Chromic acid etching metals

Fig. 5. Effect of outdoor weathering on the strength of aluminum alloy/epoxy-polyamide joints (chromic-sulfuric acid-etch metal surface pretreat-ment)61 (Reprinted from Ref. 61, p. 194, by courtesy of Gordon and Breach...

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. ... 

Several surface preparations of aluminum are widely used in the aircraft industry. " The important steps in all surface preparations are vapor degrease, alkaline clean, rinse with water, chromic acid etch (FPL Etch), rinse with water, anodize with chromic acid or phosphoric acid, rinse with water, and dry at 140-160°F. Details of the procedures for the surface preparation of aluminum and other metals and composites are available from a number of sources. 

This is a proprietary pretreatment developed by Hexcel Composites and, as such, is an alternative coating process to the traditional Iridite or Alocrom methods. It has been used in both Europe and the United States of America for several years. In the CR III process, the chromic acid etched foil is further treated to give a hard, fully hydrolysed, polymeric organic-metallic coating. This provides a surface ready for bonding with no further prelreatment necessary. 

Chemical reduction is used extensively nowadays for the deposition of nickel or copper as the first stage in the electroplating of plastics. The most widely used plastic as a basis for electroplating is acrylonitrile-butadiene-styrene co-polymer (ABS). Immersion of the plastic in a chromic acid-sulphuric acid mixture causes the butadiene particles to be attacked and oxidised, whilst making the material hydrophilic at the same time. The activation process which follows is necessary to enable the subsequent electroless nickel or copper to be deposited, since this will only take place in the presence of certain catalytic metals (especially silver and palladium), which are adsorbed on to the surface of the plastic. The adsorbed metallic film is produced by a prior immersion in a stannous chloride solution, which reduces the palladium or silver ions to the metallic state. The solutions mostly employed are acid palladium chloride or ammoniacal silver nitrate. The etched plastic can also be immersed first in acidified palladium chloride and then in an alkylamine borane, which likewise form metallic palladium catalytic nuclei. Colloidal copper catalysts are of some interest, as they are cheaper and are also claimed to promote better coverage of electroless copper. 

The reactions which take place when the mixed etch primer is applied to a metal are complex. Part of the phosphoric acid reacts with the zinc tetroxychromate pigment to form chromic acid, zinc phospliates and zinc chromates of lower basicity. The phosphoric acid also attacks the metal surface and forms on it a thin chromate-sealed phosphate film. Chromic acid is reduced by the alcohols in the presence of phosphoric acid to form chromium phosphate and aldehydes. It is believed that part of the chromium phosphate then reacts with the resin to form an insoluble complex. Excess zinc tetroxy chromate, and perhaps some more soluble less basic zinc chromes, remain to function as normal chromate pigments, i.e. to impart chromate to water penetrating the film during exposure. Although the primer film is hard... 

Hexavalent chromium-bearing wastewaters are produced in the metal finishing industry in chromium electroplating, in chromate conversion coatings, in etching with chromic acid, and in metal finishing operations carried out on chromium as a basis material. 

A 15% solution of nitric acid in ethanol was used to etch a bismuth crystal. After removing the metal, the mixture decomposed vigorously [1], Mixtures of nitric acid and alcohols ( Nital ) are quite unstable when the concentration of acid is above 10%, and mixtures containing over 5% should not be stored [2], The use of a little alcohol and excess nitric acid to clean sintered glassware (by nitric acid fizzing ) is not recommended. At best it is a completely unpredictable approximation to a nitric acid—alcohol rocket propulsion system. At worst, if heavy metals are present, fulminates capable of detonating the mixture may be formed [3], Chromic acid... 

A process has been developed for electroplating a PPA resin, modified with ethylene propylene diene monomer rubber, ethylene-propylene rubber, and styrene-butadiene rubber. As etching solution, chromic acid is used. However, it has been found that the concentration of Cr + is crucial for the success of the method. The concentration of Cr + is in the range of 50-55 gU Low levels of Cr + result in poor adhesion of the final metal plating, while high levels of Cr + can cause the formation of small blisters in the metal plating. The influence of the process parameters on the peel strength is shown in Table 12.9. 

Semiconducting nanowires, nanorods, nanodots, nanocones, nanopins, etc. are interesting due to their broad range of applications. Electrochemically, the most easily fabricated semiconductors are Il-Vl semiconductors, for example, CdS, CdSe. There are three approaches for electrodeposition of semiconductors. The first method [131] is deposition of metal in alumina nanopores, followed by etching of alumina surface by phosphoric/chromic acid to access metallic surface for sulphur or arsenic vapour to attain metal sulphide or arsenide nanostructures. The second method deals with electrolysis of sulphuric acid, causing the sulphide atoms to be deposited in pores. 

Anodic treatment may also be considered when bonding other metals. Examples inclnde the anodizing of magnesium in ammonium bifluoride solution, the anodic etching of stainless steel in nitric acid and anodizing of titanium in sodium hydroxide or chromic acid (see Pre-treatment of titanium). Alternative anodizing of copper can prodnce a Microflbrous surface (see Pre-treatment of copper). 

Before electroless plating plastic parts, the surfaces have to be treated to ensure good adhesion. The etching process usually involves the use of a chromic acid solution to provide a microscopically roughened surface to the plastic part. The catalytic process is sometimes referred to as seeding. Here very small particles of an inactive noble metal catalyst, normally palladium, are deposited into the micro-cracks created during the etching process. The palladium will act as active catalyst sites for chemical reduction of the electroless... 

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