Chromic anodizing

The adhesive thickness was about 0.1 mm. The dimensions of the bonded area were 6 in xl in (15 cmx2.5 cm). AU adherends (for shear and peel) were aluminum 2024-T3 alloy pretreated by unsealed chromic anodization. The substrates were cleaned with trichloroethylene prior to bonding. The shear and peel tests were conducted on an Instron 4481 tester. 

PAMAM No treatment No treatment Chromic anodized Chromic anodized... 

PAMAM Concn. [wt.%] No treatment Chromic anodized (no seal)... 

Relative Costs. Chromic anodize costs more than H2SO4 but less than hardcoat anodize. 

Sheet aluminium can be given a colour by a similar process. The aluminium is first made the anode in a bath of chromic acid (p. 377) when, instead of oxygen being evolved, the aluminium becomes coated with a very adherent film of aluminium oxide which is very adsorbent. If a dye is added to the bath the oxide film is coloured, this colour being incorporated in a film which also makes the remaining aluminium resistant to corrosion. This process is called anodising aluminium. 

At low temperatures, oxidation with chromic acid gives propynal [624-67-9] C2H2O (14), or propynoic acid [471-25-0] C2H2O2 (15), which can also be prepared in high yields by anodic oxidation (16). 

Dichromates and chromic acid are used as sealers or after-dips to improve the corrosion resistance of various coatings on metals. Eor example, phosphate coatings on galvani2ed iron or steel as well as sulfuric acid anodic coatings on aluminum can be sealed by hexavalent chromium baths. 

Chromium and compounds SPA Chromic acid Sodium dichromate Anodizing Cement Dyes Electroplating Paint Tanneries... 

CAA. Chromic acid anodization [74-76]. was developed initially as a treatment to improve the corrosion resistance of aluminum surfaces, but it is also used as a surface treatment for adhesive bonding especially in Europe where it is used extensively in aerospace applieations [29,77],... 

In-service issues. As mentioned previously, many early service failures of bonded structure were due to adherend surface treatments that were unstable in long-term exposure to water. A majority of these problems were resolved by the adoption of surface treatments such as chromic and phosphoric acid anodize for aluminum details. The remaining few were alleviated by the adoption of phosphoric acid anodized honeycomb core and foaming adhesives resistant to water passage. Other service durability issues such as the cracking of brittle potting compound used to seal honeycomb sandwich assemblies, and subsequent delamination, have been minor in scope. 

EPA, 1993. U.S. EPA, Office of Air Quality Planning and Standards, "Chromium Emissions from Chromium Electroplating and Chromic Acid Anodizing Operations Background Information for Proposed Standards," EPA-453[R-93-030a, Research Triangle Park, NC, July 1993. 

Nagayama, M. and Okamoto, G., The Anodic Behaviour of Passive Iron in Chromic Acid-Chromate Solutions , Corros. Sci., 2, 203 (1962)... 

Some metals and alloys have low rates of film dissolution (low /p) even in solutions of very low pH, e.g. chromium and its alloys, and titanium. In these cases the value of /p is quite low, and although it increases as the temperature increases, a maximum is reached when the solution boils. The maximum current is below and breakdown does not occur. However, in certain alloys, e.g. Cr-Fe alloys, the protective film may change in composition on increasing the anode potential to give oxides that are more soluble at low pH and are therefore more susceptible to temperature increases. This occurs in the presence of cathode reactants such as chromic acid which allow polarisation of the anode. 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

Tests for quality of sealing of anodic coatings have become internationally standardised. They include dye spot tests with prior acid treatment of the surface (ISO 2143 1981 and BS 6161 Part 5 1982), measurement of admittance or impedance (ISO 2931 1983 and BS 6161 Part 6 1984), or measurement of weight loss after acid immersion (ISO 3210 1983 and BS 6161 Part 3 1984, and ISO 2932 1981 and BS 6161 Part 4 1981). Of these the chromic-phosphoric acid immersion test (ISO 3210) has become the generally accepted reference test. 

Acid pickles Some of the acid pickles used to clean and etch aluminium alloy surfaces and remove oxide and anodic films, such as the chromic/ sulphuric acid pickle (method O of DEF STAN 03-2) and other chromic-acid bearing pickles (App. Foi DEF-151) probably leave on the surface traces of absorbed or combined chromate which will give at least some protection against mild atmospheres. 

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