Degradation of Epoxy Composite Coating Using Corrosion and Fracture Mechanics Framework
This work presents the results of a combined analytical and experimental study of the effect of reinforcement on the corrosion and adhesion ability of epoxy coatings. Firstly the corrosion behavior of 5-hydroxytryptophan (HTP), and clay particulate reinforced epoxy coatings is studied on a steel substrate that is used widely in pipelines and tanks. The corrosion behavior was studied in sodium chloride (3.5 wt. % NaCl) solutions that simulate potential seawater exposure at pH 3 and 7. X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM) were used for microstructural characterization of the samples. The thermal stability was characterized using Thermogravimetric Analysis (TGA). The underlying corrosion reactions and reaction products were also elucidated via Fourier Transform Infrared Spectroscopy (FTIR). Electrochemical impedance spectroscopy (EIS) and in-situ observations of interfacial blisters were used to study the underlying degradation mechanisms. Electrochemical impedance spectroscopy revealed that for prolonged exposure of about 90 days and above, the composite materials exhibited better corrosion resistance at a pH of 3 as seen by the higher diameter of the Nyquist plot. Fewer corrosion products were observed on the scribed areas of the HTP samples in the scribe test in pH of 3 corroding environments. This signifies improved adhesion of the coatings in that environment for the HTP/epoxy coatings. The results obtained also show that a 1 mm blister size was observed in the pristine epoxy sample while no blisters were observed in the clay/epoxy and HTP/epoxy samples exposed at pH of 3. In the pH 7 environment, the EIS experiment revealed the presence of blisters with diameters in the range of 1–4 mm, after exposure for 90 days. The implications of the results are discussed for the corrosion protection of steel surfaces with composite coatings. Secondly, the nano-indentation and Brazil Disk techniques is use to determine the Young’s moduli, hardness values and mode mixity characteristics of the composite coatings. The Young’s moduli of the reinforced composites comprising 1, 3, and 5 wt. % of montmorillonite clay particles are shown to improve respectively by about 23 %, 58 %, and 50 % while the respective hardness values increased by about 46 %, 80 %, and 88 %, relative to those of pristine epoxy. The interfacial toughness between X65 steel and the epoxy/clay coatings increases with increasing mode mixity. This is associated with crack-tip shielding by crack deflection and crack bridging. The trends in the measured mode mixity dependence of the interfacial fracture toughness values are consistent with predictions from the simplified zone, normal zone, and row models (at lower mode mixity). The insights from the observations and the measured crack profiles are incorporated into zone and row models for the estimation of crack-tip shielding. The implications of the results are discussed for the design of epoxy/clay composites with attractive combinations of mechanical properties. Thirdly, the tribological properties of epoxy composite coatings reinforced with montmorillonite clay particles are studied using nano-indentation and nano-scratch techniques. These are used to determine the nano-wear characteristics of the composite coatings. The plastic indentation resistance of the composites decreases with increasing particle loading, while the wear rates also drops with increase in re-enforcement from 1 and 3 wt.% and again the scratching experiments revealed a slight decrease in the surface damage of the coating with increasing clay loading. However, in all of the composites, the friction coefficients varied from 0.63 to 0.015. The section groove profile of each sample showed that the scratch depth reduced as clay reinforcement increased. The scratch depth of pristine epoxy was the highest ~ 150 nm followed by a depth of 100 nm for the 1% clay reinforced epoxy and 90 nm depth for 3% reinforced epoxy reinforced epoxy. There was also a general decrease with the wear coefficient, K, with hardness and increase in clay reinforcement up to 3%. The measured mechanical and tribological properties have also shown to compare favorably with predictions from composite theories of wear performance criteria. The implications of the results are discussed for the design of epoxy/clay composites with attractive combinations of mechanical and tribological properties.