Corrosion of steel pipes and related components is a continuous and almost unstoppable process. Even with the correct application of the available countermeasures, the costs for the management and controls, replacement and repair of corroded pipes are very high and turns out to be one of the worst problems in the industrial sector (but also in the private one). In addition to the intrinsic and natural corrosion of the metal, the situation appears to be worsened by the increasingly worse quality of the materials used, as well as ineffective or rare controls.
Practical examples of pipe corrosion problems include fire or heating systems, the deterioration of which can lead to drops in heat transfer efficiency, narrowed pipes, leaks (minor to severe), temporary shutdowns, operational failures, interruption of productivity, clogging of spray nozzles, personal injuries. The failure of a fire prevention system to function can lead to a very serious impact, including the destruction of the property and the loss of human life.
The most common corrosive problems found in pipes are listed
Pipe repairs take various forms, ranging from temporary clamps to replacing entire sections. In many examples, failure to resolve a corrosive problem results in multiple or incremental repairs over years, wasting valuable time that could otherwise have been used to correct the root failure and minimize the consequences. Hasty and palliative repairs can only increase the problem, without solving the real corrosive trigger mechanism.
Leaks in the threads
Each pipe thread is an inherent weak point, with approximately 50% of the pipe wall etched. Often, a leak in the thread is the first sign of a corrosion problem and requires further investigation. If not addressed the pipes could separate causing devastating damage. While the condition of large pipes is normally the biggest concern for a building or facility operator, it is the threaded pipe that typically causes the most damage.
A high pitting condition will cause failure in random areas of the threads, allowing water to pass through. For small leaks where the evaporation rate exceeds the water loss, dissolved iron oxide and other deposits will accumulate on the threads misleading the assessment of a corrosion problem that appears to originate on the external surface.
Deposits inside the pipes
Internal rust deposits represent an inevitable deterioration sentence for most piping systems. They are represented by the lighter and less dense final product of the corrosion of the steel pipes which, accumulating, gradually generates deeper and increasingly larger pitting.
The phenomenon is more likely to occur on horizontal lines and in low-flow or dead-end areas, ideal for conveying and depositing residues and hydroxides that will catalyze corrosion.
Chemical treatment is a potential solution to many internal
corrosive pipe problems. Washing the pipe with a chemical solution removes any
corrosion products, then pumping film-forming and inhibitory substances coats
and protects the surfaces. Similar treatments can be carried out both
internally and externally to the tube.
It represents one of the most damaging forms of corrosion for a piping system. Often called "cellular corrosion", it is typically very aggressive and localized and causes deep penetration of the metal surface with less overall corrosion in the surrounding areas. The triggering mechanism can be traced back to surface deposits, electrical imbalance, microbiological attack or other corrosive factors. Galvanized pipes are highly sensitive to the aforementioned phenomenon.
In some cases the pitting extends across the entire surface of the metal, giving it an irregular or very rough surface profile. In other cases, the holes are concentrated in specific areas, leaving most of the metal surface in like-new condition.
Complete rust removal and the use of chemical inhibitors are the top priority to mitigate failure. If the surface rust is light it can be easily remedied by maintenance personnel by sanding and applying an effective rust inhibitor and an external protective coating. If not removed, surface rust will develop into stratified layers against which only sandblasting will be effective for complete removal.
There are many cases in which pipes, especially those
positioned in high or poorly accessible areas, are left to deteriorate due to
poor or missing maintenance.
Of all forms of corrosion caused to piping systems, weather damage (rain, snow, weather, overspray from cooling towers) is the easiest to prevent. The pipes are exposed and accessible, with corrosive activity always visually evident.
Most weather damage takes decades to produce failure, and if it occurs, it is simply due to lack of maintenance. Smaller diameter pipes are increasingly vulnerable due to their inherently lower wall thickness.
Fiberglass insulation failure
Standard fiberglass insulation provides an ineffective moisture barrier for cold water pipes. Moisture from the condensed area produces a secondary, typically hidden, corrosion condition on the outside of the pipe. Most common in cold water and dual temperature systems, external corrosion losses far exceed internal corrosion rates by up to 10 times or more. Cold water pipes often have insufficient insulation thickness, are the wrong type, are installed incorrectly, or all three.
Over the course of decades and hidden from view, insulation
failures can destroy entire piping systems. In most examples where insulated
pipe is observable, the problem of this type of failure is very easy to
recognize. This scenario is often found in apartments and condominiums where
dual-temperature heating and cooling risers have been installed on the exterior
pillars of the building. After decades of installation, the rising pipes will
hardly still be intact, in most cases they will have been almost completely
destroyed from the outside; the only option will be the total replacement of
the pipes, with enormous costs and inconveniences.
