High Temperature Coatings for Steel

[russellcollins]

I've been hunting all over the net for clarity on what to use as a surface coating for our range of stoves made in India that meets the following criteria:

1. High Emissivity
2. High hardness at temps up to 600+C
3. Non toxic for direct food contact
4. Non toxic with regards to off gassing
5. Able to be applied in a facility in India (where we produce).

The steel to be coated is HRS (Hot Rolled Mild Steel) although we could switch to CRS (Cold Rolled) if necessary as that apparently has better bonding qualities. 

Does anyone have any tips on what coatings are being used on commercial combustion heater products? It's hard to find any clear answers on this point. I've tried a range of high temp powder coatings and paints, and all have failed the hardness test at 500-600C. 

I am wondering about plasma coatings and ceramic coatings, trying to find out whatever the industry is using to get that nice matt black finish that is apparent on most heating products on the market. 

Any ideas will be greatly appreciated, 

Russell Collins
Himalayan Rocket Stove

[Orange]

It think it would be hard to get "Non toxic for direct food contact".

I'm also interested if modern heat-resistant paints don't have any off gassing?

If not than I'd rather leave bare metal or paint it with mud or woodash/coal.

[fuegos]

" I've been hunting all over the net for clarity on what to use as a surface coating for our range of stoves made in India that meets the following criteria.
3. Non toxic for direct food contact"
I'm assuming that you're talking about a cooktop Russel ? something safe for cooking flat breads? cast iron would be a good solution although the cost of production is higher than steel plate. Maybe just an insert made of cast iron in a steel top ?

[Deleted]

www.google.com/search?client=firefox-b-d&q=geopolymer+coating

[Orange]

found one more problem - new steel is coated with some gray stuff whichs smells when heated.

It can't be removed with chemicals or burnt away so I guess only thing that works in grinding.

[josephcrawley]

Apr 17, 2021 11:14:28 GMT -8 Orange said:

found one more problem - new steel is coated with some gray stuff whichs smells when heated.

It can't be removed with chemicals or burnt away so I guess only thing that works in grinding.

en.m.wikipedia.org/wiki/Mill_scale

[Dan (Upstate NY, USA)]

It's pickled

[Orange]

thanks, I had to grind that down:



@karl is there some simple solution regular people can use? For example painting metal with waterglass?

[Deleted]

Pretty simple:
To become resistant to high temperatures waterglass must be combined with something of high alumina content
like kaolin clay or very fine grog.
See also: donkey32.proboards.com/thread/3287/basic-kaolinite-clay-geopolymer-chemistry

[Orange]

ok, I don't know if "chamotte powder" will work, it's made from fireclay and grounded clay tiles? It's the most common ingredieant for high-temp mortar when mixed with waterglass in 3:1 ratio.

and I'll have to check how well it sticks to metal, probably better than just clay.

[Deleted]

In the USA grog seems only to refer to fired and grinded kaolin clay of te highest alumina content.
Chamotte or Schamotte may be made of any kind of clay with low or high alumina content eventually containing
significant amounts of other metal oxides.
with a similar thermal expansions coefficient like the iron it is applied on it will work, else not.

[fiedia]

there are commercial protection products made out of linseed oil: for example.

I used pure linseed oil. It works perfectly, no rust, no smell. Just wipe very well before the first fire. Otherwise, you will get drops like below:

[Orange]

that linseed oil really looks good, I'm just unsure how all those paints behave at higher temps:

for above example here's Linseed Oil Paint SAFETY DATA SHEET:

Initial boiling point and boiling range Appr. 316 ºC
Flash point 216 ºC
Self ignite temperature 343 ºC
...
Emission Factor, Volatile organiccompounds, TVOC: 64 ?g/(m 2 xh) after 4 hours? drying time of linseed oil paint (white paint), 18 ?g/(m 2 xh) after 26 week drying time.

I guess all that is only relevant when oil isn't dry: www.youtube.com/watch?v=Cw5MqfrxM9g

fuegos had it tested and it obviously didn't boil or combust, only thing is unsure if it still off-gasses.

[Orange]

here's the test I did:

left: steel + waterglass
right: steel + waterglass + chamotte powder

they behave the same.

heated up to ~200C, booth look ok and stick well to metal (I guess it's not off-gassing):


heated up to ~400C, white thing formed and it peels off easily so it can't stand higher temperatures:

I may try that kaolin clay instead of chamotte powder..