The improbability of preparing an absolutely clean metal surface, even in the laboratory, is discussed. In the light of the difficulties in achieving such a goal the methods used to prepare surfaces as nearly free of contaminants as possible are presented.
The contaminants most commonly found on metal surfaces are divided into two groups depending upon their wettability. They are hydrophobic (nonwettable) and hydrophilic (wettable). The hydrophobic contaminants are, for the most part, polar organic compounds and mixtures of such materials. The polar group most commonly encountered is COOH, which is present in common oils, fats, greases, etc. These compounds are very strongly adsorbed onto metal; in fact, chemisorption is very common. The usual methods for removal of such surface contaminants are given, followed by a quantitative comparison of the various tests available for the resulting cleanliness. The hydrophilic contaminants, which can best be represented by oxides, are then discussed, including tests for their presence and methods of removal.
