1. Draw and test representative samples.
2. Know, follow, and understand test procedures and test interferences.
3. Calibrate meters – Keep batteries and reagents in date.
4. Use clean glassware – Rinse between tests.
5. Use proper sample sizes.
6. Know multipliers, conversions, control ranges, and expected test results.
7. Test in a well lit area.
8. Have established, in-plant testing program – Keep operators trained.
9. Keep complete test logs –
10. Take corrective action and document changes.
Corrosion is one of the basic problems encountered in designing cooling water treatment programs. The intent of this chapter is to outline some fundamentals, not to make you a corrosion specialist. Remember that corrosion control is just one part of a complete cooling water program. If you treat solely for corrosion, ignoring the potential effects of deposition or microbiological fouling, your program will have problems.
The Nature of Corrosion
Corrosion is the electrochemical reaction of a metal with its environment. It is a destructive reaction and, simply stated, is the reversion of refined metals to their natural state. For example, iron ore is iron oxide. Steel is refined iron ore or relatively pure iron. When steel corrodes, it again forms iron oxide. Our primary objectives in controlling corrosion are:
▪ to minimize downtime due to equipment failure.
▪ to maintain operating efficiency by minimizing fouling and loss of heat transfer due to corrosion products.
▪ to lower the initial capital investment requirement (e.g., if mild steel can be properly protected, it is much cheaper for construction than exotic alloys).
Mild steel is the primary metallurgy of concern in cooling systems. Copper, copper alloys, and other alloys are important, but they have more inherent resistance to corrosion than steel. Aluminum presents unusual problems; special consideration of the treatment program may be required.
The Corrosion Cell
The corrosion cell is set up when an electrical potential exists between two metals or two different sites on the same metal. This causes current flow in the presence of an electrolyte. The anode is where corrosion occurs (electrons are lost). The cathode is where the circuit is completed. Corrosion does not occur here, and electrons are accepted by the oxidant.
▪ Anodic and cathodic sites form for many reasons:
▪ Impurities or inclusions in the metal.
▪ Localized stresses.
▪ Grain size or composition differences.
▪ Discontinuities on the surface.
▪ Differences in local environments (e.g., temperature, oxygen, or salt concentration).
Types of Corrosion
1. Uniform — anodic and cathodic sites keep shifting on the metal surface so that metal loss is even.
2. Galvanic — this occurs when two dissimilar metals are in contact in the presence of an electrolyte. The corrosion rate of the less resistant (less noble) is decreased. The greater the distance between the two metals on the chart, the greater the potential for increased corrosion.
3. Concentration cell — this occurs when local environmental differences exist:
Under deposit corrosion – concentration differential between the deposit and the bulk water. The anodic site forms under the deposit.
Crevice corrosion – a crevice is formed when two surfaces are mechanically joined. Concentration differential between crevice and bulk water creates anodic sites in the crevice.
4. Pitting — the anodic site remains stationary, thus all the corrosion proceeds at that one spot.
5. Intergranular — localized attack begins at a grain boundary, causing disintegration of the metal.
6. Stress — usually interpreted as causing general cracking of the metal.
7. Dezincification — occurs in copper zinc alloys (usually Admiralty in cooling systems). It is the selective removal of zinc from the alloy.
8. Erosion — mechanical destruction due to high velocity, impingement, suspended solids, or turbulence.
In cooling systems, water velocity, dissolved solids, and continual aeration provide optimum conditions for continued corrosion of mild steel.
The corrosion products formed at the anodic site may remain there in the form of tubercle. The corrosion products of mild steel are a potential foulant because they are many times more voluminous than the metal itself. They may be swept away and redeposited, creating another corrosion site, or they may be complexed by appropriate deposit control agents. If corrosion products have been removed and the site is still active, the metal will appear very shiny.
An examination of corrosion products normally reveals several layers of various-colored products. At an active corrosion site, the diffusion layer next to the iron surface is composed of ferrous hydroxide, Fe(OH)2, which is greenish-black in color. The outer surfaces of the corrosion products will be orange to red/brown in color and consist of ferric hydroxide, Fe(OH)3. Ferric hydroxide may exist as nonmagnetic alpha ferric oxide (hematite) or magnetic gamma ferric oxide. A magnetic hydrous ferrous ferrite, Fe3O4 nH2O, often forms a black intermediate layer beneath the hydrous Fe2O3.
