High Corrosion Causes

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 high corrosion activity can exist at different areas of a fire protection system – but for totally different reasons.

Following 35+ 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 and design of the piping system.  Some commonly recognized high corrosion problem areas are summarized below:

• Type Of Piping System

Corrosion characteristics vary so greatly between different piping systems that they can never be evaluated as one.  Condenser water has much higher corrosion activity than chill water or hot water heating, steam condensate higher than steam, and dry or pre-action fire sprinkler significantly greater than wet fire pipe.  Pipe materials and their age also play a significant role, with the superior quality of older pipe long far outlasting its new replacements.

Corrosion activity is typically related to a combination of pipe quality, water quality, chemical inhibitor quality, water movement and especially – good building maintenance.  Other issues play a less important role.  Corrosion monitoring for any piping system, therefore, requires some basic knowledge of its common corrosion problems, combined with an understanding of the system design and operation.

• Physical Location

For larger hi-rise buildings, 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 large manufacturing facilities comprising one expansive level, process water 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 and deeper pitting at the bottom of the system.  Likewise, as most piping systems distribute further from their pumps, they reduce in size; often lowering flow rates where again any particulates will have the opportunity to settle.  Never taken into consideration in the design of any circulating piping system is the impact of rust as the piping system ages.  A seemingly reasonable and low corrosion rate of 1 MPY will actually produce tremendous volumes of iron oxide each year, which in turn then settles to produce secondary corrosion problems if not removed.  And of course – where the future impact of rust is never a consideration in piping design, neither is its removal by filtration or other means.

• Pipe Size

Every piping system has discreet components of which the most important from a corrosion perspective is its size or diameter.  With different piping materials of always different manufacturers, different pipe schedules, different joining methods and conditions present, the size distribution of any piping system becomes an extremely important factor to consider whenever a failure occurs.  For the same reason, addressing all sizes of pipe during an ultrasonic inspection is critically important.

Less of an issue for central condenser water systems where most pipe is typically of larger diameter, any heat pump, package system, chill water or other closed system is typically fabricated from the largest 12 in. or greater size pipe down to the smallest 1/2 in. drain fittings – with each size pipe presenting an inherently different level of threat.

Other than a specific reason for higher corrosion activity, such as at a dead leg or by-pass location, corrosion attacks most pipe evenly.  The same 4 MPY corrosion wall loss occurring to a section of 12 in. schedule 40 pipe beginning at 0.406 in. has far less impact than its action against a 1 in. run-out line having an initial wall thickness of only 0.133 in.  Add in the fact that almost all smaller diameter pipe is threaded, and the wall thickness holding back a failure is now cut in half.  With most catastrophic failures related to total pipe separations at the threads, their investigation during any ultrasonic investigation becomes especially important.

• Pipe Orientation

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 or lead and lag operations, horizontal piping can suffer at significantly higher corrosion rates.

• Bottom Deposits

Within horizontal sections of pipe, and often depending upon flow rate, the bottom and lower side wall areas often show significantly higher metal loss – again due to the settlement of rust and particulates.  Even under 24 hour moderate flow conditions within large diameter main riser piping, ultrasonic testing has often documented the presence of under deposit activity – as suggested by a randomly elevated corrosion rate and in some cases deep pitting.

Without the total removal of such deposits pitting will only accelerate.  Effective removal is difficult, however, and generally beyond the capability of a chemical cleanout and even full flow filtration.  Large chunks of rust, once settled, simply will not be moved by the water velocities designed for the system, nor will they ever reach a water filter take-off at the top of side of the pipe.  We have found that only high pressure water jetting can safely remove such deposits down to bare steel, although piping configuration and the ability to drain and open sections of pipe is a limiting factor.

The presence of significant differences in wall thickness from top to bottom of any same section of horizontal pipe should always provide a warning of an interior deposit problem.  Large condenser water crossover lines present the ideal opportunity for deposits to settle, as do all by-pass and water tempering lines so common to today’s piping systems.

• Random Pitting

The net result from various corrosion mechanisms is often deep and random pitting which can only be defined to its cause 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.

• Drained Conditions

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 losses are often directly proportional to the proximity of the pipe to the open atmosphere, or the roof level cooling tower.

This is a common problem for many Northern climate properties regardless of the standard lay-up precautions taken.  Increasing chemical inhibitor levels prior to draining provides virtually no benefit, as is commonly recommended, although other options have shown spectacular effectiveness.  More information on this subject is available in Technical Bulletin PI-01.

• 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.  This is for the reason that higher temperatures act as a catalyst to 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 migrating downstream, or other factors.

The bottom elbow of any condenser water return riser is a common problem area caused by heavy rust deposits and the inability of the circulating pumps to push this weight of material back up to the cooling tower.  This rust presents an obstacle to water flow, creates turbulence and erosion, initiates under deposit corrosion, and contributes and traps additional rust product – often resulting in an advanced and significant failure no one could possibly anticipate.

For chill water and dual temperature systems, the traditionally 45° F supply side temperature will condense any moisture penetrating through the insulation to produce substantially greater exterior pipe wall damage.  Although only a 10° difference below the return line, this temperature spread is sufficient to produce a significant external difference between supply and return piping.  For such older cold water systems where internal deposits have reduced heat transfer resulting in the lowering of supply water temperature to compensate, lower return temperatures will then produce the same level of external damage to the return piping and essentially double the extent of the problem.

• Pipe Quality

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 even though metallurgical experts claim it an equal product.  With ERW seamed pipe having generally replaced seamless pipe for most HVAC and fire protection applications, piping failures due to incomplete or defective ERW seams has become a new source of piping failure.

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 seemingly “innoculate” the new pipe toward higher corrosion levels.

Any new pipe should therefore always be monitored equally or even more closely than older areas of the system.  More information on this subject is available Here.

• Low Flows

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.  Bottom drain lines are a common threat wherever they exist.

• Pipe Assembly

Rarely a corrosion related factor in the early stages of a piping system, pipe construction does play a critical role in an aged system.  This is due to the fact that the end gaps of any clamped type piping system often accumulate with rust 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.  The advanced failure of all threaded pipe, therefore, is common to almost every piping system.

If the pipe has been cut grooved, an even greater threat exists.  Cutting the groove into pipe used in clamped pipe assembly, rather than rolling or swagging it, has the similar effect to threading in that it significantly reduces pipe wall life.  This wall loss, coupled with a high corrosion rate, will typically produce advanced failures.  For cut grooved pipe where corrosion has uniformly breached through the pipe wall to reach the base of the cut, failures of often 10 in. and larger diameter pipe can be in the form of a complete and catastrophic pipe separation.

• Further Examples Of Corrosion Causes 

CorrView International, LLC offers a series of Photo Galleries taken from 28 years of past ultrasonic piping investigations which address the above as well as 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.

The issue of corrosion is complex, and is 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.

© Copyright 2024 – William P. Duncan, CorrView International, LLC