Materials Performance Supplements

CORTEC 2017

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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lar VCI where applied together. Test Setup and Procedure The test method objective was to pro- vide a quantitative evaluation of the effec- tiveness of CP and an amine carboxyl- ate-based VCI when applied individually and jointly to mitigate the corrosion current in a liquid-phase corrosion "macro-cell." An oxygen concentration cell between tw o carb on st e el (C S) el e ctro d e s wa s deemed representative of the prevalent macro-cells that exist on tank bottoms. A salt water solution (3.6 L at 35 g sodium chloride [NaCl] per L) was used. A vari- able-output air pump forced air though a diffuser positioned below one of the CS electrodes to encourage a cathodic (reduc- tion) reaction and create a potential differ- ence with respect to the unaerated steel electrode. A mixed metal oxide Ti rod anode was positioned midway between the steel electrodes and powered by a variable direct current power supply. The schematic and physical arrangement of the test appa- ratus is depicted in Figures 1 and 2. Preparatory Procedure A preliminary preparatory procedure to reliably produce the macrocell consisted of the following steps: 1. The test container was cleaned and rinsed. 2. Salt water solution (3.6 L with 35 g NaCl per L) was prepared and placed in the test container. 3. Test rod metal surfaces were cleaned and sanded to NACE No. 1/SSPC-SP 5/Sa 3 1 finish. 4. Test rods were placed in solution, with- out bond, and allowed to soak for at least 16 h for each to reach a stable open-cir- cuit potential (OCP). 5. Copper/copper sulfate (Cu/CuSO 4 ) refer- ence electrodes (CSEs) were freshly pre- pared, tested to verify a <1 mV difference between them, and placed in the test apparatus. 6. The OCP of each test rod was measured and monitored to ensure their stability. 7. The test rods were bonded, and the bond current and potentials were monitored until they stabilized. 8. Aeration was started to cause a poten- tial difference between the test rods, and adjusted until a steady state potential difference of 35 to 40 mV was achieved along with an associated corrosion cur- rent (i.e., I CORR ) of 350 to 400 µA. Test 1—Effect of VCI on Active CP System Following the preparatory procedure, this test consisted of the following steps: 1. The CP arrangement was energized, and the CP current (I CP ) was adjusted to mit- igate I CORR (i.e., reduce I CORR to zero). As cathodic polarization increased, I CP was further adjusted to maintain I CORR at zero until a steady state was reached. 2. The first 3 g of inhibitor was added to the solution. The effect on the I CORR was mon- itored, and I CP was adjusted to maintain I CORR at zero until a steady state was reached. 3. T h e i n h i b i t o r c o n c e n t r a t i o n w a s increased by adding another 3 g, the effect on I CORR was monitored, and I CP was adjusted to maintain I CORR at zero until a steady state was reached. 4. T h e i n h i b i t o r c o n c e n t r a t i o n w a s increased by adding another 3 g, the effect on I CORR was monitored, and I CP was adjusted to maintain I CORR at zero until a steady state was reached. 5. CP was de-energized and I CORR was mon- itored until a steady state was reached. 6. The aeration was turned off and I CORR was monitored until a steady state was reached. Test 2—Effect of VCI Prior to Application of CP System Following the preparatory procedure, this test consisted of the following steps: 1. The first 3 g of inhibitor was added to the solution. The effect on the I CORR was mon- itored until a steady state was reached. 2. T h e i n h i b i t o r c o n c e n t r a t i o n w a s increased by adding another 3 g and the effect on I CORR was monitored until a steady state was reached. 3. T h e i n h i b i t o r c o n c e n t r a t i o n w a s increased by adding another 3 g and the effect on I CORR was monitored until a steady state was reached. 4. The CP arrangement was energized , and the I CP was adjusted to mitigate I CORR (i.e. reduce I CORR to zero). As cathodic polarization increased, I CP was adjusted further to maintain I CORR at zero until a steady state was reached. 5. CP was de-energized and I CORR was moni- tored until a steady state was reached. 6. The aeration was turned off and I CORR was monitored until a steady state was reached. Test Results The results for Test 1 are illustrated in Figure 3. The results for Test 2 are illustrated in Figure 4. Discussion of Results The Test 1 results are shown in Table 1. The Test 2 results are shown in Table 2. Conclusions The results indicate a beneficial syner- gistic effect between the VCI(x) tested and CP, where the inhibitor enhances cathodic polarization to reduce CP current require- ment, and the CP reduction reaction appears to enhance the absorption and effectiveness of the inhibitor at the cathodic metal surface. The following is a point-form summary of the conclusions drawn from this testing. 1. With respect to the VCI(x) tested: a. The VCI(x) tested is a "cathodic polarizer." b. As a cathodic polarizer, the VCI(x) tested reduced CP current require- ment, and could thereby also enhance CP current distribution. Specifically, the CP current requirement of 5.5 mA to mitigate the corrosion cell before the addition of inhibitor was reduced by 45% with the first 3 g addition, 55% with further 3 g addition, and 60% with the final 3 g addition. c. At the concentrations tested, the VCI(x) substantially reduces, but does not completely mitigate the corrosion rate (i.e., I CORR ) in a liquid-phase macro- cell. Specifically, the original corrosion 15 CORTEC SUPPLEMENT TO MP MATERIALS PERFORMANCE JUNE 2017

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