Patina Finishes for Steel
Introduction to Patinas
A patina is a surface coating on a bare metal surface formed by a chemical reaction. It is different from applied coatings like painting and powder coating in that the effect is caused by actually reacting a solution with the metal surface. Metals react with moist air on their own at a gradual pace. By using a blend of acids and/or salts, the process is greatly accelerated to a time frame that is manageable for decorative application. Also, there are non-reactive patina products available that either modify existing patinas or splash color on bare metals. One such non-reactive product is dye oxide patina.
Usually, when one thinks of putting a patina on metal they usually think of metals like copper, silver, and bronze. This is because these metal have the greatest range of reactive colors over the other metals. Unfortunately, the cost of these metals makes them practical for jewelry and not so much large artworks. Steel is the most widely used industrial metal and most inexpensive. If diligent, one can find steel even cheaper or even free in scrap form. As this article implies, there is still a good selection of finishes for steel besides the traditional rust finish.
Surface Preparation for Metals
If you are applying any finish to steel, whether it is a patina or otherwise, proper surface preparation is paramount. There are several ways to prepare the surface of steel. To get the desired results any combination of the methods mentioned here may be needed. There is mechanical debris removal with abrasives and wire brushes, acid pickle, and solvent cleaning.
There are two major kinds of mild steel, cold rolled and hot rolled steel. Hot rolled steel is shaped and rolled at high temperatures. Hot rolled steel has a black oxide coating called mill scale. To patina hot rolled, the mill scale must come off. A good and inexpensive way to accomplish this is to use an acid pickle consisting of equal volumes of water and muriatic acid. This could be hazardous due to the fumes and exposure so be sure to wear gloves and eye protection. Cold rolled steel just needs to usually be wire brushed followed by a wipe down with solvent. If the cold rolled is heavily rusted then it will more than likely need an acid pickle.
Abrasives and Wire Brushes
As far as abrasives are concerned, the most effective tool is an angle grinder with a flap wheel. Flap wheels come in a variety of grits can be had cheaply by bulk online. Another option is the sanding adapter kits sold for angle grinders. Like the flap wheels, the sand paper discs come in several grits from coarse to fine. They are beneficial as they have more abrasive surface area to work with.
Wire brushes also come as fixtures for angle grinders. Usually two configurations are found. One being a flat wheel for crevices and irregular surfaces, and cup brushes for flat sheet and plate.
When abrasives and wire brushes are insufficient to remove extensive rust or heavy millscale, an acidic pickle can be most helpful. One of the fastest acting pickle is muriatic acid. In most situations, it is best to dilute this powerful acid with water. In extreme cases though it might be best to dunk the steel in the concentrate right out of the bottle. Keep in mind this acid can blind you almost instantly, it burns skin, and the fumes are toxic with a suffocating odor. Also, when you are done pickling, you have to rinse with a lot of water to mitigate flash rusting because the steel will be in an activated state.
If fumes are a major concern, there is an acid that produces almost no fumes and converts rust into an insoluble primer base that can be painted. That acid is phosphoric acid. There are several brand name rust removers/converters on the retail market based on phosphoric acid. The principle of the reaction is the iron oxide in the steel corrosion products is converted to iron phosphate. This phosphate is relatively inert and insoluble and offers a barrier that can be painted over.
Solvent Cleaning of Steel
By far the most useful solvents for cleaning steel is acetone and denatured alcohol. These are the least expensive solvents but are excellent and removing oils and grease. These liquids are flammable so make sure to keep the containers away from ignition sources like welding equipment. Also, they mix with water in all proportions. This is helpful when water rinsing steel from an acid pickle. Acetone and alcohol will remove remaining traces of moisture then evaporate leaving dry metal.
Types of Steel Patinas
Types of Steel Patina
Warnings and Suggestions
Many cheap and accessible ingredients including: Salt, vinegar,hydrogen peroxide,chlorine bleach, muriatic acid, air, water, and sun
Bleach and muriatic acid are corrosive and toxic. Only bare,cleaned and decreased steel will rust uniformly
Gun bluing and other blackeners
Gun bluing and other commercial blackeners contain copper salts and other oxidants which are toxic. Clean shiny metal works best
Just heat and air
Bare and polished steel woorks best to give vivid colors. Be mindful of heat sources and hot metal
A slightly acidified solution of copper sulfate or copper chloride
Copper salts are toxic and harmful to the environment. Adhesion can be tricky. Thoroughly clean and degrease bare steel prior to immersion.
A rust patina is exactly as it sounds, a coating of rust oxidation on steel. One of the simplist ways to accomplish this is to leave bare steel outside in the weather. After a few weeks or months, depending on moisture and humidty, the metal will start an oxidation process eventually coating the whole surface.
