Mill Scale: What It Is & How To Remove It

Mill scale is the flaky surface of bluish-gray iron oxide that forms on hot steel as it cools after processing. While it might just seem like a cosmetic issue, mill scale can interfere with different industrial applications, especially welding, and painting. 

Besides affecting the quality of the weld, mill scale can also prevent paint and other coatings from sticking properly, which can lead to corrosion and deterioration. Therefore, understanding how to remove it is key to ensuring the longevity and durability of steel products in various industries. In this blog, the team at Red Label Abrasives explains what mill scale is, how it’s formed, and the best methods for removing it.

Understanding Mill Scale

Mill scale is a common byproduct of hot-rolled steel. It's essentially a type of iron oxide that develops during the steel manufacturing process. When steel is heated in a rolling mill, its surface reacts with the oxygen in the air to form this oxide layer. 

Origin and Formation

Mill scale occurs in the steel production line, mainly during the hot-rolling process. As steel is processed at temperatures above 1832°F (1000°C), the high heat causes the surface iron to react with oxygen in the air or water vapor, resulting in iron oxide formation. This layer forms as the outer surface of the steel cools, creating what we know as mill scale.

Primary Components

The primary component of mill scale is iron oxide, predominantly in the form of magnetite (Fe3O4), hematite (Fe2O3), and wustite (FeO), with magnetite being the most common. This iron oxide gives mill scale its characteristic bluish-black color. The composition may vary slightly depending on the specific conditions of steel production, including the type of steel and the atmosphere in the mill.

Characteristics and Physical Properties

Mill scale is typically thick, dense, and brittle. Its thickness can vary, usually ranging from a thin layer to several millimeters thick, depending on the steel production process and the time the steel is exposed to high temperatures. The color is generally bluish-black but can vary from a dark blue to a darker black hue. This layer sticks well to the steel surface initially but can become loose over time or when handled.

Impact on Metal Surfaces

Initially, mill scale acts as a barrier between the steel surface and the environment, offering a degree of protection against corrosion. The dense layer of iron oxides prevents direct contact with moisture, oxygen, and other corrosive elements, which can delay the onset of rusting on the steel surface. This protective effect is particularly relevant during the short period when the steel is stored or transported before further processing or installation.

Despite these initial protective qualities, mill scale can ultimately harm steel structures in several ways:

• Adhesion Issues:The presence of mill scale can create challenges when welding, painting, and coating. During welding, mill scale can cause poor fusion and weak welds due to its tendency to create a barrier between the joining metal surfaces. For painting and coating, it must be thoroughly removed to ensure proper adhesion. Paint applied over mill scale is prone to peeling and blistering, as the scale can detach from the steel surface, taking the protective coating along with it.
• Corrosion Underneath: If the mill scale becomes cracked or damaged, it can trap moisture against the steel surface, leading to accelerated corrosion. Over time, these areas can develop into significant rust spots that weaken the steel structure, especially in environments with high humidity or exposure to corrosive chemicals.
• Inconsistency and Unpredictability:The thickness and integrity of mill scale can vary across different sections of steel, leading to inconsistent protection. Areas with thinner or compromised mill scale are more susceptible to corrosion, making the steel structure vulnerable to localized damage.

While mill scale can initially provide a protective layer against corrosion for steel structures, its long-term presence is often more harmful than beneficial. For steel to be used in construction, manufacturing, or any application requiring durability and longevity, removing mill scale and applying appropriate protective coatings is essential. This ensures that the steel's surface is properly prepared for welding, painting, or coating.

Methods for Removing Mill Scale

The removal of mill scale is an important step in preparing steel surfaces for welding, painting, and coating. There are several removal strategies, each with its own advantages and applications. These methods can be broadly classified into mechanical, chemical, and thermal techniques.

Mechanical Methods

Mechanical methods for removing mill scale use force or abrasion to strip the layer of iron oxides from the surface of hot-rolled steel. These methods do not rely on chemicals or heat but instead use mechanical means to clean the steel. 

