Mill scale . The hea-vy oxide layer formed during hot fabrication or heat- 

 treatment of metals. The term is applied chiefly to iron and steel. 



Molality . Molality is the concentration of a solution expressed as the 

 numher of gram molecules of the dissolved substance per 1,000 grams of 

 solvent. 



Noble metal . A metal which in nature occurs commonly in the free state, 

 or a metal or alloy whose corrosion products are formed with a low nega- 

 tive or a positive free-energy change. 



Noble potential . A potential substantially cathodic to the standard 

 hydrogen potential. 



Open-circuit potential . The measured potential of a cell during which 

 no significant current flows in the external circuit. 



Overvoltage . The difference between the potential of an electrode at 

 which a reaction is actively taking place and another electrode at 

 equilibrium for the same reaction. 



Oxidation. Loss of electrons by a constituent of a chemical reaction. 



Parting . Parting refers to the selective corrosion of one or more 

 components of a solid-solution alloy. 



Parting limit . The maximum concentration of a more noble component in 

 the alloy, above which parting does not occur within a specific environ- 

 ment. 



Passivator . An inhibitor which appreciably changes the potential of a 

 metal to a more cathodic value. 



Passive-active cell . A cell the e.m.f . of which is due to a potential 

 difference between a metal in an active state and the same metal in a 

 passive state. 



Passivity . 



a^. An active metal in the e.m.f. series, or an alloy composed 

 of such metals, is considered passive if its electrochemical 

 behavior becomes that of an appreciably less active or noble 

 metal. 



b_. A metal or an alloy is passive if it substantially resists 

 corrosion in an environment where, thermodynamically , 

 there is a large free-energy decrease associated with its 

 passage from the metallic state to appropriate corrosion 

 products. 



3-7 



