Anodized Steel: Feasibility and Process Discussion
Anodizing has been considered as a surface enhancement technique for various metals such as aluminum, titanium, copper, niobium, tantalum, etc. to improve wear and corrosion resistance. Anodizing also gives different colors to metals.
However, is the anodized steel process feasible for stainless steel? The process, while promising, has some limitations. We will explore this in detail in this paper!
1.Overview of anodic oxidation technology
(1) Definition and principle
Anodic oxidation, an electrochemical process, by placing the metal as an anode in an electrolyte solution, an oxidized film will form on the surface of the metal after energization. This oxide film not only improves the corrosion resistance of the metal, but also improves its wear resistance and other physicochemical properties.
(2) Development history
Anodic oxidation technology was first applied to the treatment of aluminum, and then gradually expanded to other metals, such as titanium, steel, etc. At the beginning of the 20th century, scientists began to study the anodic oxidation of aluminum, and with the advancement of technology, this technology has gradually matured and has been widely used in industrial production. The development of anodized steel technology is relatively late, but because of its effective in improving the performance of steel.
2.Is anodized steel feasible? What are the potential problems with anodized steel?
This topic is hotly debated by industry professionals. Some say “yes”, others say “no”. Technically speaking, however, stainless steel can be anodized through certain process controls.
Steel is not a pure metal, but an alloy of iron and carbon and small amounts of other elements; different levels of carbon produce different types of steel. The most common type is stainless steel, which combines nickel, chromium, iron and small amounts of carbon (about 0.08%). Other types include high-speed steel, tool steel, mild steel, and carbon steel.
Anodized steel looks similar to anodized aluminum, but the iron in the steel alters the anodizing process. Normally, anodizing is the formation of an oxide layer through an acid bath, but here, with steel, the oxide is produced in the form of iron oxide or Fe2O3, called rust. Rust does not prevent corrosion; rather, it increases the rate at which the material corrodes.
However, anodizing steel is technically feasible using a different chemical method. This method uses zinc, aluminum or lead in an alkaline medium rather than an acidic medium. The process is carefully controlled to form a magnetite layer. Unlike rust, magnetite protects the material rather than corroding it.
3.The process of anodizing steel
Anodizing is an electrochemical treatment technique commonly used to improve the surface properties of metals and is used in stainless steel. The following is the main process of anodizing steel:
(1) Pre-treatment
1) Surface leveling:
Through mechanical methods such as grinding and polishing, the steel surface is leveled and smoothed.
2) Degreasing:
Thoroughly clean and degrease the stainless steel surface to remove any contaminants, oil or grease. Solvents such as acetone or alkaline cleaners can be used to accomplish this operation.
3) Rinsing:
To anodize steel, rinse the bare metal with deionized water and immerse it in a 50% NaOH or KOH solution.
(2) Electrolyte Preparation
Select the appropriate electrolyte composition for your needs. Use an alkaline solution such as sodium hydroxide (NaOH) or potassium hydroxide (KOH) to prepare the electrolyte. The concentration and temperature of the electrolyte are key parameters that need to be carefully controlled.
(3) Anodizing
1) Device setup:
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.
2) Energized oxidation:
Under specific process conditions (current density, temperature, time, etc.), an oxidation reaction occurs on the surface of the steel by means of an applied voltage. When steel is anodized, a coating of magnetite (Fe3O4) forms on the steel surface. Although this coating is chemically similar to rust, it provides good protection for the base metal.
(4) Post-treatment
1) Color treatment:
Depending on the need, oxidized steel can be color treated by immersing it in a solution containing dyes so that the oxidized film adsorbs the dyes to form various colors.
2) Closure treatment:
In order to improve the density and hardness of the oxidized film, a closure treatment is carried out, such as immersion in certain solutions at high temperatures.
3) Drying:
Finally the treated steel is dried, usually at a low temperature to prevent damage to the oxide film.
4.Why anodized steel?
Although anodized steel has some limitations. But it does offer some benefits:
(1) Improved corrosion resistance:
Anodizing forms a dense oxide film on the surface of steel, effectively blocking the erosion of water and corrosive media on the substrate, significantly improving the corrosion resistance of steel.
