Common Corrosion Types

Of the many different corrosion mechanisms which exist, the most common types are generally well understood.  For each, the process is complex, incorporates many factors, and varies according to metal and specific operating conditions.  Yet all still remain difficult to control, and represent a very serious threat to most piping systems as well as the buildings they serve.  Once established, most corrosion problems will produce future years of operating difficulty and expense at varying levels of severity.  We provide a basic introduction to the most common forms of corrosion and to their impact to building operations below:

• Generalized Corrosion

Generalized corrosion is the well distributed and  low level attack against the entire metal surface with little or no localized penetration.  It is the least damaging of all forms of corrosion.  Corrosion rates are typically at 0.5 MPY or less under generalized corrosion conditions – easily providing 100 years or more of service for larger diameter condenser water mains.  Even longer service is possible for closed system piping, such as chill water and hot water heating.  Generalized corrosion usually occurs in environments in which corrosion activity is inherently low, such as for high pressure steam,  or well controlled – such as for chemically treated closed circulating systems.  It is less common yet possible in open condenser water systems which are well maintained.

This is the only form of corrosion whereby the weight loss from a corrosion coupon can be assumed to represent corrosion activity – with certain exceptions of course.  The below examples represent low level generalized corrosion.

The two at left at closed chill water piping, and at right to an older condenser water system.  At a typical 0.5 MPY corrosion rate, and with the pipe interior slowly and uniformly reduced as if it were machined on a lathe, any larger diameter 12 in. standard pipe beginning at 0.375 in. will easily provide 100 or more years of reliable service.

Further examples exist in our Photo Gallery on this subject.

• Pitting Corrosion

Often termed “under deposit corrosion” or “cell corrosion,” this is a localized, deep penetration of the metal surface with lesser general corrosion in the surrounding area.  Due to moderate surface deposits, electrical imbalance, microbiological activity, coating failure, or some other initiating mechanism, all existing corrosion potential selectively and aggressively attacks specific areas.

Over time, pitting is extended throughout the entire metal surface, creating an irregular and very rough internal surface profile.  In some instances, pitting is concentrated in specific areas, leaving the majority of the metal surface in substantially better condition.  As the pipe wall is oxidized into a less dense but greater volume of iron oxide rust product, it produces what is known as “tuberculation” deposits; the size of such deposits being directly proportional to the depth and volume of the pipe wall lost.  A common misunderstanding is that such deposits are due to foreign sources or particulates captured by the cooling tower or introduced by the water supply itself.  In reality, those rust deposit represent what was previously the pipe wall.

For this reason, remote visual inspection (RVI) is the only diagnostic tool capable of identifying the largest deposits and therefore the areas of greatest threat.  Where possible, an internal remote camera inspection will provide the best assessment of a heavily deteriorated piping system by identifying the largest tuberculation deposits and then precisely identifying their location for follow-up ultrasonic testing from its exterior.

Pitting is the most common form of corrosion found where there are incomplete chemical protective films and insulating or barrier deposits of dirt, iron oxide, organic, and other foreign substances at the pipe surface.  It is prevalent at galvanized steel pipe, where any failure of the galvanizing zinc finish invokes a deep pitting condition.  Pitting corrosion may include: crevice corrosion, water-line attack, under deposit attack, impingement or erosion corrosion attack, and concentration-cell corrosion.

The below examples represent differing stages of a pitting condition.  At left heavy tuberculation deposits cover a single area of deep wall loss.  Although UT testing would identify very favorable results where the pipe is absent of deposits, it is the deep pitting along the bottom which ultimately defines the future service life of this pipe.  In our center photograph, a random cross-cut through a 12 in. section of schedule 20 condenser water pipe perfectly illustrates the impact of deep pitting against the pipe wall; a noticeable loss of wall thickness directly below the largest tuberculation deposit.  At right, pitting has advanced substantially, and now encompasses most of the pipe’s inner surface.  Highest wall loss will be under areas of heaviest tuberculation along its top, with the amount of wall loss directly proportional to the size of volume of the tuberculation deposit above.

