Development of al coatings electroplated from chloroaluminate-based ionic liquid electrolytes as environmentally friendly cd replacement sacrificial coatings for high-strength steel alloys used in aer

Résumé

Numerous alternative coatings are being developed and tested to replace cadmium (Cd) coatings with hexavalent chromium (Cr) post-treatments, mainly due to toxicity concerns with both Cd and hexavalent Cr, which are known to be human carcinogens. The objective of the thesis work of Usoa Izagirre, led by Boeing Research & Technology-Europe, is to develop metallic Al sacrificial coatings as Ni-free Cd replacement for high strength steel alloys with the objective of achieving similar or better performance than the Alumiplate¿ method using the technology of electroplating from ionic liquids (IL). Both the coatings and the plating process shall comply with environmental and occupational health and safety regulations while passing the structural and functional requirements stablished by the aerospace industry for process qualification. Three different ionic liquid-based electrolytes have been studied. The first part of the thesis focused on studying the influence of the additives used in two of the ionic liquid electrolytes in coatings¿ composition, structure and functionality, with respect to the properties of coatings electroplated from the ionic liquid-electrolyte without any additives. When plating from the ionic liquid with additives improved morphology and bend adhesion were obtained. Process pre-treatment and post-treatment procedures were also studied and optimized. The optimised electrolyte and Al plating process were further subjected to an extended qualification campaign, which included corrosion resistance tests in an acidified SO2 salt fog, corrosion resistance tests of painted panels, fluid corrosion resistance tests, galvanic corrosion resistance tests, potentiodynamic measurements, galvanic potential determination, adhesion tests of painted panels, assessment of the strippability of the coatings, further assessment of the throwing power of the process, hydrogen embrittlement (HE) and re-embrittlement tests (HRE), fatigue tests and assessment of lubricity (run-on and breakaway torque and torque-tension tests). The Al coatings behaved very close to, as good as or better than the Cd controls in all tests except the HRE. The most critical performance parameters i.e., corrosion resistance, HE and HRE performance were studied in detail and eventual failure mechanisms were proposed. At the end of the thesis recommendations to overcome these challenges and to further mature this process are given.