Surface oxidation on ductile iron castings

Most ductile iron covers and gratings are supplied with a thin transit-paint coating. This provides a uniform finish and short-term protection during handling, transport, and storage.

Once in service, the coating gradually gives way to reveal the iron beneath. This allows the surface to undergo its natural oxidation process, developing the stable protective patina which ensures long-term durability.

At first, the surface may appear bright orange. This is only a temporary stage and does not affect the product’s strength or durability. Over time, the orange colour transforms into a darker, stable finish that protects the iron beneath.

Because our products are manufactured from ductile iron, this oxidation is superficial and does not compromise long-term performance.

A patina is the thin, protective layer that naturally forms on the surface of ductile iron and steel over time when they’re exposed to the atmosphere.

For manhole covers, gully grates and other ironwork, the patina is usually made up of stable oxides such as goethite and hematite. These oxides adhere tightly to the surface and act like a barrier, slowing down further corrosion.

It’s a bit like a self-defence mechanism. Once the patina develops, it helps the product last longer without needing extra protection. This is why, in normal inland conditions, ironwork performs well. The patina dominates and keeps the metal stable.

When the initial transit coating has completed its role and the iron surface is exposed, formation of the protective patina begins:

1. Initial Surface (Transit Paint / Fresh Iron)
   1. As delivered, products have a uniform transit-paint coating.
   2. This paint is porous on sand-cast surfaces and is not intended for long-term corrosion resistance.
   3. Its role is to provide short-term aesthetic and handling protection before natural oxidation begins.
2. Early Oxidation (Bright Orange)
   1. Rapid formation of lepidocrocite (γ-FeO(OH)) and/or ferrihydrite (Fe₂O₃·0.5H₂O).
   2. These hydrated oxides are vivid orange, porous, and short-lived.
   3. This stage often appears soon after exposure to moisture, once the coating has given way.
3. Intermediate Stage (Brown/Red-Brown)
   1. Lepidocrocite and ferrihydrite transform into goethite (α-FeO(OH)), a more stable and adherent brown oxide.
   2. This explains the natural darkening from orange to brown.
4. Mature Patina (Dark Brown/Black)
   1. Surfaces become dominated by goethite and hematite (α-Fe₂O₃).
   2. Goethite forms by recrystallisation of earlier orange oxides.
   3. Hematite develops both by direct precipitation in dry, oxygen-rich conditions and by dehydration of goethite over time.
   4. Together, these phases create a dense, protective layer that slows further corrosion.

Example comparison of grey iron (left) and ductile iron (right) under a microscope

In traditional grey iron, corrosion can advance along graphite flakes, producing graphitic corrosion that undermines strength.

In ductile iron, graphite is present as rounded nodules rather than flakes. These nodules do not create significant galvanic sites, so oxidation is slower, more uniform, and remains at the surface. This ensures ductile iron retains its mechanical integrity and load-bearing capacity, even after decades in service. 

• Lepidocrocite (γ-FeO(OH)) – bright orange, metastable, transitional phase.
• Ferrihydrite – poorly crystalline, orange-red precursor.
• Goethite (α-FeO(OH)) – brown, stable, adherent oxide; typically develops by conversion from lepidocrocite/ferrihydrite.
• Hematite (α-Fe₂O₃) – dark red-brown, very stable; forms directly in dry, oxygen-rich conditions or by dehydration of goethite.
• Akaganéite (β-FeO(OH,Cl)) – chloride-associated phase, relevant mainly in marine or salt-exposed conditions.

Akaganéite is most likely to appear in chloride-rich environments, such as coastal air, areas treated with de-icing salts or in polluted atmospheres.

This matters because akaganéite is hygroscopic, meaning it absorbs moisture, and it retains chloride ions. This makes it less stable and more corrosive than the protective oxides that normally form on iron surfaces.

The impact of this is that akaganéite can slow down or even disrupt the formation of the stable patina. Instead of protecting the surface, it sustains ongoing corrosion.

For reassurance, in normal inland service conditions akaganéite is minimal or completely absent. In these environments, the protective goethite and hematite patina dominates and provides long-term stability.

In aggressive chloride conditions, however, additional protective coatings such as bitumen or epoxy are typically specified. These are applied in line with relevant standards and customer requirements.

Goethite is not only a later-forming oxide but is usually the conversion product of earlier, less stable phases such as lepidocrocite and ferrihydrite. With wet/dry cycling, these phases recrystallise into goethite. It can also form directly in moist, alkaline conditions.

Hematite originates in two ways:

• Direct formation in dry, oxygen-rich conditions
• Transformation of goethite through dehydration during long-term weathering.

Its stability and density give the patina its darker finish and further limit oxygen and moisture access, reinforcing the protective effect.