What Is Metal Oxidation? Causes, Examples, and Prevention Methods

Metal oxidation is a common phenomenon in your metal devices or parts. If you’re wondering how to maintain metal surfaces well, ensuring your metal parts lifespan in your products and your customers satisfaction, you should understand metal oxidation.

 

This article will guide you through the causes and prevention methods of metal oxidation (including some professional surface treatment methods). Whether you're an individual needing metal parts maintenance or a manufacturer seeking metal oxidation surface finishes solutions, this article will be helpful to you.

 

 

 

What Is Metal Oxidation?

 

Metal oxidation refers to the process by which a metal loses electrons. When a metal comes into contact with oxygen or other oxidizing agents (such as water, salt, and other chemical contaminants), a chemical change typically occurs on its surface.

 

In CNC machined parts, metal oxidation most commonly manifests as:

 

1. Surface darkening

2. Formation of discoloration or yellowing spots

3. Rusting and pitting

4. Reduced surface finish

5. Loss of protective film

 

However, metal oxidation itself is not always a bad thing. For example, aluminum naturally forms an oxide film to provide protection.But uncontrolled oxidation can lead to a decrease in appearance, performance, and structural strength.

 

 

 

 

 

Metal oxidation vs. Rust vs. Corrosion: What Are the Differences Between？

 

 

Have you been confused by the conception of metal oxidation, rusting and corrosion? 

 

 

Rusting of Metals

 

Metal rust (such as iron rust) is a common form of metal corrosion. Essentially, it's the electrochemical oxidation of iron in a humid, oxygen-rich environment. Therefore, rust is both a form of metal corrosion and a form of metal oxidation.

 

 

Metal Corrosion

 

Metal corrosion is the reaction between a metal and its surrounding environment; it's a broader process of material deterioration, encompassing oxidation, pitting, and electrochemical corrosion. Metal corrosion includes metal oxidation, but corrosion is a more complex and overall destructive process than oxidation.

 

 

 

Metal Oxidation

 

Metal oxidation is a chemical process in which a metal loses electrons and its valence state increases. It is a broader concept than corrosion, encompassing oxidation that naturally forms an oxide film, as well as chemical oxidation reactions and electrochemical corrosion (such as rusting). Therefore, rusting and most forms of corrosion belong to metal oxidation, but metal oxidation has a wider scope.

 

 

 

 

What Are the Factors Affecting Metal Oxidazing?

 

 

The rate of metal oxidation is closely related to the environment, the material itself, and processing methods. Understanding these factors will help you better understand how to fundamentally prevent metal oxidation.

 

 

Environmental Factors

 

1.Oxygen and Humidity (Key Conditions): Metals exposed to high humidity and high oxygen content will oxidize more quickly.

2.Acidic/Alkaline Environments: For example,metal exposed to acid rain will accelerate the oxidation of metals.

3.Salinity: High-salinity environments, such as seawater, accelerate rusting or corrosion, and speed up metal oxidation.

4.Temperature Changes: High-temperature environments generally accelerate the metal oxidation reaction faster than low-temperature environments.

 

 

Material Selection

• Highly Reactive Metals: Reactive metals such as iron, magnesium, and zinc are prone to oxidation.
• Low-Reactive Metals: Precious metals such as gold, silver, and platinum are generally not easily oxidized.
• Special Alloy Metals: Certain corrosion-resistant alloys can delay metal oxidation; for example, naval brass (Brass 464) has excellent oxidation and corrosion resistance.
• Metals that can form a natural protective layer, such as aluminum and stainless steel, whose surfaces form a protective oxide film to prevent internal metal oxidation.
• Electrochemical Corrosion (Contact between Different Metals and Electrolytes): When any two different metals come into contact with an electrolyte, a corrosion cell will form. For example, galvanic corrosion between screws and aluminum components. This type of corrosion occurs when two different metals come into contact with an electrolyte (such as humid air or water).

