The aerospace industry uses various complex equipment such as aircraft, jet engines, rockets, aircraft, helicopter, guidance and navigation systems, satellites, and many more. These equipment are designed to operate in a regulated environment and use several parts made of metals, plastic, and composite materials. Among these, the metal parts are surface finished to improve their protection against corrosion, uniform coverage, wear resistance, and friction reduction during operation. The quality of finishing is of utmost importance in this industry, because the failure of any part or component may have devastating consequences. At HPI Pro, we provide various metal finishing systems and custom tanks. Our electropolishing and anodizing lines are widely used in the aerospace industry to improve the safety and functionality of parts and components.

An Overview of Aerospace Anodizing

As the name suggests, aerospace anodizing, is the key finishing step involved in manufacturing of aluminum, titanium, and magnesium parts for the aerospace industry. A metal part is coated with an oxide to improve its sturdiness and resistance from wear and tear due to prolonged use in challenging environments. There are four types of anodizing – phosphoric acid anodizing, boric sulfuric acid anodizing, chromic acid anodizing, and sulfuric acid anodizing. All these anodizing processes offer distinct advantages and are suitable for certain materials than others. Despite these differences, they serve the same purpose – improving the usability of a part and extending its shelf life in challenging environments.

Different Steps Involved in Aerospace Anodizing
The steps below will give you a general overview of anodizing identified in the steps below.

  • Metal Finishing: Metal finishing is performed on aerospace parts and components to improve their aesthetic appeal. Sanding, polishing, buffing, and wire brushing are few common processes employed.
  • Precleaning: In this step, the parts are cleaned to remove any markings or soils, which may be acquired during any earlier manufacturing or finishing process.
  • Masking: This is done to cover the areas of the part that doesn’t require anodization. This masking is done using liquid and tape maskants.
  • Degreasing: This is done to remove grease or oil on the parts of components. Generally, degreasing is done using vapor or aqueous degreasants.
  • Alkaline Cleaning: Although the parts and components are handled carefully during manufacturing, they may still get stained due to fingerprints or perspiration. These fingerprints, perspiration or other minute contaminants are removed through alkaline cleaning.
  • Deoxidizing: The parts are deoxidized using acids. The oxides are removed by dipping the parts or components in a tank containing acid. When the part or component is removed from the tank and exposed to water or air, a layer of aluminum oxide forms naturally on the surface. This new layer of natural oxide promotes the coating of uniform appearance and thickness during the anodization process.
  • Alkaline Etching/Desmutting: Some parts or components may have smears, thick oxides, stains, shot peen residues or other contaminants, which cannot be removed by alkaline cleaning alone. Caustic formulations are used with alkaline cleaners to remove them. Smut is formed on the outer surface of aluminum parts when exposed to caustic formulants. Smut may look from black specs to dark surfaces on different types of alloys. Smut is not soluble in alkaline solutions and is removed using a highly acidic formulation known as Desmut solution. Smut is reduced during etching. The deoxidizer is used for deoxidizing and desmutting in a tank where minimal etching is performed. Desmutting helps completely remove the smut from the surfaces of parts and deoxidizing is not usually required.
  • Anodizing: In this step, the desmutted parts are placed in a tank with anodizing solution and voltage is applied to form the anodized coating. The electric current is applied immediately, else, there are chances of smut formation, which may inhibit anodization. The electric current is applied in small voltages and ramped to a steady value, which is maintained throughout the process. The current supply is stopped when the process is completed and the parts are removed from the anodization tank.
  • Rinsing: This is done to remove acid that is trapped by anodic coating during the process. If the acid is not rinsed then it can inhibit the paint adhesion in the next step.
  • Dyeing: The parts are dyed after anodization and sealed to set the dye. The dyed part is dried in the next step.

Types of Aerospace Anodizing
At HPI Pro, we provide metal finishing systems and tanks for aerospace anodizing services.