Further examples exist in our Photo Gallery on this subject.

• Galvanic Corrosion

This is an aggressive and localized form of corrosion due to the electrochemical reaction found between two or more dissimilar metals in an electrically conductive environment.  Galvanic corrosion occurs because the more electronegative material (the anode) is attacked by the more electropositive material (the cathode).  It is commonly associated between black steel anode and brass or copper cathode, and is more common at open condenser water systems than for closed piping systems.  Blue-green deposits at the brass or copper connection point, and absent the opposite steel to steel connection, provides nearly conclusive proof that galvanic activity is occurring.

Galvanic activity at threaded drain lines are the most common source for a catastrophic full pipe separation failure.  Although typically providing ample warning in the form of slow leaks and blue-green encrustation deposits at the threads, as the photos in our Photo Gallery illustrate, the lack of attention and timely repair are almost always the true reason behind of those million dollar catastrophic floods.

The most common example of such corrosion activity, widely found throughout HVAC and process plant operations, is the direct threaded connection of brass valves to carbon steel pipe, or between copper tubing and steel pipe – where the steel serves as the anode, and the brass or copper the cathode.  Carbon steel pipe, without the protection of a galvanic insulator or dielectric fitting, will show the highest rate of corrosion under such conditions – usually developing over many years.

The severity of pipe loss due to galvanic activity is often found relative to the general corrosion activity of the piping system itself – with little or no galvanic activity found where extremely low general corrosion rates exist.  A corrosion rate of under 0.4 MPY suggests little to no galvanic concern, while high galvanic activity is almost guaranteed once reaching 5 MPY or greater.  Under conditions of high corrosion rate activity above 8 MPY, galvanic losses often become aggressive – making an existing pipe corrosion problem significantly worse at the threads – its already most weakened area.  Rarely installed nor even known to exist 40-50 years ago, the far lower quality of today’s steel pipe now almost mandates the use of galvanic insulators under all conditions.

While galvanic corrosion is generally assumed to involve only dissimilar metals, millivolt potentials can actually be measured between similar metals and especially at steel pipe under certain conditions – at steel pipe supports for instance where there is a millivolt difference in the ground potential between the steel pipe and building structure.  New steel pipe installed during a repair or renovation is often more electronegative than the older existing pipe, and therefore will often suffer from some degree of galvanic attack.

Below left shows a classic example of galvanic activity at a drain line.  Highest wall loss is typically within the brass valve body; evidenced by the existing leak.  In the center photograph we illustrate the most severe form of galvanic activity common where galvanized steel is connected to brass or copper.  At right, another example showing the encrustation effect at a strainer drain as water and dissolved rust deposits slowly leak through the threads; the water then evaporating to leave a growing formation of rust and other water contained impurities.  Many engineers will misinterpret this condition as due to an external corrosion condition when in fact it represents a trough wall failure caused by internal corrosion acting against the threads directly connected at the valve.  Conditions such as this can ultimately result in significant water damage, should complete separation occur at the threaded connection.

Further examples exist in our Photo Gallery on this subject.

• Microbiologically Influenced Corrosion

Microbiologically Influenced Corrosion (MIC) is, by far, the most severe and threatening form of corrosion to HVAC and fire protection systems, with corrosion rates of 100 MPY documented. While other forms of corrosion activity are easily identified, laboratory analysis is generally required to confirm its presence.  For HVAC systems, MIC is generally associated with a lack of maintenance to the system, where heavy accumulations of rust and dirt deposits prompt the morphological transformation of aerobic microorganisms to a much more serious anerobic threat.  Fortunately, it is a less common threat.

MIC is caused by the presence of various anerobic microbiological agents under specific environmental conditions – in some cases resulting in advanced and widespread failure of entire piping systems within a few years.  Sulfur reducing bacteria are almost always identified where MIC has been established.  An MIC presence usually signals a very severe threat to the entire system – requiring extensive and repeated cleaning and sterilization at great expense.  For many affected systems, MIC cannot be eliminated, and an elevated corrosion and pitting condition will exist for the remainder of the life of the piping system.

