White rust on zinc plated parts

A. Sir:

I have seen some horrible results from shipping zinc and zinc alloy parts over the ocean. Even with containers that are supposed to be sealed well, you get condensation patterns on the parts, combined with a salt type mist on the ocean. The way we got out of the problem was to wrap all parts thoroughly and completely in plastic with no gaps or voids. Additionally, work was done on the containers on the ships.

We were very surprised at the variances of temperature that we were told occurs in the hold of a ship.

Sincerely,

A. I think your products were most probably plated in Zinc and finished in Blue Chromate Correct?

The short answer is that this finish is not suitable for the application, you need a better finish, if your customer wants the blue bright finish then you must look to applying a seal to the chromate finish, i.e., a water borne acrylic lacquer, or, a metasilicate seal.

The packaging material used also needs to be considered, as does the packaging method.

A good consultant can solve your problems quickly and relatively cheaply.

Regards,

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A. Sir,

Zinc plated and yellow or blue chromate passivated parts deteriorate in storage due to various factors like condensation of moisture due to humidity changes and also due to biological fluids from, say, rodents and also due to fingerprints (especially in the case of full (yellow)iridescent passivation).

Apart from these factors, any acidity from the packaging materials may be a contributing factor. Adding packets of silica gel will be a valuable thing to protect the parts from corrosion due to condensed moisture during the sea voyage besides taking other precautions.

Thanks.

A. Zinc plating protects steel from red rust, and itself corrodes as white rust, so that is the short answer as to why it occurs, Mr. Prabhu.

But why your product is white-rusting within 45 days of sea travel is difficult to assess based on the limited input. What is the product, how is it packaged, is it guaranteed to be fully dry before packaging, what is the thickness of the zinc, what kind of salt spray hours have you been getting before white rust shows, etc.? Thanks.

A. Dear Mr Prabhu,

Welcome to the club of white rust problems!

For Sea shipment, this is common as I reckon that 45 days sea shipment is equal to 23 days of a Salt Spray Test.

You may want to opt for Zinc Cobalt Alloy plating with yellow passivation to avoid similar problems in future. It would be wise to test parts up to 480 hours in a SST before shipping the parts again.

Regards,

A. Dear Mr Prabhu,

Your biggest problem might be the packaging. Plastic packets or sheeting is an absolute "NO". The metal will sweat and start corroding. Cardboard must not be acidic. Wooden crates with cardboard boxes and polystyrene spacers are best. Find a company that knows about packaging for zinc plating. We had a client who shipped zinc plated articles overseas for years with only 3 microns zinc and blue passivation and they shipped well. Good luck!

A. You can check the packing material because sometimes the packing material like plastic or glue used for cartons is interacting with the plating and thus it gives the rusting.

A. You can try and use top coat sealants. Plenty are available which will increase white corrosion to maximum to 200 plus hours; also improve packing material.

A. You will most certainly find that the parts have been packed wet or that the parts have been subjected to marked changes in atmospheric conditions (humidity & temperature) and the parts have sweated during transport thus getting the parts wet in the process. If the parts were packed in plastic then this will only exacerbate the situation.
Solution is to pack the parts in a container to allow free movement of air , but keep the parts dry and protect them from extremes of temperature .
Regards,

A. Dear Mr Senthilnathan,

Don't send any parts by sea unless they are alloy zinc plated well is the long and short solution. NO PLASTIC packing is the second solution.

A. Dear Sir,

There is a product available to seal your kind of plating. Thickness is 2/3 microns and applied by dip spin coating method. The product is Delta Coll from Doerken of Germany. Try it, it will work for your application. Best regards.

Hi Mukesh. As you see, we appended your inquiry to earlier discussions on the topic, and the problem is perennial. Please try to review the earlier comments and either ask for clarification on things that sound applicable, or supply the data about your situation which people have indicated is necessary to offer more specifics. Thanks!

Regards,

A. Hi Kalyan. Zinc plating can deter corrosion but it does not absolutely prevent it. And contact with the cardboard will help keep parts wet or get them wet. The cardboard can also rub off the protective chromate with time & tide; and cardboard is often high in acid or sulfides which are corrosive.

If these are very inexpensive commodity parts, and will be in boxes for a very short time and under favorable conditions, and you are sure they are completely dry, you might get away with loose packaging in cardboard like that, but it's not the right way.

Exotic elemental analysis equipment can be useful if carefully & repeatedly walking the line hasn't disclosed the source of problems ... but it is no substitute for carefully observing and controlling what is being done.

No good decision was ever made from a swivel chair -- General George Patton :-)

Luck and Regards,

A. Hi Kalyan. If the corrosion is white, it might be white rust (zinc reacting with water and oxygen); if the corrosion is rust colored, it might be red rust (iron reacting with water and oxygen; if the corrosion is black, it might be sulfide products from the reaction of sulfides in the cardboard with the parts. Send us good pictures of the corrosion please.

But what can you now tell us from walking the line and observing? You'll have to find a different traveling companion if you wish to continue down the path of trying to analyze what is going wrong from elemental scans of the surfaces or corrosion theory rather than observation of the process ... I have taught myself little about those subjects, perhaps because I've never found any success from proceeding in that direction :-)

Regards,