Any plating process performed without the presence of an external applied voltage (compare electroplating). The techniques are particularly appropriate for plating nonconducting or semiconducting surfaces with a noble metal such as gold. The surface to be plated is immersed in an appropriate solution. Immersion plating proceeds by the chemical replacement of a low electrode potential metal on the surface by the required higher electrode potential metal (see electrochemical series). The reaction ceases once a thin layer has formed. Autocatalytic plating involves the continuous chemical reduction of the more noble metal to form a coating on the base metal. The coating metal may act as a catalyst for the reduction. Both techniques are complex.