MIC produces large and deep pits due to the anerobic microorganism’s utilization of iron as an energy source (as an alternative to oxygen), and through the production of strongly corrosive metabolic by-products such as sulfuric acid – which further assists the microorganism in dissolving pipe metal.  MIC exists to varying degrees of severity, and is not exclusive to carbon steel piping systems or open condenser water systems.  MIC is a known threat to fire protection systems although frequently misdiagnosed where basic high corrosion activity is the sole cause.  MIC is less commonly found in closed systems such as chill water and hot water heating, yet has been documented to occur.  It impacts steel and galvanized steel pipe primarily, but has been known to deteriorate other piping materials as well.

Since the microorganisms produce a metabolic by-product, they typically generate a sludge or slime type of accumulation within the pipe, which is shown in the below left two pipe sections.  At right, a cleaned section of MIC contaminated pipe shows the severe pitting common under such deposits which now approaches the outer pipe wall.

Further examples exist in our Photo Gallery on this subject.

• Erosion Corrosion

This is the gradual and selective deterioration of a metal surface due to mechanical wear and abrasion.  It is commonly attributed to cavitation entrained air bubbles, suspended matter, and abrasive particulates under a flow rate of sufficient velocity.  Erosion is similar to an impingement attack, and is primarily found at elbows and tees, or in those areas where the water sharply changes direction.  A sharp increase in velocity, as can occur at flow control valves, venturis, and flow metering ports can produce the same effect.  The impact is very localized rather than across all pipe surfaces.  Softer metals such as copper and brass are inherently more susceptible to erosion corrosion than steel, with hotter water and air cavitation often involved.

After a straight run of pipe where the water becomes laminar, the slight difference in inside diameter at a soldered copper connection is all that is required to produce cavitation as the ID slightly increases.  Typically requiring higher than normal water temperature and higher flow velocity, erosion to copper hot water heating and domestic hot water pipe are the predominant forms of cavitation encountered.  Even at wastewater treatment plants moving highly corrosion and high solids sludges, erosion is less of a concern.

Of all possible erosion corrosion scenarios, the most damaging and deadly is at high pressure steam lines where erosion can and has resulted in catastrophic explosions.  This potential threat is often related to the presence of condensate in the system.

Though typically not a problem at the water velocities encountered within most HVAC piping systems, high corrosion rates and the entertainment of high volumes iron oxide particulates can produce an erosion condition under certain conditions, and especially where suspended rust concentrations are high.  Erosion at the base of elbows or after multiple sharp turns of the pipe has been documented to occur.

Below left is a classic example where water entering from below has had no impact to the copper pipe.  Immediately after the straight pipe edge, the slightly wider ID of the elbow has produced a cavitation effect now eroding the outer radius of the elbow and first few inches of the next straight run.  At center, the impact or erosion by air bubble cavitation has completely destroyed the elbow and first inch of the next straight length.  At right, another example showing a severe erosion condition initiated by the exceptionally small difference in pipe ID at the soldered elbow under conditions of changing flow direction and higher temperature.

It is this known and well documented impact to copper pipe initiated by slight changes to the water’s laminar flow that we have raised concern to some crimped forms of pipe assembly popular today.  While no such evidence exists to our knowledge, the round shape to hex shape to round shape swaged into the copper pipe at each and every joint now, and at least theoretically, presents the same opportunity for air cavitation to occur under certain operating conditions.  Time, of course, will provide an answer to that speculation.

Further examples exist in our Photo Gallery on this subject.

• Corrosion Under Insulation

Known as Corrosion Under Insulation, CUI is a significant threat to any piping system or holding tank which operates at lower temperatures in humid environments, or is subject to outdoor environmental conditions.  Arguably, the problem is due more to poorly chosen, insufficiently thick, damaged, and improperly or poorly installed and maintained insulation than insulation quality alone.  Though termed a moisture barrier by manufacturers and by even those installing insulation, in fact the thin outer paper foil covering to most fiberglass products offers very little benefit.

