Parts that have not undergone special surface treatment will rust if exposed to moisture for a long time. However, in the manufacturing industry, it is a misunderstanding to refer to all degradation of metal surfaces as rust. And as a CNC engineer, I’ve seen too many people conflate rust, corrosion, and oxidation. In order to correct everyone’s understanding, I wrote this article about the differences between the three. And after reading the entire article, you will be better able to choose appropriate materials, coatings, and develop reasonable protection strategies to mitigate risks.
What is Corrosion?
Corrosion is a gradual destruction and degradation process caused by chemical or electrochemical reactions between metal and surrounding elements (such as oxygen, salt, etc.). It can compromise the structural integrity, dimensional accuracy, and mechanical properties of materials. Therefore, in the field of CNC machining, corrosion is a major challenge for high-precision tolerances and high-quality surface finish of parts.
Types of Corrosion
In metalworking and during the use of components, we typically encounter the following five most common forms of corrosion. Below, I will systematically outline these types of corrosion for you.
Uniform Corrosion
This is the most common and most predictable type of corrosion. It occurs widely on exposed metal surfaces. Take the most widely used carbon steel material as an example. This material is prone to uniform corrosion in humid environments. The chemical principles during this period are as follows:
- Anodic reaction (iron dissolution): Fe→Fe2++2e−
- Cathodic reaction (oxygen reduction) : O2+2H2O+4e−→4OH−
- Overall reaction produces ferrous hydroxide:Fe2++2OH−→Fe(OH)2
As for countermeasures, I suggest you perform surface treatments on the components through electroplating, painting, or applying rust inhibitors. The purpose of this is to isolate moisture and oxygen.
Galvanic Corrosion
Galvanic corrosion is prone to occur when two metals with different potentials come into contact in an electrolyte solution. I assume that when metal A (more reactive, acts as anode to corrode) and metal B (less reactive, acts as cathode to protect) are in contact, then the actual potential difference is:
ΔE=Ecathode (B)−Eanode (A)
The larger the ΔE, the stronger the driving force for corrosion.
The dissolution reaction of metal: M→Mn++ne−
The reduction potential reaction is: Mn++ne−→M
Then, we can calculate the actual potential according to the Nernst equation:
$$E = E^\ominus + \frac{0.0592}{n} \log_{10} [M^{n+}]$$
In this case, you need to avoid direct contact between dissimilar metals as much as possible, or choose materials with a potential difference of less than 0.15V.
Pitting Corrosion
This is a localized and dangerous form of corrosion. Because it will form tiny holes that are difficult to detect on the metal surface of your parts, and will continue to cause damage to the interior of the parts.
Pitting corrosion is usually induced by chloride ions (Cl−). Chloride ions have strong penetrative power, which can destroy the chromium-rich passive film (Cr2O3) on the metal surface, leading to a dangerous galvanic effect.
- Rapid dissolution of metal inside the pit: M→Mn++ne−(M represents the metal)
- Hydrolysis of metal ions leads to localized acidification (a sharp drop in pH): Mn++nH2O→M(OH)n+nH+
To solve this problem, you must thoroughly clean the parts, strictly control the chlorine content of the cutting fluid, and passivate the stainless steel parts with nitric acid or citric acid.
Crevice Corrosion
As the name suggests, crevice corrosion can easily occur in tiny crevices where fluids tend to accumulate and stagnate, such as seams, seals, and overlaps of components. To eliminate this problem, you should minimize dead corners and gaps when designing, and ensure that the moisture and cutting fluid in the blind holes are thoroughly blown away by the high-pressure air gun after the parts are processed.
Lntergranular Corrosion
It is a form of corrosion along the grain boundaries on a microscopic level, caused by improper heat treatment or welding. For parts with intergranular corrosion, you will find that their appearance is intact, but the bonding between the internal grains has been lost, and the metal breaks under force.
To prevent this, you need to choose ultra-low-carbon grades for your welded parts, or solution anneal the part after machining to return the material to its microstructure.
