Mill Scale Removal

Chapter from the Metallurgy & Corrosion Design Guide.

MILL SCALE REMOVAL REQUIREMENTS

What Is Mill Scale and Why Remove It?

Mill scale is the dark oxide layer (Fe3O4 / magnetite) that forms on steel surfaces during hot rolling at the steel mill. It typically ranges from 0.003" to 0.020" thick.

Why It Must Be Removed:
1. Electrochemical: mill scale is cathodic to base steel — creates galvanic corrosion cell
• Accelerated localized corrosion under scale in corrosive environments
• Pitting initiation under disbonded scale

2. Process contamination: mill scale flakes off during operation
• Contaminates catalyst beds (iron poisoning)
• Plugs small-bore instruments, control valves, orifice plates
• Damages pump impellers and mechanical seals

3. Coating/lining failure: mill scale provides poor adhesion surface
• Coatings applied over mill scale will fail by disbondment
• Linings (rubber, glass) require bare metal surface

Removal Methods

Method	Surface Prep Grade	Description	Cost	Speed	Best For	Limitations
Abrasive blasting	SSPC-SP10 (near-white)	Grit/sand blast to remove scale	Medium-High	Fast	Vessels, piping, structural	Dust, access needed, containment
Acid pickling	SSPC-SP8	Acid immersion (HCl or H2SO4)	Medium	Moderate	Piping spools, tubing	Hydrogen damage risk, waste disposal
Power tool cleaning	SSPC-SP11	Needle gun, grinding, wire wheel	Low-Medium	Slow	Spot repair, limited access	Inconsistent removal
Chemical passivation	N/A	Citric acid or proprietary blend	Medium	Moderate	New piping systems, complete loops	Must be circulated, neutralized
Mechanical pigging	N/A	Abrasive pig through pipeline	Medium	Fast	Pipelines, long runs	Requires launcher/receiver
Water jetting (UHP)	SSPC-SP12	Ultra-high pressure water	High	Moderate	Vessels, sensitive areas	No anchor profile created

When Is Mill Scale Removal Required?

Always Required:
• Catalyst service — any piping or vessels upstream of or containing catalyst beds
• Amine service — mill scale accelerates corrosion under amine solutions
• High-purity service — instrument air, N2, H2 systems
• Any coated or lined surface
• Piping for reciprocating compressors (debris damages valves)

Usually Required:
• Hydrogen service — cleanliness for leak tightness
• Sour service — scale creates local corrosion cells in wet H2S
• Pump suction piping — debris damages seals and impellers

Not Typically Required:
• Cooling water service (with treatment program)
• Low-pressure utility steam
• Atmospheric vent piping
• Structural steel (unless being coated)

Source: Metallurgy_Corrosion_Design_Guide_v1.xlsx · sheet “Mill Scale Removal”

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