" Tannin baths, commonly using tannic acid, interact with iron and steel to form a protective, blue-black ferric tannate layer that converts active rust into a stable, corrosion-resistant surface. This process, acting as a natural chelating agent, adheres well, improves corrosion resistance, and is used for conservation and surface modification. Key Effects of Tannin Baths on Metal:
Corrosion Inhibition/Passivation: Tannins act as inhibitors, especially on steel and iron, by forming insoluble iron-tannate complexes that deposit onto the metal surface, forming a protective, dense film.
Rust Conversion: On rusted iron objects, tannic acid reacts with iron ions to turn rust into a black-blue, stable protective layer, halting further corrosion.
Surface Modification: Tannin treatment turns iron surfaces dark blue, gray, or black, which is highly desirable in art and textile applications (e.g., printing and dying).
Corrosion Rate Regulation: While tannins reduce corrosion by forming a protective layer, the effectiveness depends heavily on the concentration and the pH of the bath, with lower pH often required to enhance passivation in certain environments.
Metal Affinity: Tannic acid has a high affinity to chelate with metals, particularly iron, but can also work with aluminum alloys and other transition metals.
Common Application:
Surface Preparation: Before treatment, iron objects should be cleaned of dirt and grease.
Concentration: A typical tannic acid solution used for treating iron objects is around 2% to 10% weight/volume, often applied in water.
Disclaimer: Tannin treatments are often used to stabilize rust, but extreme concentrations or high pH can alter the coating's effectiveness."
* FROM THE ABOVE, WE CAN GO TO using phosphoric acid in place of tannic acid for it's different effect, when treating rust, or to combining tannic and phosphoric acid to take advantage of the properties of both, which is what many commercial suppliers do.
* Both tannic acid and phosphoric acid are easy to come by. The former can be made by just soaking, for example, oak chips to extract the tannins. Metal workers of old used other items to produce their tannic acid too. Even pomegranates.
*SIDE NOTE: There are some good quality commercial products too. Some both alter the rust and coat the product. Project Farm did a review and the two spendy versions worked really well (Eastwood's encapsulator and O_____).
I keep a bottle of Corrosion X around. It's pretty remarkable stuff.
Car buffs used and may still use Penetrol, and additive for oil based paints sold in paint stores or paint sections of stores, to get into and protect hard to reach places.
When I mis-spent my youth painting cars, one step after getting down to bare steel was a wipe down with phosphoric acid. It gave the surface a slight etch for primer adhesion and did any surface rust conversion. Popular for truck frames was similar products to get the rust converted before further paint work. Permatex "extend" was the product to use for that from a rattle can. Now there are a lot of others that do the same. Turns the rust into a black coating that wont flake off.
When you take on the identity of Miss. "Graffiti" Spent, I hope you're referring to your own car(s)... tagging is a greater pandemic here in Victoria (Aussieland) than C19 was.
If your first cut is too short... Take the second cut from the longer end... LBD
Heck, for "Antiquing" metals it's da bomb, been doing that forever. I guess I wasn't aware of all the other cool stuff one could do. I always like cheeep and easy, and it's that.
Sounds like a great way to prevent rust. I looked up if it would it work on cast iron table saws and band saws.
AI said it would work. If you wouldn’t mind a blue gray color to your surface. I wouldn’t mind it. It’s a lot better than constantly cleaning off the rust especially in humid places.
Splinter. Is the Art resistant to city Graffiti abatement crews?
Good write up Kelly. I have used phosporic acid on a lot of vintage tool restorations. It's simple to use and dispose of and the conversion process from iron oxide to iron phosphate is excellent corrosion protection.
I have stopped using it on anything that isn't painted though. It leaves a really dull gray patina that I don't care for on things like hand planes.
Yep. Too many solutions require a tolerance for less than pretty results.
I keep a bottle of Corrosion X spray around. Treating my cabinet saw top with it, I could set a coke can on it and not worry about the condensation leaving rust within just a few minutes, as would happen without it.
I took my table top to a mirror surface using some of my worn granite polishing pads and mineral oil. I was way purty. However, I sprayed it with Corrosion X, making it safe from rust, but all the mirror like finish was gone immediately.
And, I think that paint over treatment thing is a must. It's a bit like straight hardening oil on wood. For durability, it needs the help.
