Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.
Issue link: http://mp.epubxp.com/i/804522
57 NACE INTERNATIONAL: VOL. 56, NO. 4 MATERIALS PERFORMANCE APRIL 2017 posit" dynamic polarization curve. The "no calcareous deposits" curve was calculated in the laboratory using ASTM A36 CS and a seawater sample taken from the location of the offshore platform. A Solartron 1287 † electrochemical interface was used for the measurements, and the reference electrode was silver/silver chloride (Ag/AgCl). To ob- tain the "good calcareous deposit" curve, the CDs of the "no calcareous deposits" curve were scaled until the resulting curve passed through the long-term curve at the position of the minimum CD. Apart from the calcareous deposit effect, all the simu- lations took the anode decay effect and the anode radius into account. Optimization of SACP for Corrosion Control The offshore platform being retrofitted is located in the Gulf of Mexico. The struc- ture, illustrated in Figure 2, is a tripod com- posed of three ~20-m long elements whose bases form an equilateral triangle. The platform elements have different diame- ters, and the overall height is ~50 m. All the structural elements are made from ASTM A36 steel, and have been cathodically pro- tected over the past 20 years by Galvalum I † aluminum alloy anodes. It was determined that another 20 years of service are needed, which is the require- ment for the retrofit SACP system. When the offshore platform was constructed, 21 anodes were originally welded onto the structure. Some of those anodes were re- placed during the past maintenance pro- gram. At the start of this project, five an- odes were completely consumed, and four additional anodes were at least 80% con- sumed. The sea level is ~1.5 m from the top of the platform; so only the submerged por- tion of the platform is targeted for CP. In all simulations, the platform structure was modeled as though it was in a large box of seawater ; the surface of seawater was set as a symmetry plane so that no current flowed out of the seawater. As a critical boundary condition, the polarization cur ve of the seawater-sub- merged platform with bare structural steel was measured. The resulting curve is called the "no calcareous depot" polarization curve. According to the method used, the "good calcareous deposit" polarization curve was also obtained, as shown in Fig- ure 1 along with the long-term polarization cur ve. Calcareous deposits change with time and potential level; however, only the long-term calcareous deposit state is con- sidered here. Considering the limited height of the platform, only one seawater sample was taken, at ~25 to 30 m below the sea level. The measured polarization curve was used for the whole structure. Since the tempera- ture does not change significantly with seasons where the platform is located, the polarization cur ves were applied for all seasons. The conductivity of the sampled sea- water was measured in the laboratory as 5.6 S/m. The same type of aluminum alloy anode used previously was selected for the retrofit. Its polarization potential is –1,050 mV vs. Ag/AgCl and its original length is 2.5 m. This anode has a trapezoid cross section with an equivalent radius of ~0.218 m (based on a circular cross section with the same surface area). The separation be- tween the anode and the structure surface was 0.3 m. After all the pertinent information was prepared, the following CP optimization plans were evaluated so that the optimum quantity and distribution of anodes could be determined. In all, four schemes were simulated, with designs as follows: Scheme 1—the present actual state; Scheme 2— added anodes at five places where present an o d e c on sumption i s 1 0 0 % ; S ch em e 3— removal and replacement of four an- odes with 80% consumption; and Scheme 4—combine Schemes 2 and 3 (Table 1). The resulting distributions of structure- seawater potential are displayed in Figures 2(a) through (d), w hich correspond to Schemes 1 through 4, respectively. The dis- tribution of anodes, both added and re- placed, are marked on the corresponding figures. Thus, the effect of anode distribu- tion on the potential can be determined and the optimum scheme can be selected for the retrofit. As stated, the applicable CP criterion is –800 mV vs. Ag/AgCl. For Scheme 1, the ac- tual state, some portions of the structure are not cathodically protected and the FIGURE 1 Dynamic polarization and long-term curves for ASTM A36 CS in the seawater.