Sciencemadness Discussion Board

Any ideas to passivate stainless steel against chlorine?

Sidmadra - 16-9-2018 at 08:18

I have some stainless steel tubes/fittings that I want to pass hypochlorite/chlorine solutions through, and I've been trying to research ways on how to passivate them.

I've read an industry technique is to treat the stainless steel in nitric acid, which removes surface Iron, leaving a Nickel and Chromium oxide layer, but I then read reports that simple chloride ion will decompose/dissolve this oxide layer. I heard a suggestion of electro plating stainless steel with Silver, but as far as I am aware, Silver will react with chlorine.

Coating with polymer isn't an option due to the intricacy of metal parts. Is there any sort of tricks that could be used to protect stainless steel in this manner? Any platable metals whose oxide layers are highly resistant to chlorine?

12thealchemist - 16-9-2018 at 08:56

How feasible and how robust would a manganese dioxide coating be? I know one can make manganese dioxide-coated electrodes; and this is chlorine-resistant - Would this an idea worth considering?

Sidmadra - 16-9-2018 at 09:12

I don't think Manganese Dioxide would be ideal because I can think of several different things that would dissolve it, such as sodium bisulfite. I need the coating to be resistant to a number of different compounds. Any organics or salts, except acids really.

Magpie - 16-9-2018 at 09:37

I once worked in a paper mill with stainless steel headbox and fourdrinier. Passivating was done with nitric acid.

unionised - 16-9-2018 at 10:05

I suspect that hypochlorite would attack manganese dioxide.
I don't think there are any better options than nitric acid passivation unless you pay silly money.

What size is the project?

Sidmadra - 16-9-2018 at 11:09

I don't mind spending a bit of money for a setup to figure this out. I have tons of stainless steel or other metal things that I've wanted to coat over the years so that I could use them in reactions.

I read that bleach can attack steel that has been passivated by nitric acid, however these were anecdotal accounts. In a study I read that Nickel Oxide does not react with chlorine until about 350 c, and that is one of the main components in nitric acid passivated steel. I'm not sure about Chromium Oxide which is the other main component.


I've also considered some sort of electrostatic dipcoating. If I could dipcoat with a PTFE suspension and just cure it, that would be most ideal, but ensuring the particles stick on the inside crevices of parts could be tricky.

UC235 - 16-9-2018 at 11:13

This is why glass-lined reactors are a thing in industry. Titanium and tantalum are also used.

unionised - 16-9-2018 at 11:43

Given the OP's saying "Coating with polymer isn't an option due to the intricacy of metal parts." I guess glass lined stuff is not going to work.

Tantalum is one of the things I had in mind as "silly money"
https://www.ebay.co.uk/itm/Tantalum-metal-sheet-plate-ribbon...

Titanium isn't as bad but it's still pretty eye watering.


j_sum1 - 16-9-2018 at 12:52

You could try anodising to get a nice heavy oxide coating.
It would be simple enough to test the idea.

CuReUS - 17-9-2018 at 06:42

according to pg 60 this book - https://library.sciencemadness.org/library/books/ignition.pd... , stainless steel was passivated against fuming nitric acid using a flouride coating.Maybe the same thing could protect against Cl2 ?

Melgar - 17-9-2018 at 08:18

I like the idea of anodizing it.

Nitric acid seems to be the main way they do it in industry, so it's probably your best bet.

Silver will react with chlorine, but the solubility of silver chloride is extremely low, so silver chloride will actually form a passivating layer against further reaction. The only trouble would be that it's be extremely difficult to silver-plate stainless steel, and probably pointless.

It seems like if you're going to be running a solution through it that's a high pH, you shouldn't have much to worry about. Any metal will corrode to some degree, but parts should last on the order of years, even with continuous use.

wg48 - 17-9-2018 at 11:48

High phosphorous electroless nickel plating is used to protect pipes in the chloro alkali process. Stainless steel probably requires special preparation because of its oxide film. If you are in the USA the usual hypophosphite containing electroless nickel solutions would be a problem for you as amateurs can not purchase or own it legally I believe.

If nitric acid passivation works that would be a lot simpler.

Sidmadra - 17-9-2018 at 15:56

Quote: Originally posted by wg48  
High phosphorous electroless nickel plating is used to protect pipes in the chloro alkali process. Stainless steel probably requires special preparation because of its oxide film. If you are in the USA the usual hypophosphite containing electroless nickel solutions would be a problem for you as amateurs can not purchase or own it legally I believe.

If nitric acid passivation works that would be a lot simpler.


I didn't know about the process, I will look into it. I speculate there are other means of achieving this electroless nickel plating. Upon a cursory youtube search, there seem to be a ton of videos of random people doing this electroless nickel plating... I will report back if I figure something out.

Edit: Do you have a reference for the claim that electroless nickel plating is used to protect pipes in that process? Nickel reacts with chlorine, so how does that work? Does the electroless process form an inert alloy?



Apparently there are websites that sell "mid-phosphorous" nickel plating solutions. Doesn't look like they refuse individuals based on the amount of youtube videos I saw on the process:

https://www.caswellplating.com/electroplating-anodizing/nick...


It seems odd to claim mid-phosphorous, when the phosphorous plating content depends entirely on the ratio for how the two chemicals are mixed, and they sell the two required chemicals unmixed. Perhaps it is mid-phos if you mix how they instruct, but a higher ratio could be achieved by adjusted the mix.


