Eli5 how aluminium foil in the dishwasher makes your cutlery shinier


Eli5 how aluminium foil in the dishwasher makes your cutlery shinier

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According to a quick internet search, it is this:

“When dishwasher tablets are released in the dishwasher, the detergents found within them oxidizes with the aluminium. This oxidation then helps to remove stains and re-establish the shine that your cutlery once possessed.
The key to this process working is for the aluminium foil ball to be securely contained in the cutlery tray. If the ball is free then it’s going to bounce around your dishwasher and do very little. The ball needs to be fixed in place and in contact with as many knives, forks and spoons as possible for the best results.”

Shiny means that the surface is smooth (something the aluminium cannot change) and free of impurities. While the dishwasher can remove most kinds of dirt, metals often have an _oxide layer_, that is, rust. Sometimes this literally shows as brown decay, but often it is just a very thin layer, reducing the shine. So we want to remove that.

Aluminium is mostly a metal, but a very reactive and not very “noble” one. Generally, if something in the water wants to “oxidize” (“rust”) metals, it will pick the least noble one. This is way those metal blobs in dishwashers protect from rust and is also used in other setups to make the rust only grow at specific places instead of everywhere. The term for this is a “galvanic anode”.

But given the right circumstances, it can go one step further: the less noble metal can sometimes “un-rust” a nobler one, by offering itself to rust instead. For this it is almost necessary for them to touch, which turns the setup into a short-circuited, improvised battery. That causes a small electric potential that helps moving the rust around. (Technical reason: “oxidation” and its reverse are ultimately just accepting and donating electrons, i.e. electricity flowing to and from atoms).

In theory, there’s an alternative you could do: connect your cutlery to the minus of a battery, and whatever metal thing you don’t need anymore to the plus, then put both into impure (thus conductive) water. However, this does not work that well unless you know exactly what you are doing: too large power or voltage will eat away from the cutlery, there is also a chance for toxic products all over, and such hassles. The aluminium one is way easier for the typical person.