Many people have had the experience whereby the lid of the lunch box made of anodized aluminum, which is aluminum covered with an oxidized skin, had melted because of the acid when umeboshi were placed in the same spot every day.
アルミに酸化皮膜をほどこしたアルマイトの弁当箱では、毎日同じ場所に梅干しを乗せた場合、酸によって蓋が溶けることがあったという体験をした人は多い。