Does Carbon Steel Rust？Reasons and Prevention

Does carbon steel rust? Surely it is——all the carbon steel is prone to rust. And for the behind reason is not only for it’s primarilly composed of above 98% iron content. The microstructure of carbon steel, as well as the humidity and temperature of the environment, also affect the rate at which carbon steel rusts. Additionally, since we understand that carbon steel is prone to rust, knowing how to prevent it is essential for your carbon steel part production plan.

Four Factors : Why Does Carbon Steel Rust?

Chemical Composition

First, the high iron content of carbon steel (98-99% iron, 0.1-2.0% carbon) makes it very easy for it to react with air and moisture—leading to rust (iron oxide). Carbon steel cannot form a surface protective oxide film like stainless steel does—where over 10.5% chromium forms chromium oxide to protect the interior from oxidation. Instead, the surface oxide of carbon steel is simply rust. Without surface protection, once rust forms (which is porous and easily flakes off), moisture and oxygen are more easily absorbed, causing further rusting.

Microstructure

The microstructure of carbon steel is primarily composed of ferrite and cementite, which easily forms galvanic corrosion—ferrite acts as an anode and cementite acts as a cathode. Once a spot starts to rust, the electrochemical corrosion caused by the internal structure (i.e., accelerated corrosion) . This microstructure makes carbon steel even more prone to accelerated rusting than pure iron.

Humid or Acidic Environments

Needless to say, if you are producing carbon steel products or components, it is best to use them in a dry indoor environment. For outdoor use, seaside salt spray, or chemical acids and bases, carbon steel is clearly not suitable and will rust very easily.

High Temperature

In high-temperature environments, if combined with humidity, carbon steel rusts faster. In environments with temperatures above 40°C and high humidity (RH > 60%) but below 100°C (where moisture evaporates), the electrochemical reaction activity of carbon steel peaks, and the rusting speed increases exponentially.

When the temperature exceeds 200°C, even in a dry environment, carbon steel will begin to undergo dry oxidation (forming scale). As the temperature rises, the diffusion rate of iron atoms increases, and the oxide layer thickens and flakes off.

Does High Carbon Steel Rust?

Surely it rusts. Actually, not only high carbon steel (carbon content 0.6-2.0%, more often lower 1.2%), but also low carbon steel (the mild steel, carbon content 0.1-0.3%) and medium carbon steel (carbon content 0.3-0.6%), they are the same prone to rust.

If it rusts, how do you remove rust from the surface of carbon steel?

You can use rust removers (such as WD-40) for chemical dissolution, or remove it through physical methods like sandblasting or wire brushing. After removal, oil or surface treatment must be applied immediately; otherwise, it will rust again very quickly.

Seven Measures: How To Prevent Carbon Steel Rust?

1. The Simplest: Keeping Dry and Mild Environment and Applying Industrial Oil

As we explored, a dry environment without chemical acids or bases, or salt spray, and a normal temperature range (daily use temperature 0°C – 35°C) is the best environment. For short-term storage of machined parts (or if you need low-cost maintenance for carbon steel components), applying Industrial Anti-rust Oil is the simplest and most effective method. The oil film isolates oxygen and moisture from the air, forming a physical barrier to prevent the carbon steel from rusting easily.

2. Zinc Plating (Galvanizing)

CNC machined carbon steel parts such as fasteners, bolts, nuts, and gears usually use a galvanizing process to form an externally sealed protection (to prevent corrosion). Additionally, it serves an aesthetic purpose—galvanizing typically makes carbon steel brighter and more mirror-like.

• Process: Through electrolysis (electro-galvanizing) or hot-dipping (hot-dip galvanizing), a layer of zinc is covered on the carbon steel surface. Zinc acts as a “sacrificial anode”; even if the coating is slightly scratched, the zinc will oxidize before the iron.
• Appearance: Usually appears bright silver, light blue, or iridescent (yellow zinc), which not only improves corrosion resistance but also gives the parts an attractive look.

3. Powder Coating

Similarly, powder coating—spraying powder onto carbon steel—is also applicable to carbon steel parts produced by CNC machining and is relatively cheaper than galvanizing (for the same surface area). Additionally, for large carbon steel structural parts produced by stamping, powder coating or painting can be chosen for better cost-effectiveness. However, it is not as durable or as well-sealed as galvanizing and may flake off more easily over time.

4. Black Oxide

Black oxide is also a cost-effective surface protection process for carbon steel.

• Process: The carbon steel parts are immersed in a high-temperature alkaline salt solution, forming a dense film of ferroferric oxide (Fe3O4) on the surface through a chemical reaction.
• Appearance: Presents a deep black color, categorized into matte black and glossy black (depending on the part’s surface roughness and whether it is oiled).
• Usage Scenarios: Commonly used for precision machinery parts, tools, and fasteners because it hardly changes the dimensional tolerances of the parts.

5-7. Other Protection Processes

The above are the most commonly used methods; others available for your selection include:

• Phosphating: Forms a water-insoluble phosphate film on the surface. Often used as a pre-treatment for painting (to increase adhesion) or used for wear-resistant parts (like gears) after being soaked in oil.
• Electrophoretic Coating (E-Coating): Commonly used for automotive carbon steel components; it requires an electric current but sprays paint.
• Electroless Nickel Plating: Does not require current; a nickel-phosphorus alloy is deposited on the part surface through chemical reduction. It offers extremely strong corrosion resistance and high hardness, and the coating thickness is exceptionally uniform, but it is quite expensive.

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Through this article, you have learned about the causes and contributing factors of carbon steel rusting, as well as several surface protection processes to prevent it.

We—VMT Factory—specialize in CNC machining various precision parts according to drawings and requirements, including carbon steel parts. At the same time, our factory, engineers, and our many partner surface treatment providers can provide you with a one-stop service for carbon steel material selection, rapid prototyping, mass production, post-processing, and surface treatment.

Share the Project That We’ve Done

Project Name: Mass Production of Precision 45# Steel Drive Shafts for an Automated Transmission System

Challenges & Requirements:

The customer requested high performance while balancing cost:

• High Strength: 45# steel (medium carbon steel) must be used to meet high-load transmission requirements.
• Appearance & Tolerance: Parts need to have a glossy black texture, while dimensional tolerances must be strictly controlled within ±01mm to ensure assembly precision.
• Cost: This is a high-volume production project, and the customer is very sensitive to the unit price, requiring costs to be controlled as much as possible while guaranteeing quality.

Solutions:

The VMT engineering team evaluated the requirements and suggested a combination of CNC Precision Machining + Black Oxide + Anti-rust Oil Immersion:

• Precision Guarantee: The black oxide layer is extremely thin (only 1-2 microns), which barely increases the part size. This perfectly maintains the ±01mm ultra-high tolerance after CNC machining, avoiding the risk of dimensional deviations often caused by plating processes.
• Appearance Effect: Through precision polishing before oxidation, combined with subsequent oil immersion, the part surface achieved the deep glossy black required by the customer, providing a high-quality industrial feel.
• Cost Advantage: Compared to expensive surface treatments like Electroless Nickel or Chrome plating, the cost of the Black Oxide process is only 30%-50% of those methods, saving the customer significant processing fees in mass production.

Results:

After the delivery of the first batch of 3,000 drive shafts, the parts appeared uniform and bright as new, with 100% of critical dimensions meeting the ±0.01mm tolerance requirement. This solution met all technical indicators while helping the customer significantly reduce procurement costs, gaining high recognition and leading to long-term orders.