[Edited on 18-9-2018 by Sidmadra]

macckone - 17-9-2018 at 16:47

Hastelloy C276 is a workhorse in industry.
Others are Monel 400, Lead - sulfuric acid handling, Titanium, PTFE - 200C limit and Glass/Ceramic.

Hastelloy B is the most resistant metal to hydrochloric acid.

In a chlor-alkali plant, everything exposed to chlorine gas is plastic. Same with the sodium hypochlorite process tanks, all plastic.

The sodium hydroxide side is metal.

Sidmadra - 17-9-2018 at 17:26

Quote: Originally posted by macckone  
Hastelloy C276 is a workhorse in industry.
Others are Monel 400, Lead - sulfuric acid handling, Titanium, PTFE - 200C limit and Glass/Ceramic.

Hastelloy B is the most resistant metal to hydrochloric acid.

In a chlor-alkali plant, everything exposed to chlorine gas is plastic. Same with the sodium hypochlorite process tanks, all plastic.

The sodium hydroxide side is metal.



Hastelloy B sounds nice, but everything online tells me it is just a metal alloy, and to my knowledge, it is not possible to plate a metal alloy onto another metal. I'm not trying to fabricate metal parts from scratch here lol.

With paint, you are right that many plastics are going to resist chlorine or hypochlorite and the such, however, resistance to organic solvents at elevated temperatures is almost never a thing unless the it is a thermal-set flouropolymer, or another crosslinked polymer like crosslinked polystyrene.

In the case of polymer coatings, coating the inside of a 1/4 inch tube isn't exactly straight forward. I see no issue with spray-coating a crosslinkable styrene solution, the issue is this doesn't really get into tight crevices well.

Vinylogous - 17-9-2018 at 19:41

I've been mentally chewing on this problem for a while.

It really boils down to your use case and engineering constraints. Does it need to only last for a few hours of cold solution exposure? Just use stainless steel off the hop. Weeks of continuous operation of scalding hot bleach and chloride? You might wanna refactor to a glass system.

Putting a thick layer of nickel plate (roughly, you nickel strike bath with nickel chloride and then electroplate with NiCl2 with some NH4Cl and boric acid) will give you a fair amount of resilience as long as your solution isn't too hot.

Also unless someone knows of a way to source it that I don't, you are going to pay out the nose for anything Hastelloy in small quantities.

macckone - 18-9-2018 at 06:19

Ps. PVC, CPVC, PVDF, and PTFE seem to be the preferred plastics. Here are the operating temperatures. PVDF is flexible where the others are not.

PVC - 140F
CPVC - 200F
PVDF - 120F
PTFE - 260C (500F)

macckone - 18-9-2018 at 06:29

reading other posts, if you need 1/4 tube, get some ptfe tubing. it is more cost effective that trying to coat the inside of 1/4 stainless.

I agree that if you just need it to survive one run, just go stainless. CPVC is going to be cheaper than coating a significant amount of stainless steel.

If you are chlorinating hydrocarbons, your only real choices are PTFE or glass and the chlorinating chamber needs a quartz window for UV with the rest of the chamber being glass as the reaction is VERY exothermic.

If you want a full process solution, you need to give us the full process. But it sounds like you want the apparatus to be multifunction and that may simply not be possible.

Sidmadra - 18-9-2018 at 13:27

I'm going to try to find a company that sells PTFE lined metal tubing. A lot sell PTFE lined steel braided hoses, but I don't need thick hoses. Tubing itself is the difficult part out of everything I want to do. I am somewhat confident in my ability to PTFE coat pipe fittings, and larger pipes. Smaller tubing requires a different approach. That is true I could just get some PTFE tubing and then 1 size up metal tubing, and then afix one inside of the other. Then just use that with a compression fitting. I should have thought about that earlier, since most tubing sizes are standardized so their OD/ID's can slide in and out seamlessly.

DavidJR - 19-9-2018 at 01:58

Is there a reason you can't just use PTFE tubing directly without a SS outer layer?

Sidmadra - 19-9-2018 at 13:26

Quote: Originally posted by DavidJR  
Is there a reason you can't just use PTFE tubing directly without a SS outer layer?


I want pressure resistance of up to 300 PSI which I don't think PTFE tubing alone can handle.

nux vomica - 19-9-2018 at 21:37

We have had the internals of small stainless steel mixing tanks and parts that have to be corrosion resistant and non stick sent out to be ptfe coated .
Link to some information
https://www.chemours.com/Teflon_Industrial/en_US/products/selection_guides/coatings.html

Fulmen - 19-9-2018 at 22:20

Quote: Originally posted by Sidmadra  
A lot sell PTFE lined steel braided hoses, but I don't need thick hoses.

What's too thick for your use? Braided PFTE should be available in fairly small diameters (5-10mm), I have a couple of meters of the stuff om my bike (brake and clutch line).

AJKOER - 22-9-2018 at 20:35

Perhaps the concept of a sacrificial anode may work here.

Try inserting a thin aluminum rod in the tubing.

Install it so that it can easily be removed.

I am assuming that the corrosion process is basically electrochemical in nature.

One potential downside is that OCl- seating in the tube will be breakdown to chloride (a variant of the bleach battery with iron in place of copper being the noble metal and the NaOCl decomposing to aqueous NaCl with a precipitate of Al(OH)3). Also periodically aluminum rod will have to be replaced.

[Edited on 23-9-2018 by AJKOER]

draculic acid69 - 24-12-2018 at 02:46

Apparently there are Teflon based paints you can use to coat steels that will give chemical resistance I think someone said it was in one of festers books.