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DIY pickling of steel?

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klugesmith

Thu Jul 23 2015, 08:39PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Has anyone here had to remove black scale from a forged part, or from off-the-shelf HRS material?

I'd like to improve the smoothness of some electromagnet parts (3" round and 1"x4" rectangular HRS bars) before painting.
Could bang on them with a hammer and scrape away loosened stuff, but want to consider a chemical solution (NPI).
I have read that industrial pickling these days uses hydrochloric acid more frequently than sulfuric acid. In spite of HCl more rapidly attacking the bare steel, not to mention the effect of its fumes on the building roof and walls and workers.

The strong acids on hand are HCl ("muriatic acid") sold for swimming pools etc., H2SO4 sold for clearing drain pipes, and HNO3 used by my late mother-in-law to make fine art etchings.

Hazmatt_(The Underdog)

Fri Jul 24 2015, 04:00AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

I was unable to grind off the black scale of a steel plate. Everything I attacked it with was just useless.

I don't recommend this, because you could kill yourself, but I used my torch with a rosebud to get the steel hot, then brushed on 15% HCL and it worked. But I have to reiterate, it is nasty shit and if you don't understand what you're doing, you will probably end up in the hospital.

So yes, I have done it, I won't do it again, its too dangerous. I'll use stainless next time.

Ash Small

Fri Jul 24 2015, 10:03AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I was going to suggest phosphoric, but then I came across this:

Houdini0118

Fri Jul 24 2015, 07:52PM

Registered Member #8558 Joined: Thu Dec 06 2012, 11:38PM
Location: usa
Posts: 57

Get a cheap angle grinder and a masonry grinding disk i hear that works well and i know flap disks work as well

doctor electrons

Sun Jul 26 2015, 01:46PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

If the part is small enough you can put it in a tumbler over night with an abrasive media ;)
Its common in most all of the foundries I've worked in.

hen918

Sun Jul 26 2015, 06:22PM

Registered Member #11591 Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556

Houdini0118 wrote ...

Get a cheap angle grinder and a masonry grinding disk i hear that works well and i know flap disks work as well

I've used flappy disks to remove that kind of scale, and whilst it works, the disks don't last long.

Bored Chemist

Sun Jul 26 2015, 08:58PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

I would have gone with dilute sulphuric acid. there are pros and cons to any choice.
These people seem to know more about it than I do.

klugesmith

Tue Jul 28 2015, 12:07AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Thanks for the tips.

I'm happy to report that pole piece 1 lost almost all of its black scale, and maybe a little bit of steel, after room-temperature soaks in vinegar & then dilute HCl. Got a story and a slide show about it. Should I repeat the successful process on pole piece 2, or try H2SO4?

Here is part 1 with its pickling tank and lid, after an overnight soak in vinegar. The tank (a 32 oz floor-wax bottle) fits so closely that 4 fl oz (120 ml) of liquid is enough to submerge the workpiece. SOF! Vinegar had turned some ordinary rust on the machined surface into a slime that could be wiped away with a fingertip.

This close-up shows a bird shape in surface after vinegar and wire brushing.

Then I broke through a solid region of black scale in about 20 places by hitting it with the back of a handsaw.
Next, mixed 1 part of muriatic acid with 4 parts of water. Covered the workpiece with half of the dilute acid. When I had to leave after about 50 minutes, there were loose black flakes in the acid. I temporarily stored the bar under water in a bucket. Later, after wire brushing, it was in this state:

After another hour or so in the same partly-depleted acid, the visible end of the bar was covered with tiny bubbles. After rinsing and scrubbing, the cylindrical surface near that end looked scale-free.

The bar rested for a few more hours in the bucket of water, before a two-hour soak in fresh acid. Then rinse, scrub, and neutralize overnight in straight household ammonia. Here's how it looked this morning, next to both batches of spent acid.

One patch of black remains, perhaps where the work had been touching the tank wall.

It's worth noting that the water in storage bucket turned from black to brown overnight. Did the suspended black particles settle, or change to a different oxide of iron?

Ash Small

Tue Jul 28 2015, 01:53AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Might be worth heating the vinegar, the problems I read about with HCL and H2SO4 is reduction in size of workpiece.

I also read that all other acids apart from HCL need heating.

It may be too much to expect vinegar to remove black scale, it was a metalwork forum I found that link on, not a foundry site, but if it can get underneath the scale and is rejuvenated when depleted (volume of acid wasn't large in your floor wax bottle) and heated , given sufficient time, it may remove the scale without reduction in size of the workpiece.

Hazmatt_(The Underdog)

Tue Jul 28 2015, 03:05AM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

If you want it to shine, you're going to have to polish it. From the looks of it, it's already pretty clean. If you require further cleaning, then I'd say you're looking at several hours with the 600 grit sand paper, and a lot of buffing with coarse buffing compound.

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