CHROMFREE

Chromium free surface pre-treatments and sealing of Tartaric Sulphuric Anodizing

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 Obiettivo del progetto (Objective)

'Corrosion of Al has to be counteracted by first anodizing the Al parts and applying further protective coatings. Anodized aluminium is normally further processed with a sealing as a final step after anodizing. A hot water sealing process is one of the widely used methods. However in order to close (seal) the pores in the aluminium oxide anodized layer for corrosion protection a process involving boiling water containing chromate is still commonly used. Cr(VI)-based sealing solutions have been employed for several decades, but remain one of the most effective and commonly-used methods to improve corrosion resistance of anodized aluminium. Alternative sealing methods have also been proposed for example with Ni(II), Co(II), Ni(II) and Co(II), rare earth salts alkali metal fluorides, alkanolamine salts of phosphonic acids, Cr(III), fatty acids, silicates, etc. Kendig and Buchheit indicate that 45 of the 92 naturally occurring elements have been considered as replacements for Cr(VI) in conversion coatings on aluminium. In general these approaches have not been as successful as the Cr(VI) sealing. Also it should be noted that Ni(II), Co(II) and fluorides are not without health implications, whereas most organic molecules would be expected to have limited lifetimes under the extreme conditions (UV radiation, low pressure, large temperature range) experience by commercial aircraft during operation. Therefore, of the previously identified approaches Cr(III)-containing or silicate-forming sealing solutions are preferred options. Encouraging results were obtained with deposition of films of CeO2.2 H2O on aluminium alloys in a few minutes at room temperature with or without catalyst, though the performances still do not equal those of CCC. Detailed investigations and characterization of the obtained will be performed. The optimized sealing and pre-treatments process will be applied to a flat test panel of 384 x 742 mm.'

Introduzione (Teaser)

Aluminium (Al) is a lightweight metal used to make millions of products supporting the global economy. New eco-friendly technologies to increase corrosion resistance without the use of carcinogens will help industry stay in business sustainably.

Descrizione progetto (Article)

Al is the second most commonly used metal after iron. Applications abound in the aerospace, architectural and marine sectors as well as in domestic products. It is typically anodised (coated with a protective oxide layer via an electrolytic process) and then the pores are sealed with a solution of hot water and hexavalent chromium (Cr(VI)) to enhance corrosion resistance. Although this method has been used for decades with great success, Cr(VI) is now a known carcinogen and industry must find an eco-friendly alternative.

Recognising the critical importance to industry and EU citizens as well as the environment, the EU is funding the project CHROMFREE (Chromium free surface pre-treatments and sealing of tartaric sulphuric anodizing). The consortium is focusing on two AL alloys (AA2198 and AA2196) and developing appropriate pre-treatment, anodisation and sealing processes.

Within the first year, scientists put AA2198 through a test matrix consisting of two different pre-treatment processes and eight tartaric sulphuric acid anodisation (TSAA) processes. Scientists assessed susceptibility to intergranular corrosion (at the crystal grain boundaries) after pre-treatment and crack formation before and after sealing.

Further, they investigated wettability of the oxide layer and corrosion resistance of the anodised specimens to salt spray. The team chose one of the TSAA processes, which is currently being used in a campaign to evaluate eight sealing processes with the same tests of surface morphology, cracking, wettability and salt spray resistance.

AA2196 specimens for stringers have been investigated as pre-treatment and control samples sealed only with hot water, but no chemicals are being prepared. Fatigue tests are currently in progress.

CHROMFREE plans to deliver a chrome-free TSAA and sealing process just as effective as classic chromate conversion coatings containing Cr(VI). The technology will support the use of lightweight Al alloys in the harsh environment of aerospace applications, enabling significant weight, fuel consumption and emissions reductions. It will also enable project partners to expand into new markets for anodised Al, including the high-end electronics and automotive industries.