Studys Of Anodic Film On Cracking Behavioir And Michanism After Heating

Anodizing technology was widely applied to the surface treatment of aluminium and its alloys to improve their mechanical properties and corrosion resistance. Anodic films on aluminum alloys have excellent heat-resistance performance. However, because the difference between thermal expansion coefficients of anodic films and the substrate aluminum alloys, internal stress may be induced by thermal cycles and micro-cracks may occur in the films, which leads to decreases of thermal fatigue resistance and corrosion resistance of the anodic films. As the increasing of application conditions of aluminium alloys, it becomes more and more important to the studies on the anodic films which have excellent corrosion resistance and synthetical mechanical properties and can endure thermal recycles at high temperature. The factors affecting performance of anodic films at different temperatures involve the difference of thermal expansion coefficients between anodic films and substrate aluminum alloys, the internal stress in the films, and inherent performance of the films. If the effects of anodizing techniques and structure and composition of the matrix and the anodic films on cracking behaviors of the films are cleared, not only the science charcter of structure and property of anodic film can be cleared further, but it is possible to prepare anodic films with better heat-resistance, thermal fatigue-resistance and corrosion resistance.The effects of anodizing parameters on residual internal stress in anodic films, sealing methods and heating process on the cracking behavior of anodic films and the change of corrosion behavior before and after heating were investigated in this paper. The results showed that the pore structure of the film on pure aluminium was very regular and well arranged, while that for the films on Z1201 and LY12 alloys were irregular and disorganized. Some micro-cavities were observed in the films on alloys, which were induced by the intermetallic compounds and undoubtedly will affect the mechanical properties of the film as the presence of these flaws will decrease the mechanical properties of anodic films. There was residual internal stress in anodic films on pure aluminium and compressive stress increased with increasing of anodizing voltage. Both unsealed and sealed anodic films lost mass when they were heated induced by the removal of adsorbent water and crystal water, while the latter was more dependent on the change of temperature. Weight loss increased with increasing of heating rate. As the reactive action of sealed anodic films increased, the mass loss of sealed anodic films was more than unsealed ones. The structures of anodic films still remained amorphous structure both before and after heating in the temperature range up to 300℃. The porosity of porous anodic layer increased with increasing of anodizing voltage, which was helpful to restrain the cracking of anodic films as well as the increasing of compressive internal stress.The sealed anodic film on pure aluminium still showed an intact structure without cracks. But for Z1201 and LY12 alloys, in the films networks of un-penetrated cracks were formed. Unsealed anodic film showed relatively excellent heating resistance compared to the sealed anodic films in boiling water. Penetrated cracks were formed in sealed anodic films after heating. If the sealed anodic films were continuous, new cracks were formed after heating and the cracking behavior mainly related to internal stress and pore structure of anodic films (the cracking behavior of anodic films on pure aluminium). If there were cracks after sealing but the widening of these cracks can not release the thermal stress during heating, new cracks were formed besides the widening of original cracks (the cracking behavior of Z1201). If the thermal stress can be released by widening of original cracks, no new cracks formed (the cracking behavior of LY12). Crack densities increased with increasing of thickness of anodic films and heating rate. But the heating temperature, cooling rate hardly affected the crack density. Cracks formed during the heating process. The heating resistance of anodic films in oxalic acid was more excellent than ones in sulfuric acid.Sealing methods played an important role to the cracking behavior of anodic film during sealing as well as heating of sealed anodic films. The order of heating resistance of sealed anodic films by different methods is: cerium salt> K₂Cr₂O₇ >selfsealing>boling water> NiF₂. The sealing methods which decreased hydration and/or remain original pore structure of anodic films during sealing will retain the cracking of anodic films.The capability of unsealed anodic films to substrate hardly changed before and after heating. But for the anodic films sealed in boiling water and NiF₂ solution, the corrosion rate increased obviously. With increasing of crack density, the corrosion rate increased. On the contrary, anodic films sealed by K₂Cr₂O₇ and cerium salt can protect the substrate well even after heating.