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The recent introduction of ductile iron pipe (DIP) for snowmaking installations, along with its growing popularity, have been accompanied by claims for longer pipe life. Several months ago, corrosion engineer Hans Schmoldt of Anode Systems Company in Grand Junction, Colo., relayed his skepticism about some claims for the longevity of DIP, based on his experiences in the field. While DIP with a properly applied and fully intact coating will last longer than uncoated steel pipe, he said, there are many things that can degrade the coating and thus shorten the life of the pipe. The life of snowmaking pipe—either steel or ductile iron—depends to a great extent on the quality of external and internal coatings and other anti-corrosion steps taken, and not simply on the material itself, he added.

We asked SAM technical editor Yaroslav Stanchak to address the topic. Here’s his view.

—The editors

Snowmaking systems rely on metal pipe to deliver high-pressure water and, often, compressed air as well, to on-trail snowmaking equipment. The vast majority of this piping is either steel or ductile iron that, by their natures, are susceptible to corrosion (rust). In the past, and even in many instances to this day, the vast majority of buried snowmaking pipe has been installed without much thought given to the length of its useful service life. But that perspective is changing, and with good reason.

Unprotected, bare-surface steel and ductile iron pipe will corrode and eventually fail. The rate of corrosion depends on many factors, including but not limited to water, chemicals, bacteria, and electric currents, as well as the measures taken to combat these and other sources of corrosion. But make no mistake: both materials will eventually fail.

The question is, how rapidly will this failure occur? What can be done to slow this process down? What is a reasonable time span for the first failures to appear?

The overall goal for the minimum service life of normal snowmaking piping installations should be 20 to 30 years, while in critical areas the goal should be well over 30 years. The aim is to extend the usable service life of the pipe to a period that is cost-effective and fits into a resort’s long-term capital plans. Snowmaking pipe is a major asset, and should be treated as such.


EXTERNAL CORROSION
All soils are corrosive; however, the level or intensity of the corrosion will vary according to the soil composition and moisture conditions around the pipe. This variability has been noted by many ski areas. A buried water or air pipe that has been leaking in one specific location may show numerous holes when uncovered, while a short distance away the pipe appears nearly new. A localized corrosion hot spot has been identified!

Over the years individual ski areas have mapped out locations in their trail network and distribution piping that are corrosion hot spots. In one of these zones of doom, the life of bare ferrous pipe may be as little as five years, depending on wall thickness and pressure, while in a benign soil area that lifespan is perhaps 20 years or more.

With the understanding that metal corrosion is a relentless and ongoing process, how can it be slowed down? Many methods have been proven through years of research and testing. The simplest is to bond a non-reactive coating to the surface of the pipe, usually a long lasting polymer such as fusion bonded epoxy. Another method is a combination of bonded zinc with a polyure-thane coating; this is commonly used on ductile iron pipe manufactured in Europe. A third method favored by U.S. ductile iron companies is encasing the pipe in a heavy polyethylene bag.

All these methods can lengthen the lifespan of the pipe, but careful installation is required to prevent any major gouging/scraping of the pipe coating/ polyethylene and the metal surface.

Careful installation requires a methodical approach that lays and joins the pipe section by section. Dragging the pipe over bare ground defeats the purpose of the coating. Any dings or major flaws in the surface coating that expose the underlying metal can become a point of concentrated corrosion activity.

The same approach is required for backfilling the trench—crews must protect the surface integrity of the pipe and coating. Rocks in the backfill can scratch, and thus defeat, the pipe’s coating. In addition, the welded joints in coated steel pipe must be carefully sealed with the recommended product and procedure to prevent infiltration of moisture into the unprotected metal surrounding the joint.

Soil areas that exhibit a very high corrosion level, or piping sections in high-profile areas, require a more intensive technical approach to minimize the corrosion impacts and lengthen the service life of buried pipe. Long service life is especially desirable in base areas and main distribution lines and at road and stream crossings. The lifespan of pipe in these situations can be increased by using sacrificial anodes or cathodic protection in conjunction with external coatings. Professional technical help is recommended for these situations.


INTERNAL CORROSION
Pipes can corrode from the inside, too. Water piping will corrode internally according to the level of corrosive elements in the water. For many fortunate ski areas, this internal corrosion is a minor nuisance that is evident mainly during initial early starts. But some ski areas are plagued with severe internal corrosion and tuberculation that eats away at the inner diameter of the pipe and is unnoticed externally until failure.

It can be difficult to identify internal pipe corrosion as a contributor to pipe failure when the corroded external surface appears to be the root of the problem. But in a number of instances at ski areas, externally coated pipe has failed from a combination of external (faulty installation) and internal corrosion, only to be replaced with the same externally coated product—which then fails prematurely again. It is essential to discover the true failure mode.

How can you slow down internal corrosion? Steel pipe can be coated internally with a cement mortar, similar to the practice with ductile iron. Or, either product can be coated internally with a polymer or similar non-reactive coating.

But remember, proper handling is key! Installation of internally coated steel pipe will require careful adherence to recommended procedures to ensure a proper install.


ESSENTIAL POINTS
1. The service life goal for normal snowmaking piping installations should be 20 to 30 years, while in critical areas the goal should be well over 30 years.

2. Snowmaking pipe is a major capital asset—treat it as such.

3. In known corrosion hot spots (zones of doom), use externally coated pipe, carefully installed according to a) manufacturer recommendations and b) a liberal dose of common sense. Internally coated pipe would be a good long-term investment in these areas as well. Cathodic protection may be highly useful, too. Consult with an expert to make this install right for the choice of pipe material.

4. If you must use uncoated pipe in a known “zone of doom,” increase the wall thickness of pipe within this specific area. The yearly corrosion rate in this zone is relatively constant regardless of the pipe diameter and wall thickness, and thinner pipe will fail sooner.

5. For buried snowmaking pipes, the installation costs are typically the major expense. Coated pipe adds a small premium to the overall cost. If the designed lifespan of the installation meets the above recommendations, the extended ROI for the installation will pay for the coating many times over the initial costs.

6. Create a database of pipe installation dates and sizing, failures and repairs, and record the specific locations of known hot spots. Create an accurate map of snowmaking piping. These historical records will help your decision making and cost justifications.

7. Above-ground piping that is partially buried or in contact with wet soil needs to be approached in the same manner as buried pipe. Trail crossings are often problem areas. 8. In situations where corrosion problems are highly complex, consult a reputable snowmaking/corrosion technical professional.