Corrosion of structural materials is a serious problem for industrial and civil infrastructure worldwide, costing billions of dollars, and hampering gross domestic product. Corrosion also presents a threat to the environment and public safety, with highways and bridges losing structural integrity, and hazardous materials leaching into soils, waterways, and aquifers.
Currently, chloride-based plating baths are being widely used to electroplate aluminum on steel, brass, copper, and other substrate materials, but they have multiple drawbacks including lack of ability to form a diffused aluminum layer between the metal surface and aluminum coating due to the relatively low plating temperature. In addition, traditional approaches do not prevent the loss of functional electrolyte, thereby often producing non-ideal downstream product characteristics.
High-temperature molten salt-based electrolytes can provide a cost-effective technological solution to the manufacture of aluminum-coated metallic substrates for a variety of engineering applications, particularly in corrosive/marine environments. Methods and systems have been developed for applying metallic coatings with a quaternary bromide-containing electroplating bath and aluminum anode, primarily to develop corrosive-tolerant compositions. The process may also be used in a broad range of applications, providing high-quality aluminum coatings on both ferrous and non-ferrous metals, and divergent surfaces and geometries.
The electroplating processes involving bromide salts result in thick, uniform, and smooth metallic aluminum coatings. The processes provide protective aluminum coatings by exposing components to a high-temperature aluminum-bromide plating bath. The bromine solution — which can be prepared with lithium, potassium, or cesium — is heated and homogenized to a maximum temperature of 350 °C. A pulsed technique is used to make the coatings more adherent and robust. Post-coating, the samples are annealed at temperatures ranging up to 500 °C.
The quaternary alkali bromide electrolyte plating system produces superior-quality aluminum-plated metallic surfaces, with better corrosion mitigation properties due to the formation of a diffused aluminum layer in between the substrate metal surface and the aluminum coating. Also, the bromide plating bath can efficiently prevent the loss of the functional electrolyte that results in the formation of controlled product characteristics such as smoothness, thickness, and adhesion of coatings on various metallic substrates.
After repeated tests, samples exhibited formation of a much stronger surface layer. Moreover, the composition of the surface layer was not pure aluminum, but also incorporated the innermost substrate (e.g., stainless steel) and the intermediate substrate-aluminum composite. The process requires no new equipment or major adjustments to existing infrastructure.