Stainless steel / DS40 / Suphuric or Nitric acid

[Softus]

Hello Chaps

Happy New Year to all of you.

I've tried and failed to remove a near-total blockage of scale from within a stainless steel secondary cylinder coil.

I've been using DS-40, and it helped to extract more than half a bucket of chalky deposits, as well as whatever was dissolved in the citric acid. The flow rate, which previously was nearly nil, is now better, but I know there's still a restriction somewhere deep within because the flow rate is still below par. The restriction is at least 1 metre from each end, because I can reach it, but not budge it, with a 1/4" sani-snake.

The owner is facing an horrendous bill unless I can dissolve and/or budge this blockage, so my question is this: is there something better/stronger/quicker than DS-40 that I can use?

For example, will Sulphuric acid be better? If so, where can I get it?

Or is Nitric acid better? If so, where can I get it?

Tia

 

[joe-90]

You want hydrochloric to shift chalky deposits. BTW - where you bin?

 

[Onetap]

Hydrochloric, I'd also think, but any of these are far more aggressive to the metal. I'd think you'd do better to fill the coil and pump the DS40 solution through it. It will take time, but will be safer.

Bubbles of CO2 form on the limescale as it reacts and these shield it from the acid solution. Pumping the solution around will remove the gas bubbles and speed things up.

 

[JohnD]

I have a weaker and slower approach as an alternative

If he gets an ion-exchange softener, such as a Permutit, on the supply, it will dissolve away the limescale. After a few days you will see it starting to come away with fragments out of the hot tap, over time it will all go. I have not done it in the situation you describe, but it cleaned my kettle, WC cistern and cold water tank in reasonable time. When my old copper cylinder was replaced, I looked down the Immersion hole and saw the outside of the coil was clean, but flakes of scale were lying at the bottom of the cylinder where they had fallen off the coil and the sides of the cylinder.

I have also used weak hydrochloric acid, which is effective. However I don't know what effect it might have on the Stainless. Some chemicals attack the protective tarnish layer, and allow it to corrode as fast as ordinary steel.

BTW the restriction you mention might possibly be flakes of scale that have formed into a bit of a plug, after falling off the inside surfaces.

 

[bengasman]

Fx 40 might be worth a try

 

[Softus]

Thanks joe-90, but Hydrochloric is a no-no with stainless steel.

Thanks Onetap - DS-40 is what I've been using. Initially the results were astonishing and therefore encouraging, but I have to call it a day now because it has stopped showing any signs of improvement.

Thanks JohnD - I can't justify the cost of a softener, and there's also a technical reason not to do it. This is a heat store with a coil outlet temperature of around 80ºC, which therefore has a blending valve in the installation, and any flakes or fragments coming loose would quickly and repeatedly block said valve.

The only remaining realistic options I'm aware of are cylinder replacement or stronger acid than a DS-40 solution.

bengasman - I'm interested in FX 40, but haven't come across it before, so am Googleing now...

 

[JohnD]

Is DS40 the yellow crystals one? I used that on an old boiler, and also a different Fernox chemical (forget what). One was intended for iron corrosion, and one for limescale. Agile suggested the two. One or other of them ate my jacket.

Edit: Fernox DS3 is the Limescale remover.

 

[JohnD]

http://www.fernox.com/files/Fernox/Content/PDF/English/DS3 2kg new.pdf

still got a tub on my garage shelf

my jacket:

View media item 391 - - - View media item 390

 

[bengasman]

...bengasman - I'm interested in FX 40, but haven't come across it before, so am Googleing now...

I meant fx2.

 

[Softus]

Ah OK. FX2 appears to be a Kamco powerflushing chemical, described as follows:

Blend of phosphoric acid, organic acids, water, wetting agents, corrosion inhibitors and methyl orange colour change indicator.
Phosphoric acid (> 34%). CAS no.: 7664-38-2
Citric acid CAS no.: 77-92-9

Also: "Due to acid nature, will cause burns to exposed tissue."

This and other health warnings sound suitably dire, so it might be exactly the immensely dangerously acidic thing I'm looking for

On the other hand, it might be no stronger or more effective than DS-40.

Does anyone know about Sulphuric or Nitric acid?

 

[JohnD]

see above about Fernox DS-3 and my jacket

 

[ChrisR]

Nitric you won't get - it's too easy to make explosives with it. It's also pretty nasty stuff.
Chlorides are bad for stainless steels but it's all relative. they don't dissolve before your eyes in sea water. Most of us use HCl in secondary heat exchangers, and I've not heard of one holing as a result.

Sulphuric acid shouldn't work - wrong sort of acid, but people find it does, on some things. It's worth a try. It's also seriously nasty. It'll go straight through your jacket, and you if you're wearing it.

FX-2 is a nice combination, not a problem if you splash your hands, though it'll remind you if you have a cut

Can you heat the coil - would help enormously, as would a good flow rate of course.

One other chem you might like is Kamco's Scalebreaker FX(?). It's Inhibited HCl. That means it's supposed to passivate metals(but not the salts you're trying to dissolve). Not ideal for stainless long term though!

 

[Softus]

@ JohnD - yes I'm truly sympathetic about your jacket - I'm sure Agile will get you another one. Well, I say I'm sure, but that's really just wishful thinking.

Anyway, DS-3 is [much] less powerful than DS-40, hence I have a dust sheet that makes your jacket look like a brand new ermine cloak.

@ ChrisR - good information, thank you. I have indeed been using heat, so I realise the benefit of it, but I've reached an impasse. Amongst all the various possibilities is that there could be a foreign object lodged in the coil at, say, a narrow point, and that this can't be dissolved. I'm dying to cut the thing open and find out, but feel that I really have to try something stronger than DS-40 before giving up.

I'm thinking about pouring in a bottle (or two) of strong drain cleaner, i.e. dilute HCl, just to see if it has an effect on the obstruction that I'm meeting with the sani-snake. I know that the coil goes downwards, rather than up, because both inlet and outlet are right at the top of the heat store, below the floor of the built-in F&E cistern, but I don't know if the loops are horizontal, which would be good, or vertical, which would require much more acid to reach the obstruction. I could, and should, drain down and remove an immersion element to find out, but am feeling lazy.

 

[joe-90]

If it's an obstruction could you try blowing compressed air from the 'wrong end'? (bit like when your vacuum gets blocked?)

 

[Nige F]

Hello Softus , good to hear from you again - I vaguely remembered Hydrochloric acid and limestone chips from school chemistry - where we used to sit @ benches with all the acids mentioned here in bottles on the bench in front of us couldn`t do that now : Anyway, hydrochloric is best but Acetic acid will do - so if you ask your local chippy nicely they might give you some of theirs : In fact the "vinegar" you get on fish and chips is this acetic acid solution diluted down - I`ve seen it done Also a retailer like T*sco can`t do that - it has to be labelled Non Brewed Condiment : Vinegar must be a brewed liquid - I`ve known that for years and just waited for it to be useful to someone PS also known as ethanoic acid