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Internal Rust Deposits

The interior of any HVAC piping system can show a wide range of corrosion characteristics – those characteristics and their severity being dependent primarily upon piping service, followed by physical orientation, age, pipe size, pipe quality, and location.  Steel, galvanized steel, cast iron, and ductile iron pipe produce substantially more deposits than the material which has been lost, which then produces restrictions to flow, possible thermal inefficiency and additional operating problems.  Brass and copper pipe, on the other hand, simply dissolve away.

Corrosion problems do not appear overnight, and are generally the result of a failure to provide good chemical inhibitor protection and maintenance over an extended period of time.  While other factors, and the failure to take certain preventative measures may also apply, high corrosion rates are very often related to the volume of rust deposits which have accumulated.  The volume of such deposits, and their ultimate impact, are in turn generally dependent upon the type of piping service involved.  While there are always exceptions, the following generalizations apply:

• Steam And Steam Condensate

Steam piping wears uniformly along its interior surface, and at low corrosion rates typically near 0.8 MPY or less.  Lower rates are generally found where the steam is produced from an on-site boiler having a good chemical treatment program, as opposed to a city source or private utility.  Chemicals that “carry-over” with the steam into the condensate are especially effective at reducing corrosion losses.

Steam condensate systems wear at higher and less uniform corrosion rates, at horizontal sections rather than vertical ones (more so at the bottom of the pipe), and can suffer extreme pitting due to the generally acidic nature of steam condensate.  Chemical treatments that carry over from the steam into the condensate, while available and effective, are rarely provided.

More info on Steam and Steam Condensate, in addition to other piping systems listed below, can be found in our System Specific Issues section on our site.

• Closed Systems

Closed hot water, cold water, and secondary systems, unless neglected or periodically opened to the atmosphere for free cooling service, typically exhibit low corrosion rates of under 1 MPY, and can be maintained to under 0.5 MPY with minimal effort.  Only under unusual conditions, such as where a severe microbiological problem exists, will a secondary system produce a threatening corrosion condition.

It is especially important to note that closed chill water and secondary systems, when directly cross connected to the condenser water system through a Strainercycle or other full flow filter, will exhibit much higher corrosion and pitting levels, and should therefore be viewed as an open water system from that time onward.

For closed circulating systems, adding a side stream basket style bag filter is unquestionably the most effective and low cost solution to removing any iron oxide deposits which are naturally produced.

The use of an adjunct chemical dispersing agent to gradually remove existing deposits back into the flow for eventual capture by the filter is a necessity often neglected in any pipe clean-up plan.  Without chemical assistance, the filter cleans the turbidity from the water to everyone’s delight and assumption that the problem has been solved, while the overwhelming volume of the rust particulate problem still exists.  More information on the subject of water filtration is is available in our Technical Bulletin section under Water Filtration.

In addition, chemically cleaning and sterilizing all closed piping systems every 5 years is highly recommended.

• Condenser And Open Systems

By far, open condenser water or process water piping suffers the most widespread and severe corrosion related problems of any HVAC or process plant recirculating system.  This occurs even though the majority of any chemical water treatment budget is typically devoted to this area of building operations.  While some properties may enjoy relatively trouble free operation with a minimum of maintenance, others will see significant deterioration and wall loss after only a few years.  A look at any of our Photo Galleries related to open condenser water piping quickly illustrates the threat.

In many years of performing ultrasonic pipe testing, CorrView has documented condenser water piping with widely varying conditions – some having a wall loss of only 45 total mils over 60 years of operation, and others showing that same amount of loss within 2 years.  Corrosion rates of 25 MPY and above, not even considered possible two decades ago, are surprisingly common for newer piping installations due to a wide combination of factors.

The many different corrosion types often existing at a cooling tower system mandates close monitoring of such systems, as well as the need to take certain precautionary measures to prevent their development.

• Fire Sprinkler

Fire sprinkler piping corrosion is dependent almost entirely upon the flow of water through the pipe.  Where the system is filled and left to stand undisturbed, a small amount of corrosion takes place, oxygen is depleted, and the corrosion activity virtually ceases.  Therefore, it is not uncommon to measure a 25 mil loss of steel from stagnant fire standpipe systems after 70 years of service, and document the pipe in near new condition.

