Electrochemical and corrosion behavior of electrodeposited Zn, Zn-Ni alloy and Zn-Ni-TiO₂

Anwar, Shams (2020) Electrochemical and corrosion behavior of electrodeposited Zn, Zn-Ni alloy and Zn-Ni-TiO₂. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

Corrosion is one of the main causes of structural deterioration in process industries. It often causes process equipment failure and ruptures, which lead to severe safety problems, such as environmental pollution or even fatalities. To address this problem, one approach is to provide a preventive barrier-coating. This thesis investigates the electrochemical and corrosion behavior of the electrodeposited zinc, zinc-nickel alloys, and zinc-nickel-oxide composites. The negative reduction potential of Zn/Zn²⁺ electrode offer significant challenge to be electroplated. It also has high propensity liberate hydrogen gas. The complexation of zinc and nickel ions by complexing agent (ammonium citrate, potassium citrate and ethylene-diamine-tetra-acetic acid (EDTA)) stabilized the bath, which can extend the pH of nickel hydroxide and zinc oxide for electroplating purposes. The systematic design of the Zn-Ni alloy electroplating has been conducted using a five-variable experimental plan comprised of four steps: (1) a two-level fractional factorial design (FFD), (2) a response surface design the steepest ascent analysis, (3) a central composite design (CCD), and (4) corrosion behaviour test to optimize the factors in Zn-Ni deposition. After design of experiment, the optimum conditions were found to be a Zn/Ni molar concentration ratio 0.66, a plating temperature of 28ᴼC, an electroplating current density of 60mA/cm², an electroplating time of 13 minutes, and a citrate concentration of 0.062 mol/l. From the corrosion behaviour test of step-4 was found that the films with higher intensity of γ-NiZn₃, γ-Ni₂Zn₁₁, and γ-Ni3Zn₂₂ phases exhibited better corrosion resistance. The pure Zn, Zn-Ni alloy and Zn-Ni-nanoTiO₂ composite electroplated samples were deposited from citrate and non-citrate bath under the various level of deposition conditions from baths containing potassium and ammonium citrate were studied. Zn-Ni deposit obtained at 60mA/cm² from citrate bath exhibited lower corrosion current (Icorr), and less negative corrosion potential (Ecorr) compared to pure Zn and Zn-Ni alloy coatings from the non-citrate bath. Crystallite size of Zn-Ni coating deposited from citrate bath is 35.40 nm, and Ni content of the coating is 8.3 wt%. Zn-Ni films with smaller grain size with uniform coating had increased impedance modulus and improved corrosion resistance. The electrochemical behavior of samples shown that Zn-Ni alloy with the incorporation of 0.003 mol/l of Titania (TiO2) nanoparticles exhibited noble Ecorr and less Icorr values led to increasing impedance modulus with a less coarse, compact and stronger uniform coating of 25.84 nm grain size. The chemical composition result showed that the Zn-Ni-0.003 mol/l of TiO₂ coating electrodeposited from citrate bath at various immersion tests reduced dezincification in the coating. The most significant anticorrosion products for Zn-Ni alloy electroplating are simonkolleite, hydrozincite, zincite, and wulfingite. Additionally, Zn-Ni alloy deposited from a chloride bath containing EDTA was also investigated in this study. Polarization tests demonstrated that the Zn-Ni alloy deposited from 0.119 mol/l of EDTA bath at 20mA/cm² current density exhibited lower Icorr and more positive Ecorr values. At 24 hours of immersion time, the samples exhibit higher corrosion resistance leads to the formation of stronger anticorrosion layers. Finally, the pitting corrosion behavior of Zn-Ni alloy coatings in NaCl solutions with different chloride concentrations and pH were investigated. The pitting behavior in acidic solution with low chloride concentration was found to be significantly different from that in the neutral solution with high chloride concentration. Electrochemical analysis indicates that the corrosion behavior of the samples immersed at 0.35 mol/l of NaCl and pH = 3.0 have low impedance values in comparison to the samples immersed at 0.35 mol/l of NaCl and pH = 7.0 samples. Exclusive large pit morphology due to corrosion in acidic (pH 3.0) and low chloride concentration (0.35 mol/l) was observed. The comprehensive experimental study developed the processes for enhancing the corrosion resistant and mechanical properties of the Zn, Zn-Ni alloy and Zn-Ni-TiO2 composite coatings and investigate the stability of the electroplating baths.

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