How 7075 aluminium reacts to stress corrosion surface treatment

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7075 alumium was developed by Sumitomo, which was later used to build the Zero fighter aircraft for the Japanese navy. 7075 aluminium is widely used in the aerospace, automotive, shipbuilding and other fields of the consumer industry for its good relationship between the density, very low, and resistance, very high.

The 7075 include 6.1% zinc, 2.5% magnesium, 1.6% copper, aluminum and other metals. Its density is 2.8 g / cm3 with a Vickers hardness of 175, similar like some steels. The elongation is 11% lower compared to steel. Their fatigue strength is high, but behaves differently depending on surface treatment. This is also associated with physiological fluids corrosion, such as sweat.

A common solution to stress corrosion is the anodization process but this with the fatigue strength is decreased. It is easy for a piece that supports an effort may suffer breakages with an anodized surface. In a paper published in the Researchgate paper about the subject, it was developed a study of  surface treatments, sandblasted, anodized and untreated, to find out the response of 7075 to stress corrosion

In Investigation of corrosion fatigue durability for a 7075 aluminum alloy subjected to blasting and anodizing by N. Michailidis, F. Stergioudi, A. Ragousis, G. Maliaris used C-Rings from 7075 untreated, sandblasted, anodized and sandblasted/anodized surface ,. The corrosive medium used was 3.5% sodium chloride and distilled water. It was observed that the C-Rings with sandblasting treatment allowed for a prolongation of the corrosion fatigue. Unlike anodizing fairy detrimental effect on corrosion fatigue life sandblasted/anodized has withstood immersed in a corrosive environment very bad, while the simple sandblasted surface increased resistance fatigue. It was also noted that if anodizing is damaged for some reason, the process of fatigue rupture increased considerably and the fractured piece much earlier.

The findings of this study, applied to the treatment of surfaces for watch cases made of 7075 would be as follows,

Best surface treatment to prevent corrosion fatigue is sandblasting

Anodizing can decrease corrosion  fatigue. If used it should be the strong anodized in order to minimize scratches

The worst of all would be blasted/anodized.

Sandblasting to take effect should perfectly polished watch case, in order to avoid pores in the surface. The best would be to micro glass beads with a maximum size of 150 microns. The pressure and the distance of the application of the particles are important.

For that I have decided to make without anodized Aluminium Juntor now. We have the equipment for starting to do it and the machinery.

Technical data

Paper

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