Do crafting pipe cleaners rust?

They will rust if you submerge them and don't dry them in between dips. The wire in the core is very thin, so won't rust a lot depending on how far up the pipe cleaner you submerge and allow to rust. The rusting wouldn't occur immediately and it depends on the humidity in the atmosphere, but a good estimate would be several days for the rust to occur as long as you don't lay the pipe cleaner down on a surface. It would be best if you could suspend it in the air for best air circulation. White pipe cleaners would be the most obvious to show any rusting, so using a brown or rust colour to start out with would help camouflage the rusting wire.

Too bad it's so late. You could have made shamrocks for St. Patrick's day. Why not make huge leaves, like palm, and then put them in with flowers that you've made out of other pipecleaners ? You could use them for the leaves and for the stems on the flowers. You could even put them in small pots and wrap them around the pots for a covering. The pencil thing might have been that you wrap the pipe cleaner around the pencil from top to bottom. Then at the top you can attach a butterfly or ladybug or a flower.

Because they have flexible metal wire in the centre, they will rust if they have been in water. Because it is a thin wire, it will rust very quickly, but you may not notice it until quite a while after they are removed from the water because it is exposure to air that accelerates the rusting process.

Could you please add more details:

What are you making? Is there anything else you can use, like vinyl-coated wire?

problematic stuff. look at google or bing. that will help!