Page 43 - PCMI Journal July 2018

David Allen | Emeritus Professor | Cranfield University | UK Photochemical Machining of Aerospace Materials
5/31/18
Ferric chloride etchants for aluminium (1)
• Use of 42°Bé FeCl3 / HCl gives jagged edges, rough and irregular walls, severe undercutting and irreproducible results due to the over-vigorous exothermic reaction.
• Heat evolved on etching aluminium is 164.8kcal/g atomic weight, i.e. approximately 16 times that evolved in etching copper and 1.6 times that evolved in etching aluminium with NaOH.
• Solution needs to be rapidly and efficiently cooled or diluted.
• However, dilute solutions can produce “smut”
Ferric chloride etchants for aluminium (2):
additives to reduce “smut”
Concentration
Temperature
Additives
Reference
13.7-18.0°Bé initially →
20°Bé with additive
38 - 43°C (100 - 110°F)
Soluble persulphates to prevent deposition of iron on aluminium
US Patent 3,281,293 (Chemcut, 1966)
8 -15°Bé plus 9g HCl per litre of solution
46 - 52°C (115 - 125°F)
Hydrochloric acid to inhibit deposition of iron
US Patent 3,824,136
(US Army,1974)
Issue 131 July 2018 PCMI Journal 42
1