Development of technology for forming anodic oxide coatings on light metal alloys by plasma electrolytic treatment towards obtaining comprehensive surface protection systems
Opracowanie technologii formowania anodowych powłok tlenkowych na stopach metali lekkich na drodze plazmowej obróbki elektrolitycznej w kierunku otrzymywania kompleksowych systemów zabezpieczenia powierzchni
"Cold" plasma treatment
Considering the constantly progressing climate changes on Earth and the increasing difficulty in obtaining fossil fuels, it becomes clear that modern societies are increasingly looking for modern environmentally friendly technologies. Such solutions include, for example, reducing greenhouse gas emissions into the atmosphere by reducing energy consumption while contributing to improving the quality of life. An example of a technology enabling the implementation of the above-described activities is PEO: Plasma Electrolytic Oxidation. This process is an alternative to the anodizing commonly used in practice.
The PEO method allows the formation of thick, ceramic oxide layers, which significantly improve mechanical properties (including hardness) and corrosion resistance of light metal alloys. Thanks to protection using the PEO process, their wider use will be possible, which may influence the development of light structures in construction and transport, consequently reducing fuel costs and exhaust emissions.
However, it should be borne in mind that the process PEO is still quite a young technology. As part of the project, a technology for producing oxide layers on aluminum and magnesium alloys is being developed, which will be competitive or complementary to conventional anodizing. Actions will be taken to:
Determination of the influence of PEO processing conditions on the structural and functional features of anodic oxide coatings obtained in the process
Demonstration of the effect of additional pre-treatment on the quality of the layers formed as a result of PEO
Exploring the possibility of adding additional functionality to the layers through finishing
Taming the plasma
A special feature of PEO technology is the occurrence of small, short-lived sparks (or micro-discharges), resulting from the action of processed workpiece with high electrolysis voltage (350-600 V). By maintaining appropriate process regime it is possible to direct plasma phenomena to the state of "soft sparking".
Soft sparking obtained on aluminum alloys brings many benefits. First of all, the voltage needed to maintain it is usually 30-40% lower than in the case of the classic PEO process (reduction of energy consumption). Moreover, the coating formed in this way is enriched with an extremely hard and abrasive wear-resistant mineral - corundum (wł .alpha form of aluminum oxide). This phase is located in the so-called compact layer of the PEO coating. The faster the plasma softens, the less energy is needed to produce the coating. Therefore, the project examined the impact of pre-treatment on the PEO process, which constitutes part 2 of it. In addition, the corrosion resistance of coatings obtained in the process variant with softenedplasma is somewhat deteriorated. Hence the need for further modification, which is covered in part 3 of the project.
Control from start to end
The formation of plasma on the surface is conditioned by the presence of a barrier layer with specific insulating properties. Through appropriately designed pre-treatment, you can create a substrate that affects not only the PEO process (time, electricity consumption), but also the resistance corrosion offered by the obtained oxide layer. The project developed pre-treatment methods based on conversion layers and conventional anodizing methods.
The outermost part of the oxide obtained in the PEO process is the porous layer< /b>. It is characterized by relatively low mechanical properties due to the presence of pores. However, these can be used to load substances that enable further modification of coatings. Using the finishing treatment developed in the project, it is possible to introduce not only corrosion inhibitors into the oxide layers, but also organic dyes with a wide range of colors, which also affects decorative effect of the final product.
PEO technology with introduced design innovations compared to conventional anodizing
The PEO method is derived from conventional anodizing, where the details are immersed in solutions of acids (e.g. sulphuric, chromic; 20-40%), then incorporated into an electrical circuit as anodes, while cathodes immersed in the same bath are most often made of stainless steel. An electric current is passed through the circuit, which causes oxide growth on the surface of the details (generally 20 to 50 μm thick). Before anodizing, careful, multi-stage surface preparation is required (including mechanical processing, degreasing, pickling, decapping, rinsing), and after the process the layers are sealed. in boiling demineralized water.
In plasma electrolytic oxidation technology, diluted (max. 5%), ecological solutions with alkaline pH (no problem of corrosion of the equipment), and after initial mechanical processing, the details should be only degreased. Apart from that, the electrolysis system does not differ from conventional anodizing apart from the power source (rectifier). The process requires relatively high voltage (400-600 V) and current (5-25 A/dm^2) and is often carried out in pulse mode. >i bipolar (periodically changed voltage poles).
The higher costs of PEO processing compared to conventional anodizing are significantly compensated < b>higher technological parameters, such as hardness, adhesion, abrasion resistance, corrosion resistance.
Application
The developed technology covers elements made of light metal alloys (aluminum alloys, magnesium alloys). The solution is applicable wherever: low structure weight and surface resistance to mechanical wear and corrosion are required . Moreover, as a chrome-free technology, the method complies with EU recommendations according to RoHS and REACH.
Sport:
Bicycle frames, golf clubs, engine blocks, tribologically demanding elements (engines used in motorsport); lightweight magnesium structures are also used in modern wheelchairs
Space industry:
Counteracting cold welding (ang. cold welding, contact welding) of elements contacting in space (e.g. doors, airlocks, etc.), thermal shields, cryogenic bearings
Aerospace industry:
Helicopter gear housing, jet engine components
Automotive industry:
Braking system (brake discs - instead of heavy cast iron elements), layer in the piston/cylinder tribological pair, sealing rings, mounting washers, rollers
Other:
Construction of turbomolecular pumps (rotors with complex geometry); chemical industry (machinery elements susceptible to chemicals - e.g. pump lining); arms industry (soldiers' personal equipment, electronic device casings)
Our popular science videos about PEO technology on YouTube
External sources
Science article (Open Access)
M.Sowa et al. Corrosion Inhibitor-Modified Plasma Electrolytic Oxidation Coatings on 6061 Aluminum Alloy, Materials 14 (2021) 619
