Editing Chrome plating (section)

==Hexavalent chromium==
''[[Hexavalent chromium]] plating'', also known as ''hex-chrome'', ''Cr<sup>6+</sup>'', and ''chrome(VI)'' plating, uses [[chromium trioxide]] (CrO<sub>3</sub>, also known as chromic anhydride) as the main ingredient. Hexavalent chromium plating solution is used for both decorative and hard plating, as well as bright dipping of copper alloys, [[Anodizing#Chromic acid (Type I)|chromic acid anodizing]], and [[chromate conversion coating]].<ref name="newmoa">{{Citation|title=Pollution Prevention Technology Profile Trivalent Chromium Replacements for Hexavalent Chromium Plating |publisher=Northeast Waste Management Officials’ Association |date=2003-10-18 |url=http://www.newmoa.org/prevention/p2tech/TriChromeFinal.pdf |archive-url=https://web.archive.org/web/20110720153833/http://www.newmoa.org/prevention/p2tech/TriChromeFinal.pdf |archive-date=2011-07-20 |postscript=. |url-status=dead }}</ref>

A typical hexavalent chromium plating process is:

# Activation bath
# Chromium bath
# Rinse
# Second rinse

The activation bath is typically a tank of chromic acid with a reverse current run through it. This etches the work-piece surface and removes any [[mill scale|scale]]. In some cases, the activation step is done in the chromium bath. The chromium bath is a mixture of chromium trioxide and [[sulfuric acid]], the ratio of which varies greatly between 75:1 to 250:1 by weight. This results in an extremely acidic bath (pH 0). The temperature and [[current density]] in the bath affect the brightness and final coverage. For decorative coating the temperature ranges from {{convert|35|to|45|C|F|abbr=on|-1}}, but for hard coating it ranges from {{convert|50|to|65|C|F|abbr=on|-1}}. Temperature is also dependent on the current density, because a higher current density requires a higher temperature. Finally, the whole bath is agitated to keep the temperature steady and achieve a uniform deposition.<ref name="newmoa"/>

===Disadvantages===
One functional disadvantage of hexavalent chromium plating is low cathode efficiency, which results in bad [[throwing power]]. This means it leaves a non-uniform coating, with more on edges and less in inside corners and holes. To overcome this problem the part may be over-plated and ground to size, or auxiliary anodes may be used around the hard-to-plate areas.<ref name="newmoa"/> Hexavalent chromium is also considerably more toxic than trivalent chromium, rendering it a major health risk both in manufacturing and disposal if not handled with care.<ref>{{cite web |last1=Surface Engineering Association |title=A brief guide to the chrome plating process |url=https://www.sea.org.uk/blog/a-brief-guide-to-the-chrome-plating-process/ |website=Surface Engineering Association |access-date=21 August 2023}}</ref>