141 



of 10-* alm. corresponds to a potential R, and tlie hydrogen charge 

 of clii'omiuni does, tliereforo, not become sti'onü,er of its own accord. 

 When, however, this chrominm is catliodically pohirized in 10— * n. 

 acid, the hydrogen charge increases, the chrominm potential decreases. 

 If this fails below ]\ the chroiniiun can be activated fnrther 

 spontaneonsly till the potential has become ('. By stronger cathodic 

 polarisation the potential can become lower than U; when the 

 cnirent is l)roken the potential will, however, have to rise again 

 up to Ü. The potential itself will not stop at U, but become higher. 

 If the hydrogen generation is snfïiciently vigorous to maintain a 

 sufficient gas charge on the chromium, the potential can remain 

 between V and U. Then the chromium remains active. When the 

 hydrogen charge becomes smaller than corresponds with V, tiie 

 potential rises above V, and the activity disappears. Not only when 

 the current is broken can the potential rise above [^; it is also 

 possible tliat this already takes place in case the polarizing current 

 is weakened. When e.g. with vigorous cathodic polarisation the gas 

 charge becomes greater than V, the gas charge can become smaller 

 than T'^ when the hydrogen generation becomes feebler, and the 

 activity will disappear. This is the above desciibed phenomenon of 

 Rathert. It is likewise possible that with very weak cathodic pola- 

 rization the hydrogen charge does not become great enough to lower 

 the potential below (/. Then the metal remains passive in spite of 

 the cathodic polarization (Fladk). 



The most negative potentials that the chromium can spontaneously 

 assume in 1 n. 0.01 n. and 0.0001 n. acid, are accordingly S, T, and U. 

 These will, however, not be reached, because a vei-y great over- 

 voltage would be required for it. In reality the potentials S' , T' , 

 and U' will e.g. be observed. 



The above given considerations account, therefore, sufKiciently for 

 the spontaneous activation of chromium in acids, and the activation 

 by cathodic polarization, also in connection with the strength of 

 the acid. 



Chromium becoming more easily active in hydrochloric acid than 

 in sulphuric acid or in other acids, there must exist a specific 

 activating action of the chlorine ions. This comes to this that a 

 smaller gas charge is required for the activation in hydrochloric 

 acid, and that the line PQ must therefore be drawn more to the 

 left for hydrochloric acid. Thesame thing applies for higher temperatures. 



It appears then also from figure 1, that chromium can only remain 

 strongly active in a liquid in which it develops hydrogen, and, 

 therefore, maintains its gas charge itself. When now a fresh electrode 



