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Piping Problems

While the external signs can sometimes be subtle, learning of a corrosion problem often requires little more than a quick look into the cooling tower pans or related HVAC equipment such as refrigeration machines, pumps, and heat exchangers.  A rust build-up at the tower, fouled drift eliminators, tuberculation deposits, and flakes of scale and rust caught in the condenser water tubes are obvious indications of a corrosion problem.  Likewise, encrustations at the threaded joints, buckets of rust captured at start-up, a 10 MPY corrosion coupon result, lost heat transfer, and failed equipment should prompt the need to investigate further.

Under real world conditions defined by the laws of nature, some degree of corrosion must be expected to exist within any piping system.  However, identifying the type and severity of corrosion responsible requires far more effort, and often multiple investigative tools.  The monitoring, testing, diagnosis, control, and repair of piping related corrosion problems is a multi-billion dollar business in itself.  Yet, corrosion caused piping replacement in the United States still exceeds $75 billion per year.  Volumes of information exist on the subject and yet many corrosion mechanisms are still not fully understood.

Common Problem Areas

While hundreds of different corrosion scenarios exist, there are actually only six main forms that affect most HVAC related piping systems.  Of the many different piping systems and services available, condenser water traditionally exists under the greatest threat and suffers the most severe losses. The six most common forms of corrosion are:

• 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.

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, steam piping, 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.

• Pitting Corrosion

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, 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 example at left, 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.

• Galvanic 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).

Blue-green deposits at the brass or copper connection point, and absent adjacent steel to steel connections, as shown, 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 analysis 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 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

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 of 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, 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 usually remains hidden until severe damage has occurred to the pipe, producing telltale discoloration at the insulation itself, or failure. In many cases, CUI can exceed the degree of physical damage caused by internal corrosion of poorly treated open condenser or process cooling water piping.

• Different Levels Of Threat

While controlled generalized corrosion may take many decades to present even minor operating problems, aggressive and localized corrosion can accelerate the need for pipe replacement to as little as a few years. Typically, a visual diagnosis is all that is necessary for problems such as CUI or galvanic corrosion. Pitting corrosion and erosion usually require a reasonable degree of nondestructive testing or metallurgical analysis, coupled with the ability to predict and investigate the most problem prone areas.

MIC is unquestionably the most difficult to test and diagnose, as well as the most difficult to control. Combined with the potential to destroy piping at rates exceeding 50 mils per year, MIC is clearly the greatest threat to any piping system. For that reason, continuous, reliable and effective monitoring for any corrosion problem is required, and quick attention to correct it essential.

We provide a very extensive collection of actual field photographs from prior projects depicting the above six plus other forms of corrosion in our Corrosion Photo Gallery.

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