Can I damage an iPhone if I use a 24 watt (5volt) charger? I know that using a higher amperage charger will make a device charge faster.  But is there a limit to how high the current can be?  Could too high a current damage the battery?.  Could I use a 24watt (5v/4.1A) USB charger (intended to charge USB-C devices such as my Nexus 5x) to charge an iPhone?.  I'd use a USB-A to USB-C cable for connection.
The 6/6S can take advantage of up to 1.6 amps if the USB adapter can supply it. Using a higher capacity adapter will not hurt the phone, but won't help either. An iPad adapter is 2.2 amps, the iPhone "cube" is 1 amp. So an iPad adapter will charge an iPhone 6 or 6S series slightly faster. Not true for the 5 and earlier, which will only use 1 amp regardless of what is available.