Missing, damaged, stepped on and broken insulation defines a possible problem worth investigating. Water damage, dripping pipes, discoloration or crystallization of the exterior insulation surface are additional signs that suggest a potential problem. The aluminum and vinyl exterior coating offers little resistance to moisture migration and hides discoloration, often revealing underlying failure. The vinyl liner usually traps condensed water on the cold surface of the pipe, increasing the risk of external corrosion of the pipe.
Smaller diameter pipes are more vulnerable due to the
generally lower thickness of insulation applied and the naturally thinner pipe
wall. The tube on the supply side is always impacted worse due to colder
surface temperatures. The addition of threaded ends adds an additional layer of
vulnerability for the smaller pipes commonly found in cold water and dual
Foam insulation failure
Soft foam insulation allows moisture to infiltrate cold pipe surfaces and cause destruction to common steel pipe. In a relatively short time, soft foam insulation deteriorates, hardening, cracking and shrinking to produce large spaces where moisture can enter.
Additionally, the foam chemically degrades to a slightly acidic state, bonding to the pipe or rust layers so tightly that removing the old soft foam insulation becomes extremely difficult.
To combat the problem, you can use painted fiberglass with a high-solids coating that acts as a moisture barrier. Soft foam insulation should only be used for temporary or short-term applications.
Microbiologically influenced corrosion
Microbiologically influenced corrosion (MIC) is by far the most serious and threatening form of corrosion for HVAC piping and fire protection systems. It is caused by the presence of various microbiological agents in specific environmental conditions and can, in some cases, cause advanced and widespread failure of entire piping systems within a few years.
The presence of an MIC usually signals a very serious threat to the entire system, requiring extensive cleaning and repeated sterilization at high cost. For many affected systems, the MIC cannot be eliminated and a high corrosion and pitting condition will remain for the remainder of the system's life.
Microbiologically influenced corrosion produces large, deep
cavities due to the use of the steel tube itself as an energy source (often as
an alternative to oxygen), as well as through the production of highly
corrosive metabolic byproducts such as sulfuric acid, which assists further the
microorganism in dissolving the metal of the tube.
MIC exists in varying degrees of severity and is not
exclusive to carbon steel piping systems or open condenser water systems. MIC
is commonly found in closed chilled water lines, particularly glycol winter lines,
and has also been documented to destroy copper, brass and stainless steel
Welded pipes have a greater vulnerability to corrosion at the joints, becoming a focal point where rust and microorganisms can establish and promote corrosive activity, often leading to failures and pinholes.
The causes can be attributed to the cheaper manufacturing, which often produces an incomplete internal or external seam, in addition to the fact that the different materials used (between the tube laminate and the welding material) cause a differential in the electrical potential. Finally, the protective zinc coating on the solder is sometimes missing or incomplete, causing a very premature but defined galvanic attachment line.
Defective welded pipes are still widespread, especially in the foreign market. Under low corrosion conditions, a defective pipe can produce mild or moderate problems, but under higher corrosion conditions the consequences can become serious.
Cooling tower and exchanger corrosion
One of the first signs of corrosion in a hydraulic system usually appears in the cooling tower; White deposits often suggest a fouling condition.
Discolored, cloudy water is yet another indication that corrosive activity is high and chemical water treatment is deficient. Algae and other organic formations not only interfere with operations, but also accelerate many other corrosion processes and promote microbiologically influenced corrosion.
In many cases, cooling tower maintenance and rust deposit removal are performed without ever investigating the underlying cause of the problem and without considering that the volume of rust deposits exposed on the tower is nothing compared to the volume of rust deposits. rust still inside, firmly attached to its walls.
Most heavy rust deposits form after decades of high corrosive activity and break off for transport to the cooling tower after some form of shock to the system, such as a spring start-up or temperature change. They are rarely captured by most filtration systems or removed or dissolved by chemical aids.
Galvanic corrosion failure
Galvanic corrosion can occur when different metals (especially in an acidic environment) are joined together and depends mainly on the existing corrosion conditions and the piping system involved. It is most common, for example, in open condenser water systems, process water systems, and chilled water or fire protection systems. It commonly occurs between a carbon steel pipe joined to a brass valve or in connections between galvanized steel and brass valves.
A bluish-green deposit on the valve and the absence of leaks on the opposite steel-to-steel connection provide confirmation of the existence of a galvanic condition; can produce widespread failures and total pipe separations. This type of corrosion is insidious and is sometimes hastily misdiagnosed, automatically attributed to leaking threads between steels and brass/copper valves. Replacing the tube with dielectric insulating fittings (they act as insulators between two metals, preventing electrolysis) is expensive and is only worth it in confirmed cases of galvanic corrosion.
Microvolt differences in ground potential between building piping and structural steel (such as supports) can cause very localized failures.
Because they require highly sensitive electrical instruments to make a positive diagnosis, ultrasonic tests performed further from the fault area often show much less or even normal corrosive activity.
Although this form of electrolysis rarely occurs, it is
prudent to isolate the metal-to-metal contact, especially where the steel pipe
is exposed to water and atmospheric conditions.