The Fundamentals of Mill Surface Defects

Updated on: 08/08/2022

Surface defects are a common problem in the manufacturing industry, and they can significantly impact the quality of a product. Several factors can cause surface defects, including the milling process, the material being milled, and environmental conditions.

In many cases, surface defects can be avoided by using high-quality materials and properly maintaining the milling equipment. However, when surface defects occur, a variety of methods can be used to repair them. The most effective way will depend on the type and depth of the defect. With proper care and attention, surface defects can be corrected, so they are not visible to the naked eye.



One common problem that can occur is surface peeling or flaking. Due to poor adhesion to steel, this defect looks like the galvanized coating is peeling after forming. It is also called adherence failure or peeling galvanize. The defect can be caused by either overheating the zinc in the galvanizing pot, entering a strip that is too hot into the galvanizing pot, or low levels of aluminum in the galvanizing bath.

All of these instances can accelerate the formation of the zinc/iron alloy layer between the steel and molten zinc. This layer is brittle and causes flaking when the layer is too thick. Atmosphere problems in the heating furnace can also result in flaking by forming an oxidized layer on the steel and slowing or preventing the formation of the zinc/iron alloy layer.

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Slivers are segments of steel that have been torn from the steel and rolled onto the surface of the metal. This defect is most common in low-strength steel, which can easily tear due to incorrect or high-speed rolling processes. Slivers can also start from defects that were not previously removed by conditioning. They are commonly mistaken for shearing and laps. They can also be random, pulled up steel areas, referred to as pull slivers or lamination. A locked-up segment roll can cause this at the continuous caster, creating a random gouge in the slab.


Rust is a common problem for steel surfaces. It can cause the surface to become discolored and can eventually lead to corrosion. There are a few types of rust, but the most common are white, black, and red.

White Rust (Transit Problem)

Primarily a zinc-related defect, this defect can appear in non-linear patterns that can come and go randomly throughout the coil. This defect always has a mirror finish and happens close to the mill edge. It is also known as condensation rust or white stain. This defect occurs during the transport of the coil, where the coil gets wet in transit. The mirror finish of the defect confirms the intrusion of moisture from the sidewall. Because the moisture usually collects at the bottom, the lower portions of coils affected by this defect are generally worse in their severity.

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Black Rust (Transit Problem)

Like white rust on galvanized material, this galvanneal defect is not continuous and is random throughout the coil. It also includes a mirror finish that is close to the mill edge. This defect looks black due to the iron content in the galvanneal coating, which is absent in a galvanized coating. It is also known as condensation rust or black stain. As with white rust, the cause for this defect is exposure to moisture during transit. Black rust usually forms on painted or galvanized surfaces.

Red Rust

Red rust is the most aggressive type of rust and can cause serious damage to steel surfaces. This defect looks like oval-shaped drips of rust throughout the coil. It can be called red rust or rinse stain. The defect usually happens due to a leak in the rinse tank that sits above the pickle line’s accumulator. As steel fills the accumulator, the car passes under the dripping tank, and the rinse solution drips onto the sheet.

Steel product surface defects can occur during production, storage, or transportation. While some surface defects are cosmetic and do not affect the strength or performance of the material, others can lead to premature failure or reduced service life. It is important to identify common surface defects to take corrective action and avoid further damage.

Chatter Marks

Chatter is a pattern that can be seen on the surface of the steel coil and appears in intervals. It can be caused when the material is out of balance or formed imperfectly to create a non-circular form. Chatter marks are not a defect in the material’s structure but cannot be corrected or eliminated once they appear. In addition, Chatter marks usually show through paint that is applied.

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A scab defect occurs by splash or boiling from teeming, casting, or conditioning. Scabs are usually round or oval-shaped, have irregular surfaces beneath them, and can be seen before the rolling process. They are often mistaken as slivers, shearing, or rolled in scale. However, scabs are distinct in that they appear in only isolated occurrences.


If a part is not up to specification, a few corrective actions can be taken. One option is to re-mill the part, which means running it through the mill again. This can help to improve the tolerances and overall accuracy of the part.

Another option is to use a grinding wheel to remove any excess material. This can be a more precise way of correcting the part and leaves a smooth finish.

Finally, another option is to hand-finish the part. This involves using manual tools to remove any imperfections. While this may be a more time-consuming option, it can be very effective in terms of achieving the desired results.

It is important to understand the causes of surface defects to produce a high-quality steel product. Common problems include scale formation, rust, pitting, and staining. These defects can be prevented in many cases by carefully controlling the manufacturing process.

For example, maintaining a clean and well-ventilated workspace can help to prevent scale formation. Regular inspections can also help identify potential problems early on, allowing corrective action to be taken before defects can occur. By taking steps to reduce surface defects, manufacturers can produce steel products with a superior finish.


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