Extending the life of Chromic Acid Process Baths

The drive for this technology was the major environmental issue facing industry to replace or limit the use of Chromic Acid. Replacement of the process with a more environmentally friendly method, using less harmful chemicals is not an easy issue, particularly for the Aerospace Industry. Kurion have solved this problem by developing Chromic Acid Recycling technology.

Chromic Acid Anodising

Anodising is an anodic oxidation process used to increase the naturally occurring aluminium oxide layer on aluminium surfaces, the process is carried out in a dilute acid solution with the work piece being made the anode.

The chromic acid process is used where high corrosion resistance is required with the subsidiary requirement of crack detection. A typical process solution used in the aerospace industry has a free chromic acid content in the order of 45g/l and is operated up to a maximum total of 100g/l after which partial or full replacement of the solution is required.

The differential between the free and total chromic acid can be explained as follows. The “free” chromic acid refers to the chrome in its hexavalent or oxidised state whilst the total figure refers to the presence of the free or hexavalent chrome and trivalent or reduced chrome. The free chrome is the component that allows anodising to proceed, the reduced chrome carries out no useful function in the process.

During anodising, by-products of the electrochemical reaction, predominantly reduced or trivalent chrome build up and decrease bath performance.

Other contaminants that dictate the need for disposal, more specifically chlorides, are not exceeded before solution disposal is needed. The introduction of chlorides is usually by drag in from other processes but more likely due to water quality makeup for evaporation losses.  

Bonding Pre-Treatment

Unless treated to improve its quality, the pickling bath presents the user with an expensive disposal problem due to the chrome present and the high acidity of the solution. 

Wing Strut post Chromic Acid Anodising

- User of Chromic Acid Regeneration or

Chromic Acid Recycling 

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This page was last reviewed 30.04.08