Zinc Coating 2

[pmaher]

I would be very grateful for any help on the followling
what is white rust ?
What is the meachism behind white rust
How can white rust be stoped?
Is it just the zinc plating is broken down and becomes oxidised in the atmosphere?
We are having a few problems with plating of brackets and our subcontractor plater are telling us that there is porosity in the base metal and that is why we are getting white rust
This Plater is a bit of a chancer and has loaded us up with bull before
The plater also told us that the porosity in the base material cause the white rust on the parts.
I have never be involved in steel or iron manufacturing
when i was taking to the supplier he told that white rust come up after this is surely because he is storing the parts in a high humidity area.
a couple of day after the iron has been plated
what is basically happening is that we submit plane iron and we get plated parts back with zinc and when we receive Could you send me any information that you can on white rust and is there a way that the platers may be able to stop the white rust from forming?

any inf. would be greatly appreciated

Regards & Thanks
Paul Maher

 

[JDavidM]

"White rust" is corrosion/oxidation of the zinc metal plating. You are plating iron, and of course that has red rust. So, unless you see red, you have yet to get rid of all the zinc.

You can only get white rust if the zinc has water sitting on it. Period. End of statement.

High humidity ALONE will not cause rust. However, if the temperature goes low enough that you get dew (condensation) on the parts, you have water. And you can get White Rust.

How to keep it from forming? Keep it dry.
--You can package and store it out of the weather
--You can have an oily material sprayed on it; there are a lot of "anti-corrosion" compounds out there for just that purpose.

 

[Mechineer]

The white rust you are refering to is also known as "wet storage stain" do as search on any search engine and you will find more than enough information.

C.Cook

 

[kenvlach]

The white rust is Zn(OH)₂. As indicated above, it forms due to the presence of water. If you are only concerned, with corrosion during shipping and storage, you can use an oil or a vapor inhibitor as suggested above. But, for long term corrosion resistance, chromating and/or painting is a better approach.

A common method is to have the zinc plating chromated. Yellow chromate and clear (or slightly blue) are the most common types, and are specified as Types II and III finishes, respectively, in ASTM B633 Electrodeposited Coatings of Zinc on Iron and Steel. Types II & III should withstand a minimum of 96 and 12 hours, respectively, of salt spray testing without the appearance of white corrosion products.

For better corrosion resistance, use thicker zinc plating and/or supplemental sealants on the chromate; see thread332-42271. For even better results, plating a Zn alloy helps.

 

[frankfnnx]

[kenvlach]:The white rust is Zn(OH)2

Well, Zn(OH)2 is the unstable form of zinc corrosion products. Normally, white rust means zinc carbonate(ZnCO3), a rapidly formed corrosion product. Although, they are all white coloured powder. It won't offer corrsion resistance to coating. White Rust has the appearance of a thick, white, waxy deposit. Ideally, Basic Zinc Carbonate [3Zn(OH)2· ZnCO3· H2O] can provide further protection of the cZnCO3oating. I hope this can help you to identify the problem.

 

[kenvlach]

pmaher,
I suggest downloading ‘Protecting Galvanized Steel-Sheet Products from Storage Stain’ from

http://www.galvinfo.com/GalvInfoNote PDF's/galvinfo7.pdf

{note: paste entire URL into browser}
here is an excerpt on what white rust is and why it forms:

“Why Then is Zinc Very Susceptible to Storage Stain(White Rust)
Recall, zinc is very reactive metal. It exhibits a low corrosion rate only because a continuous passive film forms on the surface of the zinc coating. A key part of the corrosion reaction is that “the surface needs to dry in the presence of air” in order to develop the passive film. It is during the drying part of a rain cycle that the zinc carbonate/zinc oxide passive film is developed. Since the zinc coating needs to dry thoroughly to develop a passive film on its surface, and thereby, provide good corrosion resistance, what happens when the product gets wet while still in coil form or when stacked into bundles at a roll formers plant or at the jobsite? As many of you have seen, this condition can lead to the formation of an excessive amount of storage stain. What is “storage stain” or “white rust”? It is simply the chemical compound, zinc hydroxide, that forms initially when zinc is in the presence of moisture. Why doesn’t it convert to the passive film of zinc oxide/zinc carbonate? The answer is that the zinc hydroxide is never allowed to dry; nor are the tightly wrapped sheets freely exposed to carbon-dioxide containing air. Thus, the protective zinc carbonate/zinc oxide film never forms, and corrosion of the zinc surface continues to occur. Since the corrosion reaction is allowed to proceed as long as the surface is wet, a large accumulation of zinc hydroxide can form. Remember, zinc is a reactive metal in the presence of moisture under conditions that do not allow the protective passive film to form. When “white rust” does occur, there is an actual loss of zinc coating, and thus, some of the zinc that is intended to protect the coated-steel product when it is eventually exposed in its application is lost. The extent of the damage is primarily dependent on:
1. the time of exposure to moisture,
2. the temperature that is experienced during storage, and
3. the presence of accelerating corrodents such as chloride-containing salts.”

Further on, the article gives methods of preventing storage stains: Chemical passivation (chromating), corrosion inhibiting oils, vapor phase inhibitors, plastic wrapping containing inhibitor and of course, keeping the part completely dry.

frankfnnx,
The generally accepted mechanism according to the preceding article and corrosion texts (e.g., see below), the initial, rapidly-forming corrosion product is Zn(OH)₂ [which is not unstable, merely reactive w.r.t. CO₂]. The final white rust is indeed carbonate, and thus this will be generally observed, but it doesn’t indicate rapid corrosion; rather, the petrified remains of corrosion.

This is rather simplified hypothesis, but since CO₂ only makes up 0.033 vol. % of the atmosphere (exclusive of water vapor), and must perhaps undergo an adsorption step, the carbonate formation rate may be reactant-limited. It would be interesting to know how the initial reaction would differ with carbonated water under a CO₂ atmosphere. Maybe, less white rust due to more rapid formation of protective ZnCO₃?
From the article above, the absence of CO₂ certainly favors the formation of white rust.

Anyhow, here is a relevant excerpt from a corrosion text:
“…bulky deposits of porous zinc oxide (ZnO)* or Zn(OH)₂ form on the surface instead of forming a protective layer all over it. This oxide quickly takes up CO₂ to form a basic carbonate, commonly known as wet-storage stain....
The Zn(OH)₂ reacts further with CO₂ in the atmosphere, which rapidly forms a basic ZnCO₃. This film is very protective and is mainly responsible for the excellent corrosion resistance of zinc to ordinary atmospheres.” -- ASM Handbook, vol. 13, Corrosion pp. 756,757 (1987).

* ZnO forms instead of Zn(OH)₂ in water at temperatures of 60^oC or above. -- Corrosion and Corrosion Control, 3rd Edn., p. 239-240 (1985).