143 



the chromium is left in contact with liie li(|ni(l, the hvdiof^eii present 

 in the boundary surface Gt\ will for the groaler |)art pass into the 

 solution, or escape in gas form. This causes the concenlration of the 

 hydrogen at the boundary sui-face to become smaller. 



In consequence of this the hydrogen diffuses from I he metal towards 

 the boundary surface, and after a certain tiuie the concentialion of 

 the liydrogen in the chromium will be i-epresentcd by the line /I /> 6'. 

 In the part AB the coiu'entration has remained unchanged, in BC 

 the concenlration has changed tlu-ough diffusion. We shall call (he 

 layer EF, in which the diffusion is perceptible, I he dilfnsion layer. 

 The chromium now pre.sents a potential which is detoriniiied by 

 the size of FC. 



When we now polarize anodically, the first consequence will be, 

 that the hydrogen concentration at the boundary surface /XMiecomes 

 smaller. This will at any rate be the case when the solution used 

 is so little acid, that the hydrogen potential corres[)Oiuling to a given 

 hydrogen charge, is luore negalive than the chromium potential of 

 the same hydrogen charge. This applies therefore to those parts of 

 the hydrogen lines {AB, CD, etc.) that lie under the chi'omium line 

 PQ in fig. 1. For in case of anodic polarization, the potential of 

 the metal will here lie further above the eqi^ilil)rium potential of 

 the hydrogen than above that of the chromium, so that the hydrogen 

 will dissolve to a greater extent. 



At the same time chromium goes into solution. Hence the boundary 

 layer GF shifts to the left, and gets e.g. at the place G' F" . The 

 thickness of the diffusion layer has now become smaller, and the 

 concentration of the hydrogen in this layer is represented by B'C'. 

 Now the hydrogen charge at the boundary surface is F'C\ hence 

 smaller than before the anodic polarization. When the strength of 

 the current is kept constant, a stationary state will set in, in 

 which F'C' is constant, and also the concentration gradient in the 

 diffusion layer. 



When the strength of the current is increased, this stationary state 

 will be another, i.e. so, that F'C' is smaller and the concentration 

 gradient of the hydrogen in the diffusion layer greater than in case 

 of smaller strength of current, because the diffusion layer is thinner. 



During anodic polarization the potential will be more positive 

 than before, because the hydrogen concentration at the boundary 

 surface is smaller. When the current is broken, the hydrogen will 

 quickly diffuse towards the boundary surface in consequence of the 

 great concentration gradient in the thin diffusion layer. This causes 

 the hydrogen charge at the boundary surface to rise, e.g. to F' C' . 



