Dragonfire will ignite many materials used for armor, including metals.  The only way to be certain that a given set of armor will not melt under fire is by enchantment.  Do not take a merchant's word for it that a suit has been fireproofed.  Test it yourself in a blacksmith's forge before attempting to use it in combat against an actual dragon.  Remember not to wear the armor when testing it, as it may not actually be fireproof.