Cast Iron Pipe Corrosion: Scale, Channeling and What to Do

Cast iron corrodes from the inside. Over decades, rust builds up as scale and lumpy tubercles on the walls, and the flow line along the bottom wears a channel into the metal. On camera, the first looks like a rough, bumpy pipe that has shrunk. The second is a groove along the invert, and eventually a crack or hole where the bottom has thinned through.

Both matter, but they call for different fixes. Scale is a flow problem that descaling can address. A channeled or rotted-through bottom is a structural problem.

What you’ll see on camera

Scale and tuberculation

• Rough, orange-brown to black buildup coating the wall.
• Tubercles: raised lumps or nodules that stick out into the pipe and catch paper and grease.
• Reduced opening. In heavy cases the camera head barely fits, and the line runs slow long before it stops.

Channeling (“canoe” or bottom rot)

• A groove along the invert where flow has worn the metal away. It is often easier to see after cleaning.
• Longitudinal cracks along the edges of the channel.
• Holes or missing bottom, with soil, gravel or a void visible under the pipe. Water may disappear into the ground instead of flowing on.

Channeling typically shows up in horizontal runs: the building drain under a slab, and the lateral where it is cast iron. Vertical stacks lose wall more evenly.

Why the bottom goes first

In a horizontal run, the invert is where water, solids and grit move every day, and where water sits when the line isn’t running. That combination wears and corrodes the bottom of the pipe faster than the crown. The result is the classic “canoe” shape: a crown that still looks fine and an invert that has thinned or opened. That’s why a quick look at the top of the pipe says little about a cast iron drain. Get the head low and look at the bottom after cleaning.

Coding it

There is no single PACP code for “cast iron corrosion.” The observations fall into existing families. Buildup attached to the wall is logged in the Deposits Attached family (DAE is encrustation). Wall loss belongs in the Surface Damage (S) family, which includes codes such as SMW (missing wall). Cracks, fractures and holes along a channel use the normal crack (C), fracture (F) and hole (H) codes; see cracked sewer pipe. If you code to PACP, follow the manual for your version on which descriptor fits.

Descaling vs lining vs replacement

Camera finding	Typical path	Why
Light scale, wall and invert intact	Descale, re-scope	Restores flow; nothing structural to fix
Heavy tuberculation, wall intact after cleaning	Descale, then consider lining	Liner needs a clean wall to bond; lining slows further corrosion
Channel started, no holes, pipe round	Descale, then line	Liner carries the load where the invert is thin
Channel through, holes, soil visible over a short stretch	Spot replace that section, line the rest if sound	A liner has nothing to bridge onto if the bottom is gone
Widespread holes, cracks, missing bottom	Replace	Too much host pipe is lost

Descaling

Descaling uses rotating chain or abrasive heads on a drain machine to knock scale and tubercles off the wall and bring the pipe back toward its original inside diameter. It is also the prep step for lining. It does not restore metal that has corroded away.

Two cautions:

• The pipe may be thinner than it looks. Descaling a badly channeled invert can open a hole that the scale was holding closed. Set that expectation with the customer before you start.
• Scope after. The post-descaling video is what the lining or replacement quote should be based on.

Lining

Cured-in-place lining forms a new pipe inside the old one. It needs a cleaned, descaled host pipe and a reasonably intact shape. Short sections of missing bottom can sometimes be handled, depending on the lining system; large voids or collapsed sections usually cannot.

Replacement

When the invert is gone over long stretches, or cracks and holes run through much of the line, replacement is usually the better value. Under a slab that means breaking concrete or tunneling, which is why a clear video showing footages matters: it defines how much of the run must come out.

Recording it well

• Footage every change in condition: where scale starts, where the channel begins, where holes appear.
• Clock position of cracks along the channel edge (usually around 5 and 7 o’clock).
• Locate the worst spots with a sonde so the crew knows where to open the floor. See sewer cameras with a locator.
• Pick the right head size. Heavily scaled 3” and 4” cast iron can be tight; a smaller head passes where a large one hangs up. See how to choose a sewer camera.

Cast iron also fails at hub joints and cracks, where roots can get in if the line runs outside the foundation.

FAQ

Does descaling fix cast iron?

Descaling removes scale and tuberculation and restores flow area, and it is the first step before lining. It does not put back missing wall, seal cracks or stop leaks. If the invert is channeled through, descaling alone will not solve the problem.

How do I tell scale from a channeled bottom on camera?

Scale and tubercles are bumps that stick out into the pipe. A channel is the opposite: a groove worn into the invert. After cleaning, a channel shows as a smooth trough along the bottom, sometimes with a line of daylight, soil or a crack along its edge.

Can you line a pipe that has a hole in the bottom?

Sometimes, if the rest of the pipe still holds its shape and the lining system is rated for it. If the bottom is gone over a long stretch or soil is coming in, most contractors recommend replacing that section. The decision belongs to the lining contractor, based on clean post-descaling video.

Why inspect again after descaling?

Heavy scale hides the wall. You only see the real condition, including channels, cracks and holes, once the pipe is cleaned. A post-descaling video is what the lining or replacement quote should be based on.