• Sandblasting:This method uses high-velocity sand or other abrasive materials propelled by air or water to blast away the mill scale. Sandblasting is effective for large surfaces and can quickly remove thick layers of mill scale. However, it requires specialized equipment and safety measures to protect against inhaling dust particles.
• Grinding: Grinding involves using abrasive wheels or discs like our EdgeCore ceramic flap discs to wear down the mill scale. This method offers precise control over the removal process and can be used on smaller areas or for finishing work.
• Wire Brushing:Wire brushing uses a wire brush attached to a hand-held grinder or drill to scrub the mill scale off the steel surface. It is suitable for light to moderate scale removal and can be used on irregular surfaces. This method is labor-intensive but effective for smaller projects or in situations where other methods are not feasible.

Pro Tip:The best abrasives for this task are those that can remove mill scale thoroughly without causing excessive damage to the underlying metal. One of the best abrasives for mill scale removal is ceramic. Ceramic is one of the strongest and longest-lasting abrasive grains on the market. It can effectively sand steel and remove mill scale quickly. We manufacture ceramic sanding belts, as well as ceramic flap discs. In some cases, a combination of abrasives may be used to achieve the optimal result.

Chemical Methods

These methods involve using chemical reactions to dissolve or loosen the mill scale. Unlike mechanical methods, which rely on physical force or abrasion, chemical methods use substances that react with the mill scale to facilitate its removal. These processes are particularly useful for treating large surfaces or complex shapes. Here are the primary chemical methods used for mill scale removal:

• Acid Pickling:This is the most common chemical method for removing mill scale. It involves submerging the steel in a bath of dilute acid (typically hydrochloric or sulfuric acid), which reacts with the iron oxides to dissolve them away from the steel surface. After pickling, the steel is thoroughly rinsed and often neutralized with an alkaline solution to stop the acid reaction and prevent corrosion.
• Phosphoric Acid Treatments: Unlike the more aggressive hydrochloric or sulfuric acids, phosphoric acid not only removes mill scale but also leaves behind a protective phosphate coating on the steel. This coating improves paint adhesion and provides some resistance to rust. Phosphoric acid treatments are often used as a preparation step before painting or coating metal surfaces.

These methods require careful handling due to the hazardous nature of the chemicals involved. Protective equipment, proper ventilation, and adherence to environmental regulations are essential for worker safety. 

Thermal Methods

These methods use high temperatures to cause the mill scale to crack, detach, or be otherwise easier to remove from the steel surface. Thermal techniques, which are highlighted below, can be particularly effective for certain applications.

• Flame Cleaning: This method uses an oxy-acetylene torch to apply intense heat directly to the steel surface. The rapid heating causes the mill scale to expand at a different rate than the underlying metal, leading to cracking and flaking off of the scale. Flame cleaning is a skill-intensive process that requires experienced operators to avoid damaging the steel with excessive heat.
• Induction Heating: This method uses electromagnetic induction to quickly heat the steel surface. An induction coil is placed near (but not in contact with) the steel, and an alternating current is passed through the coil. The resulting magnetic field induces currents in the steel surface, heating it rapidly. This localized heating can cause the mill scale to break its bond with the steel surface, making it easier to remove mechanically or through subsequent processing.

Thermal methods offer several advantages, including the ability to target specific areas of a steel piece. However, they also require specialized equipment and can consume a lot of energy. Care must also be taken to prevent warping or altering the properties of the steel.

Preparing Surfaces Post Mill Scale Removal

Following mill scale removal, especially with chemical methods, it is essential to thoroughly wash the steel surface. This washing process, which usually involves water mixed with chemical detergents, removes any residual chemicals, dust, or debris left from the removal process. The goal is to ensure that the steel is completely clean and free from any substances that could interfere with adhesion or cause corrosion.

If mill scale was removed using acid pickling or other chemical methods, neutralizing any remaining acid is essential. This usually involves applying a neutralizing solution, such as a diluted alkaline solution, to the steel to counteract the acid. Proper neutralization stops the acid's corrosive action and prepares the steel for drying and further processing.

Applying Protective Coatings

Applying a primer is the next step after cleaning and neutralizing. Primer serves as a foundational coat that improves the adhesion of paint and provides an extra layer of protection against corrosion. After the primer, paint can be applied in one or more layers. The choice of paint and primer depends on the intended use of the steel and the environment it will be exposed to. These coatings not only protect the steel from corrosion and wear but also give it an attractive finish.

If you prefer not to paint, there are several other coating options available. Galvanizing, which coats the steel with a layer of zinc, is a popular method for providing long-term corrosion protection. Other coatings, such as epoxy coatings, powder coatings, and thermal spraying, offer additional options for corrosion protection.

