Passivate versus Chromate conversion coatings (Zinc plating on steel, ASTM B633)

[TxKent]

I'm revising an old Fe/Zn plating spec that currently calls for "Trivalent Alkaline Zinc plating per ASTM B633 Type II (yellow) SC 2." The revised spec should prohibit the use of Hexavalent Chromium, which is the intent of the word "Trivalent" now.

Would the preferred method of doing this be to call out a Type VI finish (passivate) instead of Type II (chromate conversion)? I'm finding some on-line sources that go so far as to state that a "chromate conversion coating" is Hexavalent Chromium by definition, and any Trivalent Chromium coating is called a "passivate." However, when I talked with our plating vendor he stated they couldn't do any passivates but all of their plating was Trivalent. (It's a small local operation without a coating engineer) Is he confused?

If the difference between Type II and Type VI is Hexavalent / Trivalent, why the increase in hours of salt spray? I thought that Trivalent was weaker than Hexavalent in salt spray tests, but a Type VI finish shows a 120 hour requirement, and a Type II only 96 hours.

 

[TVP]

Type V and Type VI are referred to as passivates, and do not contain hexavalent chromium. Here is an excerpt from ASTM B633:

NOTE 3—Although Types V and VI are technically not “chromate” films and they do not contain leachable hexavalent chromium ions, they are supplemental coatings that render the active zinc surface passive and provide added protection to the steel part.

Trivalent chromates/passivates come in two basic flavors: thin film and thick film. The thin film coatings (30 to 100 nanometers) do not have appreciable corrosion resistance, but the thick film (200 to 400 nm) versions do. The latest generation thick film chromates/passivates are resistant to white rust for at least 72 hours in salt spray testing. The type VI colored passivate is typically black with the coating consisting of zn plating + black chromate + organic seal, which improves corrosion resistance to at least 120 hours.

 

[TVP]

The following a draft of a European specification similar to ASTM B633 that has more contemporary information including description of sealing agents:

standardsproposals.bsigroup.com/Home/getPDF/283

 

[TxKent]

That all makes sense.

We are certainly using a thick-film passivate, but our supplier is calling it a chromate. (SC2 is 8000 nanometers minimum)

However, we aren't using any organic seal or other topcoat. That must be what's making the difference in the salt spray tests.

 

[TVP]

TxKent,

Those values I stated are for the passivate only. The Zn thickness is much greater: SC2 is for 8 micrometers of Zn.