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Aluminum turns black in the dishwasher (mander.xyz)

submitted 1 month ago by [email protected] to c/[email protected]

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I put an aluminum wok lid in the dishwasher and it came out with black marks on it. I’ve also seen other aluminum cookware come out with some kind of white powder specs on it.

So apparently aluminum is dishwasher unsafe. But obviously it’s not the water that’s the problem. It must be the detergent. So the question is, what can a lazy motherfucker like myself do? Why don’t I see aluminum-safe dishwasher detergents on the shelf?

Possibly related: Bailey’s creme liquor turns black underneath an aluminum cap. Is that a chemical reaction or spoilage, or something else?

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[–] [email protected] 14 points 1 month ago* (last edited 1 month ago)

Aluminium-safe dishwasher detergents do exist, using sodium silicate, but they're damn expensive. Sadly you're better off cleaning aluminium cookware by hand.

That tarnish happens because aluminium is way more reactive than it looks like. It's amphoteric, so both acids and bases attack it. And, in a simplified way, soaps and dishwasher detergents tend to be basic enough to attack aluminium.

Typically this is not a big deal because there's a neat layer of aluminium oxide covering metallic aluminium (that's why it looks dull, instead of shiny). But if the aluminium or the solution are hot enough, that layer itself gets quickly corroded. And, well, dishwashers heat things up, right?

Weirdly enough I believe that the tarnish and white powder are also a mix of aluminium oxide and hydroxide. But since the tarnish isn't in a neat crystalline structure, but a porous one, it gets dark. You can remove that layer of oxide with some weak acid, like vinegar; the metal will regenerate a neater one.