Smoke or burn marks on an outlet are often caused by heat resulting from an overloaded circuit, however, there are several other things that should be checked as well. Smoke or burn marks are a red flag that there may be a fire safety concern.
The source of the heat can be due to a number of causes, including the outlet having the hot (energized) wire touching the neutral or ground, or being close enough for arching to occur. Also loose wire connections that are not secured down tightly may be at fault.
Corrosion or Heavy Dust Buildup
Sometimes corrosion has built up over time and is creating the problem and occasionally, dust or dirt may be the culprit. All of these can occasionally result in heat build up or arching. Rarely are these the cause.
Outlets which are old and that have had the a lot of use over the years get worn out in the area where the metal pieces of the plug come in contact with the metal pieces in the receptacle. When this area reaches a point of where the area of contact is much smaller then when the outlet was new, then resistance builds up which creates heat. When the heat gets excessive, it may cause a burning smell and/or burn marks for smoke marks on the outlet or the cover of the outlet.
Where the wiring connects to the outlet, the screws or the wiring may have worked loose, therefore there is not a full connection and resistance and heat builds up. Resulting in smoke or burn marks to the plastic.
An overloaded outlet or circuit may also cause smoke on the face of the outlet, as well as a smoke or burnt odor. Overloading from a power surge can create the same condition.
Outlets on overloaded circuits, or with too many appliances connected and operating at the same time, may likewise have smoke or burn marks on them.
Replacing outlets that are burnt should be done. Any outlet that has smoke or burn marks on it, should be checked for damage, for wear and tear, and for proper wiring connections. Replacing the outlet would be the best idea.
Caution. When plugging in a lamp or other appliances be sure that there is a good connection. Sometimes when we plug in a lamp the plug may fall out or drop down. This is an indication that there is not a full and proper contact/electrical contact. When there is insufficient contact of the metal parts, then heat can build up.
Damage or Improper Wiring
Should an outlet become damaged from use or being hit by a hard object or an electrical cord being jerked out of the outlet, then arching may occur. Note that this is seldom the reason for burn marks or smoke on an outlet.
Improper wiring, may likewise generate the smoke. If an arc fault has occurred, the excessive heat from the arc may cause the plastic to overheat and therefore burns slightly.
Who to consult? An electrician.
Electricians can generally advise buyers on burn't outlets, do replacement of outlets and trouble shoot circuits if necessary. Also, qualified handymen can replace damaged or defective outlets.