Materials Susceptible to Corrosion
| Material Category | Primary Corrosion Risks | High-Risk Environments | Standard Protection Strategies |
| Carbon Steel / Cast Iron | Uniform Corrosion (Rusting) | High humidity, unprotected outdoor exposure | Coating, Zinc plating, Black oxide. |
| Aluminum Alloys | Galvanic Corrosion, Pitting | Contact with dissimilar metals, Salt spray / Marine environments | Anodizing, Isolation using dielectric gaskets/washers. |
| Stainless Steels (304/316) | Pitting, Crevice Corrosion, Intergranular Corrosion | Chloride environments , Stagnant fluid crevices | Nitric/Citric acid passivation, Minimizing design crevices, Using low-carbon grades for welding. |
| Brass Alloys | Dezincification, Stress Corrosion Cracking (SCC) | Ammonia environments, Hard or slightly acidic water | Specifying Dezincification Resistant brass, Nickel plating. |
Industrial Impact of Corrosion
In fact, the losses caused by the corrosion industry are unimaginable, and the most direct costs include equipment replacement, routine maintenance, and anti-corrosion coating materials. According to survey data released by the international authority AMPP (formerly known as NACE), the direct cost of corrosion worldwide is as high as $2.5 trillion per year, accounting for about 3.4% of global GDP.
Secondly, the impact of corrosion on safety and ecology is also serious. Localized corrosion, such as stress corrosion cracking (SCC) or intergranular corrosion , often weakens the strength of materials from within the metal, resulting in many safety accidents. Moreover, many oil pipelines and underground storage tanks have been exposed to extreme environments for a long time. Once corroded and penetrated, toxic chemicals or crude oil will leak, causing ecological damage.
Finally, there is the impact of corrosion on precision machining. Many components used in the aerospace or medical equipment industries require extremely tight tolerances. Even a slight surface indentation can render a part unusable.
What is Oxidation?
Causes Of Oxidation
In most cases, oxidation occurs because the metal is exposed to extremely harsh conditions.
- Oxygen & Moisture: This is the most basic and common cause of oxidation of most metals.
- Salts & Chlorides: Salt, chlorine, and other halogens can act as oxidizing agents, causing metals to lose electrons.
- Acidic Gases: The acidic gas provides hydrogen ions to accelerate the cathode reaction. It also directly dissolves the protective film on the metal surface.
Chemical Reaction Process of Oxidation
Oxidation is not an isolated physical or chemical phenomenon. According to the fundamental laws of electrochemistry, the oxidation of any substance is necessarily accompanied by the reduction of another substance. Together, they constitute the core mechanism of material transformation-the redox reaction. Its essence is the transfer of electrons between different atoms or ions.
Below I show you two typical example equations:
1. When active metallic sodium is exposed to yellow-green toxic chlorine gas, it will burn violently and eventually produce the table salt we eat every day.
Overall reaction equation: 2Na+Cl2→2NaCl
Microscopic half-reaction: Na→Na++e−
2. When a brand-new aluminum block is exposed to air, it instantly reacts with oxygen.
Chemical Equations: 4Al+3O2→2Al2O3
Unlike corrosion, oxidation is not always destructive. For some metals, oxidation can also have beneficial effects. Let me introduce to you these two oxidation effects.
- Protective Oxidation: When aluminum (Al), titanium (Ti) or chromium (Cr) in stainless steel reacts with oxygen, a very thin and dense oxide film can be formed on the metal surface.
- Destructive Oxidation: When carbon steel or low alloy steel undergoes oxidation, the resulting structure is loose and porous, which can easily lead to ongoing corrosion within the metal.
What is Rust?
The Process of Rust Formation
When it comes to the formation process of rust, I will take the rust of iron as an example so that you can understand the process more intuitively.
1. Anodic Reaction
This is the starting point of oxidation. Iron atoms are affected by the environment and turn into iron ions dissolved in water.
Chemical Equation: Fe→Fe2++2e−
2. Cathode Reaction
The lost electrons react with oxygen and water in the air to form hydroxide ions.
Chemical Equation: O2 +2H2O+4e−→4OH−
3. Ionic bonding
Positively charged iron ions and negatively charged hydroxide ions join together in water and make white ferrous hydroxide.
Chemical Equation: Fe2++2OH−→Fe(OH)2
Conditions That Lead to Rust
- Higher humidity
- Contact with oxygen in the atmosphere
- Exposure to moisture, salt or acidic environments
- In high temperature environments, the speed of chemical reactions will accelerate
It can be seen that iron metal parts used outdoors are more likely to rust than those used indoors.
