Interior Rust Deposits - The Downward Spiral of Operating Problems

The interior of any HVAC piping system can show a wide range of corrosion characteristics - those characteristics and their severity being dependant primarily upon piping service, followed by physical orientation, age, pipe size, and location.

Corrosion problems do not appear overnight, and are generally the result of a failure to provide good chemical inhibitor protection 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 affect, are in turn, generally dependent upon the type of water service involved. While there are always exceptions, the follow generalizations apply:

		Steam piping wears uniformly along its interior surface, and at low corrosion rates typically near 1 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. 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 to control condensate corrosion, while available, are rarely provided.
		Closed hot water, cold water, and secondary systems, unless neglected or periodically opened to the atmosphere for free cooling service, typically show low to moderate corrosion rates of under 1-2 MPY. 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 and treated as an open water system. For closed circulating systems, adding a side stream basket filter is perhaps the most effective and low cost solution to removing any iron oxide deposits which are naturally produced. See Technical Bulletin # W-1 regarding the use of side stream filtration to maintain clean closed systems. In addition, chemically cleaning and sterilizing all closed systems every four years is highly recommended.
		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 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. In many years of performing ultrasonic pipe testing, CVI has documented condenser water piping with widely varying conditions - some having a wall loss of only 45 mils over 60 years of operation, and others showing that same amount of loss within 2 years. Review a summary of piping quality, operating, and design changes which have occurred. 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 piping will corrode greatly dependant 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 for measure a 25 mil loss of steel from stagnant fire standpipe systems after 70 years of service. 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. Dry or preaction 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. The more recent use of schedule 10 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.
		Domestic water systems deteriorate primary based upon a combination of age and the material of construction. The common use of galvanized pipe for such services provides a service life dependant upon the water chemistry of the geographical area, its quality, 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 75 years, while properties in Chicago have been documented to show no noticeable wear within the same period of time. Brass piping generally shows a low and even rate of corrosion wear, but will eventually suffer from dezincification depending upon age and the aggressiveness of the local water supply. Though more expensive, copper is the choice of installation material today.

     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 "tubercles," 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 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 organic or microbiological material. As that corrosion activity continues over time and heavier deposits are allowed to accumulate, chemical activity beneath such deposits may develop into different 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.

The below 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 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 their 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 at the 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.

     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. See Technical Bulletin # C-2 for recommendations on maintaining good corrosion control.

     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. 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 both 12 in. schedule 40 condenser water pipes within 10 years.

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. See Technical Bulletin # C-5 for further information on MIC corrosion.

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

     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. Read more about the many limitations of corrosion coupons.

     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. Read more about the many advantages of CorrView ^®.

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 the condenser or any process 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.

Some additional photographs of the above interior deposit problem are presented in a short photo gallery from actual CVI case histories. View our photo gallery of interior pipe deposits.

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. View a photo gallery of common piping failures. In virtually ever 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.

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:

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