Validation and development of an electroplating process to deposit a black chromium coating from a trivalent chromium electrolyte
SAAB AB coats a part of their magnetron with black chromium to
enhance its ability to radiate thermal radiation. Today an
electrodeposition process that has hexavalent chromium as its main
component is used, but hexavalent chromium is carcinogenic and
will be prohibited. This project examines if an electrolyte based
on trivalent chromium can result in a black chromium coating.
The project was divided into in four experimental parts:
investigation of the adhesion on copper, the effect on color if
copper was added to the electrolyte and investigation of the
process parameters with and without cooling of the electrolyte.
It was concluded that a black chromium coating can be deposited
from a trivalent electrolyte. Heating the sample after plating and
addition of iron or copper in the electrolyte darkens the color
but addition of copper can not produce a coating on copper
substrates with good adhesion.
To examine how the coating thickness and emissivity vary with the
current density, electrolyte temperature and plating time, the
coatings thickness and emissivity were measured for different
settings. It was shown that the coating thickness increased with
plating time and current density until a critical value was
reached and the coating started to peel off. No correlation
between the emissivity and process parameters could be shown. It
is suggested that further experiment are conducted to investigate
if a variation in pH- value effects the emissivity. Based on the
results and conclusions it is recommended that the addition of
iron to the electrolyte is further examined.
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