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Inadequate Corrosion Control

Severe corrosion of HVAC piping systems and their related equipment presents the most potentially damaging losses to any private, government, industrial, or commercial property next only to the threat of fire.

It is surprising to see, therefore, the relatively low priority often provided for corrosion control and monitoring of critical HVAC and process piping by many building owners, operators, and plant engineers.  For many building properties, corrosion control is of little concern.  Whereas condenser and open water corrosion rates of 0.5 – 2 mils per year (MPY) were typical only three decades ago and easy to achieve, it is now common to find system-wide corrosion rates of 5 MPY and greater – with rates occasionally as high as 30 MPY.

While certain unavoidable factors have contributed to generally higher corrosion rates, it remains clear that the service life of most piping systems could be greatly extended by allocating further effort to corrosion control measures.

• Massive Reporting Error

A major factor in many pipe failures we see is caused by the often total reliance upon corrosion coupons. 

Corrosion coupons themselves are so inaccurate as to be virtually worthless in most applications, yet relied upon widely.  Against evidence of rust deposits in pump strainers, cooling tower pans and chiller heads, constricted pipe, thread leaks, and even piping failures – corrosion coupons are still blindly believed to report the rate of corrosion activity actually occurring within the piping system.  For a variety of reasons which we have well documented during our more than 35 years of experience in this field, a corrosion coupon produces a measurement representing only “the potential corrosivity of a fluid against new pipe steel.”  Nothing more.

This, of course, is not the understanding of those property managers and engineers receiving regular corrosion coupon reports, and assuming that such typically low corrosion rates reflect the actual wall loss occurring within their piping system(s).  In fact, reported corrosion coupon rates frequently have almost nothing to do with the rate of wall loss occurring at the pipe, and are typically in error by at least 10 times or more.  Massive failures in accuracy, examples where corrosion coupons reported 0.3 MPY corrosion rates and where actual measured wall loss through ultrasound shows rates up to 100 times higher, help illustrate the threat of relying upon a testing method so inherently inaccurate.

In a past condenser water piping investigation conducted due to multiple piping failures, corrosion coupons rates of 0.3 MPY had been consistently reported.  During the year that the newly constructed 1 in. steel corrosion coupon rack had been installed, a wall loss of 0.042 in. had actually occurred to the coupon rack itself, as measured by ultrasound.

This translated to an actual 37.5 MPY corrosion rate and more than 1,000 percent error in the corrosion coupon results!  Final proof to the fact that corrosion coupons had failed to warn building operators to the problem was in the form of thread leaks at many coupon rack threaded fittings – an impossible physical occurrence had only 0.003 in. of pipe wall been lost.

In many examples, this total trust and reliance on corrosion coupons to define corrosion activity has resulted in years of excessive corrosion losses resulting in near total piping failure.  We provide a useful handout regarding the limitations of corrosion coupons at left.

• Unclear And Confusing Terminology

To be fair, the common metal loss terminology in mils per year (MPY) poorly conveys the true extent of corrosion taking place within the piping system, and often fails to warn a plant operator of a serious corrosion problem, or its consequences.  A 2 MPY corrosion rate, which is a loss of 0.002 in., is worse than 1 mil per year obviously, but how much so in real terms?  Most building operators and engineers would consider the difference between a 4 MPY and 6 MPY corrosion rate as relatively minor, although the net effect of the higher rate over many years may be a heavy buildup of rust deposits and premature failure.

The physical size of the pipe is also a major factor to consider.  Smaller diameter pipe begins with substantially less available wall thickness to corrode, and reaches a failure point sooner – while larger diameter pipe produces a greater volume of interior deposits due to its greater surface area.

This picture changes, however, as soon as one looks at corrosion in terms of pipe mass or material weight lost, rather than a MPY value.  A moderate corrosion rate of 5 MPY at a 12 in. schedule 40 condenser water pipe, for example, while seemingly minor, actually translates to a physical loss of 64 lbs. of steel per every 100 linear feet.  At 8 MPY, approximately 102 lbs. of metal is lost.  Multiply such loss by the number of years in service, and the true magnitude of system corrosion takes on much greater significance than when reported as simply 1, 2, or 5 mils per year.

• MPY vs. Actual Weight Loss

A dramatic example showing the actual weight of steel removed from various pipe sizes at different corrosion rates is presented below in Table 1.  Although corrosion rate results are typically presented in terms of mils per year, or thousandths of an inch per year, the true impact of that measured rate can only be realized by looking at the actual volume of metal lost from the piping system.  As the piping system wears and the internal pipe diameter increases, so does the internal surface area and therefore the weight of metal lost.

