Electropolishing SS (again...) hexavalent chromium

[Munkul]

the rate of decomposition was shown to depend a lot on just how the stainless tanks were pickled and passivated. The tanks were made in the US, but in our lab tests, they were shown to be really variable one to another, some OK, some very poor.

We went out the States and it turned out that they had really detailed written procedures but the staff were not sticking to them; if they reckoned a tank didn't look right, they'd choose to process it a second time. Once they tightened up on the processing, we got good outcomes, but by then we realised the critical nature of stainless passivation and how it was going to be significant in a number of upcoming projects.

That's really interesting. If I had a bit more time I'd like to do some coupon testing to see the difference between medium rare and well-done pickling/passivating in terms of H2S/weak H2SO4 resistance.
(H2S + H2O makes weak H2SO4 and starts pitting into weak points especially in absence of O2 is what we find)
Even the metallurgical labs didn't suggest anything to do with passivation, even though it was clearly a HAZ problem that didn't affect large areas away from welds.

 

[Ross365]

I think that especially with the pickling stage, some pickling is good but too much may be very bad, so that comes down to times and temperatures (and % of HF).

I suspect the passivation step is much less critical. For your area of interest, are there no standard approaches in something like ASME? I seem to recall that when I was involved, we managed to acquire the in-house procedures of various US companies who had more experience with hydrazine, and then we formulated our own procedure, possibly based on the least harsh treatment used in the US, but my memory is a bit hazy on that. This was in the context of 300 series stainless, mostly 304L.

 

[Jonathan Webb]

Avesta pickling paste is the stuff

Haven't found an MSDS for it yet but the Avesta blurb states that Hexafluorine needs to be kept to hand. So I looked that up & it's a hydrofluoric acid countermeasure. Also states "Heavy metals from stainless steel are precipitated as a sludge, and should be sent for deposition according to local regulations" - that's back to my original question I suspect and all
getting a bit heavy, just to make parts of my boat a bit shinier...

 

[Ross365]

I'd never previously heard of "Hexafluorine", and according to this https://pubmed.ncbi.nlm.nih.gov/15461243/ it don't work. The standard safeguard used when using hydrofluoric acid is calcium gluconate cream https://www.steroplast.co.uk/calcium-gluconate-gel.html.

The calcium reacts with the HF to try prevent it attacking bones. The advice was to keep on rubbing it in until you're completely exhausted, and then keep on rubbing.... and see a medic. They will want to transplant skin off of your **** on to the burn, no matter how small the burn is.

 

[Munkul]

The labs suggested a more chromium rich alloy such as 904L (super expensive). We settled on super duplex which is far cheaper and has loads of chromium. H2S partial pressure is very low in our application, so SCC was ruled out.

I think that especially with the pickling stage, some pickling is good but too much may be very bad, so that comes down to times and temperatures (and % of HF).

I suspect the passivation step is much less critical. For your area of interest, are there no standard approaches in something like ASME? I seem to recall that when I was involved, we managed to acquire the in-house procedures of various US companies who had more experience with hydrazine, and then we formulated our own procedure, possibly based on the least harsh treatment used in the US, but my memory is a bit hazy on that. This was in the context of 300 series stainless, mostly 304L.

I am not au fault with any standards for pickling, although it sounds like i need to be. The problem with standard approaches we've looked at is that they are all based around NACE 0175 materials for sour service in O&G, which is a bit different to our industry, similar but different. We're processing different stuff at very different pressures and temperatures. CS2 is a bit of a niche chemical these days, there used to be a users group back in courtalds days but that's all gone now.

 

[Ross365]

there used to be a users group back in courtalds days but that's all gone now.

Ah, the days when British Industry used to have user groups or working groups . I was a member of a group that was focussed on the oxidation of the fuel cladding in the AGR's which used 20Cr 25Ni, niobium stabilised steel. Lab tests done before it went in to use gave results that showed lots of spalling of the (highly radioactive) oxide, but activity levels in-reactor were much lower that expected, suggesting spalling was not so bad. At the time when I left nuclear, nobody could explain why the lab data and in-reactor experience was so different, but thought it was very worrying.

