A Brief Insight into Electrochemistry

Join us for a short excursion into electrochemistry to better understand the fundamentals that make this process so important in surface technology.

Energy Source

Electrochemistry typically uses direct current sources for anodic oxidation. These are characterized by a positive and a negative terminal. At the negative terminal, an excess of negatively charged electrons is present. Nature tends to equalize this charge difference (also known as electrical voltage or potential difference). Humans have learned to route this equalization through lamps, motors, or heating elements to obtain light, mechanical energy, or heat. Nature has also made it possible to use charge equalization in surface technology: through electrochemical reactions.

Electrodes

Electrically conductive connections between the poles of the DC source and two spatially separated metal objects turn these objects into electrodes. The electrode connected to the negative pole is called the cathode, while the one connected to the positive pole is the anode. In anodic oxidation, the anode is the workpiece that is to receive an oxide layer. To build this layer, an electrolyte is required that surrounds the anode and at least wets the cathode.

Electrolyte

The electrolyte for anodizing consists of bases (alkalis), acids, or salts dissolved in water. These substances consist of electrically positive and negative ions. In water, the ions are separated and freely mobile. At first glance, it seems as if the electrolyte could conduct electricity. On closer inspection, however, the ions migrate to the electrode with the opposite electrical charge, where they accept or release electrons. In doing so, the ions equalize their own charge and undergo chemical transformation.

Chemical Reactions

In chemistry, reactions in which electrons are accepted are called reductions. These reactions occur at the cathode, where there is an excess of electrons. During anodizing, hydrogen gas is typically formed at this electrode.

At the anode, where electrons are lacking, ions release electrons. This  process is known as oxidation. In anodizing, oxygen acts as the electron acceptor and bonds with the electrode material to form a metal oxide.

What Exactly Is Anodizing? Process Sequence of Anodizing

Pretreatment as a Prerequisite for Defect-Free Layer Formation

A uniform oxide layer can only form on metallically clean surfaces. Thorough surface pretreatment is therefore essential. Surface technology provides mechanical and chemical methods that remove all types of contamination from materials.

Mechanical surface treatments remove stubborn contaminants and corrosion products. They also create a defined surface structure—such as glossy or matte—and can eliminate scratches or defects. Mechanical pretreatment typically involves brushing, blasting, grinding, or polishing.

Chemical pretreatment primarily removes contamination from previous process steps, such as oils and greases used as corrosion protection or cooling and forming lubricants from manufacturing. Common steps include alkaline degreasing and pickling in acidic or alkaline baths to remove remaining contaminants and natural oxide films.

Sequence of Anodizing

Individual workpieces are mounted on racks and immersed in the electrolyte bath. Alternatively, sheet metal wound into coils can be drawn through the bath and rewound after anodizing. A conductive connection to the positive pole of the DC source ensures that the workpiece becomes the anode. The container material often serves as the cathode; if that is not possible, cathodes are mounted on the side walls of the tank and connected to the negative pole.

Once the power source is switched on, the ions begin to migrate. Initially, a  thin, electrically insulating metal oxide layer  forms on the workpiece or semi-finished product. Because the anode continues to attract negative ions, these ions accumulate in front of the layer. The voltage rises until it reaches a value that allows the ions to penetrate the layer and reach the base metal to form new metal oxide. This process often produces visible sparks. Oxidation continues at the base metal, forming a porous layer with countless microscopic channels. The structure of the oxide layer can be varied widely  by adjusting the electrolyte composition, current intensity, and temperature.

Post-Treatment to Seal Pores and Add Color

The porous structure of the fresh metal oxide layer is ideal for absorbing dyes. However, if you choose to dye the freshly formed metal oxide layer, the pores must be sealed. This step is usually performed in a hot-water bath. At 208.6 °F (97 °C), the metal oxide reacts with water (hydration). This increases the volume of the layer and compresses the pores. This densification process is known as sealing. As an alternative, surface technology also offers pore filling using waxes.

Applications of Anodizing

Anodizing is well suited for producing corrosion- and wear-resistant oxide layers on light metals. This is primarily due to the properties of the oxides. The process is primarily used for aluminum, titanium, and magnesium. Attempting to anodize stainless steel would fail because its corrosion protection is provided by a very thin oxide film formed by chromium in the alloy reacting with oxygen in the air.

