Why we place artefacts in NAOH solution (1 Viewer)

[wrxsti]

why exactly do we place artefacts in solution of NaOH?
is it becuase OH - displace the Cl- (Desalination)?

 

[mike 1989]

Metals, mainly iron artefacts are place/soaked in NaOH solution (approx 5%) as the OH- ion reacts with the insoluble Fe(OH)Cl to form Fe(OH)2 and CL- the Cl- ion is then removed when the solution is refreshed. If left in the artefact nad allowed to dry out, the Fe(OH)Cl decomposes to form HCl which can greatly enhance the rate of corrosion.

NaOH is also used at the electrolyte in electrloysis of the artefacts.

Note: in some text books they describe the removal of insoluble chlorides and electrolysis as a singl step.

 

[Forbidden.]

NaOH has the benefit of neutralising acids present in the artefact.

Rust follows these equations in the following order.

Fe²⁺ + 2OH^- --> Fe(OH)₂ --> 4Fe(OH)₂ + O₂ --> 2(Fe₂O₃.H₂O) + 2H₂O

Using Le Chatelier's principle in one of these steps demote rust formation as you provide an excess of OH^- ions.