The Key Causes of System-Dependent Corrosion in Piping Systems
The corrosion problem is a complex issue, and rarely is one simple factor responsible for a piping failure.
In any piping system, there are a variety of factors operating alone or together that will have an effect on the rate of corrosion occurring in a pipe and thus its service lifetime. Every piping system, in fact, has specific susceptibilities, precise corrosion threats, engineering schemes and faults, and maintenance necessities. Depending on the degree of those factors, a brand new piping system will show signs of corrosive wear in as little as two years after installation.
Among the corrosion issues, a significant proportion is related to piping systems. Piping systems are used to carry a variety of fluids—corrosive or not—in an exceedingly wide selection of environments. A major lifetime cost for any industrialized plant is the cost of leaking fluids and the cost of repair of the piping system to prevent and eliminate leaks.
Corrosion watching for any system needs basic information of its common corrosion issues, combined with an understanding of the system design. Because the severity of corrosion is very dependent on the piping system, it is vital to grasp the various piping systems resulting in corrosion, and to understand system-dependent corrosion.
In Piping, Corrosion Is System-Dependent
In principle, no material is entirely corrosion-resistant. However, materials react differently depending on the conditions to which they're exposed. Beneath unfavorable conditions, corrosion may be intensive, while under ideal conditions, materials might corrode very little. (An example is copper piping, which is discussed in detail in the article If Copper is a Noble Metal then Why Are My Pipes Corroding?) Corrosion is thus not material-dependent, but system-dependent. A necessity for developing a corrosion-protection strategy that might stand up to the severest of conditions and thus prolong the lifespan of products is knowledge concerning the underlying system dependency that causes corrosion damage.
Corrosion characteristics are thus totally different between piping systems, such that they'll never be evaluated alone. For example, it has been found that condenser water has abundantly higher corrosion than chiller water, steam condensate more than steam, and dry fire pipe corrodes considerably more than wet fire pipe.
Pipe materials and their age also play a big role. The connecting pipes of varied materials and their differences in electrode potential might cause galvanic corrosion and the damage of pipes, valves and different equipment within the system. Beneath certain circumstances in environments like saltwater with free ions (discussed in An Intro to Pipeline Corrosion in Seawater), acids or bases, higher temperatures and enough oxygen, a system can deteriorate very quickly. A very common combination in piping systems is copper and low carbon steel. Such connections cause higher steel corrosion than steel alone.
Common Areas of Corrosion in a Piping System
The rate of corrosion in a piping system is determined by the following factors:
- The pH of the water
- The quantity of dissolved oxygen in the water
- The chemical composition of the water
- The quantity of galvanic corrosion from the use of dissimilar metals used in or in contact with the piping system
- The temperature of the water
- The velocity/pressure of the water within the pipe
- The elements of metals
Often, two or more individual factors are the explanation for failure or corrosion issues. Some issues are clearly associated with chemical treatment of the contents of the pipe, whereas others are associated with the pipeline's original design, selection of materials and operational conditions.
What degree of corrosion activity might exist is greatly dependent upon the kind of piping system concerned. Closed systems generally show very low corrosion and corroding activity, whereas open condenser or cooling system loops show the highest rates. An open current system also tends to show the highest variability in corrosion test results. This suggests that wall thickness may vary at supply and return and at different areas of the pipe. This will increase the chance that any corrosion coupon testing performed at one space isn't representative of the system.
For many cooling water loops, and particularly for open current systems, dramatically different conditions will exist at numerous points throughout the piping layout. Often, the particular causes of such corrosion variations, like at low-flow areas or at long horizontal runs, are inescapable. Similar variations in corrosion activity will exist at different areas of a fireplace protection system. Therefore, corrosion areas may be predicted by merely examining the physical configuration of the piping system. The following factors contribute to corrosion in a piping system.
The physical geometry of a pipe greatly influences corrosion in a piping system. For example, lower floor areas of constant piping systems generally suffer a greater degree of corrosion and corroding activity merely owing to the settling of dirt, rust, organic material and particulates. For several larger layouts, flow rate decreases furthermost from the current pumps to permit settling of even the best particulates. Higher wall loss in several cases also exists wherever piping has been reduced in size and thus has less accessible wall thickness. The corrosion rate of five MPY can really turn out tremendous volumes of iron oxide every year, which can settle to provide secondary corrosion issues if not removed.
