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23 NACE INTERNATIONAL: VOL. 55, NO. 10 MATERIALS PERFORMANCE OCTOBER 2016 Because the driving voltage cannot be adjusted, the demand for protective cur- rent must be accommodated when deter- mining the size and number of anodes. Impressed current anodes utilize an external power supply that can provide a much higher output than can be obtained with galvanic anodes. The amount of cur- rent that can be provided by an impressed current anode is limited by the anode type, the manufacturer's recommendations, and safety. Conceptual Design— Galvanic vs. Impressed Current Jetty applications can be designed using either galvanic anodes or impressed cur- rent anodes, and often it is a matter of cli- ent or designer preference. For this project, the owner requested that the CP designer look at both system types to determine the optimum design for this application based on a requested 30-year anode design life. Calculating th e C P syst em current requirements was the first step in develop- ing the CP system design. The jetty in ques- tion is constructed with four interior rows of concrete piles with an exterior row of 247 bare steel sheet piles driven to a depth of 15 m into the seabed (with 6 m in seawater) and covered with a concrete cap. When cal- culating the current requirements for the impressed current system, it was necessary to account for current going to the portion of the steel piling below the mud line as well as in the seawater, along with current requirements for the reinforcing steel in the concrete. The presence of steel-reinforced concrete interior piles had only a minor impact on the overall current requirements; however, additional care had to be taken to ensure that the reinforcing steel in the con- crete piles was electrically continuous with the metallic steel piles by providing bond- ing during the jetty construction. Additionally, with large bare marine structures, it is generally accepted practice to anticipate dif ferent current require- ments based on an initial CD (I ci ) required to polarize the structure, and a lower CD (I c ) that is required to maintain polariza- tion of the structure. After completing the TABLE 1. GALVANIC ANODE OPTION Calculated Values Unit Value I ci —initial current required A 1,023 I c —polarized current required A 623 M a —minimum anode mass required kg 96,308 Quantity of 400-kg anodes required 240 FIGURE 1 Oblique view of the jetty. current requirement calculations, deter- mining the quantity of anodes was a matter of evaluating the total anode mass (M a ) required to provide the desired design life and matching that to the anode current output based on the anode geometry and environment (seawater) conductivity, so that the anodes have a similar ratio of anode current output to net mass. For the galvanic anode option (Table 1), the conceptual design presented to the owner consisted of installing one 400-kg alu- minum anode on each of the 247 bare metal- lic piles along the entire length of the jetty. As an alternative, an impressed current design utilizing MMO anode sleds was eval- uated. MMO anodes consist of an iridium oxide catalyst and a mixture of other rare earth oxides coated onto a titanium sub- strate. These dimensionally stable, catalytic anodes have exceptionally low consump- tion rates measured in mg/A-y, provide out- standing chemical resistance, and are stable in a wide range of applications. Originally developed for use in seawater hydrolysis applications, MMO anodes are ideal for use in seawater CP service. The anode sled design for this jetty project (Figure 1) consisted of six anode sleds rated for 30 years of continuous ser- vice at an output of 175 A per sled. Each sled contained two tubular anodes. Each anode, 1.52-m (60-in) long and 25.4 mm (1 in) in diameter, weighed 0.53 kg. MMO anode life is generally linear within the operating CD used in CP, so the service life of MMO anodes will decrease in proportion to the increase in current if the CP system is operated with anode output higher than their rated output. Conversely, when the operating output of the anodes is less than the rated output, their design life increases proportionally. The 175 A-rated sleds rec- ommended for the impressed current alter- native, when operating at 60% of their rated capacity once the system was polarized,