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

Bonded components are typically etched in a sulphuric/chromic ( FLP ) solution with a make up of 15%v/v concentrated sulphuric acid and 45gl chromic acid. The process control for this solution is the free chrome content with solution replacement being dictated by the total chrome figure and the overall specific gravity of the solution, which increases with the addition of chrome and sulphuric acid. The Etch rate and quality are a direct function of trivalent chrome concentration and the acidity of the solution.

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.

The Separated Cell

Anodic oxidation can be used to continually regenerate chromic acid anodising solutions and regenerate batch volumes of chrome/sulphuric pickles.

The trivalent chrome contamination is oxidised back to hexavalent chrome and trace chlorides are converted to chlorine at the anode for subsequent fume extraction.

The separated cell system is sized upon the regeneration rate required which is a function of the mass of chrome reduced over a known period of time.

Both chromic acid anodising and chrome/sulphuric solutions will operate with a background concentration of 15-20g/l of trivalent chrome.

Commercialised Technology

• A current user of this system utilises a system on a 30,000 litre anodising tank, which requires regeneration of 24kg/day of trivalent chrome as CrO3.
• The plant has been in operation for 18 months with the following run profile having been established.
• Chromic acid additions to the process bath reduced by in excess of 90%. Makeup is now considered to be for dragout losses only.
• The chloride content has been reduced from 120mg/l to less than 10mg/l.
• The bath was requiring a 50% dumping every 3 months and a full dump every 6 months (90, 000 litres/annum). The same bath has been in operation for 23 months as it was due for dumping when the plant commissioning was undertaken.
• The catholyte circuit, has been dumped once during this period giving a disposal reduction equivalent to 99%.
• The system allows for more stable specification of free chrome content, which has improved product consistency.
• Down time for the plant has been recorded as 2 hours in every thousand hours run.
• Pay back in less than 2 years.

See how much you can save with a Cromox unit...