Where the sprinkler system is frequently drained for renovation or some other purpose, and especially where a slow leak exists, corrosion will rise dramatically, but still provide generally long service.  Required testing by NFPA and other fire codes also introduces fresh oxygenated water to help destroy the system.

Dry or pre-action systems offer a separate problem in that after testing, the water can never be fully drained, and therefore, with an abundance of oxygen, begins an attack along the bottom of the pipe.  A dry fire system is “dry” in name only – thereby the problem. More on that Here

The more recent use of schedule 10 and even thinner schedule 7 pipe has added significant threat to fire service piping since the extremely thin wall piping, half of schedule 40, leaves little room for corrosion to occur before reaching minimum standards and failure.  Reducing that limited wall thickness further is the fact that this thin wall pipe is also now approved for threaded branch lines.  Having approximately 0.014 in. of wall thickness remaining after threading, no one should ever act surprised at its failure.  The more recent use of thinwall schedule 10 pipe at threaded branch lines has further increased failures of such systems.

Combined, corrosion problems at fire systems have significantly increased.  Unlike corrosion at a condenser or chill water system, corrosion activity at a fire system can produce sufficient internal rust product to completely stop all water flow during an actual fire emergency.

Corrview has replicated a Dry Fire System to perform rust volume vs. sprinkler head flow testing.  See our Technical Bulletin FP-08 on this subject.

• Domestic Water

Domestic water systems deteriorate primarily based upon a combination of age and the material of construction.  The common use of galvanized pipe for such services provides a service life dependent upon the water chemistry of the geographical area, the quality of the galvanizing finish, and age.

Typically, once the galvanized coating is worn away, all the electrochemical forces in the area focus on that unprotected section of pipe – producing aggressive and random pitting.  Properties in New York city typically show such effects after 55 years, while properties in Chicago have been documented to show far less noticeable wear within the same period of time.

Galvanized pipe is highly vulnerable to higher temperatures which destroy the internal zinc coating.  This is why its extensive use for domestic hot water service in Chicago and some other American cities has produce such widespread problems.  Newer galvanized steel pipe is of substantially lower quality having a far lower density and thickness of zinc plating.  Its documented failure in domestic water systems within 5 years makes it unsuitable where long service life is desired.

Copper and brass pipe dissolves away rather than corrodes, and for that reason does not produce a restriction to flow.  Yellow brass piping generally shows a low and even rate of corrosion wear, but will eventually suffer from dezincification after about 80 years depending upon the aggressiveness of the local water supply.  Red brass, however, is far less susceptible to dezincification and will provide very extended service.  Ultrasonic testing of older red brass in service for 80 years has identified it still at new ASTM specifications.

TP can be found at much older building properties in older cities such as New York City, Chicago, and Boston, and is virtually indestructible.  Standing for Threadless Pipe, TP is a very pure form of copper with noticeably different elbows and tees more similar to steel fittings.  Type L copper is the choice of installation material today, but has also been documented having far less quality than the same product produced 50 years ago.

Since potable drinking water is involved, the use of chemical inhibitors is not applicable.  The addition of approved silicates as an artificial hardness to plate out on the pipe as a barrier anti-corrosion method may provide some benefit.

• An Underlying Cause

High corrosion rates, those which would be termed threatening to the piping system, depend upon a combination of many different and possibly unknown chemical / electrical mechanisms.  Very often, they are related to the accumulation of iron oxide deposits, often termed “tubercules,” at the pipe surface.  In short, an ultrasonic finding of a high wall loss exceeding 10 mils per year (MPY) means the existence of interior pipe wall deposits.  Conversely, visually observing large interior deposits and tubercles guarantees a large metal loss at the pipe.  One cannot exist without the other.

Interior pipe deposits are always the result of some form of corrosion activity, combined with captured particulates and possibly organic or microbiological material where cooling tower service is involved.  As that corrosion activity continues over time and heavier deposits are allowed to accumulate, chemical activity beneath such deposits may develop into different more aggressive forms commonly known and generally termed as “under deposit” corrosion.  Such deposits represent a far worse threat to the pipe than just a loss of heat transfer or flow.

In its least damaging form, under deposit corrosion may produce mild surface indentations over a wide area, yet leave the pipe in still serviceable condition.  But in its more severe form, under deposit corrosion will produce narrow and deep pitting – which may in turn may result in advanced pipe failure within 10 years or less.

