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Galvanic Corrosion

Numerous causes will result in the failure of a threaded pipe connection – a moderate to high corrosion rate primarily being responsible.  One fundamental and very obvious reason, of course, is the threading process itself – which removes 50% or more of the pipe wall beginning day one.

Other less common reasons include the failure of the thread sealant, poor machining of the threads, bad casting, poor quality of the pipe or fittings, vibration, stress, improper assembly, or excessive operating pressures beyond design.  In many cases, a metallurgical analysis may be required to identify the exact failure mechanism.

• Dissimilar Metals

A major cause of thread failure within a building or process plant environment is galvanic corrosion – where the carbon steel pipe directly meets a brass valve, or is transitioned to copper pipe.  Here, the microvolt difference in electrical potential of the metals will produce a small electrical current between them – the result of which is to greatly accelerate the deterioration of the more reactive and often termed “less noble” carbon steel pipe.

In effect, an extremely small DC electrical circuit is created, with the steel pipe serving as the anode, the brass fitting or copper pipe acting as the cathode, and the water serving as a weak wire connection completing the circuit.  In simplest terms, a very weak battery is created.

“Galvanic” corrosion occurs between any two dissimilar metals in contact with each other and water, and typically attacks the steel pipe to a degree somewhat dependent upon existing corrosion conditions.  Galvanic corrosion is, in fact, defined as an electrochemical reaction of two dissimilar metals in the presence of an electrolyte, typically water, and where a conductive path exists.  It is visually recognizable in its latter stages by some degree of deposit buildup where the dissimilar metals meet at the threads – creating a microfine leak.  At that point, however, most of the damage has already occurred and replacement is required.

• Effective Chemical Treatment Important

The presence of chemical water treatment, since it works by inhibiting the electrical pathway of the corrosion process itself, can be very effective at slowing down a galvanic condition.  Substantial differences in the effectiveness to slow a galvanic corrosion condition may exist between different chemical treatment programs, and even the most well maintained program can still result in threaded joint failures.

Galvanic corrosion is, however, far less likely to cause problems where chemical corrosion control is very well maintained, and where uniform corrosion rates of 1 MPY or less exist.  A piping system having a low general corrosion rate will often show no evidence of any galvanic condition at the hundreds of carbon steel to brass valves normally in service.  CorrView International, LLC has documented hundreds of examples where no greater wall loss could be found at direct steel to brass valve connections even after many decades.  Closed piping systems, almost by definition as having corrosion rates of 1 MPY or less, rarely show problems attributable to galvanic activity – moving the greatest concern always to open cooling tower systems and process plants.

In contrast, a problem condition at the pipe threads caused primarily by a high corrosion rate will be substantially accelerated due to galvanic activity – thereby turning an already bad situation even worse.  In many cases, blame is incorrectly placed upon the direct steel to brass connection as the source of problems, when a high corrosion environment is fundamentally at fault.

The cause of a thread leak can often be identified by a close look at the threads themselves.  The failure of multiple sections of threaded pipe at steel to steel joints such as at elbows, tees, and reducing bushings, without a significant increase of failures at the steel to brass or copper joints, always suggests a moderate to high corrosion condition as the underlying cause.  Again, this is especially common for cooling tower or open process water piping.

Conversely, observing multiple examples of corrosion products at only the brass valved side of a thread nipple would indicate that galvanic activity is the major force involved.  In many cases, however, a leak condition will involve some combination of both galvanic activity and normal corrosion losses.

• Misdiagnosis

While galvanic corrosion can produce widespread pipe failure, that failure is typically localized to the immediate area where both metals meet.  In most examples, higher corrosion activity may only affect a zone to approximately 1 in. away from the dissimilar connection itself.  A review of our photo gallery on galvanic corrosion, as well as the photos provided below, show multiple examples whereby a thread leak has occurred to the steel/brass connection, while absent at the steel/steel threaded connection only a few inches away.

Too often, however, the lack of dielectric fittings is blamed for a widespread pipe corrosion problem; directing corrective attention exclusively to one area while ignoring a potentially much greater threat.  In the overwhelming majority of our investigations, the presence of galvanic activity and prior pipe failure at the threads was only the first indication to a severe corrosion problem throughout the entire piping system.  Often dielectric insulators are installed and the problem then assumed to have been resolved – that is until the next piping failure.

• Other Important Factors

Pipe schedule is very important wherever threaded piping is involved, and heavier schedule 80 pipe is always recommended where a higher corrosion rate might be expected – such as at a steam condensate or condenser water system.  Schedule 80 should be specified exclusively for any threaded pipe serving a cooling tower or open process water system due to the higher corrosion rates commonly found today – this regardless of system operating pressure.  Should a galvanic condition exist, heavier pipe will offer longer service life to a degree again greatly dependent upon that level of corrosion activity.

