In the sintered type nickel cadmium batteries, the plaque has black colour, i remove the impregnated outer layer (which contains nickel hydroxide and other additives. I remove this layer by firstly etcing in acid solution then grind it). There is no shiny colour of nickel. In the beginning i think some amount of carbon is added to nickel powder but i could'nt see such a addition in the literature.
Colour of the sintered nickel plaque is a little bit darker then nickel felt. (Actually i also wonder why nickel felt is not in metallic colour)
For a somewhat related query, please check: https://www.researchgate.net/post/We_have_tried_coating_Nickel_film_on_glass_and_it_turned_out_black_Why
Impurities, such as sulfur, have the potential to affect the color of Ni. Thus, chemical analysis is required to find out the possible reasons behind the color change.
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