System-dependent corrosion may also depend on whether the pipe is in a horizontal or vertical orientation. Horizontal sections of pipe generally show a higher degree of sediment and deposit buildup, corrosion and corroding than vertical sections. Wherever a higher-than-average corrosion rate occurs, ultrasonic testing can generally document considerably larger wall loss and corrosion on its bottom surface. Coupled with low-flow conditions or the periodic loss of flow, as may occur with individual HVAC package units, horizontal piping will suffer considerably higher corrosion rates.
Bottom sediments or deposits may also influence system-dependent corrosion of piping. Inside horizontal sections of pipe, and sometimes relying on rate of flow, the lowest and lower facet wall areas typically show considerably higher metal loss owing to the settlement of rust and particulates.
The presence of great variations in wall thickness from high to bottom of any same section of horizontal pipe may act as a warning of an internal deposit problem.
Accidental corrosion on the wall of piping can also cause piping system breakdown. The net result from numerous corrosion mechanisms is commonly deep and random and, in many cases, may only be determined by metallurgic analysis.
The presence of a microbiological agent condition (MIC) is very effective at manufacturing random areas of extraordinarily high wall loss typically surpassing 25 MPY. This produces devastating results and is extraordinarily troublesome and expensive to correct.
Drainage situations have an impact on system-dependent corrosion. Piping that is drained down over the winter months, or that is packed up and drained periodically, will suffer up to 10 times more wall loss than other areas of the system. Such corrosion loss is commonly directly proportional to the pipe's proximity to the open atmosphere.
Piping of Supply and Return
The piping of supply and return can cause system-dependent corrosion. For example, the return piping at a condenser or cooling water system typically shows a higher degree of corrosion than the supply facet thanks to the marginally higher return water temperatures that favor corrosion activity and promote MIC. High temperatures quicken most chemical reactions.
Higher corrosion of return piping may also bring rust particulates originating from the supply pipe.
Quality of Pipe
The quality of pipe has a major influence on system-dependent corrosion. Because of the lower quality of steel pipe these days compared with the factory-made pipe of 50 years past, higher average corrosion rates are common. Wherever one MPY corrosion rates once existed for condenser or open water service, 3–5 MPY corrosion rates are currently expected, and 10 MPY rates aren't uncommon. Pipe made outside the U.S. also tends to be particularly prone to corrosion. (Further reading: Decline in Quality of Piping Making Corrosion Inevitable.)
For reasons not completely understood, additions of new pipe and renovations can typically show a higher corrosion rate than for the initial piping itself. This may be because iron oxide deposits from the older pipe quickly migrate to the new pipe to initiate higher corrosion conditions. Any new pipe ought to be continually monitored, and perhaps monitored more closely than older areas of the system.
Stagnant areas of flow will typically develop severe corrosion from the settlement of particulates and/or a scarcity of chemical protection. The lower flow rates existing within the distribution and run-out piping to individual package units can typically show accelerated corrosion in smaller lines that can least afford it. Pipe that ends in a seldom-used plate and frame heat exchanger is particularly liable to this result, and wall loss of up to 0.2 inches is often recorded. Dead ends, bypass lines, futures, lead and lag instrumentality, mud legs, and different no-flow areas will turn out corrosion rates well surpassing 15 MPY, and accelerate pipe replacement decades before the remainder of the system.
Assembly of Pipe
Although rarely a factor in the first stages of a piping system, pipe construction plays an important role in an aged system. Termination gaps of different clamped type piping systems typically accumulate with particulates and microbiological agents to provide localized high corrosion and corrosion losses.
Threaded pipe can nearly always leak or fail, even without the threading method, which can be attributed to wall loss of 50% or more. Cutting a groove into a pipe used in clamped pipe assembly, instead of rolling or swaging it, has the comparable result of substantially reducing pipe wall life. This wall loss, in addition to a high corrosion rate, can generally produce advanced failures.
The main ways to prevent corrosion in piping systems are as follows:
- Choice of proper piping materials
- Use of corrosion-resistant surface coating
- Use of correct geometry of piping systems
- Use of cathodic protection
In most cases, a piping system will include a mix of potentially corrosive conditions. The corrosion problem is a complex issue and is rarely simply one factor responsible for a piping failure. Reducing and preventing corrosion, therefore, involves looking at all aspects that may affect the pipeline, along with careful inspection along various points.
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