zaw, Try this to clean the contacts on your flash. The best commonly available thing to clean the camera battery corroded contacts with is distilled vinegar and Q tip/cloth/small brush then very light polish with extra fine steel wool or an ink eraser on a pen, followed by a little 90% isopropyl alcohol on a clean cloth. Then dry the contacts with a dry clean cloth. You want to avoid using a cloth that will leave lint/dust behind. Finish by blowing out with a can of compressed air to get rid of any fibers or grit from polishing. Check the attached links,instruction and guides, Good luck.

I hope this helped you out, if so let me know by pressing the helpful button.

https://www.youtube.com/watch?v=SHbimNl7...

http://cleaning.lovetoknow.com/Clean_Alk...

https://www.youtube.com/watch?v=sDd4VkUm...