Patina or rust

[Jamestyates]

I got this nakiri a while back and have been using it as my primary knife at work. I sharpened and forced a patina when I first got it. The patina has a few spots im worried about. Several spots look yellowish/orange under certain light. It's hard to get a good picture of the colors.

No residue comes off when wiping or scratching with my finger nail.

Should I use rust cleaner or is this just a patina?

[Jeff B]

Patina can come in all colors. If rags and paper towels don't show color after wiping the blade then it's just patina and I'd let it keep developing. Orange and yellow colors in patina will look translucent where rust is more solid in color. This is why you have to turn the knife a certain direct in the light to see the colors. You will also notice as you use the knife the patina and colors will change over time and sometimes with each use.

[Kekoa]

I have a White #1 Gyuto that is very reactive, it has plenty of yellow and orange patina that comes and goes. The biggest thing that helps me discern the difference from rust is that, as Jeff said, rust has more of a solid color. If I am ever suspicious there is rust I rub some baking soda on the spot while the blade is damp and rust tends to come off in short order coloring the baking soda orange. Patina tends to be stickier than rust, so if it doesn't come off after a little bit of rubbing, and the baking soda doesn't turn orange, usually it turns out it is patina on my blade.

Corrosion often isn't as dramatic as people at first assume, I actually frequently get straight up rust spots after cutting green pepper, but all I have to do is rub it off, clean it well with soap, and dry it well before storage. The key is making sure it is clean when stored so that it isn't corroding while sitting for long periods. If it isn't going to be used for a few weeks, a coat of oil will keep it from rusting do to moister.

[TheLegalRazor]

Patina can come in all colors. If rags and paper towels don't show color after wiping the blade then it's just patina and I'd let it keep developing. Orange and yellow colors in patina will look translucent where rust is more solid in color. This is why you have to turn the knife a certain direct in the light to see the colors. You will also notice as you use the knife the patina and colors will change over time and sometimes with each use.

An excellent observation. One often reads that gray, blue and purple are patina but orange is rust. This can cause needless anxiety, as not all orange is rust. The challenge is differentiating orange patina from orange rust. I find it hard to articulate the difference and just say to myself that "I know rust when I see it." This, of course, is of no use to anyone trying to figure out if they're looking at patina or rust. The observation that orange patina looks translucent and rust is more solid is a keen one and provides a good rule of thumb. Thank you for pointing this out.