September 22, 2025
Throughout the years, contractors, engineers, authorities, and customers have frequently asked questions that seem basic but actually highlight the fundamental versatility of Ductile iron pipe (DI pipe). Here are some of the most common inquiries:
Cement mortar linings are applied to Ductile iron pipes to prevent tuberculation, a process that can lead to the formation of deposits inside the pipe. The cement lining acts as a barrier between the metal and the water, preventing the absorption of minerals that could cause corrosion. Even if there are minor imperfections in the lining, they typically do not lead to significant tuberculation.
However, cement mortar linings are not necessary in sanitary sewer pipelines, as tuberculation is not a concern in such environments. In fact, if the sewer's pH drops below 4.5, the cement lining will degrade rapidly, turning into a mushy substance that washes away.
Although the exterior of Ductile iron pipe appears "bumpy," it feels relatively smooth to the touch, much like a golf ball. This texture is intentionally applied and unique to each DI pipe manufacturer. Known as the "peen," the circular dimples are machined into the forged-steel mold used during the centrifugal casting process. This pattern helps the molten iron to form the pipe wall in a productive and orderly fashion, preventing irregular movement of the molten iron within the spinning mold.
The peen pattern, which may vary between manufacturers, remains consistent along the entire pipe. This pattern also enhances frictional resistance with the surrounding soil, offering a better grip, which is advantageous when the pipe is buried underground.
The number of stripes on a Ductile iron pipe spigot has no functional impact on the pipe's performance. The stripes serve as visual indicators to maintain proper alignment during assembly, assess full insertion into a companion pipe bell, and help set the deflection limits of the joint. The difference in the number of stripes simply reflects the manufacturer and the gasket configuration used. One stripe indicates a Fastite™ gasket, while two stripes represent a Tyton™ gasket. Both types of gaskets differ in their shape, with Fastite™ being triangular and Tyton™ being semi-circular.
Friction is the primary enemy of a Mechanical Joint's ability to maintain a watertight seal. Applying the proper lubricant to the gasket reduces friction during assembly, ensuring that the gasket compresses uniformly and maintains its seal. Dry gaskets, on the other hand, cause significant friction, leading to insufficient torque and potential leaks once the pipe is filled with water. Always lubricate the gasket surfaces to avoid issues with loose or "weeping" joints.
The best way to know if you've reached the deflection limit is when the spigot of the pipe begins to push the bell into which it is being inserted. This is a reliable sign that metal-on-metal contact has occurred, and further deflection should stop to avoid damage. Unless excessive force is applied, such as from hydraulic equipment, the pipe and joint will not be damaged.
Copper, a metal known for its oxidation properties, forms a protective green patina when exposed to moist air, as seen in the Statue of Liberty. Unlike copper, Ductile iron pipes experience a similar oxidation process, but instead of forming a visually appealing patina, it leads to a cinnamon-colored, splotchy appearance. This oxide layer, though thin and barely measurable, prevents the pipe from degrading in standard conditions.
Self-Restraining Ductile Iron Pipe
Since the oxidation is not aesthetically pleasing, Ductile iron pipes are coated with a black, NSF-61 certified asphaltic-based paint called "sealcoat." This coating, originally designed to maintain moisture in the pipe's concrete mortar lining, is now optional for modern linings that no longer require it. Therefore, the paint primarily serves a cosmetic purpose, both inside and out.
The correct use and assembly of Ductile iron pipe and fittings is not complex, as long as the basic principles outlined here are followed. If you disagree with any statements or feel something was overlooked, feel free to contact us via email or phone. We're always happy to discuss any concerns.
