System Dependent Corrosion
For many cooling water loops, and especially for open recirculating systems, dramatically different corrosion conditions can exist at various points throughout the piping layout. Often, the actual causes of such corrosion differences, such as at low flow areas or at long horizontal runs, are unavoidable. Similar differences in corrosion activity can exist at different areas of a fire protection system – for totally different reasons.
Following 30 years of experience in chemical water treatment and ultrasonic pipe testing, we have been able to predict problem corrosion areas simply based upon the physical configuration of the piping system. Some commonly recognized problem areas are summarized below:
Corrosion characteristics are so different between different piping systems that they can never be evaluated as one. Condenser water has much higher corrosion than chill water, steam condensate higher than steam, and dry fire significantly greater than wet fire pipe. Pipe materials and their age also play a significant role.
Corrosion activity is typically related to a combination of pipe quality, water quality, chemical inhibitor quality, and water movement. Other issues play a less important role.
Corrosion monitoring for any piping system therefore requires a basic knowledge of its common corrosion problems, combined with an understanding of the system design.
Lower floor areas of the same piping system typically suffer a far greater degree of corrosion and pitting activity simply due to the settling of dirt, iron oxide, organic material, and particulates.
For many larger layouts, flow velocity decreases furthermost from the circulating pumps to allow settling of even the finest particulates.
Ultrasonic testing results showing a 4 MPY corrosion rate at the upper floors of a condenser water system, will often indicate higher rates at the bottom of the system. This higher wall loss in many cases also exists where piping has been reduced in size and therefore having less available wall thickness.
A seemingly reasonable and moderate corrosion rate of 5 MPY will actually produce tremendous volumes of iron oxide each year which will settle to produce secondary corrosion problems if not removed.
Horizontal sections of pipe typically show a higher degree of sediment and deposit buildup, corrosion, and pitting than vertical sections – for the obvious reason. Where a higher than normal corrosion rate exists, ultrasonic testing of horizontal lines will typically document significantly greater wall loss and pitting along its bottom surface.
Coupled with low flow conditions or the periodic loss of flow, as might occur with individual HVAC package units, horizontal piping can suffer at significantly higher corrosion rates.
The net result from various corrosion mechanisms is often deep and random pitting which can only be defined through metallurgical analysis.
The presence of a microbiological agent or MIC condition is especially effective at producing random areas of extremely high wall loss often exceeding 25 mils per year (MPY). This produces often devastating results and is extremely difficulty and costly to correct.
Piping which is drained down over the winter months, or which is shut down and drained periodically, can suffer up to 10 times greater wall loss than other filled areas of the system. Such corrosion loss is often directly proportional to the proximity to the open atmosphere.
This is a common problem for many Northern climate properties regardless of the standard lay-up precautions taken.
Supply and Return
Return side piping at a condenser or cooling water system often shows a higher degree of corrosion and pitting than for the supply side simply due to the slightly higher return water temperatures which favor corrosion activity and promote microbiological growth. Higher temperatures accelerate most chemical reactions.
Higher return side corrosion may also be due to the secondary effect of rust particulates originating from the supply side pipe, or other factors.
Due to the generally lower quality of steel pipe today in comparison to that manufactured 50 years ago, higher average corrosion rates are common. Where 1 MPY corrosion rates once existed many decades ago for condenser or open water service, 3-5 MPY corrosion rates are now expected, and 10 MPY rates are not unusual. Pipe produced outside the United States seems especially more corrosion susceptible.
For reasons not fully understood, new piping additions and renovations will often show a higher corrosion rate than for the original piping itself. Speculation is that rust deposit existing at the older pipe quickly migrate to the new pipe to initiate higher corrosion conditions.
Any new pipe should therefore always be monitored equally or even more closely than older areas of the system.
Stagnant areas can often develop severe pitting from the settlement of particulates and/or a lack of chemical protection. The lower flow rates existing in the distribution and run-out piping to individual A/C or package units will often show accelerated corrosion in those smaller lines which can least afford it.
Pipe leading to rarely used plate and frame heat exchangers are especially vulnerable to this effect, and have been documented with as much as a 0.200 in. wall loss along the bottom due to particulate deposition.
Dead ends, by-pass lines, futures, lead and lag equipment, mud legs, and other no flow areas can produce corrosion rates well exceeding 15 mils per year, and accelerate pipe replacement decades before the rest of the system. Any bottom take-off from a main service life is especially vulnerable.
Rarely a corrosion related factor in the early stages of a piping system, pipe construction does play a critical role in an aged system. End gaps of a Victaulic, Grinnell or other clamped type piping system often accumulate with particulates and microbiological agents to produce localized high corrosion and pitting losses.
Threaded pipe will almost always leak or fail prior to other areas due to the 50% or greater wall loss produced in the threading process, among other factors.
Cutting the groove into pipe used in clamped pipe assembly, rather than rolling or swaging it, has the similar effect of significantly reducing pipe wall life. This wall loss, coupled with a high corrosion rate, will typically produce advanced failures.
CorrView International, LLC offers a series of photo galleries taken from 18 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 Corrosion Photo Gallery of 27 different corrosion types and failure conditions.
The issue of corrosion is complex; typically relating to issues of maintenance, chemical treatment, pipe quality, engineering design, as well as less significant factors. Rarely is just one issue responsible for a piping failure. Investigating to the point of defining all aspects of a corrosion problem, and most importantly not excluding any area of interest, is the key to a successful resolve.