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I was travelling in China with my Macbook and USB-C Power Adapter. While plugged in at a hotel, the prongs of adapter got burnt.
Is it okay to just replace the prongs, or is it likely that the adapter's circuits have been damaged? There is no other visible damage to any other part of the charger.
Should I just replace the whole adapter to be safe?
Taking a close look at the adapter, you will notice there is evidence of melted metal on the second prong most likely caused by arcing.
What this tells me is that there was a short from the hot to neutral which (should have) tripped the breaker. What caused it? Any number of things from faulty wiring to a surge with enough voltage to cause the electricity to "jump" from one conductor to the next.
Is your adapter safe to use?
Yes, provided it works. Adapters are designed to isolate the high(er) voltage mains AC from the lower, cleaner, usable DC voltage. If anything was damaged due to a surge, the adapter simply won't work (most likely the internal fuse would be blown).
Bottom line... you can plug it in and it won't hurt your Mac. You can even used the charred AC plug connector and it will still work. Unless the prong is broken, or so physically deformed that it's causing a short, there's nothing to worry about.
If there is no damage to the body of the brick and it appears to be functioning normally, I would personally be okay just replacing the prong portion.
That said, Apple may be interested in trading out the entire power supply free of charge. They care a ton about product safety. They may be interested in dissecting it to determine if it was a manufacturing defect. You can find Apple support contact information here.
TLDR: Wrecked by series arcing, from inserting in loose socket. The pins are scrap now. Don't use AC sockets that are loose.
This is due to series arcing: power was flowing through the adapter normally, but making poor contact. AC mains voltage electricity was able to leap across the very small gap between pin and socket. This arcing acted exactly like arc welding (was arc welding) and gouged and pitted the metal, and made smoke/soot (probably from the poor grade of plastic of the Chinese socket). This worsened the contact and worsened the arcing. Arcing like this makes an impressive amount of heat - which dips the voltage to the appliance, which on a switching power supply increases current further, lather rinse repeat.
The part with the pins is scrap, since the arc damage will cause its contact with other plugs to be unreliable, which in turn could cause even more arcing. It won't hurt your Mac, but it might start a fire which would definitely damage your Mac. I would not attempt to dress the pins with a file, as they are plated, and you are exposing the underlying brass, which would then oxidize, worsening contact further. The rest of the charger is fine, you can get "just the little clip-on plug device" by buying any genuine Apple iPad charger, which use the same clip-on. I get mine on eBay for $12.
Or, look at the socket on the power block - that is IEC C8. It can mate with any common "IEC C7" appliance cord. When I had a Macbook PS that had a cord instead of a pin-frob, I left the cord behind and just grabbed a local C7 cord.
The lesson to learn here is don't use AC power sockets that are loose. If the contacts don't grip the plug securely, it is defective and don't use it.
This sort of thing is why UL and the other national testing labs (CSA, TUV, BSI etc.) have very high standards for manufacture of sockets and plugs. Both in terms of contact design and quality, and also the plastics used must resist and not accelerate fire, and not emit toxic smoke if charred by external fire. Such standards do not always exist in the third world, or are not enforced.
The pin location of this arcing is highly unusual. That is not the normal contact point for a UL-listed or CSA-listed NEMA 5-15 socket, which is on the 2 faces of each pin. I suspect you were dealing with one of the infamous Chinese "everything sockets", which make contact with every kind of plug, but very poor contact with any of them. This could not handle 0.5 amps - certainly 16A is out of the question. As such, UL and the major NRTLs will never approve "everything sockets". They cannot be made safely.
"Dual sockets" such as British shaver sockets are possible.
