Galvanising small objects

[Neeves]

Thanks Geoff,

It sounds possible but a bit messy. The problem is the temperature, I knew about the working temperature of 450 but not the fact the steel dissolves in the zinc at just above 500 degrees. Finding an old steel saucepan (which was my idea) of low silicon content (chemical analysis not listed in our department stores!) looks unlikely. If it were to be regular small lots then a small kiln, maybe pottery, would be the answer - but its then getting much more sophisticated.

A reason for the question - our local friendly galvaniser charges around Stg35 for a small piece (he is charging for the nuisance factor rather than the work or zinc used) - maybe I need to try a different galvaniser.

Armorglav (or Sherardizing) is conducted here in rotating, electrically heated, cylindrical ovens, a bit like long ball mills of various sizes. Operating temperature is lower than HDG, around 400 degrees C (needs to be lower than melting) but HT items are being conducted at lower temperatures, 370 degrees. It is said the process can operate at even lower temperatures, but I have never heard of it being done and I have to assume processing time must be extended. Costs are similar to HDG, with whom they compete directly. Armorgalv cuts out pickling, they sand blast, and there is no flux. Painted items, much chain is supplied painted as are some 'off the shelf' components, and this did cause some issues - ideally one specifies unpainted or uses paint stripper.

Jonathan

 

[Heckler]

I suspect that a rotating drum is a sophisticated improvement. The original method was a box with a bit of carbon and zinc. The rotating drum presumably avoids uncoated areas where the object was standing.

Vyv, Ive posted this before. I used to have an aluminium smelting business. Melting zinc with the ali used to be a big no no! All the scrap industry was aware of it. Apparently when it got to the point where it burned it produced some sort of poisonous gas that killed people. Ive googled it but cant find anything now.
Stu
Ah here is something http://www.anvilfire.com/iForge/tutor.php?lesson=safety3/demo
and here
https://en.m.wikipedia.org/wiki/Metal_fume_fever

 

[Neeves]

Zinc sublimates, vapourises before it melts, and this is the basis of the sherardising (and now Armorgalv) process. But the idea is to keep the vapour in the container such that it condenses and reacts with the items being processed. Apparently there is a limit to coating thickness, around 250 microns, so new ovens are simply filled with zinc powder and coated to the maximum thickness - thereafter any vapour only coats and reacts with the steel of uncoated items. Sheradising originally used sand in the drum and silicosis was the major fear - working with hot silica sand being the no-no rather than the zinc vapours. (I'm not saying zinc vapours were considered safe - just that silica was further forward in people's minds).

Because the process relies on vapourisation of zinc, from a powder, if you know the surface area of the items to be coated you can calculate fairly accurately what weight of zinc you need to add to obtain a fairly precise coating thickness. My chain was meant to be 100 microns and we managed an even coating of 95-110 microns. If you check HDG chain - there are huge variations from one supplier. And of course gal thickness is not part of chain specification (to the consumer), it is not convenient for the customer to measure and strength of adhesion (of the gal to the chain) is another issue.

Strangely - anyone who re-gals chain almost without exception says the re-gal chain coating is better than the original.

Gal determines chain life (not strength - which appears to be more than adequate) - its interesting it has not been included in the advertised specification for the consumer.

Jonathan

 

[JumbleDuck]

https://en.m.wikipedia.org/wiki/Metal_fume_fever

There have been a couple of nasty accidents resulting from people welding old point rodding - which is very heavily galvanised - used as a cheap source of steel framing.

 

[JumbleDuck]

Zinc sublimates, vapourises before it melts

That sounds unlikely, since the triple point of zinc is at 419.50 °C and 0.00064 atm. Sherardising is certainly a vapour deposition process, but I can't see how you would get sublimation above the triple point.

 

[Neeves]

The operating temperature is at 400 degrees C (and it may be 397 - I'm unlikely to be given all the trade secrets) as already mentioned. I don't know about sheradising but the zinc powder used for Armorgalv contains a catalyst (undefined) which presumably modifies the theory (or for purists - makes the theory not valid for the specific process).

Jonathan

 

[JumbleDuck]

The operating temperature is at 400 degrees C (and it may be 397 - I'm unlikely to be given all the trade secrets) as already mentioned. I don't know about sheradising but the zinc powder used for Armorgalv contains a catalyst (undefined) which presumably modifies the theory (or for purists - makes the theory not valid for the specific process).

I suspect it's just molten zinc powder being smooshed into vapour by the other stuff. Sublimation seems most unlikely.

I'm trying to track down an iron-zinc equilibrium phase diagram and a zinc phase diagram, but they are proving curiously hard to find.

 

[Neeves]

I suspect it's just molten zinc powder being smooshed into vapour by the other stuff. Sublimation seems most unlikely.

I'm trying to track down an iron-zinc equilibrium phase diagram and a zinc phase diagram, but they are proving curiously hard to find.

