Anodizing is an electrolytic process in which a protective layer of oxide is created on the surface of a metal. When steel is anodized, it's called anodized steel.
However, the presence of carbon in steel can complicate the anodizing process.
What is Anodized Steel?
Anodized steel is electrochemically treated steel for a decorative finish. The process of anodization creates a protective oxide layer over the surface of the steel which can be dyed with different colors. Depending on the type of acid and duration of oxidation the color varies.
This article describes the process of anodizing steel and tips on how to improve its durability.
Anodizing Steel: What's the Challenge?
Steel composes of around 2% iron and carbon. Based on the carbon content, steel is classified into many types: stainless steel, carbon steel, free-machining steel, tool steel, etc.
Iron content in steel causes the formation of ferric oxide (rust) when it reacts with the oxygen ions in the electrolyte. Ferric oxide easily flakes off from the steel surface and aggravates corrosion.
Hence, the anodizing method is altered by using an electrolytic bath of sulfuric acid, chromic acid, or boric-sulfuric acid, and a layer of aluminum or zinc is applied to the steel to prevent ferric oxide formation.
Though it looks promising, it has an underlying problem.
When anodized, a galvanic corrosive layer is formed between the bonded surface of steel and aluminum. It occurs due to the differences in the corrosive properties of these metals.
Galvanic corrosion occurs on metal surfaces when different metals react in an electrochemical reaction.
How to Anodize Steel: The Process
Steel can accumulate a passive oxide layer in the presence of caustic solutions like sodium hydroxide, potassium hydroxide, calcium oxide, etc.
Before anodization, the steel part is rinsed in deionized water, followed by rinsing again in methanol and air drying using compressed air.
Steel is placed in an electrolytic solution having 50% caustic solutions like sodium hydroxide (NaOH) or potassium hydroxide (KOH).
Here, the part to be anodized is connected to the positive terminal (anode), and the negative terminal of the power supply (cathode) is connected to a scrap steel piece.
Supplying power, and maintaining optimum operating conditions of hot, concentrated stirred solution results in the formation of an anodic oxide layer on the steel surface.
The adherent oxide layer formed is magnetite (Fe3O4), an iron ore.
Faster growth of the oxide layer is observed at higher temperatures and higher voltages.
Based on varying oxide thicknesses a wide range of colors is observed. The colors can range from tan and brown to light orange and pink.
The color exhibited by the oxide layer is dependent on its thickness. You can optimize its thickness by choosing the right temperature and voltage.
Apart from that, you can also use anodizing paint to add color to the anodized steel similar to the process of painting anodized aluminum.
However, all these factors directly affect the cost of anodizing, and it is crucial to optimize them according to your application to minimize cost.
You can use a similar process with different process parameters to anodize other non-magnetic metals such as aluminum, gold, and bronze.
Techniques to Enhance Steel's Properties
Anodizing is only one of the many strategies used to improve steel's properties.
Though anodizing enhances durability and corrosion resistance, the same can be achieved for steel through other methods.
Passivation is a chemical treatment process commonly used for stainless steel and aluminum alloys.
A thick metal oxide is formed on the surface of the metal, which acts as a protective layer. This increases the corrosion resistance of the treated surface.
The process removes iron from the surface of the steel and provides a uniform passive surface that increases its durability.
You can perform the passivation method in many ways. The steel can be immersed in nitric acid, a passivating bath, circulation of the chemical solution over the metal, or by spray application.
Passivating helps remove surface contamination and increases the metal's corrosion resistance.
Phosphatization is a chemical treatment process in which an adherent layer of zinc, manganese, or iron phosphate layer is formed over a metal.
A dilute solution of iron, zinc, and phosphate salts is applied via spraying or immersion to form a layer over the base metal.
The phosphate layers offer good corrosive resistance and a base coat for subsequent coatings.
Electropolishing or anodic polishing is an electrochemical process in which a thin layer of material is removed from the steel surface to obtain a smooth, shiny surface.
The metal to be polished is connected to the positive terminal (anode) and immersed in a highly viscous electrolytic solution of sulfuric and phosphoric acid.
