Chromic acid surface treatment

In order to obtain maximum corrosion protection for painted metal articles, the metal parts are pretreated with an inorganic conversion coating prior to the painting operation. These zinc or iron phosphate coatings greatly increase both paint adhesion and corrosion protection. Traditionally, a chromic acid post-treatment has been applied to these phosphatized metal surfaces to further enhance corrosion protection. 

In contrast to the relatively open cellular structures of etched and phosphoric acid-anodized aluminum, the chromic acid anodize treatment produces a very thick, dense oxide layer, consisting of solid columns, represented by the drawing in Figure 14. This presents a relatively smooth surface with no protrusions. 

This is the final neutralizing rinse after the pre-treatment to obtain a better corrosion resistance. The phosphated surfaces are treated with chromic acid-based or acidified sodium dichromate solutions which are not affected by moisture and thus protect the phosphate coating. 

While polymeric surfaces with relatively high surface energies (e.g. polyimides, ABS, polycarbonate, polyamides) can be adhered to readily without surface treatment, low surface energy polymers such as olefins, silicones, and fluoropolymers require surface treatments to increase the surface energy. Various oxidation techniques (such as flame, corona, plasma treatment, or chromic acid etching) allow strong bonds to be obtained to such polymers. 

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],... 

Although there is considerable evidence that chemical surface treatments improve the substrate bondability of stainless steels, there is no general agreement on which is the best. One etchant commonly used with stainless steels is an HNO3-HF mixture [128-131] others are chromic acid and ferric chloride/hydrochloric... 

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. 

The theoretical aspects of molybdenum s corrosion behaviour are complex and there is as yet no clear cut, generally applicable picture. There are, however, a large number of literature references which include data on polarisation, passivation and potential of molybdenum under widely assorted conditions. The electrode potential of molybdenum depends on its surface condition. For example, some tests showed an of -t-0-66V when the molybdenum was passivated by treatment with concentrated chromic acid and —0-74 V after activation by cathodic treatment in sodium hydroxide. 

Magnesium Chromate. [CAS 13423-fil-5. MgCiCC SH.o, small readily soluble, yellow cryslals. formed by reaction ol magnesium carbonate and chromic acid solution. Use Since it does not produce a fusible alkaline residue when thermally decomposed, it is used as a corrosion inhibitor in the water coolant of gas turbine engines. Insoluble basic magnesium chromates also arc available. Their potential applications arc in the treatment of light metal surfaces. 

Figure 5. Scanning electron microscope pictures of single-crystal CdSe after sev-eral surface treatments. (0001) face, Cd-side, shown. (1120) and (1010) faces behave similarly. (A) After aqua regia/chromic acid etch (B) after aqua regia etch (C) after aqua regia/photoetch.

Corona discharge is a popular method for applications on a large scale and because it can be carried out quickly in dry conditions in general it is less hazardous than chemical treatment and leaves no visible sign on the surfaces treated. On the other hand, there is some evidence that it is less effective in promoting adhesion to plastics than treatment with a substance like chromic acid. 

For some plastics, surface treatments were necessary—for example, chromic acid treatment for polypropylene. When fillers were present in the plastics the results with cyanoacrylates often were better (this was apparent in particular with polyethylene). 

Because of the solvent and chemical resistance of acetal copolymer, special etching treatments have been developed for surface preparation prior to adhesive bonding. A chromic acid etch and a hydrochloric acid etch have been suggested. Acetal parts that have been formed by heat treatment or machining should be stress-relieved before etching. 

Parts molded from polyetherimide can be assembled with all common thermoplastic assembly methods. Adhesives that are recommended include epoxy, urethane, and cyanoacrylate. However, service temperature must be taken into consideration in choosing an adhesive because PEI parts are generally used for high-temperature applications. Good adhesion can be effected by simple solvent wipe, but surface treatment by corona discharge, flame treatment, or chromic acid etch will provide the highest bond strengths. 

This is the exposure of certain plastic surfaces to a solution of reactive chemical compounds. Solutions are oxidizing chemicals, such as sulfuric and chromic acids, or metallic sodium in naphthalene and tetra-hydydrofuran solutions. Such solutions are highly corrosive thus, require special handling and disposal procedures. This treatment causes a chemical surface change, such as oxidation, thereby improving surface... 

Protective effects of chromic acid and ferric chloride treatments on surface degradation also were observed on radial surfaces. The preservation of both simple and bordered pits in woods treated by these inorganic salts was observed. At a 10% chromic acid treatment concentration, the structure of the pits retained most of the original shape after 1000 h of UV irradiation. The diagonal microchecks passing through the bordered pits in radial walls of tracheids, however, can still be observed (Figure 30). 

Influence of Acid Treatment. The presence of free alkali on the surface of glass and its influence on adsorption and allied phenomena are well known. McBain (17) pointed out the significance of acid treatment of glass vessels to remove the alkali in quantitative treatment of the phenomenon mentioned. The removal of surface alkali caused a marked change in the production of —Az (20). This last effect was studied in Siemens-type vessels, some of which were prewashed with chromic acid and boiling distilled water and dried under vacuum for 24 hours others were used as they were prepared from fresh glass. 

The treatment of the iron with concentrated nitric acid HNOj (or chromic acid, H2Cr04) leads to the fonnatjon of an oxide layer which covers the whole surface of the metal this layer prevents the diffusion of oxygen and moisture from the air into the iron lattice. However, if the surface is untreated, corrosion sets in. This is greatly favored by the high cell voltage which is built up between the base metal iron and the hydrated copper ions Cu or when the iron is treated with dilute acids (II3CV ions) as a result Fe " ions go into solution. 

Pretreatment is particularly important for structural adhesive joints such as those used in aircraft and motor vehicle construction. Aluminum adhesive surfaces are subjected to chromic acid treatment and an anodizing process, major factors in the overall joining costs. The disposal of used pickling baths is particularly demanding and expensive, especially in view of the CrVl ions resulting from the pickling process. 

---