  • Phosphoric Acid Anodizing: This is done for bonding structure adhesives in high-humidity environments. This process was developed by Boeing and is usually carried out in 10-15V. The oxide film that forms on the part or its surfaces assures greater durability in challenging environments.
  • Chromic Acid Anodizing: This type of anodizing is performed to protect joints with all types of critical structures. The oxide film formed by the acid is soft and not porous unlike other processes. The chromic acid coating is specified into two types as per MIL requirements. They are type 1 and type IB. The first type of coating is produced using 40 volts voltage, whereas type 1B is produced using a voltage of 22 volts or 20 volts depending on the specification. This is the first patented anodizing process, which has existed since 1923 and is still in demand.
  • Boric and Sulfuric Acid Anodizing: In this process, a mixture of boric and sulfuric acid is used. It is used as an alternative to chromic acid that contains toxic hexavalent chromium.
  • Sulfuric Acid or Oxalic Acid Anodizing: This process was patented in 1927 and is employed for two uses – Type II and Type III coatings. Here type III coatings are used for engineering applications throughout the aerospace industry.

Types of Aluminum Anodized Finishes in the Aerospace Industry
The aluminum anodized finishes are distinguished into two classes as per the MIL-A-8625.

  • Class 1: These parts are usually undyed and the color that remains on the part is a resultant of the alloy type, anodic treatment, thickness, and sealing parameters. You will see either bronze or gray on the part. The parts that are classified as Class 1 will have a thickness of 18 microns or 0.7 mils and are generally used for outdoor exposure.
  • Class 2: These parts are pigmented or dyed and have an anodic thickness of 10 microns or 0.4 mils. Aluminum anodized parts are used for interior applications that are not exposed to the outer environment.


What is Aerospace Electropolishing?

This is an electrochemical finishing process, where an ultra-clean surface finish is achieved by removing a thin layer of material. Electropolishing is also known as anodic polishing, electrochemical polishing, and electrolytic polishing. Electropolishing is employed in the aerospace industry for polishing or deburring parts with complex geometries or those that cannot withstand challenging environments and break easily. The process helps reduce surface roughness by at least 50%.   

Different Steps Involved in Electropolishing

Electropolishing works on the principle of Faraday’s Law. In this process, the part to be electropolished is immersed in an electrolyte solution.  The part is fitted with two electrodes – an anode and a cathode. These are connected to the power supply. The piece is fully immersed into the electrolyte solution containing the mixture of phosphoric and sulfuric acid, as well as stabilizers and other agents. The electrolyte solution is temperature controlled to achieve the proper equilibrium. When the power supply is switched on the electric current passes from anode to cathode and metal oxidation takes place. This helps remove surface irregularities and impurities.

Benefits of Aerospace Electropolishing
There are several benefits of electropolishing finishing equipment that has been perfectly leveraged by aerospace engineering services worldwide.

  • Improved Finishing: Electropolishing helps improve the finishing of a part or component by eliminating heat discoloration, stains, and minor scratches.
  • Improved Corrosion Resistance: This process helps improve the corrosion resistance of parts through removal of surface imperfections. Generally, surface imperfections allow moisture accumulation which results in corrosion.
  • Surface Preparation: Sometimes this process is used prior to anodizing to produce a smooth and clean surface of the part.
  • Improved Micro Finish: Although aerospace parts are produced with utmost detailing, they may have imperfections like microscopic peaks introduced during forming, welding, and stamping. Improved micro finish values help in increasing surface smoothness.
  • Ultracleaning: Electropolishing helps remove embedded dust or contaminants from the part’s surface without compromising its hardness.

Over the years, we have served many aerospace engineering services to achieve their targeted goals through anodizing and electropolishing. We understand that, unlike other industries, these services have specific needs for custom tanks and metal finishing tanks. Thus, our experts and engineers work closely with them to meet these requirements. They can help incorporate these systems to your existing process line or create an entirely new line from scratch. Contact our team today to discuss your requirement for aerospace finishing systems or aerospace anodizing systems. 

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