Materials Performance

MAY 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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38 MAY 2017 MATERIALS PERFORMANCE NACE INTERNATIONAL: VOL. 56, NO. 5 COATINGS & LININGS (the shifts near 1,564 and 1,496 cm –1 are bending vibration peaks of OH – ), which can be explained by a mass of OH – bonds on the c orro sion pro duct s formin g hydrogen bonds with compound molecules. 13 Accord- ing to XRD and FTIR analyses, the powder corrosion products are mostly basic alumi- num chloride (BAC) [Al 9 Cl 6 (OH) 21 ·18H 2 O]. BAC can be considered as a hydrolysate of AlCl 3 and Al(OH) 3 , with low cohesion so that it easily dissolves in NaCl solution. High HP accelerates the formation of BAC, resulting in rapid failure of Al coatings in deep sea environments. Potentiodynamic Polarization Curve Analysis Potentiodynamic polarization curves of Al coatings at different HPs are shown in Figure 4. Fitted results are listed in Table 1. The self-corrosion current density (CD) I corr is 3.0936E-6 A·cm –2 at 30 atm, which is ~10 times the value of 4.4947E-7 A·cm –2 at 1 atm. Also, the polarization resistance R p is 2.4435E4 and 5.8040E4 Ω·cm 2 at 30 and 1 atm, respectively. From the potentiody- namic polarization data, the polarization resistance of Al coatings at greater HP is significantly reduced compared to that at atmospheric pressure. Anodic polarization curves indicate a weaker passivating range. This can be explained because Al can form a thin layer of passive film, but this film is less continu- ous on surfaces of thermal-sprayed coat- ings. Furthermore, the anodic CD at 30 atm is much higher than that at 1 atm, indicat- ing that high HP accelerates anodic disso- lution. The cathodic polarization curves at different HPs perform similarly, which indi- cates that the two cathodic processes are similar. These conclusions are consistent with corrosion behaviors of Ni-Cr-Mo-V at different HPs. 14 Conclusions Corrosion behaviors of high–velocity, arc-sprayed Al coatings at high HP are dif- ferent from those at atmospheric pressure. In high HP conditions, corrosion products are mostly BAC, which cannot protect the Al coating or the steel substrate from cor- rosion. Electrochemical analysis results show that passive films of Al coatings at TABLE 1. FITTED RESULTS OF Al COATINGS POTENTIODYNAMIC POLARIZATION CURVES HP (atm) E corr (V) I corr (A·cm –2 ) R p (Ω·cm 2 ) 30 –1.1165 3.0936E-6 2.4435E4 1 –1.0620 4.4947E-7 5.8040E4 FIGURE 3 FTIR graphs of Al coatings. FIGURE 4 Potentiodynamic polarization curves of Al coatings at different HPs.

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