1.0 WHY DOES DI HAVE A LONGER LIFE CYCLE THAN STEEL?
- Corrosion Mechanisms: Corrosion occurs in both DI and steel, and the corrosion process in both systems requires oxygen. But beyond that point, the corrosion mechanisms for each material are very different.
- For DI the dominant corrosion mechanism is surface oxidation. A uniform oxide layer forms and permanently bonds to the DI surface; and, over time this oxide layer reduces the amount of oxygen that can reach the metal surface, reducing the rate of corrosion. Corrosion of DI is a gradually diminishing process over time.
- For steel there are two corrosion mechanisms: localized “pitting”; and, the formation of a loose scale of iron oxide, sulfates and nitrates on the steel surface. Over time layers of this loose scale drop off; and, are replaced by new layers of loose scale, formed in an on-going process. Corrosion of steel is a continuous process.
- Corrosion Rates: Corrosion Rates for all materials are site specific, defined by the corrosivity of the soil in which the material is imbedded. At one corrosivity level, rates are: 0.8 mil/year for DI, and 3.0 mils/yr for steel. Absolute rates often vary from site to site, caused by changing corrosivity levels. But, many field tests have shown that the differential between these corrosion rates remains relatively constant at all corrosivity levels: Steel corrodes 3-4 times faster than DI.
2.0 WHY IS DI A GOOD INVESTMENT WITH SUCH HIGH MATERIAL COSTS?
There are three (3) simple reasons why the DI system is a good investment: Longevity; Installation; and Hydraulics.
- LONGEVITY: For DI longevity is greater for three (3) basic reasons:
- Corrosion Rates: Steel corrodes 3-4 times faster than DI.
- Wall Thickness: For a working pressure of 1250 psig, wall thickness for 8″ DI pipe is 0.353 inches; and, adequate wall thickness for 8” steel pipe ranges from 0.219 to 0.250 inches. For normal operations, the wall thickness of DI pipe is ±40.0% higher than for steel pipe. For steel pipe, some clients use higher wall thicknesses to provide additional corrosion protection. This begins to equalize the material costs of the two options.
- External Coating: All DI materials in the VRS®-T system have two external coatings: A pure zinc undercoat directly on the iron surface, providing active and passive corrosion protection; and, a Polyurethane (PUR) external coat providing passive protection of the zinc undercoat. The PUR coat is similar to Fusion Bonded Epoxy (FBE) coatings frequently used on steel pipe.
- The passive protection of the zinc coating: This protection process begins while pipes are stored in the open air. Zinc reacts with rainwater and the air to form basic zinc carbonates in the pores of the PUR overcoat; and, these carbonates gradually close the pores, which gradually reduces access to the zinc undercoat; but, does not completely inhibit materials from reaching the zinc undercoat. When pipe is installed in the soil, zinc forms insoluble compounds including zinc oxides, hydrates and other zinc salts of different compositions; and, over time these insoluble compounds form a uniform impermeable crystalline layer, firmly adhering to the iron surface.
- The active protection of the zinc coating: If there is damage which extends down to the surface of the cast iron, an electrochemical cell forms at the damaged point. The exposed surface of the cast iron is cathodic, and the zinc crystalline layer on the iron surface is anodic. Zinc ions from the crystalline layer migrate to the bare cast iron and form a “scarring” layer, which over time stops corrosion of the bare cast iron. The mass of the zinc coating has been increased from 130 g/m2 to 170 g/m2; and, field tests have shown a considerable increase in the operating life of the pipes at this higher level.
- This coating system is not available from any other manufacturer.
- INSTALLATION: For DI installation is easier and less expensive for several reasons:
- With the VRS®-T gasket, alignment is not as important, an important benefit on steep terrain;
- With the VRS®-T gasket, lower insertion force is required;
- All flanged/threaded fittings in VRS®-T systems are ANSI/API specifications; no special fittings;
- Angular deflection at each joint reduces the number of fittings required, especially in areas with tight constraint; with steel systems many joints would have to be mitered/welded;
- No special handling or bedding materials required; not possible with FBE coated steel pipe;
- No thrust blocks required on any pipe sizes through DN300 (12″);
- Backfill materials may contain grain sizes up to 4″; not possible with FBE coated steel pipe;
- Assembly/Installation can be completed by in-house personnel;
- No heavy welding equipment required on site; certified welders not required for installation;
- No heavy Zap-Lok equipment required on site.
- HYDRAULIC CHARACTERISTICS: Hydraulically DI pipe is superior to steel pipe:
- In numerous Mechanical Engineering Textbooks, Roughness Coefficients (RC’s) are recommended for hydraulic design using different materials. For cement lined DI pipe the recommended Roughness Coefficient is 140. For new/clean – emphasize NEW/CLEAN – steel pipe the Roughness Coefficient is 115. As steel pipe ages, internal scale forms and the RC decreases further, to a range of 100/105. Over the life of any system, DI pipe has superior hydraulic characteristics.
- With DI pipe, any client has three (3) possible options:
- Deliver more volume with the same power;
- Provide higher residual pressure in all lines with the same power; and/or,
- Use less power to deliver the same volume at the same residual pressure.
3.0 HOW LONG WILL A DUCTILE IRON SYSTEM LAST?
- It is not possible to provide a specific answer to this question, because the corrosion rate of any material is defined by the corrosivity of the soil in which the material is imbedded. DI is not infallible, DI corrodes. At one corrosivity level, corrosion rates are: 0.8 mil/year for DI, and 3.0 mils/yr for steel. Absolute corrosion rates vary from site to site, defined by local corrosivity levels. But, many field tests have shown that the differential between these corrosion rates remains relatively constant at all corrosivity levels. That said, if a steel pipe system will last 30 years, a DI pipe system will last ±110 years, in the same soil.
In an effort to more clearly show these differences, PNP has included photos below showing the results of these different mechanisms, under actual field conditions.
Both DI and steel pipe and fittings were relocated in 2015, after seven (7) years of service, to accommodate trail widening operations. This photo shows the amount of corrosion for both materials, IN THE SAME SOIL CORROSIVITY.
Why Ductile Iron Pipe?: The collage of photos attached, shows the condition of FBE coated steel pipe, which had been installed as new clean pipe, after 7 years of service. This photo shows the “scaling” and “pitting” processes, occurring continuously in normal stages of corrosion with steel.
A 24″ .375 wall coated steel water pipeline after seven years of service.