Прокатная окалина на стали: что это такое и как её удалить

Mill scale is the hard, brittle iron oxide layer that forms on the surface of hot-rolled steel during manufacturing. Left under a coating, it is a common cause of premature coating failure in industrial, marine and infrastructure projects, and one of the most misunderstood surface contaminants in corrosion protection.

This guide explains what mill scale is, why it must be removed before a protective coating is applied, how to recognise it, which removal methods reach the cleanliness grades that coating specifications require, and how to verify the result.

What is mill scale?

Mill scale forms when steel is rolled or forged at high temperature. At hot-rolling temperatures the steel surface oxidises rapidly in air, producing a multi-layer iron oxide film. This film consists of three layers:

Scale thickness varies with section size, rolling temperature and cooling rate, and is commonly quoted in the range of tens of micrometres up to roughly 100 µm or more on heavy sections. The oxide is harder and more brittle than the steel beneath it and has a different thermal expansion coefficient, so it cracks and spalls when the steel flexes, heats or cools.

Why mill scale must be removed before coating

Mill scale looks clean and smooth, which is precisely why it is a poor substrate for protective coatings. Three mechanisms lead to coating failure:

1. Galvanic corrosion

Mill scale is electrically conductive and cathodic to the steel beneath it. Any discontinuity in the scale (a crack, a scratch, a cut edge) creates a galvanic cell between the scale (cathode) and the exposed steel (anode). Corrosion concentrates at the exposed steel and undercuts the coating from below, even when the coating surface still appears intact.

2. Adhesion failure

Protective coatings bond to steel largely through mechanical interlocking with the surface anchor profile. Mill scale is relatively smooth and does not provide the angular anchor profile that high-performance coatings require. Even if a coating adheres initially, the bond is to the mill scale, not to the steel. When the scale cracks, corrodes underneath or spalls, the coating comes away with it.

3. Moisture attack at the scale–steel interface

Moisture and chlorides that reach the steel through cracks in the scale corrode the steel at the interface. The corrosion products take up more volume than the steel they replace, lifting the scale and the coating above it. The result is blistering, flaking and undercutting that often starts at edges and damaged areas.

Key specification point: SSPC-SP 10 / NACE No. 2 / ISO 8501-1 Sa 2½ (near-white metal blast cleaning) requires removal of all mill scale, rust and coating; only light staining is permitted on up to 5% of each unit area. Every bare-metal grade, including SSPC-SP 6 (commercial blast), SSPC-SP 15 and SSPC-SP 11 (power tool), requires mill scale removal. Only SSPC-SP 2 and SP 3 (St 2 / St 3) allow tightly adherent mill scale to remain.

How to identify mill scale

Mill scale has a characteristic appearance: a blue-grey to black, smooth surface on new or recently processed steel. On exposure to weather it begins to show rust-coloured staining at cracks and edges. Identification markers:

• Blue-grey or dark grey colouration on new steel
• Smooth, slightly glossy appearance compared with bare or blasted steel
• Rolling marks in the rolling direction
• Patchy areas where the scale has already cracked or flaked, often at edges and corners
• Rust staining at cracks or mechanical damage, a sign the steel beneath is corroding while the scale around it remains intact

A copper sulphate spot test is a simple field check: bare steel plates with copper, while mill scale does not. In field practice, comparison with the ISO 8501-1 rust grade A photographs (steel largely covered with adherent mill scale, little if any rust) is the standard visual reference.

Mill scale removal methods

The removal method must reach the cleanliness grade in the project's coating specification. Common requirements are SSPC-SP 10 / Sa 2½ or SSPC-SP 6 / Sa 2 for blast cleaning, or SSPC-SP 11 where power tools are used. Four methods are used in industrial practice:

Abrasive blasting (conventional)

Abrasive blasting (sand, grit or shot) is the traditional reference method for mill scale removal. It reliably reaches SSPC-SP 10 or SP 5 and generates an angular anchor profile, typically in the 40–100 µm range, that high-performance coatings require. Limitations: it needs containment of abrasive media, creates dust and spent abrasive waste, usually requires hot-work permits or gas-free conditions in hazardous areas, and is logistically intensive for field maintenance on installed structures.

Bristle Blaster® (grit-free impact method)

The Bristle Blaster® removes mill scale by high-velocity wire tip impact rather than abrasive media. Forward-angled bristle tips, released by an accelerator bar at an operating speed of about 2,300 rpm, strike the steel and rebound. Because the scale is brittle and the steel is ductile, each strike fractures the scale and leaves a micro-crater in the steel, without any grit. On mill-scaled steel the tool produces surface cleanliness comparable to ISO 8501-1 Sa 2½ / SSPC-SP 10 and an anchor profile of 65–85 µm Rz on standard steel in documented tests. Results vary with steel grade, rust grade, belt and technique, so always verify against the coating data sheet.

