Metals are strong and have a lustrous appearance, which makes them ideal for daily use applications. However, their surfaces are vulnerable to corrosion. To address this, some metals get coated with an oxide layer through anodization.
The anodization is common for non-ferrous metals like Aluminum, but can ferrous metals like steel be anodized?
This article explores whether steel is a viable candidate for anodization and if it is, why steel anodizing is not an adopted practice in industries.
What Is Anodizing: Why Anodize Metals?
Anodizing is an electrochemical process that adds a thin oxide layer, typically 1.8 to 30 µm thick, to metals. This layer fulfills two functions: it protects the underlying surface from wear and corrosion and permits colorization to enhance aesthetic appeal.
The anodization process occurs in an electrolytic cell, usually in an acidic medium, comprising sulfuric, oxalic, or chromic acid. In this setup, the anodizing metal serves as the anode, while a similar metal or a different metal like aluminum acts as the cathode.
As current flows through the circuit, oxidation occurs at the anode. This oxidizes the material and forms a protective layer on it. All key parameters—temperature, voltage, and current—are closely monitored and controlled.
While this process is common for certain aluminum alloys, it applies to titanium and magnesium, albeit under different conditions. The situation with steel is distinct and requires a separate explanation.
Is Steel Anodizing Feasible?
Steel is a ferrous alloy that forms a layer of ferrous oxide, commonly called rust when its surface oxidizes. This rust layer, unlike protective oxides, corrodes the metal itself. Anodizing steel in a similar acidic bath, like the one used for other metals (particularly aluminum), exacerbates rusting instead of preventing it.
However, using a different chemical approach, anodizing steel is technically possible in laboratory settings. This method uses zinc, aluminum, or lead in an alkaline medium, not acidic. The process is carefully controlled to form a magnetite layer. Unlike rust, magnetite protects the material rather than corroding it.
The Steel Anodizing (The Lab Process)
Currently, labs restrain anodizing steel in controlled environments. This process is not common commercially due to its high costs and specific operational requirements, which are currently achievable in labs only.
Pre-Treatment
Pre-treatment is a requirement for both steel and Aluminum because it determines the final appearance of the coating. The surface must be free from defects and thoroughly cleaned and polished using chemical or abrasive methods.
Anodizing is the last step in manufacturing. Complete any drilling or welding operation before the anodizing process.
Setting Up Electrolytic Cell
Steel anodization requires a basic medium, typically 50% NaOH or KOH. In this setup, the process suspends the steel as the anode in the alkaline electrolyte, while using another metal, such as aluminum or lead, as the cathode. Steel anodization requires a basic medium, typically 50% NaOH or KOH. In this setup, they suspend the steel as the anode in the alkaline electrolyte, while another metal, such as aluminum or lead, usually serves as the cathode.
The solution’s temperature has to stay constant, slightly above 70 degrees Celsius, and a magnetic stirrer keeps a uniform distribution of the electrolyte.
The Anodizing Process
The electrolytic begins when we supply high voltage across the two electrodes. Over time, a magnetite film, composed of iron and oxygen (), begins to form on the surface. This black magnetite layer offers protection, unlike the typical red rust.
The thickness of the coating is proportional to the cube of the submersion time. Higher voltages and temperatures can accelerate the coating process, although specific settings vary depending on the metal.
Sealing
The top anodized layer is porous; therefore, it requires sealing to protect the finish. This is done by immersing the metal in de-ionized water at a very high temperature to close off the pores.
Advantages of Steel Anodizing
Though it’s not a common commercial process today. But it does offer some benefits:
Enhanced Abrasion Resistance
Metals often wear when they rub against a surface. This abrasion can be mitigated by a pre-existing protective layer. Unlike paint, which may scratch off easily, a chemically anodized layer, interlinked with the metal at a molecular level, makes the surface much harder to scratch.
Colorized Appearance
Although the magnetite layer is primarily black, it can produce a rainbow-like effect due to iron presence. Moreover, the porous nature of the anodized layer can absorb dyes – further offering the opportunity to customize the color according to preference.
Better Durability
A bare steel surface is susceptible to oxidation and rust. The magnetite layer not only prevents further oxidation but also provides enhanced wear resistance. As a result, the lifespan of the part gets an extension.
