Metal turned dull grey in dishwasher?

Question

I put a shiny coffee filter holder in dishwasher, and it turned dull grey. In the pic you can see the difference with filter cup, which retained silvery shine in the same dishwasher:

The holder leaves dark grey spots on kitchen towel, when I try to brush it.

I thought it is aluminum, but got confused by posts in internet stating it would turn black if washed in dishwasher. Mine is dull silvery grey.

What would the filter holder be made of, and is it safe to use it now, considering all the coffee drips through it into the cup?

Answer 1

It's cast aluminum.

Most dishwasher detergents contain lye (NaOH, sodium hydroxide) - which reacts drastically with aluminum. Fortunately the dwell time in a dishwasher is short enough that it generally only darkens, as opposed to dissolving entirely (which it will do with adequate time.)

Yes, it's safe to use (after hand-washing and rinsing throughly), no, it's not dishwasher-safe and should be hand-washed instead.

The filter cup is of some different metal (either stainless steel or something nickle-plated, at a guess.)

Answer 2

That looks like Aluminum and such items should not be washed in an automatic dishwasher. Here is an article that discusses the problem:

Aluminum and Dishwashers

Their recommendation for cleaning is:

When washed in a dishwasher, aluminum cookware can react and darken due to mineral content in the water, chemicals in the detergent or high heat from the dryer. Natural products such as lemon juice, vinegar or cream of tartar remove discoloration and provide fast, effective results without damaging the metal.