How to stop the corrosion? I have a silly question maybe.. How to protect the frame of my bike from corrosion? The bike is not new.  Are there any accessories? 
Thanks
Assuming your bike is made out of steel (since aluminium and carbon fiber don't corrode in the way you're most probably worried about):


Get "helicopter tape" to protect any areas of the bike that might get dinged (the top tube and down tube from parking the bike against parking meters, etc.). Or if you're cheap or worried about theft, use ugly duct tape / pipe insulation / electrical tape/etc. I use retroreflective tape which serves two purposes -- keeps me lit up at night and protects me against scratches.
Make sure your rear right chainstay has a chain protector on it.
Get some touch-up paint from either your bike dealer or from an auto-parts shop and make sure to quickly touch up any dings and scratches. This is where corrosion will start.
Get some Boeshield or frame corrosion protection spray (JP Weigle Frame Saver is another brand) and spray the inside of your frame. The seat tube and top-tube can be sprayed if you take the seat off. The down-tube often has a drain hole but can be hard. Read the directions of what you're using to make sure it's safe for your bottom bracket or headset. 
Spray boeshield or another waxy protection film on all exposed bolts.
Try to never leave your bike out in the rain and if you do, wipe it down as soon as possible. 
Keep your bike clean. Dirt and salt attract water and aid rusting.
Occasionally make sure all of your frame drain holes are clear. 
Don't worry about a little rust, wear it with pride.


Many of the answers here are also applicable: How do I make my frame water tight?