Why do those viral Stanley cups stain and discolor so easily?

MA Master US 🛠️ Specialist 👁 85 ⚑ Report Chemistry

I got one of those Stanley thermoses everyone's obsessed with last year, and after a few months of normal use with coffee and tea, the interior has these weird brown stains that won't come off no matter what I do. Is it just how the coating breaks down, or is something actually reacting with the metal inside? I've tried soaking it with vinegar and baking soda but nothing works.

stanley thermosstainingmetal corrosionwater discoloration

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KE Kevin_38 US 🛠️ Specialist 💬 129 answers ⚑

the staining you're seeing is pretty much inevitable with those Stanley thermoses - it's tannins from coffee and tea bonding to the stainless steel interior, and once they set in, they're nearly impossible to remove because they're not just sitting on the surface. the interior coating on those bottles can wear down over time with repeated hot liquid and scrubbing, which actually makes the staining worse since the tannins have more of a porous surface to cling to.

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