Advantages over blasting for mill scale removal: no abrasive containment, no spent media disposal, and portable use on installed structures, pipelines and offshore platforms without a blast enclosure. The Bristle Blaster® Pneumatic is ATEX-evaluated (2014/34/EU, Category 2) for zone 1 gas (group IIA) and zone 21 dust. For small patches see how to remove mill scale from small steel areas, and for the procedure see how to achieve an SSPC-SP 10 result without sandblasting.

Chemical pickling

Acid pickling (SSPC-SP 8) dissolves mill scale by immersion in hydrochloric or sulphuric acid. It is effective for new fabricated steel in a workshop, but not practical for field maintenance or installed structures. It requires neutralisation and effluent treatment, and produces a clean surface with little or no anchor profile, so a separate profiling step is needed before high-build coatings.

Flame cleaning

Flame cleaning (historically SSPC-SP 4) uses an oxy-fuel torch to expand the mill scale differentially so that it cracks and loosens; the loose scale is then wire-brushed. It does not produce a bare-metal grade or an anchor profile suitable for high-performance coating systems, carries fire risk and can distort thin sections. It is rarely used for protective coating work today. The differences between descaling mechanisms are covered in mechanical descaling of mill scale.

Choosing the right method for your application

Verification: how to confirm mill scale is fully removed

After treatment, mill scale removal is verified by three checks used together:

• Visual assessment: compare the treated surface against the ISO 8501-1 reference photographs for the original rust grade (grade A for new steel with mill scale). No blue-grey scale should remain, only the matte grey of clean, profiled steel.
• Anchor profile measurement: use replica tape (ASTM D4417 Method C) read with a calibrated micrometer, or a depth micrometer (Method B). A profile well below the range expected for the method can indicate scale left in place.
• Soluble salt test: even after mill scale removal, soluble salts (chlorides, sulphates) may remain. Use the Bresle patch method (ISO 8502-6 with ISO 8502-9) and compare with the limit in the coating specification. The US Navy, for example, allows a maximum of 3 µg/cm² chloride for immersion service and 5 µg/cm² for atmospheric service.

For full verification of a near-white result, see our SSPC-SP 10 complete technical guide, and for the waste and exposure side, environmentally friendly mill scale removal.

Need help specifying the right mill scale removal method?

Our technical team can review your substrate condition, coating specification and site constraints, and recommend the right tool configuration and verification protocol for your project.

Frequently asked questions

Can you paint over mill scale?

Not for any coating system intended for long-term corrosion protection. Coatings applied over mill scale bond to the scale, not the steel, and fail when the scale cracks, corrodes underneath or spalls. Coating manufacturers specify a bare-metal grade for high-performance systems, typically Sa 2½ / SSPC-SP 10 or as stated on the product data sheet.

Is mill scale the same as rust?

No. Mill scale forms during steel manufacturing at hot-rolling temperatures and consists of wüstite, magnetite and haematite. Rust forms by electrochemical corrosion at ambient temperature in the presence of moisture and oxygen. Mill scale exists before any corrosion occurs; it is a product of the hot rolling process, not of service exposure.

Does mill scale prevent corrosion?

No. Intact mill scale looks protective, but it is cathodic to bare steel and accelerates localised corrosion at any discontinuity. In practice mill scale always has cracks, thin areas and damage, especially at edges, weld toes and corners. These become initiation sites for localised corrosion that undercuts the surrounding scale and any coating applied over it.

What is the best mechanical method for removing mill scale?

For local areas, an impact tool such as a rotating bristle tool. It fractures the brittle scale and leaves a 65–85 µm Rz anchor profile in the same pass in documented tests, so the steel is coating-ready without a second process. Grinders and wire brushes tend to polish the scale instead. For heavy scale, a Tercoo® disc first and a bristle finish second.

How fast does a Bristle Blaster® remove mill scale?

MontiPower documents about 1.1 m² per hour with a single-belt Bristle Blaster® on API 5L X42 pipe at rust grade B, and around 3 m² per hour on flat surfaces with the Ultimate Electric Double. Rates on full mill scale vary with steel grade, scale condition and technique. For large-area new fabrication, automated shot blasting is more economical.

How can mill scale be removed locally in an environmentally friendly way?

Mechanically, without media: no spent grit to dispose of, no silica from abrasive, no water or chemicals, and essentially only the removed scale itself as waste. Grit-free is not dust-free, because removed scale still becomes particles, so capture it with a dust extraction hood and wear suitable PPE. Cordless tools add battery operation on the CAS platform.