While trying to do some repair welding on Al-5086 of an aluminum ship, there has been a strange phenomenon. When Al was heated it changed colour like if it was saturated by fuel oil, or some type of oil. Then an oily looking spot started to appear where in some cases was as big as a 15 inch diameter circle. Any idea what this is? Could the Al be "soaked" with fuel oil, and when heated the oil appears on the surface? Any suggestions?