Higher corrosion resistant process of hard-anodising aluminium with bubble test resistance of over 8 hours

High tech products must be resistant against aggressive process chemistries as well as protected against wear. Aluminium parts without a protection are not durable for such applications. Hard anodised surfaces create a corrosion resistant layer. The corrosion resistance can be measured and compared with a so-called bubble test: A 5%-hydrochloric acid (HCl) acts on the surface and reacts with the aluminium substrate; this chemical reaction leads to gas bubble formation. As long as the anodised layer protects the substrate, there is no bubble formation and only the oxide layer is attacked. But as soon as the protective oxide layer is dissolved, the aluminium substrate is attacked, which leads to the characteristic bubble formation. The process can lead to first bubbles after several minutes. The oxide layer is further dissolved and more and more bubbles form, until a constant bubble stream appears. On standard hard-anodised layers, such constant bubble stream appears after 10-30 minutes. The time between first contact of HCl and a constant stream is measured. The longer the time the better the corrosion resistance (bubble resistance). The innovative coating of the Swiss company is a new anodising process that enhances such bubble resistance by more than factor 10 to more than 8 hours until a constant bubble stream appears. The new anodising process has then been optimised and industrialised with the support of design of experiments software (DOE). Validations of reproducibility have been made for 4 different corrosion resistance classes. All other properties of the anodising process are at least equal or significantly better than those of standard hard-anodising for each of the four classes (abrasion and wear resistance, electrical isolation properties, minimal increase of surface roughness, salt spray test resistance, etc.). Results of the validation for bubble resistance are shown in Figure 1. - Class 1: Bubble resistance higher than 120 minutes (2 hours) - Class 2: Bubble resistance higher than 240 minutes (4 hours) - Class 3: Bubble resistance higher than 360 minutes (6 hours) - Class 4: Bubble resistance higher than 480 minutes (8 hours) The Swiss company has a long expertise in processes for aluminium finishing of components. The new hard-anodising coating meets a wide variety of requirements and offers new areas of application. Products for anodising line: The new process is run on a fully automatic anodising line which is capable of handling parts up to 2000 x 1500 x 500mm with a maximum weight of 300kg. With sulphuric acid anodizing and hard anodizing the line provides large production capacity and high-quality according to MIL-A-8625F Type III (hard-anodising / hardcoat). Manufacturing agreement is thought with manufacturers of high-tech products requiring a process surface that is resistant to aggressive media, i.e wafers in the semiconductor industry, high-wear parts in corrosive environments such as gear wheels, valves, slides, carriages, etc. The innovative process enables completely new areas of application that were previously unthinkable with ordinary aluminium surfaces.