Ironically, the higher the corrosion rate, the more deposits produced – and the more deposits produced, the higher the corrosion and pitting rate.  It is a difficult to stop downward spiral, requiring substantial effort and cost to stop.

• Clear Relationship Exists

The bottom side by side comparison shows a section of domestic water galvanized steel pipe uniformly covered with a 1/4 in. to 1/2 in. layer of iron oxide deposits.  Removing the deposits using a high pressure water jet revealed widespread though shallow depressions into the pipe wall, and no significant threat to the remaining service life of the pipe.  Ultrasonic thickness testing proved the pipe suitable for further service.

The top and center photographs show other examples of the level of random and deeper pitting typically located under internal rust deposits.  Where internal deposits exist, deep pitting is present.  Conversely, where ultrasound identifies deep pitting, internal deposits will be found.

Mild Pitting Caused by Moderate Surface Deposits

Heavy Generalized Deposits	Underlying Deterioration

• Underlying Deterioration

The more serious scenarios of “concentration cell” and “oxidation cell” type corrosion often define themselves as having a much heavier buildup of corrosion product at isolated areas, and little to no corrosion product elsewhere.  Instead of attacking the entire pipe surface, such corrosion types focus all of its activity at specific points.  In such cases, it is common to measure some existing wall thickness values at or near new pipe specifications, and then locate areas of high deterioration having perhaps a 75% or greater wall loss. This is especially common at galvanized pipe.

Shown by the below right photograph after physically removing the iron oxide deposits from the photo at left, deep 0.125 in. depressions exist at this 12 in. ASME A 53 steel condenser water pipe.  Yet the surrounding areas show virtually no wall loss whatsoever.  It is this high degree of localized corrosion activity, resembling the action of a drill bit, which will dramatically shorten the service life of any affected piping system.

Deep Pitting Caused by Heavy Surface Deposits

Isolated Heavy Deposits	Severe Pitting Results

• Deposits Always A Threat

Whether eventually creating shallow and generalized wall loss, or deep and localized pitting, the buildup of iron oxide deposits always represents a serious threat to any piping system.  Therefore, a high importance should always be placed upon removing such deposits in order to stop or minimize any underlying corrosion activity.  Preventing the establishment of such deposits is obviously a preferred maintenance or operating strategy, and can be generally achieved using a combination of good water treatment, supplemental filtration, and chemical dispersant.

Our experience, both in ultrasonically testing pipe at hundreds of building properties, and by inspecting metallurgical samples, has shown a very clear relationship between the extent and profile of the interior deposits, and the wall loss damage incurred.  In all examples, heavier deposits will define a more severe pipe loss.

The danger of deep pitting caused by heavy rust deposits or tubercles is further illustrated in the below photographs.  Here we show generally even and acceptable remaining wall thickness throughout most of the pipe wall, but with severe and localized pitting hidden under those areas where the heaviest interior deposits exist.  This is shown at the left side of each photograph.  A settlement of particulates at the bottom of the pipe proved to be the cause of this problem – resulting in the total penetration of random sections of the 12 in. schedule 40 and schedule 20 condenser water pipes within 10 years.

Pitting Proportional To Deposit Volume

Deep Localized Pitting	Deep Localized Pitting

• Microbiological Threat

Accumulated deposits may also create conditions favorable to extremely destructive microbiologically influenced corrosion, or MIC – the general term for a wide group of microorganisms which often produces their own acidic environment, and have been documented to virtually dissolve steel pipe at corrosion rates exceeding 50 MPY.

Interior deposits provide attachment points, nutrients, insulation from microbiocides, and other benefits all encouraging the growth and spread of this most serious form of corrosion attack.

More information on this subject is available in Technical Bulletin CT-05.

