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Topic: Let's make a rust spray  (Read 2193 times)

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Offline applez

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Let's make a rust spray
« on: May 30, 2018, 09:07:53 PM »
I work in construction and have noticed that the paintwork on our site vehicles are getting rust stained within a month of exposure to our groundwater.  The groundwater chemistry is saline (conductivity around 35000ms), pH ~ 5.7 and contains dissolved iron and manganese.  I actually like the look of rust staining (patina) on vehicle paintwork.  Best part is it can be cleaned off with a cutting compound or scourer pad if needed.

I am trying to replicate this type of effect with the hope of speeding up the process.  It would be great to spray a solution onto the paintwork that only takes a few days to show signs of a patina paint and rust effect.

Note:  This is not going to be like other rust activator liquids because we do not have an exposed metal substrate to oxidise.

My initial thoughts:
- Try to dissolve super fine 0000 grade steel wool in white vinegar solution.  The vinegar should strip the oil coating off the wool.  The problem is unless oxygen is introduced and thus oxidation occurs I can't seem to dissolve the steel wool.  I could possibly grind the steel wool down so it is super fine or add a certain type of iron tablet.
- Add some manganese like this https://www.bunnings.com.au/manutec-500g-manganese-sulphate_p2961522
- Dilute the mixture with salt (sodium chloride) and tap water solution.

I want the water in the bottle to be mostly unoxidised and then when it is sprayed and the water evaporates, oxygen is introduced and the iron / manganese should rust on the paint surface.

I've even considered heating up the car panel with a hair dryer before applying.

Your knowledge and assistance will be greatly appreciated.  I am not a chemist.  My knowledge in this field is limited to google searches. 

Thanks in advance  :)

Offline pcm81

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Re: Let's make a rust spray
« Reply #1 on: June 03, 2018, 05:02:20 PM »
To get what you want you are actually dealing with couple different problems:
1. Fe2O3 vs Fe3O4 formation (ratio determines the colour you get)
2. keeping the oxide layer attached to the part.

Short answer: Buy some iron oxide powders, mix haematite and magnetite in a ratio that gives you the look that  you want. Add the mixture to clear lacquer and paint it on.

Long answer:
Look into weathered steel. Basically a 1%-2% Chromium steel.It turns brown, but does not flake off like normal haematite would due to chromium content. Sounds to me this is the look you are after.But ofcourse that means you need to change the surface of the part to weathered steel and let it age.

Offline applez

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Re: Let's make a rust spray
« Reply #2 on: June 04, 2018, 07:48:05 PM »
So are you suggesting the rusting reaction will still happen on the car, this time with the introduction of salt water or similar as an activator?


Offline pcm81

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Re: Let's make a rust spray
« Reply #3 on: June 04, 2018, 09:15:12 PM »
So are you suggesting the rusting reaction will still happen on the car, this time with the introduction of salt water or similar as an activator?

Any time the iron is exposed to oxygen the rusting reaction will take place.
The difficult part is to form enough magnetite Fe3O4 to keep the layer of rust stable. Haematite Fe2O3 is a lower energy state, so very slowly even magnetite will transform to haematite.

Usually when rusting process happens quickly because of abundance of oxygen, moisture (moisture adds oxygen... or elevated temperature you will form haematite, not the preferred magnetite. Cleaning the surface of oils will expose the virgin metal causing it to rust faster. gasses like chlorine will accelerate rusting process as well ( a cup of HCl next to the part will rust the hell out of it).

Your goal should be to clean the surface and then play with oxidation accelerators to form the correct ratio of haematite to magnetite to make it visually appealing without compromising the strength of the deposit. Rust it too fast and you get loose flaky deposit. After you created the colour you want, dry it up with a low heat source, say hair dryer and oil it up to seal up the pores and stop further rusting.

Offline applez

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Re: Let's make a rust spray
« Reply #4 on: June 04, 2018, 09:36:08 PM »
I appreciate you working with me on this :)

I have built some old vehicles using this paint https://www.dulux.com.au/products/designeffects/overview/rust-effect 

It paints on dark grey to black and then the activator is light green to blue in colour. 

What I don't like, is it's still a paint.  It can be brushed on and is designed to stick like a paint.  It doesn't look like a natural patina effect as its built up. 

If the glue/paint holding the haematite/magnetite could dissolve away.  Any ideas on what may work as a glue in that way?

What would you use as an activator (salt water / vinegar mix) or even dilute the Hcl in water until the pH drops to low 5's say?

Thanks

 

Offline pcm81

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Re: Let's make a rust spray
« Reply #5 on: June 04, 2018, 09:57:53 PM »
Simple answer:
play with dilute HCl or salt+vinegar solutions and find the correct rusting parameters for your environment. Temperature, humidity, cleanliness of the part, alloy of the metal etc will affect the rate of rusting, so i can't tell you exact parameters for the mixture... Start dilute, say 1% hcl and build up.

Research rust bluing: https://en.wikipedia.org/wiki/Bluing_(steel)
that is basically the process you are after if you want to do it by hand and not a "paint" finish.

The main key thing to remember: the faster you rust it the faster it will fall off.
Look at some videos on rust bluing on youtube. You will see that they "card" the part to knock off loose rust. The longer you spend between cardings or faster you rust it, the more of the produced rust will get carded off and go to waste. Brushing actually works better than carding, but the common term is carding.Usually people are after the black finish which is mostly magnetite. Your goal for brown deposit is to produce more haematite but still have enough magnetite to hold the layer together.

Offline applez

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Re: Let's make a rust spray
« Reply #6 on: June 18, 2018, 12:49:54 AM »
Thanks for sharing your knowledge.  I've ordered the powders.  As soon as they arrive ill be working on it.

Cheers  ;D

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