How to blue steel (and, how to remove zinc electroplating before bluing steel)

AKA - a deep-dive into the nuts and bolts - and science - of making

It’s Science Friday! Well, every day is a science day. But I’m sharing a bit more detailed account of something very sciency today, and it is Friday, so there. Enjoy (if this is your geeky type of thing). ~JRC

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Zinc-plated, bare, and blued. Science in action.

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If you're a curious person, you've undoubtedly been here too. You start out trying to learn one thing only to realize you need to do something else first, which leads you down a rabbit hole searching for something you didn't even know you needed to know - only to resurface again, ready and more knowledgeable, but nowhere near where you wanted to be when you started in your initial pursuit. 

This very thing happened to me as I explored the dark & mysterious, chemistry-laden world of steel bluing. 

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"Bluing" is the process of oxidizing steel to create a durable, corrosion-resistant layer. Metal parts that look black or blue-black are usually blued (rather than painted). Drill bits and gun barrels are common examples. There's also blued hardware available, like nuts, bolts, screws, and hinges. 

Bluing is accomplished in two ways: heat bluing - heating the metal and then submerging it in oil, and cold bluing - using a chemical solution that darkens the surface at room temp. Either way, bluing protects the metal and increases its application and versatility. It's an oxidative process that coats the steel with a thin layer of iron oxide (technically magnetite or "black rust," as opposed to yellow and brown rusts). The coating is very thin and smooth, making it ideal for steel objects that move or cut things. Best of all, bluing doesn't continue to rust like other iron oxides do and thus protects the steel underneath from corrosion. 

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I wanted to understand the process better, so I gave heat bluing a try. My initial attempt worked - sort of. I knew I needed clean steel, but I wasn't aware of how clean.

At first, I did only a quick brush with a Scotch pad and a wipe with acetone before heating. While some of the metal blued after using this method, it was blotchy and inconsistent. There was too much of something in the way. And it was shining right in front of my eyes.

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Caution: tangent ahead - removing zinc electroplating

Problem: all shiny silver-looking hardware has been zinc coated to keep it from rusting. Raw steel is grey metallic in color, a lot less bright, and rusts easily. Since the zinc coating is an anti-oxidant and bluing is oxidative, we have, as stated, a problem. Simply rubbing it off as I tried isn't enough; there's always some left.

Solution: zinc reacts with acid. Hydrochloric works well but is dangerous and overkill. Weak organic acids, like vinegar and - better still - citric acid - work fine and are far safer. 

You can use vinegar to remove zinc, but it takes forever. Use citric acid instead. It's what makes lemons and other citrus sour, hence the name. 

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Citric acid is available in powder form for baking and cooking. When exposed to zinc, it reacts, forming a zinc salt (zinc citrate) and hydrogen gas. This chemical reaction "eats" up the zinc coating, leaving the exposed steel. 

There are lots of YouTube videos and written how-tos on zinc removal using citric acid. But none of these (at least the ones I saw) gave any actual specifics on how much to use. Some instructed utilizing a couple of tablespoons per gallon. Another video recommended as much as a cup. The worst tutorials said to use a few scoops in a bucket (say what?). And all that I watched and read indicated that if it wasn't working, add more citric acid powder. 

None of this sat well with me. I'm a scientist, after all, not some guy fumbling around "experimenting" in his garage (oh wait, I'm that too). No, these imprecise concentrations were causing me great consternation.

Thankfully I did okay in chemistry class. Not great; just okay. But this isn't life-or-death research here, so it'll do.

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A fundamental truth in chemistry: reagents interact in known ratios. These chemical formulas and equations balance out like other math problems if you know all the values. What this means is there is an ideal citric acid to zinc ratio. All I had to do was uncover it. 

I'll spare you all the details, but I found a primary research paper on this very subject. It turns out there's a lot in play with this reaction (alas, it's organic chem, not simple arithmetic). Too little citric acid, and the reaction is slow (as expected). Adding more helps, but there is such a thing as too much with citric acid and zinc. 

At a point, adding more citric acid will slow the reaction, even more so than with too little in solution. The reaction basically stalls because (and I quote), "sorption capacity of the solid surface...may sterically protect the solid surface from proton attack and hinder the dissolution rate" (read more on this enthralling finding in Boukerche et al., Environmental Research & Technology, Vol. 1 (3), pp. 11-18, 2018). In other words, too much citric acid will get in the way of citric acid doing its job eating up the zinc. 

It's like a bunch of people crowded around a buffet, but no one's eating because they're all in each other's way. Or something like that. 

The authors conclude that 0.5 M (molar concentration) citric acid is ideal for zinc removal. 

Now, I won't pretend to understand all this paper shared. And no, it's not published in Science or Nature or even a well-known lesser journal. But at least it's peer-reviewed and recent. Merits aside, the paper bestowed what I was most looking for - a definite concentration I could test (instead of this "a scoop or two in a bucket" malarkey). 

My task next was to convert molarity into a real-world concentration I could apply.

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Citric acid comes in the monohydrate form (citric acid and an attached water molecule) with a total molecular weight of 310.1. This translates to a molar mass of 310.1 g/mole, so a 0.5 M citric acid concentration is half the molar mass or 155 grams in a liter of solution. Back in the day, while a lab rat, I would have stayed metric and made up the solution. 

But this is America, and we do things differently here (and I'm in my garage and not an actual lab). So I converted this concentration to standard volumes for the average DIYer on this side of the Atlantic. I came up with 1.3 oz (weight) per cup (volume). Handily, I have a scale and measured out the 1.3 oz of citric acid. It equals a very shy 1/4 cup. 

So, after all this, I can now share the "ideal" concentration of citric acid for zinc removal in a way most anyone can apply. Americans: It's slightly-less-than-a-quarter-cup citric acid per one cup water. Everyone else: see above.

Me? I've since tested it, and the concentration works like a charm.

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Back to our regularly scheduled program: bluing steel

Soak zinc plated parts in a 0.5 M citric acid solution before bluing. It takes about 30 minutes to remove all the coating, give or take. Cheaper bolts (like those from Harbor Freight) de-zinc quickly. Quality hardware is better coated and will take longer. Note that some parts will also have oil or grease on them. Remove this first with a solvent as it gets in the way of the reaction. 

You'll know it's working well if you see a steady stream of bubbles rising from the parts; that's hydrogen escaping. Yeah, SCIENCE!

Next, pull the now-bare steel from the bath, but don't touch it. Instead, grab it with forceps or a spoon and pat it dry with a clean cloth. After that, blue away. 

• Heat: torch it until it starts to glow lightly, and drop it in oil (carefully).

• Cold: brush on the bluing solution, wait about 30 seconds or more, then rinse off the excess with cold water. Repeat as needed. 

Wow, that was a long haul to get some parts blued. 

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Lesson learned here? To do a job right, do what it takes to clean your parts first, even if this means taking a deep dive into the nuts and bolts - and science - of making.

Until next time.

JRC