Where the
world gathers for metal finishing
Q&As since 1989


-----

Die caster's zinc chromate is much too dark

Dear Guillermo,

Is Zinc dichromate the same as that of Chromate, since our zinc chromate parts is much darker than the zinc dichromate parts supply from customer. Or can we use chromate method to provide a more golden colour?

Joe Fok
OEM die casting - Hong Kong, HK, China
2003

---

First of three simultaneous responses --

Chromate method to provide a more golden colour. Method 1) remove silicon - HNO3 + HF 2) ZINC plating on products 3) control of element. But can't achieve perfect golden color.

SE DO JANG
- KOREA
2003

---

Second of three simultaneous responses --

In my language, zinc dichromate is a chemical used in primers. Then there is chromated zinc, which is probably the coloration you refer to. The exact color or hue is a function of the chemical formulation used, the pH , concentration, temperature, and time in the bath. Activation of the zinc may also affect the hue as it affects the speed of the initial reaction.

James Watts
- Navarre, Florida
2003

---

Third of three simultaneous responses --

To contribute a little book knowledge, but little practical and useable information . . .

The chromate ion is actually Cr04 and is yellow, while the dichromate ion is Cr2O7 and is orange. As you reduce the pH you drive it toward dichromate; as you raise the pH you drive it towards chromate. This is probably why a chrome plating tank is so very deeply orange whereas an alkaline cleaner/stripper contaminated with chrome is a nice lemon-icing yellow.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003