Forms Of Corrosion

Common Forms Of Corrosion

Many different corrosion mechanisms 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.  Once established, most corrosion problems will produce future years of operating difficulty and expense at varying levels of severity.


Common Categories


Generalized Corrosion


formofc 01Generalized 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 1 MPY or less under generalized corrosion conditions - providing easily 100 years or more of service for larger diameter condenser water mains.  Longer service is provided to closed system piping.

Generalized corrosion usually occurs in environments in which the corrosion rate is inherently low or well controlled - such as for chemically treated closed circulating systems, and in some well maintained open water systems.

It is the only form of corrosion whereby weight loss or metal loss data from corrosion coupons or ultrasonic testing can be used to accurately and reliably estimate corrosion rates and future pipe life expectancy. 




Pitting Corrosion


formofc 02Often termed "under deposit corrosion," this is a localized, deep penetration of the metal surface with little or normal general corrosion in the surrounding area.  Due to surface deposits, electrical imbalance, microbiological activity, coating failure, or some other initiating mechanism, all existing corrosion potential attacks a select number of individual sites.

In most cases, pitting is extended throughout the entire metal surface, creating an irregular or very rough surface profile.  In other instances, such as in the above example, pits are concentrated in specific areas, leaving the majority of the metal surface in like new condition.

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 pitting condition.

Pitting corrosion may include: crevice corrosion, water-line attack, under deposit attack, impingement or erosion corrosion attack, and concentration-cell corrosion.




Galvanic Corrosion


formofc 03This is an aggressive and localized form of corrosion due to the electrochemical reaction often 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.

Blue-green deposits at the brass or copper connection point, and absent adjacent steel to steel connections, as shown above, provides strong indication that galvanic activity is occurring.

The most common example of such corrosion activity, widely found throughout HVAC and process plant operations, is the direct 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. 

Under conditions of high corrosion rate activity, galvanic losses often become aggressive - making an existing pipe corrosion problem significantly worse at the threads - its already most weakened area.

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.  New steel pipe installed during a repair or renovation is often more electronegative than older existing pipe, and therefore may suffer from some degree of galvanic attack. 




Microbiologically Influenced Corrosion


formofc 04Microbiologically Influenced Corrosion (MIC) is, by far, the most severe and threatening form of corrosion to HVAC piping systems, with corrosion rates of 100 MPY documented.  Laboratory alanysis is required to confirm its presence.

MIC is caused by the presence of various microbiological agents under specific environmental conditions - in some cases resulting in advanced and widespread failure of entire piping systems within a few years.

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 system life.

MIC produces large and deep pits due to the microorganism's utilization of iron as an energy source (often 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.  It is a frequent problem to fire protection piping.

MIC is less commonly found in closed chill water piping, in hot water heating and domestic water systems, and has been documented to destroy copper, brass, and stainless steel pipe. 




Erosion Corrosion


formofc 07This is the gradual and selective deterioration of a metal surface due to mechanical wear and abrasion.  It is commonly attributed to entrained air bubbles, suspended matter and particulates under a flow rate of sufficient velocity.

Erosion is similar to impingement attack, and is primarily found at elbows and tees, or in those area where the water sharply changes direction.  Softer metals such as copper and brass are inherently more susceptible to erosion corrosion than steel.

High pressure steam will often contribute to the erosion of carbon steel, and especially where condensate is present. 

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




Corrosion Under Insulation


formofc 06Known 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, insufficient, damaged, and improperly installed insulation than the insulation alone.

In the absence of an effective moisture barrier and a protective pipe surface coating, any available moisture will penetrate commonly used fiberglass or foam insulation to condense at the cold pipe surface. 

Often, moisture can accumulate sufficiently to waterlog the insulation and cause its total deterioration.  This effectively creates an untreated water condition at the outer pipe surface, and produces a corrosion problem acting against two fronts.

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.

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, vapor barrier, material used, natural corrosion resistance, 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.

CUI corrosion usually remains hidden until severe damage has occurred to the pipe, producing telltale discoloration at the insulation itself, or failure.  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.




Brass Dezincification


formofc 08Brass pipe is produced from an alloy of primarily copper and zinc.  Depending upon the local area water quality and composition, 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.

Soft surface water supplies, such as provided to New York City, are weak in zinc concentration, and therefore the water pulls or leaches the zinc it wants from the brass pipe; typically producing very random whitish granular or powdery deposits to its outer surface.  Eventually, a leak or fracturing of the pipe will result.

Scraping away the deposits and cleaning the surface typically reveals what appears to be an inlay of copper into the brass itself, as illustrated above.  This is a more porous and weaker form of copper, however, which will fail. 

Dezincification is more of a chemical issue based upon time and water conditions than it is a corrosion related event.  For brass pipe installed to New York City properties, a service life of no more than 80 years should be expected before failures begin.


Additional Examples

corrosiongalleryCorrView International, LLC offers a series of photo galleries taken from 20+ 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.  View our extended Corrosion Photo Gallery of 25 different corrosion types and failure conditions.

