[Bucket]

Hello to you all.
I'm an enthusiastic "model maker" and left school a very long time ago.
My knowledge of chemistry reached the zero point.
Not because I didn't like it, but just by not using it anymore.
At the moment I'm busy making a self propelled bucket dredger (scale 1:25)
Including two engines and steam boiler.
Of Course she will be radio controlled and therefore almost fully self controlled in the engine room.
Using different sensors and a PIC, all self designed.
After learning how to use a 3D drawing program, I managed to get a dredge bucket to be printed.
That resin-print-model is used to create a mold in heat resistant silicone.
In that mold I poured liquid ZAMAC (Alloy of zinc,aluminium,magnesium and copper) and created a wear resistant "riveted" model bucket.
The weight is about 70 grams, capacity 27 mL.
Major problem is that the buckets are shining like cutlery.

Using:
Kalium Polysulfide (K2S3) someone advised me
or
Ferro 3 chloride (FeCl3) anotherone advised me
Did not work.



On the left the test, right the untreated.
It looks like the outer scin is to "hard", the parts that I worked on with a file became gray.
In the barn I had some pickling paste for stainless steel, consisting HNO3 and HF.
Drowning the two zamac parts  for 20 seconds was enough to "clean" everything.
After rinsing in clean water, I brushed the black ("dust") from the surface.
After that I put both parts in the Ferro3chloride and left it for 2 hours, nothing happened.
Then one part I put in the Kalium polysulfide for 10 minutes and after that again in the FeCL3 for another hour.

This is where I got lost (I did not write down what axactly I did and for how long).
The second part eventually became nicely brown (iron rust look).



Does Kalium polysulfide have any influence?
Is it necessary to add some piece of iron in the FeCL?
I did have a test with some steel glued on the zamac-bucket and for me it looked better/faster to color the zamac.
(the steel is part of the chain, connecting the buckets to eachother, so it has to be on the bucket anyway)

Can someone please explain what is the proper way to color the zamak in the way that it look like very old rusted iron?
In a way that I can reduce the time getting the result (the last part 5 hours, and I have to do 51 buckets)  and more important: That I know what I'm doing and why.

[Burner]

Not sure with the chemistry behind it.
However, out of curiousity I googled 'iron rust paint' and found something that maybe useful to you.
https://www.google.com.hk/search?q=iron+rust+paint

[Intanjir]

For an authentic look, you could electroplate a nice layer of iron metal and then just rust however you like.

Here is a very nice overview of electroplating with iron:
http://www2.bren.ucsb.edu/~dturney/port/papers/Modern%20Electroplating/11.pdf

[Bucket]

Thanks for answering.

Burner: thanks, I'll check that out.

Intanjir:
Electroplating (electrolysis?) I have tried yesterday.
I only used a pre used bucket (it has been in the HNO3 and was pretty oxidized).
A lot of iron was transported from a piece of scrap-iron.
I could lift the bucket with a magnet.
Only problem was that the new layer of iron did not stick to the zamak.
I made a rotating device to spread the iron more equally over the surface.
Next I try with an new zamak bucket which only has been degreased.








cracks touched with my fingers


after cleaning with toothbrush



I'll study the PDF you gave me.

[Borek]

Making metals stick to each other during electrolysis can be challenging. Know way of dealing with the problem is to plate your objects first with another metal that is sticky to both what is below and what is needed on the surface. From what I remember copper is often a good intermediate - but this is based on the reading I did thirty years ago in Polish (Galwanotechnika domowa by Stefan Sękowski if memory serves me well), so don't expect any details from me.

[Intanjir]

Nice pictures!
I think that starting with a clean piece of zamak might be enough.
Iron and Zinc should be fairly compatible with one another.
You might also try roughening up the surface. As it is your buckets are fairly shiny and thus rather smooth on small scales.
If it isn't enough then a good intermediate would be to electroplate iron and zinc simultaneously as an alloy.
The pdf has a couple such compositions that they use for galvanizing steel.

BTW, electrolysis is the same basic set up but the goals and conditions are different.
In electrolysis you want to break up 'lyse' H₂O while keeping your electrodes pristine.

[Bucket]

Yesterday (my time)  I cleaned the zamac in vinegar for 15 minutes and after that the water didn't make wet "isles" and dry places on the surface. The bucket was completely wet.
The bucket was a rotating cathode.

Half the side of the bath (not heated ferric chloride) was a steel plate, connected to the +.
Amperes flowing this time 0.5. Duration 1 hour.
The zamac turned gray and was only weak magnetic.
Next test will be for a longer time.

Any ideas how to rust the "new iron" layer, without eating it away completely from the zamac.?
In the pdf they wrote: "Electrodeposited iron is relatively resistant to corrosion, as would be expected from its high purity".


Test in the rain/snow.


My rotating device.

[Intanjir]

Ah that is the beauty of this set up.
If your current sacrificial iron electrode has the right sort of 'rustiness' to it then you can probably duplicate that effect by just reversing the polarity. 

Alternatively spritzing with hydrogen peroxide + dilute acid and letting it dry and repeating would work.

Iron has a very intricate collection of oxides and hydroxides and oxide-hydroxides.
Greens, yellows, oranges, reds, browns, and blacks are all possible.
 
Whatever method you use will initially make some kind of iron hydroxide or oxide-hydroxide.
You can fully convert it to the corresponding oxide at 200°C.
This should still be red if it started out red and I would think it would prove somewhat more durable.

[Bucket]

Almost ready.

[billnotgatez]

We are looking forward to your report.