What Is Internal Lining? Cement vs Epoxy

Internal pipe lining is a protective layer applied to the inner surface of steel or ductile iron pipe to prevent internal corrosion, reduce friction, and maintain water quality. The two most common internal lining types are cement mortar lining (CML) and epoxy lining. Selection depends on the fluid transported, pipe diameter, operating pressure, and design life requirements.

When Internal Lining Is Used

Internal corrosion occurs when the transported fluid attacks the pipe wall from inside. This is common in:

• Water pipelines: Dissolved oxygen, chlorides, and low pH cause internal tuberculation and pitting of unlined steel and iron pipe
• Gas pipelines: Wet gas with CO2 and H2S causes internal corrosion in carbon steel
• Produced water lines: High chloride and dissolved solids content accelerates internal attack
• Seawater injection systems: Highly corrosive, requiring reliable internal protection

Internal lining reduces corrosion rates, prevents contamination of potable water, reduces pumping costs (smoother surface = lower friction losses), and extends pipe service life by 30-50+ years.

Parameter	Cement Mortar Lining (CML)	Internal Epoxy Lining
Material	Portland cement + sand + water	FBE or liquid epoxy resin
Thickness	6-16 mm (varies by pipe size)	0.3-1.0 mm (FBE) / 0.5-2.0 mm (liquid)
Application	Centrifugal spinning	Electrostatic spray (FBE) / airless spray (liquid)
Max temperature	50-60°C	80-110°C (FBE) / 60-90°C (liquid)
Fluid compatibility	Potable water, wastewater	Water, gas, oil, chemicals
ID reduction	Significant (6-16 mm per side)	Minimal (< 1 mm per side)
Cost	Low	Moderate-high
Standards	AWWA C205, EN 10298, ISO 11298	AWWA C213 (FBE), AWWA C210 (liquid)
Pipe sizes	DN 80 - DN 3000+	DN 25 - DN 1500
Repair in field	Trowel application	Brush/spray application

Cement Mortar Lining (CML)

CML is the industry standard for ductile iron pipes and large-diameter steel water transmission mains. The lining is applied by centrifugal spinning: the pipe rotates at high speed while a lance deposits the cement mixture onto the inner surface. Centrifugal force compresses the mortar into a uniform, dense layer.

CML protects by maintaining a high-pH environment (pH 12-13) at the steel surface, which passivates the metal. Even if small cracks form during handling, the alkaline environment inhibits corrosion.

Standard CML thicknesses per AWWA C205:

Pipe Diameter	Minimum CML Thickness
4”-12” (DN 100-300)	6 mm
14”-24” (DN 350-600)	8 mm
26”-36” (DN 650-900)	10 mm
38”-48” (DN 950-1200)	13 mm
50”+ (DN 1250+)	16 mm

Internal Epoxy Lining

FBE and liquid epoxy internal linings provide a thin, smooth, chemically resistant barrier. They are preferred over CML when:

• The fluid is not water (gas, oil, chemicals)
• ID reduction from CML is unacceptable
• Operating temperature exceeds CML limits
• Enhanced flow efficiency (low roughness) is required

Common Mistake

Cement mortar lining is not suitable for soft water (low alkalinity, low pH) without proper water treatment. Aggressive water can dissolve the cement lining over time, increasing water hardness and pH downstream while reducing the lining thickness. Always check the Langelier Saturation Index (LSI) of the water supply before specifying CML. Waters with a negative LSI (aggressive/corrosive) may require epoxy lining instead.

For more on internal and external protection systems, see the guide to pipe coating, lining, and cladding.

Read the full guide to pipe coatings