When the current is passed, a thin layer of material (from the anode) gets released. As a result, the ions from the steel plate get dissolved in the electrolyte.
Blackening is a conversion process in which the base metal is converted into a thin layer of adhered coating.
In this method, steel is immersed in a hot bath of nitrates, sodium hydroxide, and nitrites to form a thin black oxide layer over its surface known as magnetite.
Advantages of Anodized Steel
Anodizing steel in caustic solutions causes the formation of magnetite instead of ferric oxide, which triggers rusting.
The magnetite layer reduces the corrosiveness of steel, allowing it to exhibit better corrosion resistance in both deionized and salt water.
The oxide film magnetite (Fe3O4) offers a better protection layer when compared to coatings of other metal oxides. Thus, the adherent layer offers a better base for the bonding of coatings.
Apart from that, the anodic coating also enhances the aesthetics of steel workpieces and can be used to attain a smooth surface finish on steel workpieces, especially for forged steel workpieces.
Improved Abrasion resistance
Anodizing in the presence of caustic solutions improves the adherence of magnetite to the steel, offering good abrasion resistance.
It must be noted that anodizing is the final process that should be performed after finalizing the machining of the steel workpiece.
Limitations of Anodizing Steel
Specific Oxide layer
Anodizing steel with zinc or aluminum oxides is possible, but it is prone to form a galvanic corrosive layer between the base metal and oxide layer.
Thus, steel can only be anodized using caustic solutions to form an oxide layer interrelated to steel, which is magnetite (Fe3O4).
High operation Conditions
The magnetite layer only begins to form only at temperatures higher than 70oC. So maintaining proper operating conditions is essential to anodize steel.
Using special electrolytes like caustic solutions and maintaining high temperatures boosts the processing cost. This is why anodizing steel is not considered a commercially viable option.
Anodized Steel vs Stainless Steel
Often the terms anodized steel and stainless steel are misinterpreted. To make it less ambiguous, you'll have to know their properties and composition.
Stainless steel is a type of steel having 1.2% of carbon along with other alloying elements like chromium, nickel, etc.
Whereas, anodized steel is steel on which a metal finishing is using anodization.
Stainless steel originally has corrosion resistance because of the presence of chromium.
But, anodized steel can only achieve corrosion resistance when anodized using caustic solutions.
Stainless steel can be easily molded into varying shapes and sizes. This allows it to be used in a wide range of applications, from making surgical instruments to the exterior cladding of buildings.
Comparatively anodizing complex steel shapes is difficult. This limits the application of anodized steel.
In terms of cost, anodized steel is priced higher than stainless steel because of the extensive processing it goes through.
An anodized metal shows enhanced mechanical properties. This is widely established in the case of aluminum and titanium.
But, in the case of steel, it is not. Anodizing steel without developing any corrosive layer is still a challenging problem.
Though experimental results show that steel can be anodized without affecting the base metal, the cost and time involved are not feasible.
Hopefully, future research can provide a solution to this issue and revolutionize manufacturing.
Frequently Asked Questions (FAQ)
Can steel be color anodized?
Yes, steel can be color anodized. When steel is anodized at high temperatures in an electrolytic bath of caustic solutions, the newly formed dichroic magnetite layer shows a range of colors depending on its thickness.
How can the lifespan of a finished metal be estimated?
You can estimate the lifespan of a finished metal by evaluating the thickness of the coated surface layer. As the coating layer's thickness increases, the finished metal's lifespan also increases.
Is anodized steel strong?
Yes, anodized steel is strong. It has a higher strength than regular steel. Also, the anodizing process enhances the durability of steel.
Thanks for the good explanation:
I have an Question open.
When I treat normal steel Pipe 1,2 mm Wall thickess, ⌀ 20mm, and 1m long made out of Q235: or S 235 as following
A: e-Plating or Vacuum-plating.
B: NITI:with 4µm NI nickel and 3µm Ti Titanium
After this like to anodise the TI on top with to achive a new color.
can I then use the anodising Procedur for TI Anodising which is done in a solution of trisodium phosphate (TSP) or various salts.
Will the steel Pipe inside rust and loose the strength ? And if can it be avaoided.