Influence of Hydrothermal Sealing on the High Cycle Fatigue Behavior of the Anodized 6082 Aluminum Alloy

Abstract

For aluminum alloys, anodizing is a common electrochemical surface treatment to allow for protection against corrosion and wear. The produced conversion layers are first sealed in industrial processes to further enhance the corrosion protection by closing the coating surface pores. In their lifetime, anodized components often undergo cyclic loadings. However, despite the relevance of a sealing treatment, there is a lack of systematic studies regarding its influence on the fatigue behavior of anodized aluminum components. In this work, a 6082-aluminum alloy was anodized in sulphuric acid and the effect of the anodizing treatment with and without further hydrothermal sealing on the fatigue strength was investigated. The thickness and Martens hardness of the coatings were determined and the coating appearance in non-sealed and sealed conditions was analyzed by scanning electron microscopy prior to and after cyclically loading at R = −1. The fatigue strength was significantly decreased by the anodizing treatment, when compared to the bare substrate. However, hydrothermal sealing had a positive influence as the anodized and sealed condition attained a fatigue strength in the range of the bare aluminum. Distinct differences regarding the coating appearances, thickness, and hardness were not observed when comparing the non-sealed and the sealed conditions. After fatigue loading, numerous pronounced radial cracks were present in the anodic coating, but the number of cracks were significantly lower for the hydrothermally sealed coating. Fatigue failure occurred due to propagation of one crack from the coating towards the substrate, resulting in single-point crack initiation, which was similar to the fatigue fracture behavior of the bare aluminum substrate.

Fatigue Resistance of an Anodized and Hardanodized 6082 Aluminum Alloy Depending on the Coating Thickness in the High Cycle Regime

“…Further, the tested oxide coatings in their study were hydrothermally sealed, which can also have a major influence on the fatigue resistance. [26] The observed failure behavior of the anodized specimens was also shown by Vie et al [46] They combined acoustic emission and microstructural analysis confirmed that only after few cyclic loadings numerous cracks are already initiated in the brittle anodic coating and that after 15% of the fatigue life the crack number reaches a saturation value. Due to further cyclic loading, although there are multicrack initiation sites in the coating, only one main crack propagates toward the substrate leading to fatigue failure and fracture.…”

“…The experimental determined fatigue data points were curve‐fit using a logarithmic model provided by Rateick et al [ 9 ] This empirical model allows for a high compliance of the experimentally determined data points with the calculated progression of the fatigue life as it was developed by Rateick et al [ 9 ] for a hardanodized aluminum alloy and was successfully used in previous own studies representing the fatigue curve of coated specimens. [ 22,26,28 ] …”

“…The anodized specimens were not sealed to prevent an additional influence of the sealing treatment on the mechanical properties, as it is shown in an own previous study. [ 26 ]…”

“…For the subsequent electrochemical treatment, the used electrolytes, bath temperatures, and current densities are listed in Table 3. These parameters were chosen according to previous own studies [7,26,28] showing their suitability to form functional coatings to enable a comparability. Anodizing and hardanodizing was performed in aqueous sulfuric acid due to the industrial relevance of this electrolyte.…”

“…Further, as Cirik et al [21] and Wragg et al [24] used sealed specimens, the comparability to non-sealed specimens is limited due to the significant influence of the sealing treatment on the high cycle fatigue behavior. [25,26] Furthermore, the influence of the coating type and the coating thickness on the failure behavior under fatigue loading needs to be addressed with regard to the preliminary damage of the coating. It is necessary to understand the relationship between these influencing factors and the fatigue resistance, as there are application fields where the corrosion and wear resistance enabled by an anodic coating are needed but the components are also undergoing cyclic loading.…”