Whereas controlled generalized corrosion may take many decades to produce even minor operating problems, aggressive and localized corrosion, such as 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 often suggested by measured corrosion rates exceeding 5 MPY, or a highest to lowest wall thickness variation of over 0.050 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 may be need in order to correctly identify the cause and extent of a piping failure problem.


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  Installed in an isolated loop,
                                             corrosion coupons never suffer the same environmental effects as the pipe
                                             itself, and rarely provide accurate test results. Hardened deposits, electrical
                                             activity, under deposit corrosion, micro biological buildup, flow effects, and
                                             other common environmental factors typically do not exist for corrosion

A flow requirement, by definition, prevents their installation
                                             in precisely those locations traditionally showing the highest corrosion

In addition, installing corrosion coupon racks at multiple
                                             points throughout a circulating system is not practical and is almost never
                                             performed - thereby forcing the unlikely assumption that the test results shown
                                             for one specific location are representative over the entire piping







  Periodically cutting out samples of
                                             pipe for metallurgical analysis is extremely expensive, usually requires a
                                             system shutdown, is rarely carried out for large diameter piping, and for any
                                             critical or 24/7 operation - is virtually impossible to perform. Combined
                                             maintenance and metallurgical costs can easily exceed $4,000 per

Metallurgical analysis does offer valuable information
                                             unavailable through any other means, and is especially useful in order to
                                             identify the cause of a corrosion condition, but is generally limited in use
                                             due to its cost and inconvenience.

Metallurgical analysis usually plays
                                             an inportant role in defining a corrosion problem, rather than discovering







  Spool pieces, which are nothing more
                                             than removable sections of actual pipe within the system, provide valuable
                                             information regarding the actual net effect of corrosion activity against the
                                             pipe surface. Unfortunately they are only applicable for smaller diameter
                                             piping separate from the main lines.

Properly installed, spool pieces
                                             offer a true inside look at deposits, surface pitting, inhibitor and cleanout
                                             effectiveness, as well as provide samples for micro biological cultures. Like
                                             corrosion coupon racks, however, they are rarely installed throughout a piping
                                             system and enjoy limited use.







  Ultrasonic wall thickness testing
                                             provides the greatest volume of reliable data, and will typically produce a
                                             thorough corrosion evaluation as long as a sufficient number of test points are

Ultrasound is often used as a prerequisite to other testing
                                             methods due to its low cost and wide coverage, or as a confirmation that wall
                                             thickness conditions known to exist in one area do or do not exist elsewhere
                                             within the piping system.

It is most often used as a tool to identify
                                             the extent of an already recognized leak or rusting problem, and long term
                                             corrosion monitoring using ultrasound requires establishing specialized testing
                                             procedures. Read more about
                                             ultrasonic pipe testing.







  A wide variety of electronic
                                             techniques eixst to produce an estimate of corrosion rate generally based upon
                                             the principal of Linear Polarization Resistance, or LPR.

LPR provides
                                             the benefit of an immediate corrosion measurement that can be routed to
                                             monitoring electronices, or data logged for download, and offers an extremely
                                             useful corrosion measurement tool.

LPR is generally expensive to install
                                             and maintain. Regular celaning and calibration is often required, and even
                                             then, results may not approximate true corrosion activity - expecially if
                                             underdeposit corrosion or MIC is active.







  The insertion of a remotely
                                             controlled camera into the pipe offers a valuable thought very limited
                                             inspection option. Its use requires the system to be out of service and
                                             drained, and is greatly limited by access into the piping system. Pipe size,
                                             physical configuration, internal conditions, and length of travel offer further
                                             restrictions in its use.

Remote Video Inspection (RVI) cannot provide
                                             any wall thickness data, but can quickly locate those internal indications that
                                             wall loss has occurred - such as tubercular deposits, deep pitting, or
                                             suspected MIC growths.

Combined with ultrasound or metallurgical
                                             testing, RVI can quickly and cost effectively document whether similar problem
                                             conditions exist in other areas of a piping system.







  X-ray offers limited application for a
                                             piping evaluation primarily due to its high cost and safety concerns. While
                                             x-ray can provide the wall thickness values necessary for a true pipe condition
                                             analysis, it is most often used for the inspection of weld integrity or for
                                             identifying cracks, voids, or a major localized deterioration in a pipe

Cost, health, and environmental issues severely restrict its
                                             use in all but the most critical of applications.







  Similar technology also exists for an
                                             excellent but rarely used tool in measuring a wide variety of piping related
                                             problems. Its use of safe, low powered gamma radiation rapidly identifies areas
                                             of higher wall loss - quickly locating those areas in need of further

By detecting variations in metal density, this hand held
                                             device can also detect pipe blockages, identify wet insulation, show liquid
                                             level, or confirm pipe schedules, etc.

Company Info


CorrView International, LLC

P.O. Box 8513
Landing, NJ  07850
Ph:    973-770-7764
Fax:  973-770-6576

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