In order to retard the deterioration of the highly corrosion susceptible carbon steel pipe commonly specified for HVAC systems and process cooling waters, chemical water treatment programs of varying degrees of complexity and effectiveness are maintained. The present condition of any facility is therefore greatly dependant upon not only the current water treatment contractor, but on the actions or inactions of all others beginning from the time the piping was first installed, and possibly before.

     While typically receiving blame for all corrosion problems, system condition is not totally dependent upon the water treatment contractor alone - with operator actions, proper maintenance, environmental issues, budgetary limits, equipment, controls, and engineerging and design issues often playing critical roles.

     A successful and well maintained chemical water treatment program will hopefully minimize general corrosion activity to within an acceptable level - thereby eliminating any threat to the piping infrastructure. It should also minimize any secondary effects, such as under deposit pitting caused by a buildup of corrosion product and MIC. Ideally, the annual measured loss of metal for any piping system will be far less than the tolerable mil per year (MPY) corrosion limit calculated for the life expectancy of the system.

     Just how much the corrosion rate impacts a piping system can be easily estimated. For example, the service life of a 12 in. diameter schedule 40 open condenser water system operating at 150 PSI will be the difference between its minimum acceptable wall thickness dimension of 0.175 in. (typically), and its original wall thickness of 0.406 in.

     At a low rate of 1 mil per year (MPY), the life expectancy of such a piping system is easily 200 years, or virtually limitless. However, at 12 MPY, and under the high pitting conditions which would accompany such elevated corrosion activity, its useful life is dramatically reduced to only 19 years or less - a significant and intolerable loss of service to any building or plant operation. See Technical Bulletin C-1 regarding the pounds of steel pipe lost at different corrosion rates.

     For most typical HVAC or process cooling waters, the corrosion activity of a condenser or open water system will exist somewhere between such high and low corrosion limits. Unlike low corrosion rates which tend to remain uniform over decades, however, high corrosion conditions generally tend to worsen at an accelerating rate if not quickly and effectively addressed.

     While corrosion rates will vary depending upon the type of piping system involved, its material of composition, and chemical water treatment effectiveness, there are some established guidelines to follow.

     In general, an open water carbon steel corrosion rate of 1 MPY or below is considered excellent, 2-3 MPY is considered good, and a rate of 4 MPY is borderline acceptable. Corrosion rates at or exceeding 5 MPY warrant immediate investigation. Rates of 10 MPY and greater signal a serious threat to system safety, reliability, and building or plant operations.

     Any corrosion rate above 15 MPY generally indicates a severe pitting condition, an under deposit or cell corrosion condition, or possibly MIC. In most cases, such high corrosion rates dramatically reduce service life, and strongly raise the probability of a premature piping failure. Corrosion rates above 25 MPY are not uncommon where under deposit corrosion and MIC are involved.

     Closed system corrosion rates are considerably lower, and are far more easily maintained due to the lesser external influences on the system. They are also treated at much higher chemical concentrations. Corrosion activity against a typical closed steel piping system should never exceed 1-2 MPY, and can often be maintained to half that amount with only minimum attention and expense. See Technical Bulletin # M-12 regarding why closed and open piping systems are treated to different chemical concentrations.

     Our experience has shown that a good water treatment program, well applied and rigidly maintained, along with some additional preventative actions such as full flow or side stream filtration, regular chemical cleanings and sterilizations, and a supplemental chemical dispersant, will achieve excellent corrosion control in most cases. In many examples, however, the currently achieved corrosion rate is dependant upon all previously applied chemical treatment programs, their effectiveness, and the interior pipe wall conditions they have produced.

     Where extremely low corrosion rates are reported of less than 0.5 MPY for steel piping, suspicions should be raised toward the method of testing employed, and of whether representative samples have been taken, or the correct procedures used. This is especially true for open water systems - those most often employing corrosion coupon testing.

     The under reporting of corrosion rate by corrosion coupons is a common occurrence due to a large variety of factors. Read more about the many limitations of corrosion coupons. Questions regarding a questionable corrosion rate determination can often be resolved using ultrasonic testing or metallurgical investigation.

     Due to the awareness that corrosion always exists to a certain degree, it is assumed that consulting engineers, building operators, and mechanical contractors have accounted for this annual loss of wall thickness in their original equipment and piping design. Our experience in testing hundreds of such systems, however, shows this may not always be the case.

