IU) was this condition fulfilled, while with the three higher test concentrations the reaction is not linear. Commercially available hyaluronates can be contaminated with chondroitin sulfates. They are more acidic than hyaluronic acid itself and hence can form better protein complexes and influence the turbidity. In a suitability test of the USP [133], the substrate must pass both an inhibitor content test and a turbidity-production test. The assumption is made that... 

The industry has always attempted to standardize materials and their applications. Despite great and protracted efforts to standardize test procedures, and there are many standard corrosion tests there are no standard tests for corrosion inhibitor evaluation. The reasons for this are complex as may become clear below. However, basically it had been thought that inhibitors, once evaluated under a specific set... 

To illustrate the above, inhibitor evaluations by means of the so-called "Wheel Test [iO] fail all three quality criteria. This test has in the past been used extensively to evaluate corrosion inhibitors for use in oilfield applications. The results are always based on weight loss and are, therefore, not unique. They are not relevtmt, because as the test bottles turn on the wheel to which they are affixed, the flow regime or agitation is not defined. The results are also not predictive, because the controlling variables do in general not reflect field conditions. Disregtirding these most basic quality criteria has in the past been, tind still is, a major resison for costly failures in the field of oil tmd gtis production. 

Corrosion testing and corrosion inhibitor evaluation are done (apart from mechanistic studies) for the purpose of determining... 

The effectiveness of inhibitors is affected by a number of environmental, physical, and metallurgical parameters. These variables interact with each other in unpredictable nonlinear fashion, and moreover, such interactions are inhibitor specific. This state of affairs negates the validity of screening or standardized testing. Relevant and predictable inhibitor evaluation must be carried out under conditions simulating as closely as possible those of ultimate usage. 

Acidizing treatments are used to stimulate production by the acidic dissolution of undesirable materials on the walls of the producing formation or by pumping acid into the formation to improve permeability. Hydrochloric acid (HCl) is used for limestone formations. Hydrofluoric acid (HF) is added to the HCl for silicate formations. Inhibitors are added to the acid to protect steel tubing during the short period of exposure to acid during injection, and the following period when partially spent acid is returned to the surface. Suppliers use a variety of nonstandardized tests for acid inhibitor evaluation. 

Transport of gasoline and other refined products in steel pipelines may result in corrosion products that can create a product contamination problem. Internal corrosion of the pipeline can also have an adverse effect on pipeline capacity. Corrosion results from condensation of a water film on the pipe wall plus dissolved air or SRB in the product. Corrosion control is commonly achieved by adding a corrosion inhibitor. Evaluation of inhibitor performance can be done using NACE Test Method for Antirust Properties of Cargoes in Petroleum Product Pipeline (TM0172). This test method is a modification of ASTM D 665, Test Method for Rust-Preventing Characteristics of Inhibited Mineral Oil in the Presence of Water. 

Rust problems during usage of certain refined products, such as steam turbine oils, are controlled by addition of corrosion inhibitors. Evaluation of the performance of these inhibitors is done using ASTM D 665 or ASTM D 3603, Test Method for Rust-Preventing Characteristics of Steam Turbine Oil in the Presence of Water (Horizontal Disk Method). 

Inhibitor selection begins with the choice of physical properties. Must the inhibitor be a solid or liquid Are melting and freezing points of importance Is degradation with time and temperature critical Must it be compatible with other system additives Are specific solubility characteristics required This list can be extensive but is important because it defines the domain of possible inhibitors. It must be the first step of the inhibitor evaluation for any new system. The physical measurements are those routinely done as part of minimal quality acceptance testing. 

The challenge in inhibitor evaluation is to design experiments that simulate the conditions of the real-world system. The variables that must be considered include temperature, pressure, and velocity as well as metal properties and corrosive environment chemistry. System corrosion failures are usually localized and attributed to micro conditions at the failure site. Adequate testing must include the most severe conditions that can occur in the system and not be limited to macro or... 

The nominated inhibitor(s) should be evaluated (tested) with regard to function before selection for a system. Test(s) should include laboratory test(s) as well as field or operational tests. 

Ailor, W. H., Handbook on Corrosion Testing and Evaluation, John Wiley, New York (1971) Bregman, J. I., Corrosion Inhibitors, Collier-Macmillan Co., New York (1963)... 

NACE Publication ID 182, Wheel Test Method for Evaluation of Film Persistent Inhibitors for Oilfield Applications, Mater. Perform., 21, (12), 45 (1982)... 

Britton, C. F., The Selection, Evaluation and Testing of Oil Field Corrosion Inhibitors , Seminar The Development and Use of Corrosion Inhibitors , Oyez Scientific and Technical Services Ltd., London (1983)... 

Apart from these two Vertex compounds, only one other caspase inhibitor, BDN-6556, has been used in clinical trials. This compound belongs to the class of oxamyl dipeptides and was originally developed by Idun Pharmaceuticals (taken over by Pfizer). It is the only pan-caspase inhibitor that has been evaluated in humans. BDN-6556 displays inhibitory activity against all tested human caspases. It is also an irreversible, caspase-specific inhibitor that does not inhibit other major classes of proteases, or other enzymes or receptors. The therapeutic potential of BDN-6556 was first evaluated in several animal models of liver disease because numerous publications suggested that apoptosis contributes substantially to the development of some hepatic diseases, such as alcoholic hepatitis, hepatitis B and C (HBV, HCV), non-alcoholic steato-hepatitis (NASH), and ischemia/reperfusion injury associated with liver transplant. Accordingly, BDN-6556 was tested in a phase I study. The drug was safe and... 

I. Drela, P. Falewicz, and S. Kuczkowska. New rapid test for evaluation of scale inhibitors. Water Research, 32(10) 3188-3191, October 1998. 

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