By thoroughly washing and neutralizing the surface, then applying appropriate protective coatings, the steel is protected against corrosion and prepared for use in a wide range of applications. Proper surface preparation lays the foundation for the steel's performance, appearance, and longevity, making it an essential process in steel production and fabrication.

Safety Considerations and Best Practices

When removing mill scale and preparing steel surfaces, safety comes first. Handling and disposing of hazardous materials, wearing appropriate personal protective equipment (PPE), and minimizing environmental impact are key aspects of a safe and responsible operation.

Handling and Disposal of Hazardous Materials

The use of chemicals, such as acids in mill scale removal, introduces the need for careful handling and disposal practices. Used acids and other hazardous materials must be treated with respect to prevent harm to workers and the environment. It's essential to:

• Use secure containers for transporting and storing chemicals to prevent leaks and spills.
• Follow local regulations and guidelines for the disposal of hazardous waste. This may involve neutralizing acids before disposal and ensuring that waste is disposed of at a certified facility.
• Implement spill response procedures to quickly and safely address any accidental releases, minimizing the risk of exposure and environmental contamination.

Personal Protective Equipment (PPE) Requirements

Personal protective equipment protects workers from chemical exposure, flying debris, and other hazards associated with mill scale removal. Essential PPE includes:

• Protective goggles or face shields to protect against chemical splashes and flying particles.
• Gloves resistant to chemicals used in mill scale removal processes.
• Respiratory protection when working with dust-generating methods or hazardous fumes.
• Protective clothing to prevent skin contact with harmful substances.

Training workers on the proper use and maintenance of PPE is as important as providing the equipment itself.

Environmental Impact Considerations

The processes used to remove mill scale and prepare steel surfaces can have significant environmental impacts, particularly when chemicals are involved. To minimize these impacts, take the following steps:

• Opt for environmentally friendly chemicals and methods whenever possible. For example, choose less harmful acids or mechanical removal methods that don't generate hazardous waste.
• Implement waste reduction and recycling practices. Reuse or recycle materials whenever feasible, and reduce the use of single-use items.
• Use water treatment systems to clean wastewater from washing processes before it is discharged, ensuring that it meets environmental standards.

With these guidelines, steel fabrication and processing operations can ensure the well-being of their employees and the environment.

Choosing the Right Removal Method

Selecting the right mill scale removal method is key to achieving the best results. Various factors influence this decision, from the type of metal and thickness of the mill scale to the desired finish and the resources available for the job.

Type of Metal

Different metals react differently to mill scale removal processes. For example, softer metals might require gentler mechanical methods or specific chemical treatments to avoid surface damage. In contrast, harder metals might withstand more aggressive abrasive sanding without compromising the integrity of the material. Knowing the properties of the metal you're working with is the first step in selecting an appropriate removal method.

Scale Thickness

Mill scale can vary in thickness from a thin film to several millimeters. Thicker layers might require more aggressive removal methods, such as abrasive blasting with steel grit or flame cleaning. Thinner layers might be effectively removed with abrasive sanding, wire brushing, or chemical treatments, which can be less damaging to the metal surface.

Desired Finish

How you remove mill scale depends on what the metal is going to be used for. If a smooth, polished surface is required, finer abrasives and detailed finishing methods might be necessary. If the surface finish is less critical, more cost-effective and time-efficient methods can be employed.

Cost-Effectiveness

It's important to consider your budget when choosing a mill scale removal method. Mechanical methods like grinding and wire brushing might require less upfront investment compared to setting up chemical treatments or purchasing specialized equipment for abrasive blasting. However, the labor costs and time investment should also be considered, as some methods might be more time-consuming than others.

Remove Mill Scale From Your Projects With Red Label Abrasives

Removing mill scale is a critical step in preparing steel surfaces for further processing, whether for welding, painting, or applying other coatings. Proper removal is essential not only for aesthetic purposes but also to ensure the longevity and durability of the steel. 

At Red Label Abrasives, we manufacture top-notch sanding belts, sanding discs, and other abrasive products designed to meet the diverse needs of our customers. Our products are crafted to assist in effectively removing mill scale, ensuring your steel surfaces are perfectly prepped for any task at hand. If you're uncertain about which abrasives are best suited for your needs or if you're ready to place an order, we're here to help. Please contact us by calling 844-824-1956 or filling out our contact form today.