(2) Enhance wear resistance:
Oxide film has a high hardness and good abrasion resistance, can significantly improve the wear resistance of steel, extend its service life.
(3) Decorative:
By adjusting the composition of the electrolyte and process parameters, different colors and textures of the oxide film can be prepared to enhance the decorative effect of steel and meet the needs of different application scenarios.
(4) Insulation performance:
Under certain conditions, the anodic oxidized film can have good insulating properties, which is suitable for the preparation of insulating materials and electronic devices.
(5) Strong antistatic and antifouling properties:
Anodic oxidized film has anti-static and anti-fouling characteristics, the surface is not easy to adsorb dust and dirt, easy to clean and maintain.
(6) Strong hardness:
The hardness of anodized steel is significantly enhanced, which can effectively resist friction and scratches, further enhancing the durability of the steel surface.
5.Performance Evaluation of Anodized Steel
(1) Corrosion resistance test
The corrosion resistance of anodized steel is evaluated by methods such as salt spray test and immersion test. In the salt spray test, the anodized steel samples are exposed to a high salt spray environment, and the corrosion resistance is evaluated by observing the surface corrosion. In the immersion test, on the other hand, the sample is immersed in a corrosive solution, and the corrosion resistance is evaluated by measuring its weight change and surface condition.
(2) Abrasion resistance test
Abrasion resistance is one of the important properties of anodized steel, which is usually tested by a friction tester. During the test, an anodized steel sample is rubbed against an abrasive material, and its abrasion resistance is assessed by measuring the amount of wear of the sample. The test results show that the anodized steel has excellent abrasion resistance and can significantly improve the service life of the steel.
(3) Other Performance Tests
In addition to corrosion resistance and abrasion resistance, other properties of anodized steel, such as hardness and electrical conductivity, are also evaluated by corresponding test methods. The results of these tests comprehensively reflect the overall performance of anodized steel and provide a scientific basis for its application in different fields.
6.Alternatives to anodized steel
(1) Passivation
Passivation is the chemical formation of a dense oxide film on the surface of a metal to improve its corrosion and wear resistance. The passivation process is usually simpler and less expensive than anodizing.
Passivation differs from anodizing steel because it uses nitric or organic acids to remove any free iron from the metal’s surface, creating a stable oxide film.
The by-product of this reaction is an oxide layer that protects the metal underneath from external elements.
(2) Phosphatizing
Phosphatizing involves applying phosphoric acid to steel to form a zinc, iron or manganese phosphate layer. This coating also protects against corrosion and reduces friction on moving parts.
This film not only provides good corrosion resistance, but also improves lubricity and paint adhesion on metal surfaces.
(3) Electrolysis
Electrolysis is the use of the principle of electrolysis to deposit a layer of metal or alloy on a metal surface to change its surface properties.
By controlling the composition of the electrolyte and the electrolysis conditions, different properties of the coating can be obtained, such as abrasion resistance and electrical conductivity.
Steel is connected to the anode of a DC power supply and submerged in an electrolyte solution of phosphoric and sulfuric acids.
As current passes through the anode, tiny iron ions are oxidized from the surface and dissolved into the solution. This process removes micron-sized material.
7.Application case study of anodized steel
(1) Application in Construction Industry
In the construction industry, anodized steel is widely used in curtain walls, doors and windows and other decorative materials for its excellent corrosion resistance and decorative properties. For example, Shanghai Center Building adopts anodized steel as the curtain wall material, which is not only beautiful and durable, but also can effectively resist the corrosion of the marine climate.
(2) Application in automobile industry
In the automotive industry, anodized steel is mainly used to improve the wear resistance and corrosion resistance of parts. For example, anodized steel is used in the manufacture of engine parts, which can maintain stable performance in high temperature and high pressure environments and extend the service life of the engine.
(3) Electronic Product Applications
In the electronics industry, anodized steel is used to manufacture electronic components and equipment shells, with good protective performance and aesthetics. For example, anodized steel is used to manufacture cell phone shells, which are not only wear-resistant and corrosion-resistant, but also provide a variety of color options to meet the individual needs of consumers.
7.Summary
Overall, anodized steel shows certain feasibility in different application scenarios. However, some technical difficulties and cost barriers still need to be overcome to realize its large-scale popularization and application.