Age is also an important factor – since even a moderate corrosion rate will not protect a decades old piping system from thread failure.  A section of 2 in. schedule 40 pipe having an initial wall thickness of 0.154 in., less its thread cut of 0.072 in., leaves only 0.082 in. of available pipe wall for service over its entire lifetime.  At a moderate corrosion rate of 3 MPY, such pipe will last roughly 20 years before completely wearing through.  Yet, the first signs of leakage can be expected years earlier – normally as the interior wall wears roughly to within 15-25 mils of the threads.

Internal pressure is still another factor.  While plant process piping may vary greatly in pressure, cooling water systems usually operate at low to moderate pressures.  Higher operating pressures exceeding 300 PSI rarely exist except at many high rise commercial office properties – but add greatly to the leak potential of any threaded connection in such applications.  Therefore, failures of any lower level threaded piping should not be assumed an isolated event, but rather a preview of a system wide weakness that will extend to the upper floor areas given sufficient time.

The below gallery of threaded pipe failures are due primarily to the galvanic action of the brass or copper attacking the carbon steel pipe.  Brass to steel problems far outweigh in terms of number that of copper to steel.  For the overwhelming majority of examples shown, a high corrosion condition exceeding 5 MPY was also found – greatly accelerating the galvanic effect.

A general rule to follow is that the worse a leak site looks, the worse it probably is.  Once a pinhole leak is produced at the threads as shown below, the leak often temporarily seals itself.  High corrosion is still proceeding its course, however, and should not be assumed to have stopped or slowed.  The consequences of a 2 in. line failure operating at 200 PSI can be devastating to any building property or plant operation in terms of water damage, and dictates the need to immediately address any such problems found. Total separation of a 2 in. threaded line under 150 PSI water pressure for even an hour at an office building can easily top $1 million in water damages alone, as we have seen.

Galvanic Induced Corrosion

• Even Greater Threat

By far a worst condition exists where galvanized steel is threaded to copper pipe or brass.  Here, the internal zinc finish increases the galvanic potential between the dissimilar metals.  Once the internal coating fails in areas, it allows a more aggressive corrosion attack against those localized sites.  Although typically used as a safeguard and assumed preventative measure against a corrosion threat, galvanized pipe will typically fail far sooner than carbon steel under equal conditions.

Galvanized pipe always demonstrates a far greater wall loss at the threads below the brass or copper connection, and presents a greater potential for catastrophic failure.  Even in some examples such as for fire sprinkler systems, the mixture of galvanized steel pipe and carbon steel elbows and tees has been documented to result in greater wall loss to the galvanized pipe due to galvanic forces.  Galvanized steel storm drain and sanitary waste lines show very similar loss at the threads where connected to carbon steel fittings – thereby raising the need to use similar metals throughout all installations.

Galvanized Steel To Brass

It should be noted that standard galvanized pipe looks very similar to silver dielectric insulators, and have been found installed as dielectric insulators at some facilities.  Whether by mistake or otherwise, both items are vastly different.

• Insulators Rarely Used

While galvanic activity is well recognized as a potentially serious threat to any piping system, dielectric insulators have only recently become more common to new piping specifications.  Most often, they are in response to a prior failure at a threaded valve connection, and frequently the wrong response where a high system wide corrosion condition is actually at fault.  Nevertheless, with generally higher corrosion rates throughout the HVAC industry and new steel pipe seemingly more susceptible to corrosive attack, the installation of dielectric insulators for open condenser water systems where steel meets copper or brass is always recommended.

Properly Installed

What’s Wrong With These Pictures?

CorrView International recommends establishing a strict specification for the use of dielectric fittings, as well as a close review of all construction work to ensure their proper installation.  The above set of photos represent the improper installation of the dielectric installed between the brass valve and copper pipe, rather than between brass and black steel – a surprisingly common occurrence and misinterpretation of its purpose by many piping contractors.  Attack is always from the brass or copper metal against the carbon steel, and this is where electrical isolation must exist.

In the top left photo, placing steel pipe between two brass fixtures sets up likely the most aggressive galvanic corrosion cell possible – only exceeded had galvanized steel been installed.  None of the above galvanic insulating fittings, installed at added expense, are providing any benefit, and have only added a false sense of security to system operation.

In short, the direct connection of dissimilar metals should be upgraded wherever possible to include dielectric fittings even though no external indication of a galvanic problem may exist.  This is even more critical where high corrosion activity is known or suspected.

• Further Examples

CorrView International, LLC offers a series of photo galleries taken from years of past ultrasonic piping investigations, which address the above and additional corrosion conditions.  A review of the different types of corrosion is often helpful in initially determining the likely corrosion cause.

In many cases, however, a combination of conditions will exist within the same piping system.  View our extended Photo Galleries of different corrosion types and failure conditions.

© Copyright 2023 – William P. Duncan, CorrView International, LLC