Dichromate-etched metal, acid

On the other hand, aluminum metal also has an oxide coating. Smith (29) found that the layer is approximately 5 nm thick on ethanol-HC104-electropolished metal and about 20 nm thick on acid-dichromate-etched metal (also determined by ellipsometry). However, adsorption of virus on aluminum is considerably stronger than on AI2O3 and is quite characteristic of metals as predicted by the Lifshitz theory. 

There are many references in the literature to this method of surface preparation of aluminium. The name refers to the US Forest Products Laboratory, which published a specification based on etching the metal at c. 65 °C in a solution of sodium dichromate and sulphuric acid, sometimes loosely referred to as chromic acid . In the United Kingdom, a similar Ministry of Supply Aircraft Process Specification known as DTD 915 has been widely used. Over the years, a number of slightly different solutions ( chromic acid etches ) and procedures have been developed. Clearfield gives recipes for six such solutions. ... 

Fortunately, a great deal of work has been accomplished in a short time, and notably by aircraft manufacturers as well as adhesives suppliers. There are several important contributions in this area. First, in the area of FPL etch, the important consideration is what kind of bonding surface is provided by the preparation method. The chromic acid/sulfuric acid not only removes air oxide and leaves base metal it also has a chemical potential which produces a very thin anodic type oxide layer of the surface. This oxide layer is porous, due to the dissolving action of the strong acid mixture, and thus the surface produced may be characterized as a thin, porous anodic oxide. (A. W. Smith compared it to a 3V chromic acid anodize based on impedance measurements.) The optimum conditions for this etch as to time, temperature, and composition have been studied at Fokker and by Smith and generally a somewhat higher concentration of sodium dichromate or chromic acid was recommended than was commonly used. 

Samples of 0.030 in 2024 T3 bare aluminum were pre-cut, to suitable size for mounting in the spectrometer, with a tab to facilitate handling. The metal was subjected to 10 minute etch at 62-63 C in a sodium dichromate-sulfuric acid solution (per Section One, Handbook of Adhesives, Bloomingdale Aerospace Products, Havre de Grace, Maryland 21078), rinsed ten minutes in running, deionized water, and dried one hour at 63 C. One sample was then additionally subjected to the phosphoric acid anodization as outlined in BAG 5555. ... 

The chromate-free treatment based on the sulfuric acid-ferric sulfate etch provided an improved joint strength compared with dichromate-sulfuric acid etching, alkaline etching, or mechanical abrasion. This increase is associated with the porous oxide layer formed, hut it depends on the adhesive nature used. The joints with Al—Cu—Mg alloy substrates generally presented higher adhesive strength values than those with pure aluminum adherends because of the selective etching of other elements and inter-metallic compounds, which have different electrochemical potential. 

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