Table 1

For a typical commercial building property of 33 floors, having a cooling tower at the roof and a 5,000 ton refrigeration plant in the basement, we can estimate approximately 1,000 linear feet of 24 in. supply and return piping in service.  From the above table, and based upon a moderate 5 MPY corrosion rate commonly found today, we can then estimate that approximately 1,210 pounds of steel will be lost from the pipe wall itself and distributed into the system for Each Year of service due to corrosion losses.

For a typical 30 year old property, the weight of metal lost into solution and blown down, filtered out, settled, or deposited somewhere within the system is enormous.  And this value does not include the hundreds of feet of smaller distribution piping generally involved.

• Deposits A Secondary Problem

The damage caused by a high corrosion problem extends past just the possibility of a leak, however.  In its oxidized form, steel produces approximately 10 to 12 times its original volume in iron oxide or rust product.  This by-product is often found in horizontal lines, headers, by-pass configurations, dead ends, and at low flow areas – often accumulating in sufficient volume to produce under deposit corrosion, heat transfer loss, flow rate problems, and ultimately failure.

Top left is an example of a 10 year old 12 in. condenser water line suffering from a moderate corrosion rate of approximately 5-8 MPY, and a buildup of “tuberculation” deposits along its bottom.  The top right cross-sectional photograph documents the deep pitting into the wall, and the creation of iron oxide tuberculation over the area of wall loss.  The net result from rust deposition being severe underdeposit corrosion.  Adding to the threat, measurements made through ultrasound identified that thinner schedule 20 pipe had been substituted by the HVAC contractor.  Another example at bottom left shows an accumulation of rust deposits from other areas of the system which have migrated and settled likely due to flow conditions or elevation.

Below right is a removed section of 4 in. condenser water distribution piping to a package unit where 8 years of higher than normal corrosion produced sufficient rust deposits to completely fill its entire length.

Once established, and without effective filtration, chemical cleaning, and other preventative steps, such deposits are virtually impossible to remove – thereby preventing any benefit from the chemical treatment program and accelerating corrosion activity further.  See Technical Bulletin CT-04 about the problems associated with interior deposits.

Table 2 below provides an estimate of rust related debris created by the oxidation of the metal lost, and shows that in general, a tremendous volume of corrosion product is produced.  Using the same 33 floor office building as an example, we can estimate that as much as 30 cubic feet of rust and iron oxide deposits will be created from the 1,210 pounds of steel rusted away for Each Year of service.  Over a 30 year history, a significant volume of particulates will be produced to create various secondary operating problems.  For any piping system, an effective water filtration system is mandatory.

Table 2

For condenser or open process water systems, much of this corrosion product will be lost in the cooling tower blowdown, some will settle in the tower basins and condenser heads, and some will remain attached to the pipe wall surface.

Corrosion products are often a much greater threat to closed piping systems, since no outlet exists to remove the deposits, and due to the much smaller distribution lines which are more susceptible to deposition.  Closed systems generally do not reveal their corrosion products like an open system, and therefore receive little attention.  In addition, increased deposits present ideal opportunity and nutrients for microbiological organisms to grow.

Not only is the structural integrity of the piping itself threatened by excessive corrosion, but the resulting corrosion products generally cause secondary problems in the form of lost heat transfer, biological fouling, microbiologically induced corrosion (MIC), clogged pipes, and abrasive wear to pump seals and other components.

• Effective Chemical Water Treatment Critical

Most corrosion problems can be avoided by specifying a comprehensive chemical treatment program using a reliable water treatment contractor.  Fully automatic water meter activated chemical feed control and dual biocide feeds are an absolute necessity, and not an option.  A supplemental corrosion testing program and the frequent review of its results are critically important to ensure satisfactory corrosion control.  In addition, effective water filtration, properly installed, is mandatory.

Proper start-up and passivation of a condenser water system is absolutely critical, and many examples of extremely high corrosion loss have been traced back to poor start-up procedures, with the actions of the mechanical contractor, or an inadequate low bid water treatment package are often revealed at fault.  In short, poor planning and coordination between the water treatment company, mechanical contractor, and building owners or operators can result in operating problems for many years following.

While the evaluation of any corrosion rate is typically presented in mils per year (MPY), knowing the true physical loss of pipe in actual weight places a new perspective on any wall loss evaluation.  As the above tables dramatically show, there is a very significant difference in threat level between a 1 MPY, 5 MPY, and 10 MPY corrosion rate.

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