We are most interested in modification of the environment to control and retard corrosion rather than the use of protective coatings or changing properties of the metal.
Inhibitors or passivating agents are used to modify the environment. These materials act as either anodic or cathodic inhibitors, i.e., they function by reducing or slowing either the anodic or cathodic reaction. Normally, the treatments we apply are combinations of both anodic and cathodic inhibitors for optimum protection. These materials tend to passivate by promoting some type of barrier film. For instance, a very localized pH increase at the cathodic site is responsible for precipitation of certain materials, thus forming a barrier or cathodic film The mechanisms of various materials that are used as corrosion inhibitors will be discussed in later chapters.
Corrosion Inhibitor Functions
The anode is where corrosion, an oxidation reaction, occurs and is the point where the corroding metal goes into solution. The cathodic area is where a reduction reaction takes place and hydroxyl ion is formed.
The anodic and cathodic areas are not static; they constantly change position. Accordingly, an area that is anodic in nature can become cathodic.
Chromate, molybdate, and nitrite — catalyze the reaction between the metal and oxygen to form a passivating film. They also become a part of the gamma iron oxide film. Chromate and nitrite are the only anodic inhibitors that function in the absence of oxygen.
Orthophosphate — also catalyzes the reaction between steel and oxygen to form a passivating gamma iron oxide film. Oxygen must be present in water for orthophosphate to function as an anodic inhibitor.
Polyphosphate — exhibits some anodic properties but functions primarily as a cathodic inhibitor.
Cathodic inhibitors sense the highly localized elevation in pH and form a microscopic coating.
▪ Zinc hydroxide
▪ Zinc phosphate
▪ Calcium carbonate
▪ Calcium phosphate
Most methods of corrosion control are based on forming a film that acts as a barrier to stifle corrosion. The rate at which the film or barrier forms will largely determine the effectiveness of the treatment. The rate at which the film forms is related to the inhibitor concentration.
The function of pretreatment is primarily to permit rapid film formation to stifle the corrosion reaction immediately by formation of a uniform impervious film. Under these conditions, the low treatment levels will maintain the film intact and avoid the accumulation of corrosion products.
The low treatment levels normally used for corrosion control in open recirculating systems should be viewed as the quantities required to maintain the film intact and to heal the slight breaks that may occur from minor variations in environment. Whenever any serious changes in environment occur that cause destruction of the film, corrosion products can accumulate before the film is reestablished by the low treatment levels. Under these conditions, in order to secure maximum corrosion protection and to minimize accumulation of corrosion products, treatment levels should be increased to reestablish the protective film as rapidly as possible.
Pretreatment is required:
- for all new bundles
- whenever the exchangers are acid cleaned
- whenever low pH is encountered
- immediately following start-up
- immediately after inspections
|PRETREATED VS. NON-PRETREATED
||0.5 – 2.0
||1.5 – 7.0
||1.0 – 3.0
||5.0 – 15.0
||0.7 – 3.0
||3.0 – 10.0
||0.5 – 3.0
||2.0 – 7.0
||0.5 – 3.0
||5.0 – 10.0
In today’s Water Treatment Programs, there are many different types of controllers and a wide variety of system control enhancing options that improve treatment control and effectiveness. Here is a brief explanation of some of the more common controllers and accessories that are in use today.
- Feed and Bleed Controllers – This type of controller monitors the system conductivity through an in-line sensing probe, and when the unit senses the conductivity of the system is at the predetermined control point, it activates two circuits. One circuit energizes to open the system bleed valve to allow for conductivity reduction. Simultaneously, a second electrical circuit activates the treatment pump to replace the inhibitor that is lost during bleed and thereby maintain inhibitor in the prescribed control range. This type of system is by far the most common cooling tower controller in use today. One draw back to this controller is that it does not account for uncontrolled water loss from the system.
