Galvanitic/electrolytic reaction

Hi

I have just ruined 4 cylinders for my latest engine while pickling them and I am hoping someone on here can tell me what went wrong

I had to silver solder some base flanges to the cylinders, so far so good

I then pickled them to remove the flux ect

Now at this point I should say my pickle is 20-1 water sulphuric acid, it was previously used to clean my allchin boiler throughout it's construction and was a deep shade of blue

I hooked the cylinders up with stainless wire and dunked them in overnight when I went back the pickle was fizzing with copper particles floating on it

Removing the cylinders revealed the devastation, severe pitting and even large parts of the steel eaten away

A long time ago someone told me that steel in contact with copper would cause the steel to rot

Can anyone on here shed any light on this, was there enough copper dissolved in the pickle to cause either galvanitic or electrolytic dissolving of the steel

I have ordered some new steel and don't want to make the same mistake

Thanks

Dougie

The most I ever pickle anything is an hour or two. 'Overnight' may be your problem.

Assuming your sulphuric acid was indeed, actually diluted 1 part in 20 parts water then cleaning your boiler would have neutralised some of the acid as it reacted with the carbonates, as hard water scale, in the boiler. The blue colouration at that dilution was due to the reaction between copper oxide residue left during the original brazing and not from a reaction with the metallic copper.
By now, your oriiginal sulphuric acid would be much weaker (in terms of hydrogen ion potential).

Perhaps oddly, mild steel does not react much with concentrated sulphuric since an impervious coating of ferrous sulphate forms on the carbon steel. However, when dilute sulphuric is used, the ferrous sulphate dissolves in the water component and steel corrosion proceeds rapidly.

How does that help you?
Don't use dilute (or concentrated !) sulphuric acid as a pickle after Ag-soldering mild/carbon steel.

The corrosion is much less with dilute hydrochloric acid and I confess to using that sometimes on Ag-soldered steel to soften flux but never leave the item immersed for more than a couple of minutes.

Safest is to use water and pick at the encrusted flux with a sharpened rod followed by a wire brush.

I've done overnight before but never seen this before

Dougie

Thanks Fowlers Fury

We Don have hard water here in Scotland so not much carbonates in the water

I won't do it again but I just wondered if the combination of stainless steel, the cylinders, en1a and the copper created the perfect storm

Dougie

As a professional steam boiler maker I use the same solution as the OP for copper, often leaving a boiler in there for several days. I donr put brass in as the process strips zinc and leaves a patchy copper/brass finish which looks bad...I digress, I put a boiler in the pickle a few days ago with a piece of hose and jubilee clip attached, When I took it out the acid the jubilee clip was completely rusted through. A combination of acid, water and contaminants makes short work of anything ferrous. I have used dilute citric acid for ferrous parys in the past but they go very black.

I soak parts I have silver soldered in boiling water to remove the flux.

I'm no chemist (last real chemistry at school, to university entrance level), so could be talking rubbish. I think your suggestions are likely correct, and the presence of metallic copper is the clue. Copper sulphate solution in contact with iron causes copper to be deposited onto the iron. Copper and iron are at different levels in the electro-chemical series, thus, with an electrolyte, make a cell. Your pickling solution was copper sulphate solution. Perhaps, at a nano-scale, metallic copper attached to the iron-containing substrate, and the resulting millions of electro-chemical cells caused accelerated corrosion. Hopefully, a real chemist will tell us what happened.

Edited By Kiwi Bloke on 01/10/2022 21:39:19 (typos, and more typos)

Edited By Kiwi Bloke on 01/10/2022 21:45:01

Edited By Kiwi Bloke on 01/10/2022 21:45:49

Sulphuric acid, which is a strong mineral acid will attack iron and steel fairly vigorously, even at quite large diutions. That's fairly basic chemistry. Copper sulphate solution is also bad news for steel but I think that the acid was the main culprit. Perhaps best to stick with fairly short immersion time in a weak acid such as citric acid, hot water or mechanical removal.

Concentration is the major culprit here.

Dilute sulphuric acid is extremely corrosive to steel, and corrosivity decreases with increased concentration due to formation of sulphate salts.

Using a 20:1 dilution of acid, with an acid which was probably less than 33% before you started diluting (I am assuming you didn't go diluting 98% suphuric, which is a dicey business), you're going to have a solution in the region of 1% - 1.65% which is in fact about the peak concentration for corrosion of steel.

When commercial operators use a Sulphuric based pickle for Carbon Steel, you'd expect to see a concentration of between 10-25% and a temperature of 50C - 100C, along with a short duration of pickle followed by an immediate rinse.

