Search

Ni-P∖alginate microgels coatings, as potential metallic protective coatings with self-healing properties, were deposited by the electroless method. The alginate microgels contained nickel chloride and sodium hypophosphite. It was proven that the reduction of nickel ions released from the microgels is possible on the steel and Ni-P coating surface. The self-healing effect of this system was studied by X-ray fluorescence (XRF), chronoamperometry and scanning vibrating electrode technique (SVET). An improved corrosion protection observed here is attributed to the reduction of nickel ions to metallic nickel on the tested surfaces. Differences in the surface concentration of nickel and phosphorous species in the corrosion tested coatings with and without microgels, as evaluated using X-ray Photoelectron Spectroscopy (XPS), provided substantial evidence for the formation of a Ni-P coating from the compounds included in the microgels.

Hexavalent Chromium has been successfully employed for corrosion protection purposes in aerospace coatings for decades. However, legislation will restrict the use of Cr6+ in the future and therefore the aerospace sector needs to identify alternative environmentally friendly coatings for corrosion protection. Before implementation of newly developed systems into actual components is possible, rigorous and time-consuming testing practices are required to ensure the new systems can achieve the strict aerospace standards requirements. The emerging number of coating systems being developed, and the vast research conducted on the subject worldwide, make the selection of suitable replacements for industrial application challenging. In this work, differently pretreated aluminium AA2024 alloy surfaces are coated with conventional Cr6+ containing coating and compared to a number of industrial alternative coatings. Corrosion performance is assessed by real-time imaging while immersed, by EIS, and by standardised salt-spray tests. Results reveal that the performance ranking acquired by salt-spray tests (SST) can be readily replaced by short-term immersion tests, and the time to failure in SST can be estimated from key corrosion indicators arising from EIS measurements at specific immersion times.

The internal topology of epoxy resins is, for the first time, shown not to be the determining factor for small molecule transport. Whilst epoxy resins comprise the matrix component of many high performance composites, coatings and adhesives, the nanostructure and transport properties of these materials are not well understood. Here, peakforce AFM imaging, in-situ FTIR cure analysis and nanochemical AFM-IR imaging are used to establish the effects of reaction selectivity and stoichiometry on the nanostructure of epoxy-phenolic resins based on bisphenol-A and diglycidyl ether of bisphenol-A. In the presence of excess epoxy, resins transition from exhibiting homogeneous internal nanostructures to the familiar nodular morphology characteristic of epoxies. This occurs as a result of lower reaction selectivity in the presence of increasing catalyst concentrations. Surprisingly however, chemically similar stoichiometric resins with a heterogeneous nanostructure display improved resistance to corrosion breakdown (ion transport) and lower water uptake than the homogeneous resins.

Water absorption is believed to be one of the main causes leading to the deterioration and degradation of protective organic coatings. Water uptake in coatings on metal may be measured, for example, by changes in electrical impedance (e.g. capacitance and resistance) as a function of time. Generally coating capacitance is expected to increase during initial stages of water uptake (due to the greater dielectric constant of water) while coating resistance is expected to decrease (due to the lower resistivity of water compared to most polymers). However, here we present evidence that water ingress is not the main determinant of damage in epoxy-phenolic can coatings. Thus, as the degree of cure is increased, the water saturated coating capacitance and resistance both increase while the time-to-failure also increases. We suggest that these observations are due, respectively, to an increase in the free polymer volume (permitting more water uptake) and to an increase in the charge transfer resistance at the metal-polymer interface (due to a higher density of polymer-to-substrate bonding). These results and interpretations are supported by local electrochemical impedance spectroscopy which has confirmed water absorption, coating failure and increased coating resistance for highly cured capacitive systems, at the microscale.