46006

Rough and lumpy finish on galvanized parts  

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We are a sheet metal parts manufacturing company that has to supply hot dip galvanized parts to our customer. The parts are generally of 3 mm thickness and upto 250 mm long.The material is hot rolled steel Some are flat, others are in the form of a U channel.Some parts have a weld

We have tried a few local galvanisers but are not satisfied with the finish. It is rough and has excess deposition near holes and sometimes even in flat areas where it seems that the zinc accumulates and solidifies.This has to be removed by filing , which leaves its own marks and results in poor aesthetics.

 

What can be done to improve the appearance? The thickness of galvanising is approx 80 microns.One of the parties has a centrifuge but even these parts have problems. What should the rpm be?

Bomy Dabhoiwala
Owner -sheet metal parts supplier - Mumbai, Maharashtra, India

Sir,
this sort of surface defect is rather usual for hot dip galvanizing and is associated with both the quality of the steel surface and the process itself.
Unfortunately, hot dip galvanizing is mainly a corrosion protection system and shall not be intended as an aesthetic treatment. However, you could obtain better appearance by:
- making sure surface preparation is ok
- making sure the hot dip galvanizing line used is well kept, especially for iron levels in flux and subsequently in the zinc bath
The small particles seen in your picture indicate the presence of floating drosses that would be reduced by reducing Fe in flux and then with proper mechanical drossing in the zinc kettle.
Regards

Mario Ubiali
- Brescia, Italy

As Mario says, Galvanizing is not an aesthetic finish, but there are things to do to improve this.
As you are not the galvanizer, you are not in control of these matters, but...

With the lower availablity of GOB zinc in recent years, some zinc baths have ended up with lower Pb levels (down to 0.1%), and this can have the effect that dross is left in suspension rather than sinking. Pb at about 0.8% seems to cause the dross to increase density and sink to the bottom where it is periodically removed. SHG zinc, the more readily available, is very low in impurities, including Pb.
Dross is an undesirable alloy of Fe and Zn that forms in the zinc and in small crystals. These lumps might easioly be dross grains floating in the Zn, attaching to the steel part as its removed from the zinc.
Its quite normal to file these off.

Geoff Crowley    galvanizing &    powder coating shop Glasgow, Scotland

Thanks for the suggestions.What is the correct level of Fe in flux. Geoff indicates that a PB content of aleast 0.8% would be reight. What is the max limit.

We have pretreatment line of 7 tanks for powder painting. I am just wondering whether we can add a "kettle" to do the galanizing ourselves. We would need a capacity of 15 tons per month. The parts are all small 10 to 30 cms and all of 3 mm. In terms of number of parts it would be
about 40,000.

We have small furnace for a die casting machine, so to some extent we are familiar with handling of molten zinc.
What is the kettle material? For the die cast furnace we use a cast irn pot, but it is quite small in size.
Would appreciate any comments you have on the above.

Bomy Dabhoiwala
- Mumbai, Maharashtra, India

Starting your own Galvanizing plant for 15t/month is very unlikely to be economic. Even 15t / day might not be worthwhile!!
There is no "correct" level of Fe in flux. You might say that none at all is an objective, but an unrealistic one!
Your Powdercoating pretreatment is unlikely to be at all useful in pretreating for galvanizing. For Galv you need degreaser, rinse, HCl (possibly several for capacity), rinse, flux, zinc, passivate (optional).
Zinc kettles are made from special steel (there was a question on this back a couple months). Use the right steel and it might last 10 years, use the wrong and you might get only months life from it.

Your material is only 3mm thick you say. Have you considered using pregalvanized sheet then cut and form your articles? Do you need welding? If so, then this is not a good suggestion, but if you could press or fold from sheet, then you'd be able to get a mirror finish. It would not be so thick in Zn, but would be smooth. And far cheaper than building your own galv plant.

Geoff Crowley    galvanizing &    powder coating shop Glasgow, Scotland

Once again thanks. Starting a small unit was a shot in the dark -- obviously it's not a good idea!
We seem to have some luck in finding a few more galvanisers who are relatively better though not quite what we want. I guess we will have to work with them and improve the quality.
Regarding your suggestion of using pregalvanized sheets we have thought about it. Unfortunately we make the parts for someone else who has to approve the change and that is not an easy job! These parts are for brackets used in antennae that are exposed to all sorts of environment. They would obviously have sheared edges and holes punched in them, which can cause rusting to start easily. Also some of the brackets are welded, so in those parts hot dip is the only answer.What is the normal thickness of galanising for pregalvanised sheets?

Bomy Dabhoiwala
- Mumbai, Maharashtra, India

From you description of the use, pregalv sheet is NOT for you. Its thickness of zinc is often only 1/4 of that from Hot Dip Galv.
On these brackets you could expect a coating thickness of 50-80 microns (1 micron = 1/1000 mm), in pregalv sheet perhaps 15 microns, and as you say, not coating on cut edges, and welds uncoated.
Talk to your galvnanizer. Explain your needs. They CAN get them smooth, but it takes extra work. They might charge more for a smoother surface.

Geoff Crowley    galvanizing &    powder coating shop Glasgow, Scotland

Sir:
Because the product in the photo on the left has "spangle" (zinc crystals), I expect the lead in the zinc may be around 1% which is quite satisfactory. The photo on the right is quite out-out-of focus, however the "bad" spots seem to be circular. I expect the flux solution is not proper and is not "burning-off," or "cooking-off" properly. I have found many commercial fluxes in India to be of very poor quality. A mixture of 1.6 parts ammonium chloride and 1 part zinc chloride (e.g. quadraflux with chemical formula ZnCl2.4NH4Cl) would work much better. Each chemical should be 99%+ pure. Moisture content is not important.
Regards,

Dr. Thomas H. Cook, Galvanizing Consultant Hot Springs, South Dakota, USA