Sai and rust

[MatsuShinshii]

Stainless steel shouldn't rust on its own. If it does, I suspect it's in contact with an electrically dissimilar metal, which causes a galvanic action.

When it comes to doing anything that causes dust, it's always wise to wear a mask - asthma or no.

You do not need galvanic action for stainless steel to rust. Good old water and time will do it the trick.

Although there have been gains in the metallurgy of stainless, it's not rust proof. If you leave a stainless steel blade outside in the rain you will find surface rust especially if it's been used. A polished piece of steel will take longer to rust than a raw piece or a scratched piece. The small divots, pits, scratches and deformations leave a place for moister to accumulate and form rust. Other elements other than moister can also attack and weaken the steel and allow rust to develop.

Stainless steels use several elements to reduce corrosion such as Chromium, Molybdenum, Columbium, Copper, Titanium and Nickel to name a few. However these elements do not aid in hardening the steel. For this you must have carbon. If carbon is present so is rust. There are other elements that aid in harden-ability and edge retention but carbon is the key component.

Stainless means it will stain LESS. Not that its stain proof. Any metal that contains carbon will rust over time if subjected to moisture.

All steel that contains carbon will rust. The added elements reduce the effect as apposed to high carbon steel like 1095 or even tool steel that contains some of the same elements. But make no mistake given enough time, surface deformations and moisture, stainless steel will rust.