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.


     A quick review of the six forms of corrosion commonly found at HVAC and process cooling piping systems is often helpful in understanding their end result. They are:


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. 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 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. View a photo gallery of various corrosion types.



Often termed "under deposit corrosion," this is a localized, deep penetration of the metal surface with little general corrosion in the surrounding area. Due to surface deposits, electrical imbalance 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, 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 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.



This 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. It occurs because the more electronegative material (the anode) is attacked by the more electropositive material (the cathode).

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, 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 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. View a photo gallery of galvanic corrosion examples.



Microbiologically Influenced Corrosion (MIC) is, by far, the most severe and threatening form of corrosion to HVAC piping systems. Corrosion rates of 100 MPY have been documented. 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 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.

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. See Technical Bulletin # C-5 for more about the threat of MIC.



This is the gradual and selective deterioration of a metal surface due to mechanical wear and abrasion. It is 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. 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.



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. In the absence of an effective moisture barrier and a protective metal coating, any available moisture will penetrate commonly used fiberglass or foam insulation to condense at the cold metal 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 on 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 install sealants at the overlap of the hard metal outer shell.

CUI is commonly found at cold water domestic piping, free cooling condenser water systems, and especially at chill water piping - being most severe at the colder supply side piping. The degree of CUI type corrosion depends upon a combination of pipe temperature, insulation thickness, vapor barrier, and 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. View a photo gallery of under insulation corrosion examples.


     Of the above mentioned corrosion conditions, microbiological MIC, under deposit pitting, and galvanic corrosion are, by far, the most damaging corrosion types, since they localize their aggressive action against the pipe metal at one or a few specific points to produce extremely high wall losses and failure. They are also the most difficult to resolve. Access other Internet sites offering valuable information on corrosion issues.


     CorrView International offers a series of photo galleries taken from 10 years of past ultrasonic piping investigations. 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 system - therefore requiring sufficient monitoring over enough locations to provide a reliable corrosion rate assessment.






     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 a piping failure problem.