If time is of the essence, then we need to speed things up chemically. Thankfully, most formulas for rusting steel can be made from readily available products. One very simple method uses just salt water and warm sunshine. Just repeated application of salt water and allowing the warm sun to work takes several days instead of weeks. If it is winter or you live in a predominently cloudy conditions, then perhaps a little more active solution is needed. A cheap but very effective recipe involves a mix of salt, vinegar, and peroxide. This solution is of low toxicity and fast acting. It gives the most intense chile red color on contact with steel. The proper proportions are in the recipe below. It is best to make a fresh batch each time. If you want more information, check out the instructable.
Vinegar/Peroxide Patina Formula
This is a formula derived from all easily accessible materials and is of low toxicity. Dissolve the salt in the vinegar and once dissolved, add the peroxide until total liquid volume is 16 ounces.
- 2 tablespoons of rock salt or kosher salt
- 2 ounces or 1/4 cup of 5% distilled vinegar
- 3% Drugstore Hydrogen Peroxide until total volume is 16 ounces
Other Rust Patina Formulations
The above paragraphs give a description of the easiest, most cost effective, and safest route to obtain a fairly consistent rust patina. There are numerous other ways to accomplish the same results but they require corrosive and/or noxious chemicals. One method is a strong acid like hydrochloric or sulfuric acid diluted to less than 5% and a trace of copper to accelerate rusting by galvanic corrosion. Bleach with peroxide is rather effective but you have to cover your clothing and eyes to keep bleach out. Also, the mixture fizzes profusely so you can only add bleach to peroxide a little at a time.
Gun Bluing on Mild Steel
There is a product called gun bluing that is sold whereever sporting goods are sold. Typical use is to give a handsome bluish black finish to gun barrels. It is a poisonous light blue acidic liquid that turns mild steel black on contact. It's active ingredient is selenium dioxide which oxidizes the iron to magnetic black iron oxide and also deposits elemental selenium. Apply to clean steel and burnish to desired results then promptly rinse. If you do not rinse this patina off thoroughly, the metal will start rusting under the patina. Also, it should be know that gun bluing is ineffective on stainless steel alloys.
Black Patina on Cold Rolled Steel
Birchwood Casey Super Blue For Bluing Mild Steel.
This is the gun bluing I personally use to blue and blacken mild steel. This is the most economical package. It can be used full strength or diluted with water. The finish can be burnished with fine steel wool to even out the color.
Your Ideal Steel Finish
What type of finish looks best on steel wall sculpture?
Heat Patina on steel
Heat Coloring of Steel
If you have a shiny and freshly poolished piece of steel, you can create an impressive pallet of color just by heating in air. What happens is the oxygen from the air plays upon the hot steel and starts forming oxides. The layers of oxides are thin at first and the transparent colors that result are an interference effect. The colors are difficult to control. Although you can use a torch, an even heat source is best. A ceramic support over an electric heating element or in an oven will give more consistent results. Placing a few drops of water or oil on the metal before heating can create some interesting patterns. Many of the blues, magentas and violets cannot be clear coated because of the thickness of the clear coat added to the oxidation changes it optic properties. The golden and bronze finsihes can be clear coated but will eventually darken with age. A periodic application of renaissance wax are recommended for protection but also the coating is thinner and would be helpful for preserving more delicate coatings.
Copper Patina on Steel
It is possible, with diligent care and preparation, to obtain a copper finish on bare steel. This is not the same as electroplating and is not nearly as durable. In fact the process is more like electroLESS plating or immersion plating.
Copper is less chemically active than iron, if you have copper in compound form dissolved in water, then some of it can be displaced by more active metals like the iron in steel. It is usually necessary to add a small amount of acid to the copper solution so to "activate" the steel surface by removing residual oxides, and assist in keeping the copper salt dissolved in the process.
Once applied, the copper patina solution must be rinsed with clean water and sealed by a clear coat at once unless the steel will rust from underneath. A 10% solution of copper sulfate with a few drops of sulfuric or muriatic acid added is a good place to start. There are also a few companies that market proprietary products that have been tested extensively.
Copper Patina on Mild Steel
Questions & Answers
Do you have any treatment suggestions that will reduce/eliminate corrosion of a steel structure that will be outside exposed to the Minnesota weather year round? This piece will be exposed to extreme temperatures (1000 + F) so I am doubtful that any clear coats would function well. Do you have any suggestions for the best solution to create an interesting effect and reduce corrosion?
Hopefully it is made from Corten or special weathering steel. If not, make sure no snow or water is caught in pockets or recesses. I would recommend blasting thing a torch until bright orange-yellow (2000 degrees F) to allow fire scale to form. Fire scale is just black magnetic iron oxide and forms a nonflaking oxide coating. A thinner black oxide can be had with gun bluing. Never leave bluing on steel and remove excess solution promptly or it may accelerate rusting.Helpful 2