A fundamental cause of CUI, beyond the inadequacy of the insulation itself, is the mistaken belief that insulating the pipe will fully protect it from corroding.  For that reason, bare steel pipe is never protected by any form of coating or rust inhibiting paint before the insulation is installed.  This massive problem would be barely an issue if the simple and low cost step of applying a high solids waterproof coating to the pipe was performed prior to the insulation.  It is not, however, and after billions of dollars in mostly preventable losses, the same mistake continues on.

In the absence of an effective moisture barrier and a protective pipe surface coating, any available moisture will eventually penetrate commonly used fiberglass or foam insulation to condense at the cold pipe surface.  Most fiberglass insulation is entirely too thin for the application – specified based upon thermal insulation requirements or R factor, rather than moisture limiting needs.  Often, moisture can accumulate sufficiently to water log the insulation and cause its total deterioration.  Discoloration at the surface, and especially effervescence or crystallization at the outer paper surface, are sure signs that the insulation requires replacement.  Ignoring a wet insulation condition often leads to mold, given that moisture and cellulose are two of its requirements for growth.

Waterlogged insulation essentially produces an untreated water condition at the outer pipe surface, and produces a corrosion problem now acting against the pipe at two fronts.  Water saturated insulation also eliminates any R value benefit, and substantially increases heat or cold transmission.

In outdoor environments, moisture, rain, snow, and ice can also penetrate the insulation due to physical damage, wear, or by the failure to use sealants at the overlap of the hard metal outer shell.  Rarely sealed to the degree necessary, most outdoor insulation will deteriorate to the point requiring replacement in under 25 years.  Add maintenance foot traffic and external damage, and that time frame can be cut in half.  Water treatment chemicals also negatively impact any aluminum outer jacked – turning it porous and essentially worthless over time.   Pipe hangers and support are always vulnerable areas both difficult to insulate and subject to easy moisture infiltration.

CUI is commonly found at cold water domestic piping, free cooling condenser water systems, dual temperature piping, and especially at chill water piping – being most severe at the colder supply side lines.  The degree of CUI type corrosion depends upon a combination of pipe temperature, insulation thickness, an additional vapor barrier or overcoat of paint, material used, natural corrosion resistance of the pipe, and area humidity.  In extreme examples of high humidity, CUI corrosion will even occur on typically warm condenser water piping.  Conversely, the extremely cold temperatures of a brine or ammonia refrigeration plant can create substantial exterior pitting even from a relatively dry atmosphere.

Rarely is insulation inspected for its CUI threat, and it is very rarely investigated during any building acquisition.  Instead, corrosion usually remains hidden until sufficient enough damage has occurred to the pipe to produce a failure.  More commonly, however, the presence of discolored insulation, crystallization at its surface, mold, even water droplets to the floor below are ignored.  In many cases, CUI corrosion can exceed the degree of physical damage caused by internal corrosion of poorly treated open condenser or process cooling water piping.

Below left we show a 30 year old 4 in. chill water riser insulated using only nominal 1 in. fiberglass.  At center, a smaller diameter dual temperature riser in severely deteriorated condition, and again with insufficient insulation; its outer “moisture barrier” having totally failed its purpose.  At right, the failure of soft form insulation – a material widely favored due to its supposedly superior performance over fiberglass.  In fact, some of the worst examples of CUI can be found where soft foam insulation has been installed.  Soft foam insulation is actually somewhat porous to moisture infiltration, hardens over time, becomes brittle, shrinks, cracks, and splits.  A final negative feature is that it actually produces a weak acid as it

An Important Note:  Simply re-insulating rusted pipe will not stop the exterior corrosion from progressing – only hide it from view.  Instead, the pipe must be rehabilitated.  All rust must be removed and a rust reverter coating applied to stop any further corrosion.  A high solids waterproof marine type paint is next applied followed by the insulation and an effective and properly sealed moisture barrier.