 

[Misterg]

CS2 is a bit of a niche chemical these days, there used to be a users group back in courtalds days

Totally OT, but this tale was told to me by an ex Courtaulds employee. Not sure if it will stand being written out, but the original was told with a very heavy Welsh accent:

"Dai" was a sweeper upper or something similar in the Courtaulds factory in Greenfield, North Wales. One day, he went to the boffins and asked:

"This CS2... is it good for starting fires?"

The boffins winked at each other and said "Oh yes, VERY good."

"Well, you see, I live in an old stone cottage with my 80 year old mother and every morning she gets up early to make the kitchen fire so that the house is warm when I wake up. It's her birthday next week, so I would like to give her a surprise and have the fire going for her when she gets up"

"Would I be able to have some of this CS2 so I can get the fire going?"

So Dai ended up taking a bottle of CS2 home.

The next week, the boffins asked Dai how he got on...

"Well, I put everything ready the night before. Joe in the fitting shop gave me some newspaper so I didn't have to use Mother's supply, and Bill in the joiner's shop cut some kindling for me".
"After my mother went to bed, I let the fire go out and carefully cleaned the grate"
"I set my alarm clock early and crept downstairs to the kitchen as soon as it went off"
"The grate was cold, so I crumpled the newspaper and put the kindling on top of it."
"I didn't want to wake mother with the noise of the coal scuttle, so as quietly as I could, I put a few lumps of coal on top of the kindling."
"I poured the CS2 over the top - so that there would be a good fire."
"Then I struck the match..."

"The next think I knew, I was in the garden!"
"I looked back at the cottage and I could see into the kitchen because the door was off its hinges!"
"The whole kitchen was black with soot and I could see mother standing there"
"I couldn't hear anything because my ears were ringing, but I could tell she was crying..."

True story (Or so I was told)

 

[Munkul]

Totally OT, but this tale was told to me by an ex Courtaulds employee. Not sure if it will stand being written out, but the original was told with a very heavy Welsh accent:

"Dai" was a sweeper upper or something similar in the Courtaulds factory in Greenfield, North Wales. One day, he went to the boffins and asked:

"This CS2... is it good for starting fires?"

The boffins winked at each other and said "Oh yes, VERY good."

"Well, you see, I live in an old stone cottage with my 80 year old mother and every morning she gets up early to make the kitchen fire so that the house is warm when I wake up. It's her birthday next week, so I would like to give her a surprise and have the fire going for her when she gets up"

"Would I be able to have some of this CS2 so I can get the fire going?"

So Dai ended up taking a bottle of CS2 home.

The next week, the boffins asked Dai how he got on...

"Well, I put everything ready the night before. Joe in the fitting shop gave me some newspaper so I didn't have to use Mother's supply, and Bill in the joiner's shop cut some kindling for me".
"After my mother went to bed, I let the fire go out and carefully cleaned the grate"
"I set my alarm clock early and crept downstairs to the kitchen as soon as it went off"
"The grate was cold, so I crumpled the newspaper and put the kindling on top of it."
"I didn't want to wake mother with the noise of the coal scuttle, so as quietly as I could, I put a few lumps of coal on top of the kindling."
"I poured the CS2 over the top - so that there would be a good fire."
"Then I struck the match..."

"The next think I knew, I was in the garden!"
"I looked back at the cottage and I could see into the kitchen because the door was off its hinges!"
"The whole kitchen was black with soot and I could see mother standing there"
"I couldn't hear anything because my ears were ringing, but I could tell she was crying..."

True story (Or so I was told)

I believe it... Cs2 auto ignites at around 85 degrees. There's a lot of stories I've heard about it from over the years... Some of those stories rather more recent too.

 

[Jonathan Webb]

I don't know. I presume there's a degree of auto-passivation after removal of surface contaminants, but it also makes claims to leave a protective wax layer. Personal experience is that it gave lasting results on stainless fittings in a salt-water environment, for very little effort.

I found a tube of Autosol on the boat, so I tackled it amd made it nice and shiny. A week later, it looked like this...

 

[Ross365]

Just to prove that fitting is an austenitic stainless (like 316), I'd be interested for you to confirm that it doesn't attract a magnet.
That picture makes me think of what chrome-plated steel often ends up like.

 

[Misterg]

A week later, it looked like this...

Oh dear

Could anyone ever have used a grinder anywhere near it?