Abstract
The anodic oxidation process is an established means for the improvement of the wear and corrosion resistance of high-strength aluminum alloys. For high-strength aluminum-copper alloys of the 2000 series, both the current efficiency of the anodic oxidation process and the hardness of the oxide coatings are significantly reduced in comparison to unalloyed substrates. With regard to this challenge, recent investigations have indicated a beneficial effect of nitric acid addition to the commonly used sulphuric acid electrolytes both in terms of coating properties and process efficiency. The present work investigates the anodic oxidation of the AlCu4Mg1 alloy in a sulphuric acid electrolyte with additions of nitric acid as well as oxalic acid as a reference in a full-factorial design of experiments (DOE). The effect of the electrolyte composition on process efficiency, coating thickness and hardness is established by using response functions. A mechanism for the participation of the nitric acid additive during the oxide formation is proposed. The statistical significance of the results is assessed by an analysis of variance (ANOVA). Eventually, scratch testing is applied in order to evaluate the failure mechanisms and the abrasion resistance of the obtained conversion coatings.
Highlights
The anodic oxidation process is a suitable means for the surface refinement of aluminum and its alloys
Anodic oxide coatings with a low porosity and high hardness and abrasion resistance can be achieved by anodizing in sulfuric acid electrolytes at low temperatures beneath 5 ◦ C
As can be seen in the diagram, the voltage amounts to about 23 V for the base in the diagram, the voltage amounts to about 23 V for the base electrolyte without additives after the electrolyte without additives after the process initiation
Summary
The anodic oxidation process is a suitable means for the surface refinement of aluminum and its alloys. The formation of an oxide ceramic coating under anodic polarization in an acidic electrolyte leads to an increased corrosion and wear resistance, enhances haptic-visual properties and depending on the process regime, provides certain other surface property alterations like electrical insulation. The so-called “hard anodizing” process itself is costly and demands the input of substantial amounts of electrical energy for both the process itself and the temperature control of the electrolyte. Another means of reducing the coating porosity lies in the addition of organics to the commonly used sulphuric acid electrolyte. Giovanardi et al [1]
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