Galvanic Corrosion in Tap Water? 1

[ddowns46]

Hi Guys,

I have an aluminum tray with a drop-in steel structure that sits at room temperature. The tray will be filled with just plain tap water and the steel and aluminum are untreated for corrosion. Will there be a galvanic cell formed between the aluminum and the steel if there is no electrolyte in solution? I know if there were it would eat away the thin aluminum tray rather quickly but my hope is that with just plain tap water the steel will rust instead (perfectly acceptable for this application).

Thanks!
D

 

[EdStainless]

It depends on your water. Given that he water has some conductivity (it isn't high purity) there will be a galvanic cell.
The corrosion products on the Al will tend to slow the attack.
If this is point contact (like from feet) where there is some pressure then perforating the Al is likely.
Why not small silicone or poly caps to provide some isolation?

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[Compositepro]

You will get corrosion in tap water. Deionized water is typically pretty corrosive to metals without any galvanic action.

 

[moltenmetal]

Be aware of the difference between corrosion of sufficient quantity to make deionized water conductive and contaminated perhaps beyond your ability to use it, and the amount required to remove large quantities of material. Deionized water has a low buffering capacity so it doesn't take much acid to make it acidic, but other than that there is nothing which makes it magically corrosive.

 

[IRstuff]

Tap water tends to be slightly alkaline. DI water, which starts off neutral, for about a nanosecond, quickly becomes mildly acidic when exposed to air and it incorporates CO2. It's a weak acid, to be sure, but it's been the bane of many unaware scientists, including the poor soul that thought he invented "polymerized water" back in the 70s, only to discover that it was DI water actually eating at his glass beakers.

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[moltenmetal]

The low buffering capacity of DI water would actually make it less corrosive toward glass than tap water, given that glass dissolves via silica dissolving as hydrosilicic acid. But the weak buffering capacity does mean that the water can become acidic easily by dissolution of CO2 etc. which can be hard on metals, especially if the water is oxygenated and fresh oxygen is replenished on a continuous basis. I think a lot of people are surprised by just how quickly glass dissolves in hot, frequently replenished water such as that in a boiler, which is why boiler sight glasses generally need a mica shield. Dissolution of 10 mils/yr on an unprotected boiler glass is not unexpected.