 

 

Improper Processing

 

1. Residual cutting fluid causes "yellow spots" on aluminum parts.

2. Lack of post-sandblasting treatment causes aluminum parts to darken rapidly.

3. Residual cleaning agents cause "brown spots" on stainless steel.

4.Unprotected direct contact with air leads to oxidation of polished parts.

 

 

 

 

Examples of Metal Oxidation

 

Here I list some examples of metal oxidation based on the reaction mechanism for you to check.

 

Table 1: Examples of Metal Oxidation Based on the Reaction Mechanism

 

 

 

 

 

 

What Are the Effects of Metal Oxidation?

 

 

 Metal oxidation can cause structural damage, performance degradation, and aesthetic deterioration in metal parts, increasing maintenance costs. Furthermore, even slight surface oxidation can affect the fitting accuracy and functionality of CNC parts.

 

Here are some examples of common metal oxidation phenomena and their effects.

 

Table: Common metal oxidation phenomena and their effects.

 

 

 

Structural Damage

• Rust (Fe₂O₃·nH₂O): Strong structural damage, including spalling, pitting, and strength reduction in the ferrous metal.
  
• Verdigris: Light structural damage, with gradual thinning of the copper metal.
  
• Zinc Oxide:Light structural damage, with the protective properties of the zinc plating compromised, increasing the risk of structural failure.

 

Performance Degradation

• Silver Sulfide (Ag₂S): Moderate performance degradation.Silver surfaces lose their luster and form a black sulfide film, reducing conductivity.
  
• Titanium Oxide (TiO₂): Slight performance degradation. An excessively thick TiO₂ layer can lead to an increased surface friction coefficient, affecting precision assembly or friction performance.
  
• Chromium Oxide (Cr₂O₃, on Stainless Steel): Slight performance degradation. Damage to this film in chloride-containing environments can lead to localized corrosion and reduced durability.

 

Appearance Deterioration

• Silver Sulfide (Ag₂S): Severe appearance deterioration. Silver surfaces darken significantly and lose their luster; this is one of the most obvious types of surface discoloration.
  
• Verdigris: Moderate appearance deterioration. Green or bluish-green deposits form; while decorative, they are usually considered a sign of aging or contamination.
  
• Aluminum Oxide (Al₂O₃): Slight appearance deterioration. A thin, uniform oxide film forms on the aluminum surface, making the metal appear slightly dull, but without powdering or peeling.

 

Cost Impact

• Rust (Fe₂O₃·nH₂O): It requires frequent rust removal, repainting, and structural replacement.
  
• White Rust / Zinc Oxide (ZnO): It requires removal of white rust and re-protection of galvanized parts; otherwise, it will accelerate corrosion of the underlying steel, affecting maintenance cycles.
  
• Aluminum Oxide (Al₂O₃) & Chromium Oxide (Cr₂O₃): Although the self-passivating film is protective, the darkened appearance may require cleaning, polishing, or repassivation, increasing maintenance costs.

 

 

 

How to Prevent Metal Oxidation?

 

In industrial production, machinery manufacturing, electronic equipment, automotive, and aerospace fields, effectively preventing metal oxidation is a key technology for ensuring product performance and lifespan. 

 

You can improve the corrosion resistance and stability of metal parts and slow down metal oxidation by addressing multiple aspects, including environmental factors, material selection, surface treatment techniques, and chemical methods.

 

 

1. Material Selection

 

You can choose to use rust-proof metals, corrosion-resistant metals or alloys, and avoid using reactive metals in highly corrosive environments.

 

 

2. Environmental Control

 

You may keep metal surfaces dry and reduce contact between metals and corrosive substances such as sulfides.

 

 

3. Chemical Inhibition

 

Chemical inhibitors adsorb onto the metal surface can reduce reactivity.

 

 

4. Surface Finishing

 

 

 

Proper surface treatment can significantly extend the lifespan of parts.

• Physical Barriers: Paints, clear sealants, varnishes (e.g., marine varnishes), oil/grease-resistant coatings, and powder coatings (e.g., aluminum powder) help prevent metal oxidation.
  
• Electroplating: Using metals such as gold, silver, and chromium to improve appearance and corrosion resistance.
  
