Treatments other than Chromic

Chromates have been known to be inhibitors since 1907, and the use of zinc chromate in primer coats used to prepare aluminium for painting dates back to 1931. Given their high efficiency, it was not until the end of the 1980s that replacing them began to be considered. 

However, the toxicity of hexavalent chromium and the cost of its elimination from surface treatment baths have generated a tendency to replace chromate-based conversion treatments. Several products are presently being evaluated as a replacement for chromates [11]  

The idea to thicken the natural oxide layer was first mentioned by Buff in 1857 [17]. He 

This idea was taken up at the beginning of the 20th century, and a first patent was filed in 1911 by the Frenchman de Saint-Martin, who proposed an anodising process in a sulphuric medium containing iron sulphate. 

Several processes and variants thereof have been developed since 1923  

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Hydroxymethylation of ketone (155) was followed by protection of the aliphatic hydroxy group (2-methoxypropyl ether) and addition of an a-benzyloxymethylene group at C-4. Acidic workup at the last stage of the reaction sequence produced (156). Its transformation to aldehyde (157) was carried out by successive treatment with methoxypropyl ether, acetic anhydride and pyridine, hydrochloric acid and methanol, and finally chromic acid, pyridine and hydrochloric acid. Dehydration of (157) led to the formation of (158) in 20% yield. Reagents other than the mentioned produced appreciable quantities of the cis A/B isomer. The butenolide (159) was finally synthesized by oxidation and hydrogenolysis. In order to complete the synthesis of triptolide it was necessary to introduce the... 

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. 

In 1873, it was first reported that cellulose that has been oxidized with chromic acid or potassium permanganate is weakened in alkaline solutions, and several years later these findings were confirmed. By 1899, it was known that cellulose that had been oxidized with nitric acid, or bromine in the presence of calcium carbonate, afforded oxidized celluloses from which D-glucoisosaccharinates were formed on suitable treatment with lime-water.Hydrocelluloses, alkaline-hypobromite oxycelluloses, and alkaline-hypochlorite oxycelluloses have no internal, alkali-sensitive links. Cellulose oxidized with neutral hypochlorite, acidic hypochlorite, potassium dichromate and sulfuric acid, potassium dichromate and oxalic acid, periodic acid, or metaperiodate gives products which are alkali-sensitive. Periodic acid oxycellulose and metaperiodate oxycellulose are much more alkali-sensitive than the others. ... 

By treating the surface with powerful chemical solutions. In the original application natural rubber was treated with concentrated sulphuric acid for a few minutes then thoroughly washed and dried. A modified finely crazed surface was formed. More recent treatment of other polymer types usually involves chromic acid solutions. It will be appreciated that this can be a rather hazardous process more suited to industrial conditions than for the amateur. 

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