Discussion
The most suitable reagent for chemical etching of aluminum alloy surfaces in cases of restoration of erased number is :
Answer: B
The key's answer is Villella's reagent.
When a serial number stamped in a metal has been ground away, the stamping leaves a deformed zone below the surface that an etchant can attack at a different rate from the undisturbed metal, which is how the number is brought back. Forensic course notes name Villella's solution (30 ml glycerin, 20 ml hydrofluoric acid, 10 ml nitric acid) as the best reagent for aluminium alloys such as engine surfaces and vehicle identification plates.
Fry's reagent (cupric chloride in hydrochloric acid) is the standard reagent for steel and iron, not aluminium. I could not match Thomas's reagent or William's reagent to a standard etchant. The same notes mention a ferric chloride etch for aluminium, which is not among the options. In metallurgy generally Villella's is described for ferrous alloys, so the forensic convention rather than metallurgy decides this item.
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