• Serious Negative Impact

Typically not indicated through corrosion coupon monitoring, the most common form of corrosion testing employed, under deposit corrosion is often identified after its presence is noticed through other means.  Overall, the damaging effects of under deposit corrosion include:

• • • High corrosion rates exceeding 25 MPY
    • Produces conditions favoring MIC
    • Higher maintenance costs
    • Higher operating costs
    • Premature piping failure
    • Obstruction at cooling tower drip pans
    • Fouled cooling tower basins
    • Clogged condenser tubes
    • Overloads many non-automatic filtration systems
    • Prevents chemicals from reaching the base pipe metal
    • Reduces heat transfer efficiency
    • Clogged heating and cooling coils
    • Promotes galvanic corrosion near brass or copper
    • May cause abrasive wear in high velocity areas
    • Premature refrigeration equipment failure
    • Restricted water flow
    • Unpredictable shutdown of equipment on high head
    • Damages expensive mechanical pump seals
    • Damage to heat sensitive equipment
    • May prevent isolation valves from operating
    • May prevent actuators and control valves from operating
    • May produce false sensor readings

• Effective Monitoring Important

Under deposit corrosion activity can be generally identified without the need to remove samples of pipe through the use of ultrasonic testing.  Due to the very random nature of such pitting, a significant amount of testing is often required to ensure that such conditions are identified, if they exist.  A metallurgical examination of a removed sample of pipe is also recommended to confirm any suspicion, and a laboratory culture or DNA analysis of the under deposit growths is recommended in order to determine if MIC is present.

The best solution, of course, is to prevent such deposits from developing in the first place.  This, however, is easier said than done.  In many cases, such problems reveal themselves only after producing a water flow loss, heat transfer problem, obvious rust sediment in the strainers or pans, or a leak condition.  Buckets of rust and scale following spring start-up or after a shock to the system are clear indicators of a problem.  In some examples, a service repair or renovation may allow entry into the piping, thereby revealing the extent of the problem for the first time.  Frequently, such problems are inherited from previous building owners, operating personnel, or an HVAC contractor.

As discussed previously, strict reliance on standard corrosion coupons often allows many years for under deposit corrosion to develop and worsen, while at the same time providing metal loss results only applicable to the corrosion coupon itself.  In many cases, we have been advised of corrosion coupon rates well below 1 MPY, and yet, through extensive ultrasonic testing, have found interior corrosion conditions similar to the above examples with actual corrosion rates exceeding 10 MPY.  It is a very common scenario well documented in this internet site.

Corrosion coupons will show whether a water treatment chemical exists, and whether or not it is effective to control the corrosive attack against a new coupon sample.  Rarely, however, does it reflect actual corrosion at the pipe surface, and in fact, we highly discourage relying exclusively upon corrosion coupons as a measurement of corrosion control.  The CorrView monitor has been designed specifically with such limitations in mind, and is intended to provide a more realistic corrosion rate assessment.

• General Recommendations

Obvious recommendations to prevent such problems include a strict and automated chemical water treatment program, good water filtration, better corrosion monitoring, and added dispersants or cleaners to control those deposits which will inevitably result.  While there is no absolute guarantee for trouble free condenser water operation, the below recommendations will significantly reduce the possibility of being surprised by a severe pitting condition.

• • • Do not rely on corrosion coupons to indicate corrosion activity
    • Maintain a strict water treatment program
    • Monitor the chemical treatment program closely
    • Automate all chemical feeds and bleed-off
    • Install a 3 in. to 6 in. inspection spool piece in at least two locations
    • Perform a visual inspection of the pipe whenever possible
    • Verify that water flow rates are adequate to prevent settlement
    • Eliminate any low flow areas
    • Use borescope or robotic camera to inspect the pipe interior
    • Provide flow by-pass through any A/C units when off
    • Perform ultrasonic testing to establish actual corrosion rates
    • Add full flow or side stream water filtration
    • Avoid draining down the piping
    • Properly lay-up the pipe when drained down
    • Consider the services of an outside chemical consultant
    • Chemically clean the condenser or open water piping twice per season
    • Chemically sterilize any open water piping twice per season
    • Add a dispersing agent to the regular chemical program
    • And of course, add CorrView monitors at specific areas within the piping system.

• Further Examples

CorrView International, LLC offers a series of photo galleries taken from over 28 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.  More photos on this subject are available Here.

In many examples, a combination of conditions will exist within the same piping system – therefore requiring sufficient monitoring over enough locations to provide a reliable corrosion rate assessment. In virtually every case we have been involved, better precautionary measures, combined with more thorough and accurate corrosion monitoring and faster and more effective corrective actions, would have prevented such problems from occurring.

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