I think maybe you need to look at the Sherardizing and Armorgalv processes - operating temperatures are 'well' below melting point (for example my chain was coated at 370 degrees C). The process is called 'thermal diffusion galvanising'. Maybe my use of the word sublimation is incorrect - but its shorter than saying the zinc metal vapourises without the zinc melting and condenses and reacts with the steel being coated. I think my use of the word condenses might also be wrong - but its as close as I can get. My understanding is there is no liquid phase for the zinc (or anything else in the oven) in the process. Its a dry process. I believe there might be a number of operators in the UK of sherardizing but the only one I know is Bodycote - and they are operate the Armorgalv process under licence (as does the operator in Oz) and I think Bodycote call the coating 'sheracote'. I've seen reference to sheradizing being operated in China.

The US Navy are keen and have been coating the chains they use on assault craft to tie down military vehicles, tanks and the like on landing craft. Their spec is 70-80 microns - but the application is different to anchor chain, obviously. There is a chain coating facility in Norway or Sweden. Because it operates at low temperatures it finds particular application with Q&T HT chain (and other HT components).

But JD - if you nail down the process, not sublimation - let me, us, know

Jonathan

 

[vyv_cox]

Vyv, Ive posted this before. I used to have an aluminium smelting business. Melting zinc with the ali used to be a big no no! All the scrap industry was aware of it. Apparently when it got to the point where it burned it produced some sort of poisonous gas that killed people. Ive googled it but cant find anything now.
Stu
Ah here is something http://www.anvilfire.com/iForge/tutor.php?lesson=safety3/demo
and here
https://en.m.wikipedia.org/wiki/Metal_fume_fever

As far as I remember, it was a long time ago that I studied this, the box was steel. I do recall the emphasis that the box was closed - sounds like good advice.

 

[JumbleDuck]

I think maybe you need to look at the Sherardizing and Armorgalv processes - operating temperatures are 'well' below melting point (for example my chain was coated at 370 degrees C).

Very interesting. You have piqued my curiosity and I shall investigate. Perhaps thermal diffusion simply means that the hot zinc dust adheres to the steel (via formation of an intermetallic?) and then sinters to form the protective layer. In other words, it could be happening without any vapour being involved.

 

[Neeves]

The holders of the licence, for Armorgalv, and the operator here tell me its a vapour process - I take them at face value. There is a, I think, similar process called Greenkote - but it is a mixture of zinc and aluminium metal powders (again plus pixy dust) - and closed rotating ovens

But there are number of simple references to zinc metal being easy to sublimate - though I cannot find anything that might tax your background.

The coatings appear to be ZnFe alloys, which are said to be harder than the alloys formed from HDG. Certainly the tests I've run suggest this is correct and if the coating is thick enough, or of a comparable thickness to HDG, it will have greater longevity.


But returning to the original thread - home galvanising does not want to be conducted in an aluminium vessel!

Jonathan

 

[JumbleDuck]

The coatings appear to be ZnFe alloys, which are said to be harder than the alloys formed from HDG.

I think the whole point of hot dip galvanising is to form an iron-zinc intermetallic at the interface.

 

[huldah]

My first job was working in a small factory making stainless steel tanks. They then took on a job of welding a 30 foot tank made of 1/4" galvanized steel. The factory was filled with white smoke. The welder was unwell and was given free milk. I do not know whether this helped.

 

[MM5AHO]

Sheradizing isn't really a vapour process, it's a solid phase diffusion into the steel. It forms alloy totally (no coating of "pure" zinc as in galvanizing), so it will be a harder coating that the average hardness of galvanizing. But standard galvanizing also has these same alloy layers, at the steel interface, just that they comprise less of the coating's thickness than in sheradizing where they are all of it.
Heating zinc beyond its melting point - way up around 800C (I forget exactly) it then fumes forming copious quantity of zinc oxide. The most common sight of this is in welding galvanized steel, where dense whits "smoke" comes off. It's not good for humans to breathe. (Though quite good for the skin). It's a common old time remedy for welders to drink milk before and after welding galv. The sickness produces when breathing this fume used to be called "Galvy flu".

The lowest cost way to get small quantities of perhaps small items of steel galvanized is to find a friendly fabricator, and add you items to his next order. The bigger the fabricator, the lower the price he's likely paying, and so you should get a good rate without a minimum order charge. It's true and understandable that galvanizers charge a lot for a tiny order. The cost of raising the invoice can exceed the income at a standard rate.

On Sheradzing using blasting to clean steel, that's true and while it might be thought to be cheap, blasting typically costs more that acid pickling. That's why galvanizers don't blast everything, they acid pickle it. It costs far less. A main reason for blasting costing a lot today, is the cost of compressing air. An air compressor is about 20% efficient (80% of the input comes out as heat), and that combined with today's environmental controls, means high costs. Acid pickling isn't suitable for high carbon high tensile steels. (risk of embrittlement).

 

[Neeves]

Geoff,

its possible (they say) to coat voids with Sheradizing or Armorgalv as long as there is an orrifice, tubes closed at one end, boxes, hollow beams. How do you get the zinc into the empty interior space to effect the coating.

