General

Accelerated testing of catalyzed titanium for use as CP anodes in soils or natural waters is intended to provide an indication of the anode's ability to perform satisfactorily for a specific number of years. Because such anodes can fail with little or no weight loss, lifetime expectation cannot be extrapolated from operation at normal operating current density. Testing must instead be conducted at high current density until an acceptable amount of charge has been accumulated. This test is considered appropriate for catalyzed anodes based on film-forming metals, such as tantalum, niobium, and zirconium, when used for applications in soils or natural waters. Catalysts may be mixed-metal oxides (MMO) or precious metal/metal oxide cermets.

The anode shall survive the accelerated total charge density that is equivalent to the total charge density for the anode design service requirements. The relationship between life and current density is not linear for these anodes, but rather a log-log relationship, as shown in Figure 1. The total charge density for the accelerated test is numerically smaller than the total charge density calculated for the design service requirement.

Total charge density is simply the anode life multiplied by the current density. For the accelerated total charge density, it is the current density of the test multiplied by the minimum test duration. For the total design service charge density, it is the current density for the anode in the intended service times the desired service life for the anode.

For example, for an anode intending to be put in service for 20 years at 100 A/m² (9.3 A/ft²), the design service charge density would be 2,000 A-y/m² (190 Ay/ ft²) or 17,500,000 A-h/m² (1,630,000 A-h/ft2). The accelerated total charge density may be determined from the manufacturer's information.

The electrolyte used for this test shall be an aqueous solution of 1.0 M sodium sulfate. Because this electrolyte is aggressive toward these types of anodes, the results of this test may be considered to be conservative.

Testing shall be conducted using a full-wave, singlephase, unfiltered power supply with a minimum of 40% ripple. This condition will simulate the power supplies commonly used for CP in the field.

An anode specimen with an active surface area of ~50 mm² (~0.078 in²) should be used for this test, and shall be operated at a test current density so as to verify that the specimen meets the manufacturer's life versus current density curve. If this size or shape of anode specimen is impossible or inappropriate, a different specimen size may be used and current adjusted to provide the current density required.

At least two specimens of each anode shall be tested in separate cells under identical operating conditions.

The cell current and voltage, together with anode potential, shall be monitored either automatically or manually during the test period. Anode failure can be detected by a significant rise in cell voltage and anode potential, while operating at the required constant current density.

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