• Galvanizing: The zinc layer provides sacrificial anodic protection, particularly suitable for steel. You may consider this effective and economical-acceptable way if required for metal surface finishes industrially.
  
• Anodizing: Thickening the oxide film through electrochemical processes, such as anodized aluminum.
  
• Passivation: Forms a dense, stable, and non-peelable oxide film on the surface for protection. If your metal parts are made of stainless steel or aluminum, passivation is recommended.

 

 

5. Advanced Surface Protection Technologies

• PVD/CVD Coatings (Titanium nitride, Chromium nitride, etc., with extremely high corrosion resistance)
  
• Ceramic Coatings (with extremely high hardness, high temperature resistance, and corrosion resistance)
  
• Polymer Coatings (Epoxy resin, Polyurethane, suitable for marine or chemical environments)
  
• Aerospace-grade Surface Treatments (Chemical Conversion Coatings, Chromate Treatment)

 

 

 

Comparison Table of Common Metal Materials and Surface Treatment Processes

 

 

In the production of precision parts, surface treatment of metal materials is a crucial step in preventing metal oxidation, improving appearance, and extending service life. Different metal materials have different compositions and properties, and therefore require different surface treatment methods. The following are typical surface treatment solutions for common metal materials for you to check.

 

 

Table: Commonly used metal materials and surface treatment processes

 

 

 

 

 

 

 

How VMT Can Help

 

High-quality metal CNC machining parts should match high-quality surface finishes to prevent oxidation and maintain both aesthetics and performance.

 

VMT's professional team possesses extensive experience in challenging surface finishes such as polishing, multi-color anodizing, painting, sandblasting, electroplating, and etching. We are always committed to innovating cutting-edge surface protection processes to ensure components meet stringent safety and quality industry standards. You will find the perfect metal surface treatment solution for your project at VMT. (VMT CNC machining factory also provides prototyping and production services for CNC machined metal parts). 

 

If your project is seeking surface treatment solutions for metal parts or products, please contact VMT experts for a free consultation and quote.

 

 

 

 

FAQs

 

 

Is there any metal oxidation in CMOS (complementary metal oxide semiconductor)?

 

No. The semiconductor silicon is oxidized rather than a metal. The core of CMOS technology lies in using the 'metal' (refers to electrical materials such as polycrystalline silicon and metal gates) and 'oxide' (refers to silicon dioxide insulating layer) for manufacturing transistor fabrication. 

 

 

What happens when lead metal is oxidized？

 

When lead metal is oxidized, it forms grayish-white or white lead oxides (PbO, PbO₂, Pb₃O₄, etc.). These oxides weaken the metallic luster, but do not cause severe structural corrosion like iron.

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What is a non metal oxide？

 

Non-metal oxides are compounds formed by the reaction of non-metals with oxygen (O₂).​

 

 

How to remove oxides?

Oxides can be removed from metal surfaces using methods like abrasive cleaning, acid washing, or electrochemical treatments. Common techniques include using sandpaper, wire brushes, or chemical rust removers.

 

 

How to determine if a metal is oxidized?

You can determine if a metal is oxidized by inspecting its surface for discoloration, such as a reddish-brown color on iron or greenish patina on copper. Additionally, tarnishing or a dull finish may indicate oxidation.

 

 

How to remove oxides from metal surfaces?

To remove oxides, you can use abrasive pads, steel wool, or a chemical oxide remover. Vinegar, citric acid, or phosphoric acid are also effective for dissolving the oxide layer, especially for aluminum or copper.

 

 

What causes metal oxidation?

Metal oxidation occurs when a metal reacts with oxygen in the environment, forming a chemical compound, such as rust in iron or patina in copper. This is often accelerated by moisture, heat, or exposure to air.

 

 

Which metals are prone to oxidation?

Metals like iron, steel, copper, aluminum, and zinc are particularly prone to oxidation. Iron forms rust when exposed to moisture, while copper develops a green patina. However, stainless steel and gold are more resistant to oxidation.