Anodizing Aluminum

Anodizing aluminum is the most widespread and best-studied variant of anodizing. It has acquired its own name: *Eloxal*. Aluminum is used in aerospace, automotive engineering, machinery, equipment manufacturing, construction, and architecture.

Anodizing Titanium

Titanium is an ideal material for aerospace and aviation due to its low density, high mechanical strength, and excellent heat resistance. These properties, combined with outstanding chemical resistance, make anodized titanium valuable for the chemical industry and medical technology.

Anodizing Magnesium

Because magnesium is extremely lightweight yet strong, it is gaining interest in the automotive and equipment manufacturing sectors. However, anodizing magnesium is challenging due to its high reactivity. It requires special approaches both in preparation and during anodic oxidation.

Der Beitrag Surface Treatment by Anodizing erschien zuerst auf Kluthe Magazine.

Why Is Surface Pretreatment Necessary?

Every surface treatment process includes pretreatment of the base material. In metalworking, forming and cutting fluids are essential, and components are often preserved with corrosion protection oils. These oils work because they spread evenly across the workpiece and adhere well, preventing moisture and oxygen in the air from reaching the metal.

If a metal surface is to effectively receive a conversion coating, paint finish, or electroplated layer, the process chemicals must reach every part of the material. Any oil or grease residue from metalworking would block certain areas and contaminate the process chemicals. Pretreatment ensures these residues are removed. To do this, different degreasing agents and methods are available.

How Do Degreasing Agents Differ?

In general, there are three main types of degreasing agents: organic solvents, water-based cleaning solutions, and cleaners that contain enzymes. Organic solvents can fully mix with and dissolve greases. They work very quickly, even at low temperatures, and usually evaporate without leaving residues along with the dissolved oils. However, this evaporation can release harmful substances into the environment, pose health risks for workers, and contribute to climate change. OSHA workplace safety standards require proper ventilation and air filtration systems when using organic solvents to protect employee health. These control measures can reduce emissions but cannot eliminate them entirely. Because of these drawbacks, organic solvents—despite their effectiveness—are used less and less.

Water-based degreasers are becoming increasingly popular. In these solutions, surfactants bind to the grease and disperse it in the water. Surfactants are chemicals with molecules that have a water-attracting end and a grease-attracting end. The grease-attracting end bonds with the oil particles, while the water-attracting end makes the whole structure mix with water. This process often works best at elevated temperatures, but more and more cleaning solutions are effective even at lower temperatures.

Which Methods Are Used in Surface Pretreatment?

Degreasing agents are applied to metal surfaces using either spraying or immersion methods. Manual degreasing with cloths and brushes is rare today. In spraying, the cleaning solution is forced through nozzles at high pressure onto the metal surface, where the chemical cleaning action is supported by mechanical force. This dissolves and removes the greasy residue. To improve sustainability, used cleaning solutions are normally collected and treated so that they can be partially reused. This reduces chemical consumption and waste.

However, with metal parts that have complex shapes, holes, or threads, the spray method may not reach every surface area. In these cases, immersion methods are used. Parts are submerged in large tanks filled with a degreasing solution. Cleaning takes more time but it can be accelerated by agitating the solution, moving the parts, or using ultrasonic cleaning. During circulation, some of the fluid can be filtered and treated, which extends bath life, reduces wastewater, and saves process chemicals.

How Are Degreasing Residues Removed from the Parts?

The pretreatment process includes multiple steps, including both rinsing and drying. If any degreasing agent or water remains on the surface and dries there, it can cause defects in subsequent surface treatment processes. Coatings could peel, conversion coatings might not form properly, and paint flaws could occur. To prevent this, degreasing agents and cleaning residues must be completely rinsed away.

Rinsing is done in multiple stages. The final rinse uses fully deionized water, because even tap water contains minerals that could interfere with coating adhesion. To save water, rinsing almost always follows a cascade system: as the parts move through each tank, the rinse water flows in the opposite direction. The freshest deionized water enters at the final tank and is still clean enough by the first tank to perform effective rinsing.

Summary: Degreasing Agents in Surface Pretreatment

Metalworking leaves oily and greasy residues on parts, which can interfere with surface treatments. Degreasing agents remove these contaminants—an essential step for ensuring high-quality results in subsequent surface finishing processes.

