How to Remove Mill Scale: Step-by-Step Guide

Mill scale is a common surface characteristic that can be seen in steel fabrication and processing. When hot-rolled steel cools down in the presence of oxygen, a thin, firm coating of iron oxides is created on the surface. Although initially it may appear like a protective covering, it gradually causes problems for further procedures, including painting, welding, and long-term corrosion resistance.

The first crucial step in managing mill scale is to learn what it is,  its formation, and its inherent qualities. Apart from this, learning how to eliminate it is just as crucial to maintain the durability and quality of steel products and structures. This blog will give a brief definition of mill scale, explain why it must be removed, and then give an overview of the many mechanical, chemical, and thermal techniques used to produce a prepared and clean steel surface.

WHAT IS MILL SCALE?

Mill scale is a thin layer of iron oxides ( mostly hematite, magnetite, and wüstite), typically bluish-black in color, that builds on the surface of hot-rolled steel while manufacturing. This fragile coating, which is formed when heated steel reacts with ambient oxygen, is initially adhering but is susceptible to flaking and breaking as a result of handling or environmental conditions. 

Although mill scale offers some temporary protection against uniform corrosion while intact, it is usually considered harmful for further processing due to the following reasons:

• Provide a poor base for coatings: applying paint or other coatings over a mill scale might not work because the scale will break, letting moisture and air enter the surface and cause corrosion.

• Interferes with welding: mill scale serves as an electrical and thermal insulator, causing arc instability, reduced puddle fluidity, and possible porosity in welds 

• Causes uneven corrosion: Because of its electrochemical characteristics, the exposed steel may corrode more quickly if the mill scale coating is broken.

HOW TO ELIMINATE MILL SCALE?

There are various methods of removing or eliminating mill scales each having its advantages and disadvantages. Following is the list of techniques used to remove the mill scale:

1. Mechanical Methods

This method of removing the mill scale is based on physical force and abrasion to separate the oxide layer from the surface of the steel. following are the techniques used under the mechanical method:

• Abrasive Blasting: this is a highly effective approach for removing mill scale from large surfaces. The process involves high-velocity abrasive materials such as sand, steel grit, or smashed glass, driven by air or water to blast away the scale.

• Grinding: mills can be eliminated by using abrasive wheels or discs with power wheels particularly from smaller areas. It is advised to use silicon carbide discs as they are less likely to clog compared to other types. 

• Wire Brushing: this approach involves using a wire brush which is usually attached to a power tool, to eliminate the mill scale. It's perfectly ideal for removing lighter scale and uneven surfaces, although it can be time-consuming.

• Bristle Blasting: in order to eliminate mill scale and add a fresh layer of adhesive, this approach involves the use of a rotating brush with particularly designed curved tips that strike the surface.

These mechanical techniques have differing degrees of efficiency. They are frequently selected according to the steel structure's size and complexity, the mill scale's thickness and adhesion, and the required surface polish.

2. Chemical Methods

For eliminating the mill scale, the chemical method involves using acidic solutions to react with and dissolve iron oxide layers. Popular techniques that come under the chemical approach include:

• Pickling: this process involves the soaking of steel in a diluted acid solution that reacts with and dissolves the iron oxide. ensure to rinse and neutralize the steel thoroughly after pickling.

• Phosphoric Acid Treatment: apart from eliminating mill scale, phosphoric acid leaves behind a protective layer of phosphate which helps in increasing paint adherence and provides some resistance to corrosion.

• Vinegar: this process involves submerging small parts of steel in white distilled vinegar for about 24 hours, which helps in loosening the mill scale, and then it is easier to scrub away the mill scale.

These chemical methods effectively eliminate scale, especially for fine details and complicated structures, but need careful handling of chemicals, appropriate ventilation and proper disposal of waste materials. 

3. Thermal Methods

When exposed to quick and high heat, the steel substrate and the mill scale layer expand and contract differently, which is why thermal techniques for mill scale removal use this property. 

• Flame Cleaning: when the steel surface comes in contact with high heat from an oxy-acetylene torch, the mill scale can expand differently than the base metal, which results in cracking and flaking. This needs skills to prevent damaging the steel. 

• Induction Heating: the bond between the mill scale and the steel can also be broken by quickly heating the steel surface using an electromagnetic induction. 

SELECTING THE RIGHT METHOD

Selecting the correct method for eliminating mill scale is an essential choice that has an immense impact on the effectiveness, cost, and quality of procedures that follow. following are the key factors that should be considered while making a decision:

Scale of operation and Size/shape of the steel object

The scale of the operations and physical features, including the size and shape of the steel object, are crucial factors in selecting the right method. Abrasive blasting is frequently the most feasible and economical technique for major steel constructions, such as bridges, ships, or massive tanks, because of its rapid coverage of immense surface areas. pickling is also suitable for large parts, but it requires huge immersion tanks and material-handling machinery.

For smaller or delicate shapes, pickling can be an extremely amazing technique as it provides smooth and consistent scale removal, even in hard-to-reach areas. for smaller quantities or targeted removal, grinding, and wire brushing are perfect. 

While mechanical procedures may find it difficult to reach narrow crevices or interior corners, chemical methods are often excellent at reaching all surfaces of complicated forms.

Thickness and Adherence of the Mill Scale

When choosing the best approach, the mill scale's thickness and adherence are quite important. The amount of work required to remove the mill scale coating is directly correlated with its thickness. In general, thicker scales call for longer treatment durations or more forceful techniques. 

Similarly, adherence to the scale is also crucial- that is how well it sticks to the steel. Tenaciously or strongly bonded scale requires more aggressive techniques, such as abrasive blasting with coarser media. a loosely adhered scale, on the other hand, requires gentler methods such as wire brushing or milder chemical solutions. 

Hence, selecting a perfectly efficient removal method requires evaluating the thickness and how effectively it adheres to the steel. 

Desired surface finish

The final use of the steel determines the desired surface finish after mill scale removal. For the perfect or best coating adhesion, a bit roughened profile (anchor pattern) usually obtained by abrasive blasting or bristle blasting, is required. For welding, a clean and plain metal surface that is free from any contaminants is crucial and can be achieved by most methods. certain applications may have distinctive smoothness or texture requirements that prefer specific removal techniques. 

Available Equipment and Resources

The effectiveness of a mill scale removal method greatly depends on easily accessible equipment like blast cabinets, compressors, grinding tools, and essential protective gear. Additionally, the available resources like the availability of skilled workers who are able to operate the machinery safely and handle materials. as well as the essential infrastructure for power supply, ventilation, and waste disposal. Practical and economical mill scale removal requires selecting a technique that works with the infrastructure, available skills, and equipment already in place.

Cost considerations

There are a number of factors, including cost considerations in mill scale removal. Initial investments involve the cost of buying or renting tools and establishing facilities. operating costs include consumables such as abrasives—chemicals, energy, and fuel, as well as labor expenses. Time efficiency is also a cost consideration since quick removal minimizes labor hours and project deadlines.  

Also, expenses for disposing of waste materials and byproducts should also be considered. Choosing the most cost-effective approach requires balancing these expenses with the size of the operation and the necessary level of cleanliness.

Ensure to test your selected method on a scrape material to check if it generates the required outcomes without causing damage to the material being worked. ensure to follow proper safety precautions while using these techniques.