I am a little confused by a catalytic reaction

[fly_boy_bc]

When Sodium hyroxide is used to remove aluminum from a steel sputtering mask the reaction is energetic at first but after a while the solution gets dirty and the reaction slows down until we need to replace it. As I understand it the caustic soda is a catalyst in this reaction and is not consumed or changed by it. So why does it lose effectiveness? Is it "poisoned" by the steel? Is there another material I could use for the mask that would not poison the Lye? (I am amused by the fact that it has so many names).

Or is the water too saturated with aluminum?

Gary

 

[Borek]

NaOH is not a catalyst, it is chemically dissolving Al.

 

[fly_boy_bc]

Well! That would explain THAT then wouldn't it? Hmmmm I guess I figured it was a catalytic reaction just because using it as an electrolyte to separate water is catalytic. Dumb guy thank chemistry ppl for help! ugh!

 

[lightarrow]

Well! That would explain THAT then wouldn't it? Hmmmm I guess I figured it was a catalytic reaction just because using it as an electrolyte to separate water is catalytic. Dumb guy thank chemistry ppl for help! ugh!

The reaction is not so simple as it seems. With little amounts of Al hydroxide dispersed in the solution and little concentrations of $$OH^-$$, I'd say that $$OH^-$$ acts a bit as a catalyst:

$$2Al\ +\ 6H_2O\ -(basic\ solution)\rightarrow 2Al(OH)_3\ +\ 3H_2\uparrow$$

But if the concentration of $$OH^-$$ is enough and there is enough hydroxide dispersed, $$OH^-$$ reacts with the aluminum hydroxide:

$$Al(OH)_3\ +\ OH^-\ \rightarrow Al(OH)_4^-$$

and so the base is consumed.