Jonathan

 

[MM5AHO]

I imagine that the hot zinc powder that surrounds the steel item within the rotating drum will end up with zinc inside through the hole. In a similar way to HDG, where provided holes into the void, the various liquids can enter and alloy with the internal surfaces. I've once opened up some experimental hollow items (not customer's work!), to see how effective the coating is internally, and was pleasantly surprised. The turbulence created by the drum rotating holding the zinc powder and items being coated should be enough to get the powder inside.

 

[Neeves]

Sherardizing and the Armorgalv process have some similarities but are different. I asked the owners of the Licence for Armorgalv for a quick comment on what actually happens. This is their response:

The zinc alloy we developed causes the Zinc to sublimate in the process retort at temperatures just over 250C, well below the melting point of Zinc. The Zinc vapor or Gas, reacts with the steel to create the Zinc/iron alloy.

The rest of the email described their marketing success and I did no think that would be so acceptable on the Forum.

The Sherardizing process required the zinc metal to be mixed with silica sand and I can understand that there may be, or is, some solid state diffusion (though if zinc sublimates so easily then I cannot see why it is also not part of the Sherardising process).

Jonathan

 

[JumbleDuck]

The zinc alloy we developed causes the Zinc to sublimate in the process retort at temperatures just over 250C, well below the melting point of Zinc.

Absent actual evidence, I simply don't believe that. Sorry. I smell marketing, or possibly just confusion. ArmorGalv themselves think that they have a thermal diffusion process (http://www.armorgalv.com/about) - the idea that it's vapour diffusion comes only from their Australian licencee (http://www.armorgalv.com.au/page15957/Home.aspx), as far as I can see.

The Sherardizing process required the zinc metal to be mixed with silica sand and I can understand that there may be, or is, some solid state diffusion (though if zinc sublimates so easily then I cannot see why it is also not part of the Sherardising process).

In general, nothing sublimates easily at temperatures or pressures above its triple point which for zinc is, as I wrote, 419.50 °C and 0.00064 atm. Water, as so often, is an exception as it will slowly sublimate at most pressures as long as it's below its triple point temperature of 0.01°C. It would take actual evidence to convince me that zinc could be persuaded into the gaseous phase at 1 bar and 250 °C.

 

[Neeves]

The quote I provided came from Distekna in America who hold the license and is owned by the developer of the process. I would guess the Australian's, who are independent of Distekna other than they run the process, are simply repeating what they have been told.

It may be marketing (though I fail to see why it makes any difference as to the why when the users are interested only in the end result). So fabricating something that could be disproved that actually has no impact on the end product looks to lack logic and would impact negatively if proved otherwise.

I have had some items coated with a 20mm x 3mm hole drilled through - the internal surface has been coated (though I cannot measure whether to 100 microns) it would be difficult to achieve this with powder.

If you can prove otherwise I'll query with them or provide you the contact - but saying you don't believe it has as much logic as their saying that's how it works. Whilst I do understand where you are coming from - they have been running the process for years now - and if the statement were wrong I am sure someone would have mentioned it. But come up with the evidence that what they say is rubbish and I would be interested in their reply.

I'm not sure that is matters - as Geoff has mentioned the end result is a layer of Zn/Fe alloys, no raw zinc, which can be fabricated to thicknesses in excess of 100 microns, is harder than the HDG coating, operates at a lower temperature (so good for HT steels) and can coat intricate shapes with precision. They cannot coat very large items, the ovens are too small.

It appears to be working very well with my 6mm G80 chain - but I have only been using it for 15 months now - in 5 years time I will have a better idea. Making as I have done saved a large amount of money in comparison to buying locally made 8mm G30 (and it is stronger, lighter and (the big question mark) should last longer).

Jonathan

 

[JumbleDuck]

The quote I provided came from Distekna in America who hold the license and is owned by the developer of the process. I would guess the Australian's, who are independent of Distekna other than they run the process, are simply repeating what they have been told.

I think they may be repeating what they think they have been told. I have had a look at Distek N.A.'s rather amateurish website, and they seem to be thoroughly confused about whether they are selling a version of Sherardising or a vapour process. The ArmorGalv website does not appear to mention vapour anywhere.

I have had some items coated with a 20mm x 3mm hole drilled through - the internal surface has been coated (though I cannot measure whether to 100 microns) it would be difficult to achieve this with powder.

Why not? Powder can get into all sorts of tiny spaces. In Sherardising, for example.

If you can prove otherwise I'll query with them or provide you the contact - but saying you don't believe it has as much logic as their saying that's how it works.

I really don't have anything to prove - it's they who are making claims which appear to contradict both the inventors of the process they use and basic materials science. Of course I am open to be convince, but the idea that they can get zinc in a vapour state at 250 ^oC and 1 bar is, on the face of it, rather unlikely.

Please note that I am ot doubting that the process exists, or that it provides good protection ... just that it involves sublimation.

I'm not sure that is matters - as Geoff has mentioned the end result is a layer of Zn/Fe alloys, no raw zinc, which can be fabricated to thicknesses in excess of 100 microns, is harder than the HDG coating, operates at a lower temperature (so good for HT steels) and can coat intricate shapes with precision. They cannot coat very large items, the ovens are too small.

Does the Zn-Fe intermetallic produce the same cathodic protection that zinc does?