Patina. A green coating, consisting principally of basic sulfate and 

 occasionally containing small amounts of carbonate or chloride, wliich 

 forms on the surface of copper or copper alloys exposed to the atmos- 

 phere for a long time. 



pH . A measure of hydrogen ion activity defined by pH = log-j_Q(l/aH+) where 

 aH = hydrogen ion activity = the molal concentration of hydrogen ions 

 multiplied by the mean ion activity coefficient. 



Pickle . A solution or process used to loosen or remove corrosion products, 

 such as scale and tarnish, from a metal. 



Pitting erosion . (See cavitation erosion. ) 



Pitting factor . The depth of the deepest pit resulting from corrosion, 

 divided by the average penetration as calculated from weight loss. 



Polarization. The production of a counter e.m.f. by the products formed 

 or by the concentration changes resulting from passage of current through 

 an electrolytic cell. 



PPM. Parts per million. 



Positive electricity . A body is said to possess positive electricity 

 when it has a deficiency of electrons. 



Potential . When a point is said to be at a certain potential, the 

 meaning is that there is a voltage difference of that amount between 

 the point concerned and a given reference. See also standard electrode 

 potential. 



Prime coat . A first coat of paint, originally applied to improve ad- 

 herence of the succeeding coat, but now frequently containing a corrosion 

 inhibitor. 



Reaction limit . The minimum concentration of an alloy component below 

 which appreciable attack of an alloy takes place in a given environment, 

 but above which the alloy is corrosion resistant. 



Redox . The term "redox" is an abbreviation of oxidation-reduction 

 potential, used to determine whether soil is aerobic or anaerobic. The 

 redox-potential is obtained between a platinum electrode and a calomel 

 half-cell. 



Reduction . Gain of electrons by a constituent of a chemical reaction. 



Relative humidity . The ratio, expressed as a percentage, of the amount 

 of water present in a given volume of air at a given temperature to the 

 amount required to saturate the air at that temperature. 



B-8 



