Current Requirement Tests

The current needed to protect a given buried metallic structure may vary over wide limits, depending on the nature of the environment and whether or not it has a protective coating, and if coated, on the quality and effectiveness of the coating as applied. For example, if it is assumed that a steel structure to be protected is buried in corrosive soil and has an exposed surface area of 90 m the current required, assuming reasonably uniform current distribution, could range from about 3 A if the structure is bare, to as low as 30 //A or less if the structure has a superior coating. This means that a bare structure may require 100,000 times as much current as the same structure would if it were well coated. 

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Successful application of cathodic protection depends upon the selection, design, installation, and maintenance of the system. Before designing the cathodic protection systems, adequate field data must be collected, analyzed, and evaluated. Nature and conditions of the soil are reflected by field measurements like soil resistance, hydrogen ion activity (pH), and the redox potential. To understand the nature of the pipeline, potential measurements, coating resistance, and meaningful design current requirement tests must be conducted. 

Figure 13.32 Coating resistance and CP current requirement tests.

When current requirement tests are made on an all-bare structure, results are typically quite different Assume, as shown by the illustration in Fig. 13.33 that DC is forced to flow from a temporary ground connection to the system section being studied, and determining how much current will be needed to protect that section. 

It is necessary to experimentally conduct a current requirement test for the structures to establish the magnitude of power requirement for the system to be installed. 

In case of a bare structure, or a pipe, a current requirement test is made by placing the electrode at a position remote to the structure. The setup for a current requirement test of a bare structure is shown in Fig. 5.43. The most important condition is that the structure must be fully polarized to a potential —0.85 V for complete cathodic protection. It is to be noted on the current off position, the potential must show —0.85 V immediately. This value should not be reached gradually on... 

Figure 5.42 Current requirement test using close CU/C11SO4 electrodes. (From TEXACO Cathodic Protection - Design and application school, Texaco Houston Research Center, Training Manual. Reproduced by kind permission of Cheveron, USA)...

However, due to the minor variations in the phase impedances, all the phases may not be subjected to identical severity of faults. For instance, in the above test each phase has recorded a different fault cunent. To evaluate the fault level from these test data, the general practice has been to consider the phase that has recorded the highest fault current as the base, which may occur in any of the phases. In the above test, it has occurred in phase Y. For this fault current, the test duration is adjusted to achieve the required severity of fault in terms of thermal effect (50 x 1 in the above case). 

Coatings of less noble metals than the substrate metal (e.g., Zn on Fe) are only protective if the corrosion product of the metal coating restricts the corrosion process. At the same time, the formation of aeration cells is hindered by the metal coating. No corrosion occurs at defects. Additional cathodic protection to reduce the corrosion of the metal coating can be advantageous. Favorable polarization properties and low protection current requirements are possible but need to be tested in individual cases. The possibility of damage due to blistering and cathodic corrosion must be heeded. 

Fig. 11-1 Determination of protection current requirement at a gas station using a drainage test.

Minimum Igniting Current (MIC) Ratio The ratio of the minimum current required from an inductive spark discharge to ignite the most easily igni table mixture of a gas or vapor, divided by the minimum current required from an inductive spark discharge to ignite methane under the same test conditions. 

Estimate of current required The surface area of the structure is calculated and the current density required for the particular environment is selected (Table 10.26). In the case of an existing structure the condition of the coating may be unknown and the application of a temporary cathodic-protection system may be necessary to determine the amount of current required for protection, as established by the potential. Such a test to determine the... 

High-voltage coating-testing equipment When cathodic protection is applied to a structure which has a protective coating, the current required is proportional to the bare metal area on the structure. Thus whenever a protective coating is applied it should be of good quality, with very few failures or pin holes in it, so that the cathodic-protection system may be economic. 

As mentioned previously (and discussed in detail in Sec. IX), contact lens products have specific guidelines that focus on compatibility with the contact lens and biocompatibility with the cornea and conjunctiva [75], These solutions are viewed as new medical devices and require testing with the contact lenses with which they are to be used. Tests include a 21-day ocular study in rabbits and employ the appropriate types of contact lenses with which they are to be used and may include the other solutions that might be used with the lens. Additional tests to evaluate cytotoxicity potential, acute toxicity, sensitization potential (allergenicity), and risks specific to the preparation are also required [75-77], These tests are sufficient to meet requirements in the majority of countries, though testing requirements for Japan are currently much more extensive. 

In summary, new brominated flame retardants have essentially met the challenge for thermoplastics that do not meet current requirements (e.g. a V-0 rating) without additives. More stringent flammability requirements for advanced materials and applications, coupled with improved test methodology (e.g. lOa-b) may shift the focus of research to other approaches. 

Freshwater media based on the OECD 203 ecotoxicity testing medium for fish and daphnia have been used in all T/DP testing of metals, metal compounds and alloys in the pH range 6-8.5 to date. However, the composition of a marine medium is also given in the T/DP section of the GHS, and by implication, a method for marine T/D testing is open for development and validation. While not currently required for REACH dossiers, T/D data in marine media and attendant classification proposals may be required in the future for marine shipping. 

The dental industry is currently accepting the International Standards Organization (ISO) standard for Polymer-based filling and restorative materials. This standard will require testing of the following properties ... 

The resin secreted by Cannabis indica and Cannabis sativa, varieties of hemp, is known variously as marijuana, hashish or bhang and is abused as a hallucinogenic drug. It appears however to have some beneficial properties and is currently under test as an antiemetic in cancer therapy. The secretion contains a number of interrelated oxygen heterocycles, some of which are shown in Scheme 281, which attempts to indicate their biosynthetic relationships (70MI22401). The cannabinoids are probably derived from a monoterpene unit based on p-menthane and 5-n-pentylresorcinol (olivetol), acting the part of a polyketide. 2,2-Dimethylchromene biosynthesis also requires the intervention of an isoprene fragment. 

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