Total Interface Current Density of Four Edges

From Figure 8a, the current density of four edges hardly has a difference in the first straight section. However, in the elbow section, the current density on the interior and exterior edges has a sharp fluctuation respectively. The current density on the exterior edge has a relatively positive fluctuation in the first half of elbow, and the current density on the interior edge has a negative and a positive fluctuation respectively. In the second straight section, the current density of four edges has a small fluctuation around 0 A/m<sup>2</sup> .

× <sup>−</sup>

× <sup>−</sup>

−

− −

Anode Current Density of the Four Edges

From Figure 8b, the anode current density of the top and bottom edges is maintained about 4.4 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> throughout the pipeline. However, the current density on the interior and exterior edges of the elbow section has a sharp fluctuation, especially on the interior edge. The anode current on the interior edge increased sharply to about 6.1 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> in the first half of the elbow, then was maintained at about 4.4 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> , and finally decreased to 4.1 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> in the second half of the elbow. The anode current density on the exterior edge decreased to 3.9 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> in the first half of elbow, and then maintained a stable value at around 4.4 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> .

**Figure 8.** The current density distribution of four featured edges on the tube. (**a**) Total interface current density distribution; (**b**) anode current density distribution; (**c**) cathode current density distribution.

Cathode Current Density of the Four Edges

− × <sup>−</sup> − × <sup>−</sup> − × <sup>−</sup> − × <sup>−</sup> From Figure 8c, the cathode current density of four edges in the first straight section maintains at about −4.4 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> . However, in the elbow section, the current density on the interior and exterior edges has a sharp fluctuation respectively. The current density on the interior edge in the first half of elbow has a relatively negative fluctuation (about −9.0 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> ) and then in the second half of the elbow has a relatively positive fluctuation (about −2.7 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> ). The current density on the exterior edge in the first half of elbow has a positive fluctuation (about −2.4 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> ). In the second straight section, the current density on four edges has a small fluctuation around −4.5 × 10 <sup>−</sup><sup>4</sup> A/m<sup>2</sup> .

#### − × <sup>−</sup> Current Density Distribution on the Wall

Figure 9 shows the current density distribution caused by electrochemical reactions under turbulence throughout the pipeline. From Figure 9a, the serious part of the total interface current density is mainly concentrated at the intersection of the elbow and the straight sections. From Figure 9b, the most serious metal loss caused by the electrochemical corrosion is concentrated at the junction of the elbow and the first straight section. From Figure 9c, the serious part of the cathode local current density is similar to the part of the total interface current density.

**Figure 9.** Current density 3D map on the wall. (**a**) Total interface current density; (**b**) anode local current density; (**c**) cathode local current density.