Further examples of CUI exist in our Photo Gallery on this subject.

• Brass Dezincification

Brass pipe is produced from an alloy of primarily copper and zinc.  Yellow brass is produced from about 60% to 70% copper and 30% to 40% zinc, along with trace amounts of tin and zinc.  Red brass contains a much higher 85% copper and lesser 15% zinc, and very little tin and lead.  This higher percentage of copper makes red brass look red, although in the field this differentiation may not be so obvious.  Yellow brass is far more common over red brass, and has been widely installed for older domestic water piping systems.  The issue is commonly associated with domestic water systems and is relatively rare in HVAC applications.

Brass pipe generally provides excellent corrosion resistance and long service life.  It was widely used for domestic cold water and especially domestic hot water service prior to the plumbing industry’s transition to mostly copper pipe.  Even though it was threaded, brass pipe still provided long service.  Depending upon the local area water quality and composition, however, zinc can be leached or removed from the brass pipe – thereby returning it to a more porous and weaker form of copper.  The process is also known as dealloying or more appropriately – “Dezincification.”

Soft surface or rain based water supplies, such as provided to New York City or Chicago, are weak in zinc concentration, and therefore the water naturally pulls or leaches the zinc it needs from the brass pipe; typically producing very random whitish granular or powdery deposits on its outer surface.  Eventually, a leak or fracturing of the pipe will result.  With the resulting copper pipe wall extremely weak, larger longitudinal splits are common, and produce the more dramatic failures.

Dezincification is more of a chemical issue based upon time and water conditions than it is a corrosion related event and is predominantly associated with yellow brass pipe.  Ultrasonic testing may show some evidence of lesser wall thickness to indicate an issue, although metallurgical testing is required for a confident determination.  For yellow brass pipe installed to New York City properties, a service life of no more than 80 years should be expected before failures begin – a fact many such older properties from the 1940s and before are now realizing.

This problem has been traditionally associated with very old yellow brass piping installed into much older buildings, and therefore relatively more and more rare as those older properties undergo pipe replacement.  At the same time, and to the surprise to many in the building management and operations industry, its incidence has been accelerating at new properties due to various industry changes.

• • • The manufacture of brass components such as valves having substantially higher zinc content
    • Over “softening” of water to turn it more aggressive in terms of leaching zinc
    • An increased use of “reverse osmosis” water purification – again creating a more aggressive water
    • Increased foreign produced brass products of lesser quality and higher zinc content
    • The demand for greater water flow control resulting in localized higher water velocities
    • Excessively high water temperatures which accelerate deterioration due to the above conditions

Below left we show the characteristic growing white chalkish deposits common at 84 year old yellow brass pipe.  At center, multiple failure sites are present at pipe in service at an 85 year old NYC building property.  At right, scraping away the deposits and cleaning the surface characteristically reveals what appears to be an inlay of copper into the brass itself.  This is a more porous and weaker deposited form of copper, however, which will inevitably fail.  Where the condition extends over a larger surface area, a lateral split in the pipe will frequently occur.

Further examples exist in our Photo Gallery on this subject.

• Additional Examples

CorrView International, LLC offers a series of Photo Galleries taken from 25+ years of past ultrasonic piping investigations which address the above as well as additional corrosion conditions.  For many, 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.

Whereas controlled generalized corrosion may take many decades to produce even minor operating problems, aggressive and localized corrosion, such as erosion, under deposit, and MIC, can accelerate the need for pipe replacement to as little as a few years – sometimes with little noticeable indication that such a problem exists.  A pitting condition is always suggested by measured corrosion rates exceeding 10 MPY, or a highest to lowest wall thickness variation of over 0.100 in., and should be addressed immediately.

It should be noted that some mechanical, engineering design, and age related factors can also produce or contribute to failures similar to those caused by a high corrosion or pitting rate alone.  Therefore, various investigative tools, such as metallurgical investigation, may be needed in order to correctly identify the cause and extent of a piping failure problem.

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