Article | Red Rust vs. Black Rust

Rust Problems in Factories

Iron is widely used in factories—for buildings, heavy machinery, and production equipment—because it is inexpensive and easy to process. However, those involved in factory maintenance are well aware that iron always comes with the issue of rust. This article discusses why rust forms, what problems it causes, and what to do when it occurs.

Why Does Rust Happen?

Rust forms when iron is left exposed to moist air. While many people know this, fewer understand the detailed chemical reactions involved. Looking at the process at a microscopic level reveals several stages:

1. Iron atoms (Fe) release electrons (e⁻) in water and become iron(II) ions (Fe²⁺).
2. Oxygen molecules (O₂) and water molecules (H₂O) in the water accept electrons and form hydroxide ions (OH⁻).
3. Iron(II) ions (Fe²⁺) and hydroxide ions (OH⁻) react to form iron(II) hydroxide (Fe(OH)₂).
4. Iron(II) hydroxide (Fe(OH)₂) reacts with oxygen (O₂) and water (H₂O) to form iron(III) hydroxide (Fe(OH)₃).
5. Iron(III) hydroxide (Fe(OH)₃) undergoes partial dehydration and turns into iron(III) oxide (Fe₂O₃), commonly known as red rust.

The key point is that rust forms as a result of electron exchanges between iron and oxygen in water. Therefore, applying a coating of oil or paint on the iron surface to prevent contact with water and oxygen can stop the reactions and suppress rust formation.

What Problems Does Rust Cause?

Iron is strong and flexible, but iron(III) oxide formed through rusting is brittle. Rusted iron loses the strength needed to support equipment or personnel. For example, rusted scaffolding can collapse or give way, leading to serious accidents. Additionally, the smooth surface of unrusted iron becomes rough when rusted, which increases its ability to attract water and oxygen—accelerating further rusting. This is commonly known as the phenomenon of “rust begets rust.”

What Should Be Done When Rust Occurs?

When rust appears, the first step is to remove the iron(III) oxide. This can be done physically with a wire brush or chemically with an acidic cleaner that dissolves the rust. However, what happens after rust removal is even more important.

After rust is removed, the exposed metal surface is highly susceptible to further rusting. Many acidic cleaners can actually accelerate rust if not properly neutralized. Therefore, it is critical to apply coatings or rust inhibitors such as paint or rust prevention oil to prevent recurrence after removing rust.

Another Approach: Black Rust

Another approach, aside from removing rust, is to utilize a different kind of rust—black rust.

Red Rust vs. Black Rust

Red rust is iron(III) oxide (Fe₂O₃), while black rust is magnetite (Fe₃O₄), a black-colored compound. Unlike red rust, black rust forms a dense, adherent layer that protects the underlying metal. This principle is used in Japanese traditional crafts like Nambu iron kettles.

These kettles undergo a process called kanake-dome, where they are heated over a 900°C charcoal fire. Iron reacts with water vapor to form Fe₃O₄ and hydrogen gas (H₂), changing the silver-colored surface to black. This black rust layer protects the kettle, allowing it to last a lifetime with proper care.

Under normal conditions, forming a stable black rust coating in factories is difficult. So, how can its protective properties be practically utilized?

Rust Conversion for Protection

One realistic method is rust conversion. This involves applying a special chemical agent to transform red rust into black rust, halting the corrosion process.

The main advantage is that the rust does not need to be removed. When rust is widespread, physical removal is labor-intensive, and using acid carries safety risks and may damage surrounding parts. Rust converters reduce these risks and costs. Unlike acid, they don’t require neutralization, and the remaining rough surface is ideal for painting.

However, rust converters are less effective on small, isolated rust spots and perform best on large, rusted areas. Additionally, their chemical reactions may not work well at very low temperatures.

Effective Rust and Corrosion Maintenance Solutions

Choosing the right rust prevention method depends on the on-site conditions. NCH offers a wide range of advanced solutions, including acidic cleaners that suppress rust recurrence, non-flammable water-based rust converters, and quick-drying protective aerosol coatings. Leveraging our expertise and product lineup, NCH provides tailored, effective rust prevention strategies to meet customer needs.

Kanake-dome

Ironware removed from the mold is fired in charcoal at a high temperature of approximately 900°C, forming an oxide film on the surface to prevent rust. This is a traditional technique unique to Nambu ironware.