Blackening of electrical connection occurs by sulfur or others. Sulfuration of connection
makes it from brown to black due to the thickness of the membrane of silver sulfide from
the gas in the air. The cause of sulfuration is hydrogen sulfide from foul water, polluted
river, human, exhaust gas, etc, which sulfurates alloyed silver. Silver sulfide is a
semiconductor membrane, which is not problem with usual situation or switching frequency,
however, it is the cause of bad contact with switching small current or low voltage. Sticking
a foreign substance causes the discoloration, black from carbon dissolved by arc, and
brown being scattered from oxide. Because silver oxide is not strong insulation membrane
like membrane of copper oxide, weak to heat, it can be pyrolyzed at 250℃ and destroyed
by low voltage, contact resistance is not matter in the circuit over 24V.