Commonly Affected Materials
| Material Category | Representative Material | Rust |
| Highly Susceptible | Carbon Steel | Extremely High (Flaky red rust) |
| Highly Susceptible | Cast Iron | High (Surface rusts easily) |
| Highly Resistant | Stainless Steel | Very Low (Rusts only in extreme conditions) |
| Self-Protecting | Corten Steel | Surface rusts, interior is protected |
| Cannot Rust | Aluminum Alloys | Surface oxidation only |
How to Prevent Rust?
Through the above introduction, I believe you have understood the principle of rust. In order to eliminate the problem of parts rusting, the underlying logic of rust prevention is already very clear. So what are the ways?
Coatings and Paint: You can paint or spray polymer coatings on metal surfaces to isolate the metal from contact with moisture and oxygen.
Oil and Grease: Lubricating oil and anti-rust wax are extremely hydrophobic and can both lubricate and isolate moisture.
Galvanizing: You can apply a layer of zinc to the surface of the metal components.
Alloying: Use stainless steel or alloys containing chromium.
Of course, you can also use rust inhibitors or desiccants.
Key Differences between Corrosion, Oxidation, and Rust
It can be said that when we talk about the degradation process of metals, these three are the most frequently mentioned terms. However, they actually represent completely different physico-chemical levels. The following table will give you a more intuitive display.
Core Difference Comparison Table
| Concept | Scientific Nature | Applicable Scope | Is Oxygen Required? | Ultimate Impact |
| Corrosion | Environmental degradation of a material | Engineered materials | Depends | Destructive |
| Oxidation | Loss of electrons at the microscopic level | All matter | No | Neutral |
| Rust | Specific electrochemical decay | Only Iron and its alloys | Absolutely | Devastating |
Common Industries Affected By Corrosion And Oxidation
Nowadays, metals have been widely used in multiple industries, so it is very necessary for you to understand which industries are affected by corrosion and oxidation. Check if your industry is among them.
1. Aerospace
The aerospace industry has extremely stringent material requirements.If the degradation of aircraft metal is not managed properly, the consequences can be catastrophic. Aircraft fuselages, landing gear and other components are easily affected by corrosion or oxidation.
2. Automotive
Cars are exposed to various complex and changeable climatic environments for a long time, so you must also pay great attention to the problem of metal degradation.
3. Marine
The marine environment is extremely harsh, and chloride ions accelerate the occurrence of corrosion and oxidation reactions. Therefore, for ships, subsea pipelines, and offshore platforms, it is essential for you to develop detailed protection strategies.
4. Construction
Structural integrity is critical to building safety. So you need to be very careful about corrosion and oxidation of steel frames and other metal parts.
5. Electronics
Once the circuit board of an electronic device is exposed to humid air, oxides will be formed on its surface, which can easily lead to problems such as short circuits on the board.
6. Energy Industry
Equipment in the energy industry is generally exposed to various extreme environments for a long time. If such equipment suffers from corrosion or oxidation, it can cause catastrophic safety leakage risks and huge equipment replacement costs.
What To Consider When Selecting Materials And Coatings?
After reading all the above content, you must have clarified the scientific boundaries between oxidation, corrosion and rust. But how should you use this knowledge in future engineering design or product manufacturing? I will sort it out for you and help you make the smartest engineering decisions.
1. Anti-corrosion strategies
Corrosion involves many industries, especially in marine, chemical or highly polluted environments. You must pay more attention to anti-corrosion design.
Material Selection
In terms of materials, you must be extremely cautious about galvanic corrosion and chloride ions. First of all, when designing, you must not directly connect two metals with huge potential differences. This will cause severe galvanic corrosion and accelerate metal corrosion.
Second, components intended for use in environments rich in chloride ions are susceptible to pitting corrosion, even if made of standard stainless steel. Therefore, you need to choose to use 316 stainless steel containing molybdenum (Mo), or directly use titanium alloy.
Coating Selection
Insulation isolation: You can choose to use special rubber or Teflon (PTFE) gaskets to isolate different metals.
Special anti-corrosion paint: You can add zinc-rich powder or chemical corrosion inhibitors to the paint. In this way, the coating is waterproof and can also neutralize corrosive ions.
