What Is Anodized Steel And How Does It Work?
Anodized steel is ordinary steel with an extra oxide coating to protect it from corrosion and prolong its usage. But what is anodizing? And how does it work?
Anodizing is a crucial step in making metal products and as an experienced engineer, I’ve seen anodizing process in every CNC shop.
In this guide, I’m going to cover what is anodized steel and how does it work? What’s anodizing? And why anodizing is important for steel parts.
What is Anodized Steel?
Anodized steel is a chemically treated steel that has its surface covered with an oxide layer. The main purpose of this protective layer is to enhance corrosion resistance but it also functions as a decorative finish.
Anodizing process originated with aluminum parts. Anodizing is an electrolytic process and an aluminum oxide layer is deposited on the surface of aluminum using sulfuric acid and metal reaction. The key here is that the thicker oxide layer is grown from the aluminum substrate.
In anodized steel, the protective oxide layer is deposited in a different way.
How Is Anodized Steel Different From Anodized Aluminum?
Steel is not a pure metal. It is a mixture of iron and carbon and depending on the carbon percentage inside steel, it can be classified into different types.
The most common is stainless steel which is an alloy of nickel, chromium, iron, and 0.08% carbon. Other steels include high-speed steel, tool steel, mild steel, and carbon steel.
Anodized steel has a similar appearance to anodized aluminum but the iron content inside steel changes the anodizing process. Anodizing involves depositing an oxide layer using an acid bath, but with steel and its iron content, the oxide layer generated is ferric oxide. This is an iron-based oxide (Fe2O3) which is more commonly known as rust and offers no protection from corrosion and metal degradation.
Instead, steel is anodized using another metal like zinc or aluminum which prevents any rust formation. This is an electrolytic process and involves
The coating metals
Chromic acid, sulfuric acid and boric-sulfuric acid
Electrolytic solution
Cathodes and anodes
Galvanic Corrosion In Anodized Steel
The process of anodizing steel using electrolytic bath results in galvanic corrosion. Aluminum coating adheres to the steel surface and due to a difference in corrosion potentials, a galvanic layer develops between the two bonded metals.
How quickly the corrosion takes effect depends on the steel surface area in contact with aluminum or another metal.
Galvanic corrosion stops steel from making a strong bond with the oxide layer of another metal. For this reason, stainless steel is more commonly used in anodizing with aluminum layers as it forms better bonds.
How to Anodize Steel? The Process
The most effective method of anodizing steel is to use caustic solutions like sodium hydroxide.
Since any anodizing process aims to grow anodic films on the surface for corrosion resistance, the most optimum anodized coatings for steel come from sodium hydroxide or potassium hydroxide.
The parameters to be controlled during the anodizing process are
Caustic solution concentration and composition
Time
Voltage
Temperature
Pre-treatment
To anodize steel, rinse the bare metal with deionized water and submerge it in a 50% solution of NaOH or KOH.
Connect the steel to be anodized to the positive terminal (anode) of the power supply and use another counter electrode made of steel, platinum, or nickel.
Next, maintain the solution temperature and use a magnetic stirrer to disperse the electrolyte solution evenly.
Using a power supply, pass current through the electrolyte concentration mixture and a protective layer develops on the steel surface.
Anodic coating thickness depends on the concentration and submerged time of the metal part. Thicker films tend to develop when metal parts are submerged for extended periods.
Anodizing steel results in a magnetite Fe3O4 coating with a chemical composition similar to rust but offers lasting protection to the base metal. The newly anodized surface displays multiple colors from different angles and this aesthetic is one reason steel utensils are anodized.
The color variation comes from the thickness of anodized coatings and the colorful nature of dichroic magnetite. The rainbow colors under different lighting conditions is one of the cosmetic effects of Fe3O4 coating.
Other metals can be anodized in a similar way using different voltages. Higher voltages and temperatures lead to quicker coatings. However, an optimum voltage and temperature are selected to increase cost-effectiveness.
Advantages of Anodized Steel
Steel is anodized inside a machine shop for its anodized finishes. However, it’s not strictly a surface finishing process and has many other advantages.
Anti-corrosive Properties
Anodizing steel gives it corrosion protection from rusting agents like moisture. The magnetite itself is a black oxide but its iron compound gives it a rainbow appearance despite having a composition similar to rust (Fe2O3).
The magnetite film passivates the surface offering corrosion resistance.
Enhanced Durability
Naturally protecting the base metal using another oxide coating will result in a more durable part. The magnetite layer offers better wear resistance than a normal steel surface and is able to withstand harder materials due to increased Rockwell hardness ratings.
Improved Abrasion Resistance
Abrasion is the term used when a metal piece is rubbed against another surface resulting in some kind of wear.
Steel anodizing is a surface-finishing process that results in a porous surface. Thick porous coatings can absorb dyes and provide a more resistive finish. This makes it harder to scrub off paint from abrasions, let alone the anodized layer.