- Proportional Feed Controllers – This type of controller controls the conductivity and inhibitor levels independently. The conductivity is monitored through an in-line sensor and when the conductivity is at the predetermined control point, activates the system bleed valve for conductivity control. The inhibitor level is maintained by receiving a signal from a water meter in the make up water line. Inhibitor feed is initiated through a timing circuit on the controller panel for a preset amount of time based on the quantity of make up water delivered to the system. This is a very common controller and provides consistent inhibitor control even when there is uncontrolled water loss.
- pH Controller – pH controllers are designed to monitor the system pH and initiate the addition of acid, and in some cases caustic, to maintain the system pH and control the scaling or corrosive tendencies of the treated water. This type of controller is used in conjunction with conductivity and control and in some cases is incorporated on either type of controller described above. A flow switch is mandatory on this type of controller.
- Flow Switch – This device is placed in either the controller sensing line or in the recirculating water line to monitor system water flow. When there is system flow, the switch allows for bleed and inhibitor feed, but when flow has been lost or secured, the flow switch prevents controlling actions from taking place. Many newer controllers have flow switches integrated into the sensor plumbing assemblies, but virtually any system can be retrofitted. Besides preventing controlling action during no flow or idle system situations, another benefit to a flow switch is that it allows the electronics in the controller to remain energized, reducing wear and tear on the electrical components.
- Biocide Timers – These devices are sometimes built into your controller and are adjustable timers that will automatically feed biocide to the system on selected days for a specific amount of time to provide a consistent microbiocide control program. The timer is set based on your system’s specific biocide retention time. A biocide timer may be as simple as a timer that is completely independent of the conductivity controller. Timers may also be incorporated in the controller to accomplish biocide treatment. Timers may also lock out controller operation while biocides are being added, alternate biocide feed automatically, and lock out bleed for a predetermined amount of time and to increase biocide contact time for more effective control.
The styles and types of control equipment utilized in Water Treatment are varied. The level of automation that is available using these systems has become extremely reliable and can be used to free up man hours while still providing consistent and accurate control. If you have any questions or would like additional information on any of these controllers or accessories, ask your Water Treatment Consultant.
If you could see the different service reports written each month, you would see many different control ranges. If you were to ask why this is, the answer would be that it is due to the quality of make-up water.
One of the purposes of water treatment is to extend the usefulness of the water. In order to do this, we must control the deposition of hardness salts onto the heat exchange portions of the systems, and control corrosion of the metal in the system.
Cooling Tower control levels have a broad spectrum of control ranges, which is due to the challenge of maintaining higher levels of hardness salts in solution and the vast differences in make-up water quality.
Most boilers, on the other hand, are normally controlled at about the same treatment levels. This is attributable to the fact that most boilers are operated with soft feedwater. These levels are maintained in case you should experience an upset with your water softener. Although the amount of treatment added to the system will change from time to time, it is the quality of the make-up water that determines the base amount of product to add.
Corrosion is controlled with both anodic and cathodic corrosion inhibitors — some are natural and some have to be added to the water to protect the metal.
Protection from scale is controlled with the use of Phosphonates and polymers as well as the use of scaling indices. The Langlier Saturation Index measures scaling potential by evaluating calcium, carbonates, conductivity, pH, and temperature of the water. The product that will best meet your needs is selected based on the LSI and make-up water characteristics. Each product has guidelines that it must be controlled within for it to perform to its potential. The LSI also allows us to determine what is happening with the water.
There are specific products for soft water and hard water as well as products for use with high levels of Phosphate. All of these factors have to be considered during the product selection process.
Water Treatment Consultants are frequently asked about the best place to get a water sample. While there are many options, this list outlines the most desirable places/methods for obtaining testing samples.
- Softeners – Sample downstream of the softener. Each softener should be sampled individually.
- Boiler Feedwater – Obtain a sample from the storage section of the deaerating heater or from the feedwater line between the feedwater tank and boiler.
- Boilers – The preferred location is the continuous blowdown line. The water column is acceptable provided the column is thoroughly blown down before sampling. It is also preferable to take the sample through a cooler to prevent sample flashing.
- Condensate – Samples should be obtained upstream of traps and some distance away from main stream lines. As with boiler water, it should be gathered through a sample cooler
- