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Galvanising S G iron castings

We have a client who has sent a number of SG castings for hot dip galvanising. Normally there isn't a problem with the coating. However in this instance the zinc will not adhere to the casting surface and by all accounts the casting goes a black colour.
Any ideas as to why this is happening?

Stuart Horner
Foundry supplier - United Kingdom
February 16, 2010

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Sir:

I had to put S G Cast Iron into the internet AGAIN to see what it is. Apparently the graphite is in the form of spheres rather than flakes. Cast iron typically has an iron "shell" and within the casting is substantial carbon (graphite). If the castings are "over pickled" then the graphite is exposed (black) on the surface of the casting and molten zinc cannot alloy with the iron because there is too much carbon on the surface. Perhaps the inhibitor that you use in the acid (H2SO4 or more likely HCl) is not properly protecting the bare iron once exposed.

To correct the problem so as to allow galvanizing a light grit blasting may work, or proper inhibitor in the acid or perhaps using HF pickle to remove sand if sand casted. Be aware that HF is extremely dangerous, causing mortal damage to the lungs.

Regards,

Dr. Thomas H. Cook
Galvanizing Consultant - Hot Springs, South Dakota, USA
February 19, 2010

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February 19, 2010

Galvanizing cast irons and steels is often the most unpredictable. The reason is that galvanizing is an alloying process, (not just a coating) and as a foundryman you'll appreciate that the cast chemistry could be very variable, and usually unknown to the galvanizer.
So in effect he's asked to form an alloy on an unknown (to him) chemistry metal.
Of particular importance is the Si and P concentrations. These have a huge impact on the rate of alloy growth.
The method of casting can have impact too, especially sand moulded castings, where Si bearing sands are sometimes used.
If the chemistry of the cast in known, its possible to galvanize most castings, but not all galvanizers know what to do. Many avoid the hassle by just avoiding this work, which typically isn't a large market share.

Where the alloy doesn't form on the cast surface, it will usually go black.
More info needed to be more specific.

Geoff Crowley
Crithwood Ltd.
Westfield, Scotland, UK