​

Which brings us to the minor culprit, duration of pickle, "overnight" is excessively long to leave any steel part in a strong acid (strong in the chemical sense of fully dissociated in water, not the colloquial sense) without repeated inspection and monitoring.

If you want to pickle steel overnight then a Phosphoric or Hydrofluoric acid pickle at room temperature would be more appropriate, or a poorly dissociated organic acid like Citric Acid (not Acetic/Ethanoic - AKA: Vinegar) at an elevated temperature.

Thanks everyone for the replies

Jelly, it was 98% acid that I have and used in the pickle, I won't use it again for any steei parts

One interesting thing is the flanges I soldered on were a different steel than the cylinders and they had little or no corrosion but they were not in contact with the stainless steel wire holding the cylinders

Thanks

Dougie

.

Forgive me for jumping-in please, Jelly … but is that [my emboldening] a typo ?

This is an extremely nasty chemical, and I would certainly avoid using it.

**LINK**

https://en.wikipedia.org/wiki/Hydrofluoric_acid#Health_and_safety

MichaelG.

The combination of Copper Sulphate and Sulphuric Acid is indeed something of a perfect storm when used to clean Steel.

Dilute Sulphuric Acid reacts with Copper Oxide (the gunk on a copper boiler), but doesn't react with pure Copper. Thus Copper items can be left in Sulphuric Acid pickle for a long time without trouble.

However, once used on Copper, the pickle is contaminated with Copper Sulphate and as explained by others Copper Sulphate and Dilute Sulphuric Acid both react with Iron, causing rapid corrosion. And unlike dilute Sulphuric Acid and Copper, the reaction doesn't stop when bare metal is reached.

So don't:

• Put steel items into Pickle contaminated with Copper, or
• Leave steel soaking in pickle. Steel must be removed as soon as it's clean and washed with clean water.

The reaction between Copper Sulphate and Iron is useful. Slightly acidulated Copper Sulphate was widely used in the past as a marking out fluid. Wiped over steel it leaves a thin sheen of Copper that highlights scratch marks. Works reasonably well except the liquid is toxic and corrosive; keep it off skin and don't spill or drink it! Blue dye is safer.

On a much larger scale, Copper Ore is tipped into a pond full of dilute Sulphuric Acid and allowed to stew. The resulting acidic Copper Sulphate solution is filtered into a second pond, where steel scrap is added. Pure Copper is precipitated as the scrap dissolves and Iron Sulphate can be sold too.

Dave

Dougie, I hope you have a EPP licence for your acid.....

https://www.gov.uk/government/publications/licensing-for-home-users-of-explosives-precursors/licensing-for-home-users-of-poisons-and-explosive-precursors

Robert G8RPI.

Here are some pics of the damage, note the base flange which looks ok

I have always used citric acid, much safer than traditional hard acids, Sulphuric and Hydrochloric, the only downside it is slower acting but hey us modellers have plenty of time. Dave W

Thats what I was taught to do, no acids and should only take a few minutes, just a wire brushing before rinsing and drying.

Edited By old mart on 02/10/2022 19:19:05

Anyone any thoughts on why the square flanges haven't corroded?

Probably the flanges are a slightly different alloy composition, and higher up the electrochemical series.

That was what I intended to post, as it's a very good picking and etching agent in the 0.25% - 2.5% concentration range and has wide applicability to steels (mild, carbon, alloy and stainless), aluminium and superalloys with excellent surface passivation properties post-pickle for stainless and aluminum.

I will admit that a career of familiarity with all manner of potentially nasty chemicals (including a period dealing with fluorine chemistry, including F2 gas) leaves me with much less horror of the potential risks than most people.. 

So any recommendation should really have been bookended with a warning to use it only in dilute form, read an SDS before buying it and take what is read very seriously.

​

HF does have some rather specific (but not quite unique) handling risks, but in dilute form is reasonably safe with similar precautions to any other acid...

The level of risk has been inflated in many reports due to the reputation as a "contact poison" (something which Phenol is in fact rather nastier for in my estimations), which catches the imagination in a way that other routes of exposure to toxic substances just don't. There's a good review paper here which does still read somewhat alarmingly, but no more so than an article about injuries from handling 98% sulphuric acid would!

Specifically It's only over 5% concentration that it's liable to exhibit systemic toxicity (though <5% could still cause a particularly nasty localised injury), and with anything other than pure HF you need a really big burn to cause systemic toxicity as long as you seek treatment.

To this end, you will find that many commercially available alloy wheel cleaners have 0.5% - 1% HF in them and are available to Joe Public, with no serious harm related to their sale having come to light (so far)!

Edited By Jelly on 02/10/2022 21:52:50