[pcm81]

Recently i've been curios about chemical rust removal and corrosion prevention via passivation. I have been playing with a ~12% by weight citric acid bath as well as a sulphuric + phophoric acid mixture bath.

What I got was not just rust removal, but i also developed a nice, even layer of magnetite on my steel parts submerged in citric acid bath. After a bit of digging i determined that my citric acid bath is essentially doing the job of steam bluing. In steam bluing gun smiths of the past would allow the gun to rust in steam, then boil it in water to dissolve haematite (Fe2O3), leaving just magnetite (Fe3O4) coating. I am guessing my citric acid bath is basically doing the same thing. Water reacts with iron, producing magnetite and haematite, then haematite dissolves back into solution by citric acid.

The problem with this or the traditional steam bluing method is that the thickness of the magnetite coating is very thin and limited by corrosion of the steel. Once magnetite layer is thick enough that steel can no longer rust the growth of the magnetite layer stops. The resulting thickness of magnetite is thin enough that even a small amount of abrasion can remove the magnetite layer.

This got me thinking... can I use an iron donor in my citric bath to continue deposition of magnetite into the part? Naturally the haematite will form as well but then it will dissolve back into citric acid solution. While dumping a powdered haematite into the solution would seem like the natural choice for iron donor, i am thinking that this additional, powdered haematite will be converted into magnetite powder but not magnetite coating on the part in the solution.

Anyone has any thoughts, ideas or suggestions? I am interested in learning the chemistry behind this process, so please do not suggest to just buy off the shelf blueing solution.

Thanks ahead

[Enthalpy]

On aluminium, the method to grow a thicker oxide layer is electricity. It's called anodization. I don't remember of it being used for steel.

On screws, the black layer is mainly FeO and Fe₂O₃ if not phosphate
https://de.wikipedia.org/wiki/Br%C3%BCnieren (the English article isn't the same process)
https://schraube-mutter.de/schrauben/schwarz-brueniert/
Not very protective against corrosion but very resistant to abrasion. Despite the huge contact pressure, it doesn't rip off. Phosphate give a better protection against corrosion.

I only wonder if any time is meaningfully invested in the corrosion protection of steel.

[pcm81]

On aluminium, the method to grow a thicker oxide layer is electricity. It's called anodization. I don't remember of it being used for steel.

On screws, the black layer is mainly FeO and Fe₂O₃ if not phosphate
https://de.wikipedia.org/wiki/Br%C3%BCnieren (the English article isn't the same process)
https://schraube-mutter.de/schrauben/schwarz-brueniert/
Not very protective against corrosion but very resistant to abrasion. Despite the huge contact pressure, it doesn't rip off. Phosphate give a better protection against corrosion.

I only wonder if any time is meaningfully invested in the corrosion protection of steel.

Living in Florida, corrosion is an annoyance to me, hence looking to find interesting ways to combat it. I have messed around with electoplating before and the problem with electroplating is dealing with odd shaped parts due to variations of current densities and deposition rates. Chrome plating is the hardest of the bunch because it requires 1.4A+ per square inch current density. Electroless nickel plating is so far my favorite, but also reasonably expansive way of protecting parts against corrosion. Hence the interest in chemical process rather than electrical process. Odd geometries and large surface area is difficult to deal with using electoplating for metal deposition. Also electro driven processes are a pain when dealing with large number of small parts.So far most of the acid bath bluing (blackening) processes i found produce a rather loose magnotite coating, only had 4 pieces, accidentally, get coated by black rust, that is durable enough for me to consider usable.  Been trying to reproduce that coating with no major success so far... The closest quality bluing to those 4 accidental rods i got so far is simple boiling in water, rather than any type of a chemical bath.