Materials Performance

MAY 2015

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.

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55 NACE INTERNATIONAL: VOL. 54, NO. 5 MATERIALS PERFORMANCE MAY 2015 ±50 mV criterion, we can rule out the pres- ence of any dissolution-controlled SCC mechanisms. Additionally, the active-pas- sive transition potential peak increases with higher scan rates, which is consistent with electrochemical activation control. However, the proximity to the recorded OCP implies that SCC could occur if the oxidizing potential of the environment increases. O CP displacement to higher (more anodic) values can occur due to tem- perature increases or incorporation of oxi- dizing species (CO 2 ) in the injected gas. EIS results indicated that the electro- chemical response could be attributed to Faradaic activation. Using a Randles equiv- alent circuit model, the polarization resis- tance (R p ) was measured to be 272 Ω·cm 2 . The solution resistance was subtracted from this value to determine corrosion resistance. Using the corrosion resistance value and Tafel slope values from cyclic polarization (b A 58.90 × 10 –3 V/decade, b C 88.60 × 10 –3 V/decade), the corrosion rate was found to be 1,575 mpy. The results also indicated the absence of a coherent surface film, which explains the high corrosion rate. Cyclic polarization for UNS S41000 SS (Figure 2) indicates a fully passive state, with an OCP of –870 mV vs. SCE, which is in the passive region . Therefore cracking caused by active dissolution is unlikely. The presence of th e passive l ayer on UN S S41000 SS was verified using EIS. The results indicated that the R p value is 8.5 kΩ·cm 2 . The Tafel slope values were b A 1 × 10 15 V/decade and b C 68.40 × 10 –3 V/decade, with a corrosion rate of 0.1 mm/y (4 mpy). The high b A values indicate passivation control. The EIS data correlates with a con- stant phase element equivalent circuit model. The resulting capacitance of 5.54 × 10 –4 F, along with the R p value, are consis- tent with the presence of a protective pas- sive layer. The cyclic polarization test on UNS S41000 SS performed at 0.1 mV/s (Figure 2) revealed current spikes along the potential range associated with th e presence of metastable pitting. Metastable pitting can lead to SCC if the pits propagate. However, chrono-amperometric testing (not shown here) indicated that no propagating meta- stable pits can be produced in the SS at these conditions, w hich means SCC is unlikely. FIGURE 3 Photos showing tested corrosion coupons after cleaning in inhibited acid. FIGURE 4 Pitting profles from proflometry analysis of the coupons shown in Figure 3. Weight Loss Testing Following the seven-day exposure to the test solutions in the four glass cell reac- tors, the corrosion coupons were removed for weight loss measurements. All four cou- pons had black corrosion product adhering to the surface. The test solution also turned black. This was more pronounced in the CS tests than in the SS ones. The pH of the solution was measured after degassing, the coupons were cleaned in inhibited acid, and the weight of the dried coupons was measured to determine general corrosion rates. To evaluate pitting corrosion, pro- filometry analysis was done. The number of pits was counted and maximum penetra- tion rate determined. As per typical user definitions, a pit is designated as a defect if it is larger than 50 µm in diameter and is deeper than a 50 mpy corrosion rate. These values trans- late to ~10 µm in depth for a seven-day test. Corrosion Studies of Scale Dissolver in a Carbon Dioxide-Containing Environment

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