Potential Issues with Steel Anodizing
Although anodizing is a common and accepted technique for aluminum, it is not as popular or widespread for steel due to multiple challenges related to feasibility, cost, and scalability.
Expensive Process
Anodizing is chosen for aluminum because it is cost-effective nature compared to other finishing processes. In comparison, making anodized steel is not economically viable due to the high costs of chemicals and the need for a tightly controlled, high-temperature environment.
Process Complexity
Steel anodizing requires a bath with a caustic solution. This process produces a single magnetite layer, requiring consistent maintenance of specific voltage and temperature conditions. This process can only produce a single magnetite layer, which demands specific voltage and temperature conditions to be maintained consistently.
Since the components are to suspend within the solution, this necessitates the inclusion of hooks in the design to hold the steel substrates.
Strict Operating Conditions
The anodizing process for steel components requires the solution to stay above 70°C at all times to ensure proper layer formation. When processing multiple parts simultaneously, the time and conditions must remain constant; any deviation could lead to uneven layer thicknesses and compromised quality.
Alternatives to Anodize Steel
Anodization of steel may is not a common surface treatment as these alternatives are more practical:
Passivation
Passivation involves treating the stainless steel surface with a mild oxidant, usually nitric acid, to remove free iron and other contaminants from the surface. This treatment helps in forming a thin, protective oxide layer that is less reactive and more resistant to corrosion.
Phosphatization
Phosphatization (or phosphating) is the application of phosphoric acid to steel to form a layer of zinc, iron, or manganese phosphate. This conversion coating prepares the metal for further coating or painting. It also provides corrosion protection to an extent and reduces friction in moving parts.
Electropolishing
Electropolishing or ‘reverse electroplating’ is a process that uses an electrochemical solution to remove the outer layer of metal, thereby smoothing and streamlining the microscopic surface of the steel.
Steel vs Aluminum Anodizing
Anodizing aluminum results in aluminum oxide (Al2O3), a non-magnetic and hard layer that enhances corrosion resistance and can be dyed in various colors. Anodizing aluminum creates aluminum oxide (Al2O3), a non-magnetic and hard layer that enhances corrosion resistance and allows for dyeing in various colors. Whereas, anodized steel has a magnetite (Fe3O4) layer or black rust. This layer is magnetic with protective qualities, though different from aluminum oxide.
To anodize aluminum, we use an acidic bath where aluminum acts as the anode. Whereas to anodize steel, we use a basic (NaOH) solution. Unlike aluminum, anodized steel is not typically commercially viable and is reserved for specialized laboratory conditions. Unlike aluminum, anodized steel lacks commercial viability and is primarily used in specialized laboratory conditions.
How RapidDirect Can Support Your Anodizing Projects
For aluminum alloys, anodizing is the best choice. It protects the metal from corrosion and provides options for colorization. If you want to make your aluminum projects more durable and aesthetically appealing, RapidDirect’s anodizing services are the solution. We offer both Type II and Type III anodizing for aluminum.
Our team of dedicated engineers is here to support and advise on your engineering projects. We are ready to handle all customization needs – contact us today for an instant quote.
Conclusion
Even corrosion-resistant metals like aluminum are subject to wear and tear. Anodization provides a cost-effective solution for aluminum alloys, enhancing their durability. However, the same process applied to steel can lead to rust formation, which does more harm than good.
Steel anodization is possible, but in controlled lab environment settings, which are not cost-effective on a larger scale. Therefore, industries commonly treat steel with alternative surface methods such as passivation, electropolishing, and phosphatization, which offer more practical solutions for industrial use. Therefore, steel is commonly treated with alternative surface methods such as passivation, electropolishing, and phosphatization, which are more practical for industrial use.
FAQs
Can titanium be anodized?
Yes, titanium anodizing is as common as aluminum anodizing. For that, titanium gets dipped in a trisodium phosphate solution. Type 2 and type 3 titanium anodizing are more prevalent. The former is for wear resistance and the latter one for color addition.
Which is better, anodized stainless steel or aluminum?
Anodized aluminum is a more common choice as it provides a durable, corrosion-resistant coating that can be easily colored. Whereas, stainless steel does offer similar characteristics but the process is difficult to achieve.
What metals cannot be anodized?
Ferrous metals like iron and steel cannot be traditionally anodized because their oxide layers have rust, which degrades the material rather than protects it.