Materials Performance

DEC 2018

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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25 MATERIALS PERFORMANCE: VOL. 57, NO. 12 DECEMBER 2018 lem. 5-11 However, there are many types and chemistries of corrosion inhibitors that affect the electrochemical reactions at the metal-electrolyte interface. Each of these has specific properties that may or may not be compatible with CP or other corro- sion prevention measures. For example, a study done by Abed, Panchal, and Gandhi 12 showed that an amine carboxylate-based VCI has a synergistic effect on the corro- sion rate when combined with CP and out- p e r f o r m e d e a c h sy st e m w h e n i m p l e - mented as standalone. The findings of this study were further confirmed by another work done by Pynn & Abed, 1 which showed an amin e carboxylat e-based corrosion inhibitor worked as a cathodic polarizer and resulted in reduction of corrosion cur- rent and cathodic corrosion current. The study also suggested a synergistic effect of CP reduction reaction in enhancing the absorption and effectiveness of the inhibi- tor at the cathodic metal surface. The following experimental work inves- tigates mutual compatibility of three VCIs when jointly applied with CP. Experimental Procedure An oxygen concentration cell between two carbon steel electrodes was deemed representative of the prevalent macrocells that exist on tank bottoms. A salt water solution (35 g sodium chloride [NaCl] per L) was used. The schematic and physical ar ran gem ent of th e t e st app aratu s i s depicted in Figure 1. A preliminary preparatory procedure to reliably produce the macrocell was fol- l ow e d , w h ere th e t e st c ont ai n er w a s cleaned and rinsed , salt water solution (35 g NaCl per L) was prepared and placed in the test container, and test rod metal surfaces were cleaned and sanded to NACE #1/SSPC.SP 5/Sa.3 13 finish. The test rods were placed in solution, without bond, and allowed to soak for at least 24 h for each to reach a stable open circuit potential (O CP). Copper/copper sulfate (Cu/CuSO 4 ) reference electrodes (CSEs) were freshly prepared, tested to ver- ify <1 mV difference between them, and FIGURE 1 Test setup schematic. placed in the test apparatus. The OCP of each test rod was measured and monitored using an automatic data logger to ensure stability. The test rods were bonded, and the bond current and potentials were mon- itored using the automatic data logger until th ey stabi li zed . O nce stabl e O C P was achieved, aeration was started to cause a p ot enti al di f feren c e b etw e en th e t e st rods—and adjusted until a steady state potential difference of 30 to 40 mV was achieved along with an associated corro- sion current (i.e., I CORR ). Results In the three following tests, the CP arrangement was energized, and the CP current (I CP ) was adjusted to mitigate I CORR (i .e., reduce I CORR to zero). As c at hod ic pola r i z at ion increased , I CP was further adjusted to maintain I CORR at zero until a steady state was reached . The VCI was added to the solution at a dosage rate of 1 g and the effect on the I CORR was monitored. After 20 h, another 1 g was added and the effect on I CORR was monitored. A third dose of 1 g of the inhibitor was added, the effect o n I C O RR w a s m o n i t o re d , a n d I C P w a s adjusted to maintain I CORR at zero. Volatile Corrosion Inhibitor (A) Test Results Figure 2 shows the following: 1. Both coupons were bonded and left to stabilize to –790 mV. 2. A macrocell was created by aera- ti on to a chi e v e a st e a dy st at e p ot enti al dif ference of ~40 mV between coupons and an I CORR of ~ 4 0 0 µ A . Th e a e ra t i o n c a u s e d potentials of both coupons to shift electropositive because of cathodic depolarization. 3. The application of CP with I CP = 8 mA caused the cathode coupon to cathodically polarize toward the pot ential of th e anode coupon , thereby reducing I CORR to 0 µA and effectively mitigating the macro- cell. CP current was left to stabilize for 20 h. 4. CP current was adjusted to I CP = 12.5 mA to maintain zero I CORR. 5. One gram of VCI (A) was added to the solution and I CP and I CORR were monitored over 20 h. Both coupon potentials shifted electronegative, indicating they were being cathodi- c a l l y p o l a r i z e d . T h e c a t h o d i c

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