2. Oxidation Prevention Strategies
Oxidation is not all bad. Skilled engineers know how to use the oxidation reaction of metals.
Material Selection
You can give priority to aluminum alloys and titanium alloys. When they come into contact with air, an extremely hard and transparent oxide film will form on their surface.
Coating Selection
Anodizing: Anodizing makes metal surfaces extremely wear-resistant and can also inject dyes into micropores.
High-temperature-resistant ceramic coating: For high-temperature components such as the automobile engine compartment and turbocharger, you must apply a high-temperature ceramic coating to isolate oxygen intrusion in extremely hot environments.
3. Anti-Rust Strategies
Rust prevention is very important for iron-based parts
Material Selection
In terms of material selection, you need to evaluate the cost and service life of good materials. For low cost, you can choose ordinary carbon steel, which has excellent mechanical properties and is cheap. For high costs, you can choose stainless steel, which will generate a dense Cr2O3 passivation film on its surface.
Coating Selection
Physical barrier coating: You can choose to use an epoxy or polyurethane primer. These coatings can physically prevent exposure to moisture, oxygen and iron substrates.
Sacrificial anode coating: Hot-dip galvanizing.
Conclusion
Well, that’s all the content. I believe you will gain a lot after reading it. I believe you will gain a lot after reading it. In addition, our DSR company is an excellent CNC machining manufacturer and can produce various types of parts according to your plans. Of course, we will also perform appropriate surface treatment for each material to ensure the quality of your parts. So if you have any custom CNC machined parts needs, please contact us quickly.
References:
1. Encyclopedia Britannica: Oxidation-Reduction Reaction.[Link]
2. ScienceDirect: Industrial Corrosion Types [Link]
FAQ:
Q1: What metal doesn’t rust or oxidize?
A: A very small number of precious metals such as gold(Au) and platinum(Pt).
Q2: What metals can you not shower with?
A: Iron-based alloys, copper alloys such as copper and brass, 925 sterling silver, etc.
Q3: What role does oxygen play in oxidation?
A: Oxygen is the main reactant in oxidation reactions, and it reacts with metals to form oxides.
Q4: Can oxidation occur without corrosion?
A: Yes. Oxidation can occur without corrosion. This is the well-known passivation phenomenon.
Q5: What are the indicators of oxidation in metals?
A: Decrease in metallic optical reflectance and color change.
Q6: What is the fundamental difference between corrosion, oxidation, and rust?
A: In short, oxidation is the principle of electron transfer, corrosion is the outcome of material damage, and rusting is merely a specific manifestation of oxidative corrosion in ferrous metals.
Q7: What two metals should not be used together?
A: Aluminum alloy and pure copper, carbon steel and stainless steel, zinc coating and copper.
Q8: Recommended protective coatings for outdoor metal structures against rust?
A: Hot-dip galvanizing, zinc-rich epoxy primer, aliphatic polyurethane, fluorocarbon.
Q9: Can you recommend industrial solutions to control corrosion and oxidation in pipelines?
A: For the exterior of the pipeline, you can use 3PE / FBE coatings and cathodic protection. For the interior of the pipeline, you can use chemical corrosion inhibitors and internal linings.
Q10: What are the most effective rust removal products for vehicles?
A: One is a chemical rust remover, and the other is a rust converter.
Q11: Does WD-40 really remove rust?
A: WD-40 is not a true rust remover, but an excellent rust inhibitor and penetrating lubricant. It does not have the chemical ability to dissolve or transform deep-seated iron oxide.
Q12: Will rust stop if kept dry?
A: Yes, if you can keep the metal completely dry, the rusting process will stop entirely.
Q13: What are the four types of rust?
A: Red rust, yellow rust, brown rust and black rust.
Q14: What metal can’t corrode?
A: Metals that are almost impervious to corrosion include iridium, platinum, and gold.
Q15: What’s the worst metal to weld?
A: Titanium and its alloys, as well as high carbon steel.
Q16: What metal does not oxidize?
A: Gold.
Q17: What is the healthiest metal to wear?
A: Titanium, platinum and high-purity gold.
Q18: Which metal does not tarnish?
A: Pure gold and platinum.
