By Expert No. 8966, PE
Among a homeowner’s more traumatic experiences, especially if the home is fairly new, ranks the telltale wet spot on the living room carpet (if the plumbing is in the slab), or the soggy bits of not-so-dry-wall (if the plumbing runs overhead.) An inspection is likely to reveal more than just a single trouble spot, and the usual verdict is an expensive replumbing job. The homeowner is upset and wants to be made whole, so he sues the most likely culprit – the water company. After all, it was the water that caused the problem. Or was it not?

The corrosion engineer, in trying to determine the most probable cause of a plumbing system failure, must look for, not just one, but two factors: 1) What initiated the problem, and 2) what propagated it.

There are some aggressive (corrosive) waters which will dissolve metals with which they come into contact, just as there are some saturated ones that deposit heavy scale in water heaters and hot water systems. An aggressive water will usually cause general corrosion. That is, it will dissolve metal at a uniform rate throughout the internal pipe surfaces. The loss of metal at any one point is thus very minor, and the system generally attains its design life. If the same loss of metal were to be confined to just a few small points in the system, however, the loss of metal at those ‘pits’ may result in perforation of the pipe and catastrophic failure of the system due to ‘pitting corrosion’. While there are some waters with a greater propensity to cause pitting, the great majority of pitting corrosion is initiated by factors other than the water.

Some of the initiating factors include manufacturing defects such as internal surface imperfections, poor workmanship in the installation of the pipe (flux runs, excessive heating, stagnation-formed films in copper pipe, overthreading in steel pipe, faulty joints, direct contact of dissimilar metals [galvanic corrosion], etc.) and wrong (too small) pipe sizes, resulting in excess water velocity thus causing erosion corrosion.

So does the water play any role in this? Of course it does. For corrosion to occur, the corrosion curent must flow from the anode to the cathode. This requires a conductive medium – the water. All waters (with the exception of pure distilled water) are capable of carrying the corrosion current. And some waters are indeed more aggressive than others. Frequently, the water is implicated only inasmuch as it sustains the corrosion reaction – but the water is not the primary cause for the corrosion to occur.

Internal Corrosion of Plumbing

Simplified Galvanic Cell

Note that areas A and B are located on the inner pipe surface.

Title 22 of the California Code of Regulations (formerly CA Administrative Code) has for a long time, required water purveyors to deliver a “non-corrosive” water to their consumers, while the (federal) Safe Drinking Water Act specifies that the water shall not be “excessively corrosive”. Neither regulation is what might be deemed quantitive.

The number of truly ‘aggressive’ waters has been severely curtailed, and may be in the process of being eliminated by a more recent federal and state regulation called the “Lead and Copper Rule” (LCR). The LCR severely limits the amount of lead and copper (either naturally occurring or present due to corrosion) the water may contain, as delivered to the consumer. To achieve compliance with the LCR, water utilities must optimize their treatment so as to minimize corrosive tendencies of their waters. The LCR has gone a long way toward controlling generalized (uniform) corrosion. The fact that localized (pitting) corrosion is (and will probably continue to be) a problem, is indicative of factors other than the water as being the initiating causes for internal corrosion failures of domestic, commercial and industrial plumbing systems.

Thus, in order to determine the most probable cause of a corrosion failure, the investigator must not only possess, or have access to, expertise in corrosion science, electrochemistry and metallurgy, but also expertise in water chemistry and unit operations of water treatment.

Expert No. 8966, PE is a registered corrosion engineer, certified water treatment operator and certified water quality analyst. He is highly experienced as a consultant and expert witness specializing in analytical and process chemistry, water & wastewater analyses, hazardous waste management, corrosion processes & prevention, project & operations management, swimming pool sanitation equipment & swimming pool water chemistry, water quality & treatment, wastewater treatment & disposal, food & drug chemistry, environmental sciences & engineering, and laboratory management.

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