     The significantly higher corrosion rates typically found today at open cooling systems suggest placing far greater emphasis on methods to control corrosion. It also suggests to better prepare for those installations where, for whatever reason, corrosion control may not be adequate. While higher corrosion rates would suggest the interest to install heavier piping schedules, we have actually found the opposite - with thinner schedule 20 and schedule 10 stock now commonly recommended as cost cutting alternatives. Access other Internet sites offering valuable information on corrosion issues.

     For some piping systems, such as galvanized steel, copper, or brass pipe carrying domestic water, steam supply and steam condensate systems, gas or oil lines, etc., the chemical inhibition of corrosion is not available, not permitted, or not desired - thereby making the corrosion of such systems entirely dependant upon water chemistry and corrosivity, environmental factors, metal quality, coatings or barrier protection, and design engineering. Close monitoring of such piping systems can therefore rate an even higher priority.

     Brass, copper, and other non-ferrous metals also suffer from specific types of metal loss such as dealloying or dezincification - whereby the selective removal of one component of the metal may take place over extended time, and eventually leading to its embrittlement, cracking, and failure. Microbiological attack is also always a serious concern. In such cases, metallurgical analysis is usually required in order to identify the cause.

     Ultimately, the quality of the water treatment receives either the credit or blame for the internal condition of any piping system. While additional factors exist to influence the condition of any piping system, some general recommendations toward providing an effective corrosion control program are presented below:

• Maintain a strict water treatment program from a reliable vendor. Avoid low bidding as the sole criteria for awarding contracts.
  
  
• Automate all chemical feeds and blowdown. Chemical inhibitor feed should be based upon water meter makeup, biocide feed based upon an alternating timer, and blowdown based upon water conductivity.
  
  
• Install a side stream filtration to reduce overall particulates - a significant and well documented cause of all corrosion problems. Filtering even 10% of the flow rate will greatly help in reducing the buildup of dirt and debris. A wide variety of manual and automatic filters are available. Full flow filtration is always preferred, and is far more important for open circulating systems. Review Technical Bulletin W-4 for a comparison of different filter types.
  
  
• Review all test reports carefully and question any changes in result.
  
  
• Chemically clean and sterilize all open water systems twice per operating season.
  
  
• Chemically clean and sterilize all closed systems every three to five years.
  
  
• Consider employing an outside consultant to review the water treatment program on a regular basis.
  
  
• Perform independent chemical and corrosion testing through an outside laboratory.
  
  
• Clean and flush the cooling tower basins regularly. Corner or low flow areas are especially susceptible to particulate settlement, and for the potential of MIC or under deposit corrosion to develop. Cover or shield all distribution pans and basins from the sun in order to minimize biological growths.
  
  
• Incorporate a supplemental chemical dispersing agent into the water treatment program. Combined with filtration, this will help prevent the settlement of particulates, and redistribute settled material for gradual removal.
  
  
• Perform regular corrosion coupon testing. While their under reporting of true corrosion rate activity is a common concern, coupons well document the presence and general effectiveness of an inhibitor, as well as allow trending of relative corrosion activity.
  
  
• Install multiple "spool pieces," or test sections of pipe which can be periodically removed for visual and metallurgical examination and returned to service. Spool pieces are ideal for evaluating the effectiveness of any deposit cleaning program.
  
  
• Include monthly testing for biological cell counts in the water treatment program. Biological contamination can develop within a few days period of time under favorable conditions. Currently used non-toxic inhibitors, and their regulated dosages, place an added emphasis on the effectiveness of biocides.
  
  
• If possible, prevent the buildup of dirt and debris by eliminating low flow areas. This is especially common in secondary water lines having long horizontal runs. Some studies have shown low flow conditions to reduce the effectiveness of water treatment inhibitors as well as allowing dirt to deposit. Booster pumps or by-pass lines are two viable and easy options to increase flow rate.

     For many property owners and operators, just the existence of a chemical water treatment contract is assumed to mean the elimination of that area of concern. In reality, there are far more steps required in order to ensure a trouble free operating system.

     Following 10 years of ultrasonically testing hundreds of commercial and industrial properties, CorrView International has well documented the failure of such logic. Even the most effective and rigidly maintained chemical inhibitor program, provided by the most reputable water treatment contractor, still requires additional steps by the building or plant operators in order to provide the best corrosion protection possible.