Metal finishing Q&As since 1989
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Aluminium corrosion and sodium stannate
⇦ (tip: readers rarely show interest in abstract questions;
actual situations usually prompt responses) 
Q. WHY IS SODIUM STANNATE ADDED TO SODIUM HYDROXIDE ... TO REDUCE THE CORROSION OF ALUMINIUM OR ENHANCE THE CORROSION?
K S SENTHILKUMAR- TRICHY India
August 13, 2022
Hi SENTHILKUMAR. I like James Watts' answer the best: it doesn't matter because stannate isn't stable and quickly deteriorates to troublesome stannous in either case :-)
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
⇩ Related postings, oldest first ⇩
Q. Hello!
I have a problem in protecting aluminium electrode from corrosion: I have a water bath 9N NaOH and I have tried Sodium Stannate but it seems there are solubility problems. What would you suggest to protect my aluminium electrode? Do you think I should use potassium stannate into potassium instead ?
What other component could be use instead, at this kind of pH and alkaline concentration?
Thank you very much
PCA - Paris FRANCE
2002
A. I could not understand what you are trying to do. The information that you left out is required to make suggestions. Aluminum in a high pH solution is an absolute NO_NO! Why did you try it?
James Watts- Navarre, Florida
A. Low phosphorous electroless nickel will serve well as an anode in sodium hydroxide solutions. Plate with EN 2-5% P
Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [on Amazon or AbeBooks affil links]
and "Plating ABS Plastics" [on Amazon or eBay or AbeBooks affil links])
A. Well yea aluminum will dissolve in 9N sodium hydroxide; we use caustic as aggressive etch for removing aluminum prior to plating, anodizing, chromating, etc. Electrode should be protected by some sort of barrier, perhaps electroless nickel, since I'm presuming it has to be conductive. If not, barrier coating such as Teflon or powder coat might suit.
Milt Stevenson, Jr.
Syracuse, New York
Sorry, I give you more details:
This electrode is for a battery AgO/Al
The electrodes are in sea-water to which we add NaOH (9N) and sodium stannate in order to protect the aluminium electrode. For the past 3 years, I have problems with sodium stannate because it dissolves badly in concentrated alkaline solution and blocks the filter for no special reason.
I suppose there is a stannate process which protects aluminium. I have the idea of using potassium stannate which is more soluble but I was wondering if I could change for a zincate process (cheaper but efficiency and solubility?) or the electrode itself?
thank you again
- france
2002
A. Hi Alexis,
When it comes to electrodes I can not be much help. I am a little confused by its construction, but I can not help much anyway. A thought: Can you live with it Electroless nickel plated, or will that mess up the EMF. A second thought would be can it be heavy silver plated?
Your problem with stannate is that it is not a stable form of tin. It converts to stannic which will precipitate out and plug your filter.
Sorry that I could not be of any positive help.
- Navarre, Florida
A. A drawing may help here, I am not sure where the filter comes into this, also as a battery the aluminium electrode should dissolve to provide the power as the AgO gets converted to Ag the aluminium goes into solution.
The problem (as I see it) is that the aluminium rod is going too fast. The stannate is added to slow the reaction down. The stannate would deposit tin on the electrode so you would have an AgO aluminium/tin battery. One answer may be not add the stannate as salts but tin plate (with a thin coat) the electrode.
Please can you provide more details so I can see if I am on the right track with my answer.
Martin Trigg-Hogarth
surface treatment shop - Stroud, Glos, England
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