Limitations of Anodizing Steel
Even though there are many advantages to anodizing steel, corrosion resistance being the obvious one, anodizing steel is not a common industry standard.
Here are a few reasons other methods of enhancing steel properties are used.
Specific Oxide layer
Anodizing steel is only viable when caustic solutions are used. In other words, the magnetite layer is the only oxide layer that can protect the bare metal (steel).
While it is possible to use other metals like zinc and aluminum alloys in various acids like chromic acid, sulphuric acid, and nitric acid, the metal oxide coatings form a galvanic layer with steel.
High operation Conditions
Operating parameters are crucial to the surface finish and coating thickness.
To develop a consistent oxide layer, it is necessary for the solution to be over 70°C to develop a magnetite layer. If multiple metal parts are coated then the submerged time must be equal, otherwise, coatings of different thicknesses and colors can form resulting in uneven surface properties like wear resistance.
High operation Conditions
Operating parameters are crucial to the surface finish and coating thickness.
To develop a consistent oxide layer, it is necessary for the solution to be over 70°C to develop a magnetite layer. If multiple metal parts are coated then the submerged time must be equal, otherwise, coatings of different thicknesses and colors can form resulting in uneven surface properties like wear resistance.
Expensive process
Anodizing aluminum is widely accepted as a cheap strengthening alternative. You’d expect to see something similar with steel, but anodizing steel is not economical.
Since it is an electrochemical reaction that requires a continuous supply of chemicals, current, and temperature, the cost of anodizing is significant when compared to methods like alloying, increasing carbon content, or stress relieving.
Other Techniques to Enhance Steel's Properties
Steel is used in industries like construction, fabrication, infrastructure, and transportation. It is one of the few metals with such wide applications which means it has to undergo different processes for each industry.
Here are some other similar processes used to enhance steel’s properties.
Passivation
Passivation is another surface treatment process that can prevent rust and oxidation.
Passivation is different from anodizing steel because it uses nitric acid or organic acids to remove any free Fe from the surface of the metal.
The byproduct of this reaction is an oxide layer that protects the underlying metal from outside elements.
Phosphatization
Phosphatizing or phosphate coating is a metal-finishing process that forms a layer of protective coating over metal parts.
Metal is first cleaned and then submerged in a solution of phosphoric salts that quickly covers the surface of the metal.
After a final rinse in a neutralizing solution, the part is air-dried and stored.
Electropolishing
Electropolishing is an electrolytic process that can be considered the reverse of electroplating.
In electropolishing or anodic polishing a thin layer is removed from the steel giving it a smooth shiny finish.
Electropolishing is a similar process to passivation but the working principle is different. Electropolishing uses electric current discharge whereas passivation does not.
Steel is attached to the anode of a DC power source and submerged in the electrolyte solution of phosphoric acid and sulfuric acid.
When the current passes through the anode, tiny Iron ions oxidize from the surface and dissolve into the solution. This process removes material at the micron scale.
Anodizing Preconditions
Anodizing is a term popularized with aluminum. While it is possible to anodize metals, there are some prerequisites that the material and specimen must meet for ideal anodization.
Anodizing is not commercially profitable for steels, although it is being done on titanium, aluminum, zinc, and other handful metals.
Anodizing is largely a chemical process that requires you to completely submerge the metals. This means adding design features to support hooks and chains for the part. If these features are not made, the metal will not be anodized completely.
Anodizing changes the thickness of your part. Anodizing adds a layer of coating which can interfere with close tolerances on intricate parts. It can also change the machining finish and surface roughness.
It’s not possible to get thin transparent coatings with anodization. Another thing to keep in mind is the 70 ℃ temperature which can be expensive to maintain in large tanks.
FAQs
Q: Can You Anodize Steel?
A: Yes, steel can be anodized using a caustic solution and a power source. The electrolytic process deposits a layer of magnetite which anodizes steel.
Q: What Is The Purpose Of Anodizing?
A: Anodizing is done to improve wear resistance, rust resistance, and surface finish of metal pieces.
Q: Can Steel Be Anodized?
A: Steel can be anodized using NaOH or KOH solution. This is different from anodizing aluminum as no acid is used in the process. The anodized steel has a layer of protective magnetite oxide.
Q: Does Anodizing Improve Steel’s Resistance?
A: Anodized steel has a surface coating that offers rust and wear resistance. The new oxide layer is harder and more resistant to abrasions.
Q: What’s The Difference Between Anodizing Steel And Anodizing Aluminum?
A: When aluminum is anodized, an oxide layer of aluminum is grown from the substrate metal (aluminum). When steel is anodized, a layer is deposited using a caustic solution which is different from the Iron (III) oxide which offers no protection to the steel.
Q: Can You Anodize Titanium?
A: Anodizing titanium is essential for its many applications. Titanium is anodized using an electrolytic process to develop an oxide layer. Hardened titanium surface prevents titanium dust formation from abrasions.