Modern facilities must also comply with EPA air quality standards and waste disposal regulations when selecting and implementing degreasing systems. Because of this, degreasing is an integral part of surface pretreatment, which is typically done utilizing spraying or immersion methods. Degreasing agents include organic solvents, water-based cleaners with surfactants, or enzyme-based solutions.

Der Beitrag How Degreasing Agents Are Used in Surface Pretreatment erschien zuerst auf Kluthe Magazine.

Purpose of Pre-Treatment

Both coating and bonding rely on the materials ability to allow a liquid or powder to spread evenly, penetrate micro-irregularities, and create mechanical interlocking as it hardens.

The primary goals of surface pre-treatment are therefore cleaning and degreasing. To achieve this, both mechanical and chemical techniques are used.

Mechanical Pre-Treatment Methods

Mechanical pre-treatment aims to remove stubborn contaminants such as rust, scale, or old coatings, to degrease the part, and to roughen the materials. In some cases, this treatment also strengthens the material just beneath the surface.

Brushing

The most straightforward method of mechanically removing rust, scale, or old paint is brushing—either manually with a wire brush or using rotating brushes mounted on a grinder.

Large-scale industrial systems may guide parts past rotating brushes. Depending on the setup, these brushes may leave scratch marks on the surface—sometimes intentionally, sometimes they are removed later through sanding.

Grinding

Grinding removes material from a surface using abrasive tools like grinding wheels or sandpaper. A key factor here is the grit size: higher grit numbers correspond to a greater density of finer abrasive grains, resulting in a smoother finish. This process is used to create precisely defined texture or roughness.

Blasting

Blasting is a widely used mechanical method for surface cleaning. It involves directing a high-pressure stream of material at the part to remove contaminants. One of its key advantages is its ability to reach cavities and hard-to-access areas. Depending on the blasting media and the impact velocity, this process can clean or also work harden the subsurface layer of the material.

Common blasting media include:

• Steam
• Water
• Crushed nut shells or hardwood granules
• Glass beads
• Sand
• Steel shot
• Aluminum oxide (corundum)

When hard blasting media like steel shot or aluminum oxide strike the metal at high velocity, they cause micro-deformations in the surface. These permanent changes strengthen the material and create a finely textured finish that is ideal for subsequent coatings.

If cleaning agents are added to the water during water blasting, mechanical and chemical processes act simultaneously.

Chemical Pre-Treatment Methods

Chemical surface preparation includes degreasing and cleaning with chemical agents, pickling, surface activation, and passivation of the metal.

Cleaning and Degreasing

Oils and greases are often used as temporary corrosion protection. Before further processing, these residues must be thoroughly removed to ensure coatings or adhesive bonds adhere properly. Cleaning is done by immersion or spraying. Preferred cleaning agents include surfactants and organic solvents—with surfactants generally being favored.

A benefit of many solvents is that they evaporate without leaving residue. In contrast, surfactant-based cleaning requires multiple rinse cycles to remove any remaining residue. The final rinse is typically done with deionized water, preventing mineral deposits and water stains during drying.

Pickling and Surface Activation

Pickling uses strong acids or bases to remove oxidation products like rust. A clean, oxide-free surface is critical, especially for electroplating or adhesive bonding.

This step is often followed by surface activation, preparing the material for passivation coatings that serve both as corrosion protection and as an adhesion base for paints or finishes.

Passivation

Passivation involves creating a protective chemical layer on the surface of a metal. Specialized chemicals react with the substrate to form a dense, uniform layer that protects against moisture and oxygen, thus improving corrosion resistance. If a coated surface is scratched or damaged later, the underlying metal is less likely to corrode.

This protective layer also improves paint adhesion. Pre-treatment technology primarily uses phosphating and chromating to create such layers.

Because the base material reacts chemically to form the layer, passivation layers are also called conversion coatings, and the process is referred to as a conversion treatment. Post-passivation, parts are rinsed thoroughly—first to remove residual chemicals, then in a final rinse using deionized water to prevent spotting. The process concludes with controlled drying.

Der Beitrag Surface Pre-Treatment erschien zuerst auf Kluthe Magazine.