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.
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 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.
Often 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.
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.
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
Microbiologically 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.
This 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
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, 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 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.
CorrView 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.