Fruit acids oxidise steel?

[Bendik]

Yesterday I was peeling a kiwi with a knife I made a while ago. When I was done I washed the knife with some water, and as I was drying it I noticed that where the steel had been in contact with the kiwi there had appeared tempering colors. From blue through purple to yellow. Is this normal when steel comes in contact with acids? I have never experienced it before.

[HWooldridge]

The Filipinos etch their kris knives in fruit juice. Can't vouch for colors but it will raise the pattern.

[ThomasPowers]

Yes this is normal with the alloy and the fruit working together on how much will occur---why 19th century "fruit knives" often had a silver blade. The plain carbon steel blades left a bad taste on the fruit.

Thomas

[Kurr]

I got an uncle an every time he buys a new knife like a case folder, he sticks the blade in a potatoe an leaves it overnight, maybe a day to blacken the blade