Since being introduced alongside the iPhone 5 in 2012, Apple's Lightning cable has been criticized for its poor durability, as it is prone to breakage and fraying near the adapter.
It appears the Lightning cable may be vulnerable to corrosion as well, according to a report from ZDNet's Jason O'Grady, who noticed corrosion on the gold contacts of his Lightning cable after it had been failing to charge his iPhone.
About a month ago the Apple Lightning to USB cable (PN: MD818ZM/A, $19) that I keep in my vehicle for charging my iPhone 5s began to fail. At first charging was intermittent, then within a week the cable wouldn't charge my iPhone at all. A closer look at the male end of the Apple Lightning to USB cable reveals that several of the gold contacts have become corroded.
In addition to O'Grady, several other users on Apple's Support Communities have experienced corrosion issues as well, with reports dating back to 2012. Several users have been told by Apple that it is likely that the cables were exposed to moisture to cause the corrosion, but several reports suggest there was no contact with liquid before the corrosion appeared.
Apple Community user "Brockap3" supplied photos of his Lightning cable under a microscope and suggested that the corrosion was happening primarily on the VBUS/V++/Power pin on the Lightning cable. He offered two explanations for the issue:
1) two gold electrodes placed in an aqueous solution with a positive voltage on one wire and ground applied to the other will corrode the positive gold electrode away
2) electrical arcing causing the corrosion (originally I dismissed this as it is just USB - i.e. 5 V /2 A max)
After examining the male and female sides, it would seem both are likely culprits, but you would have to have some pretty serious condensation on the pins to get this to happen with steam.
Apple does appear to be exchanging Lightning cables that are unusable due to the corrosive deposits, so it is likely that users experiencing issues can have their cables replaced at an Apple retail location or through an Authorized Service Provider.