Apple 20W Charger prongs seem to have arc marks. Is it safe to use?

Just noticed these identical marks on my charging adapter’s prongs (Please see the images below). I searched online and found that they could be arc marks. I checked the lightning cable prong and there’s no corrosion or similar marks. The phone is working alright. Is it still safe to use?

Side 1

Side 2

As much as I’d like to replace it for peace of mind, I don’t want to put down $20+ since I just got it and my iPhone.

I only plug it into a grounded power strip with working surge protection, so I’m not sure why this happened. Thanks in advance!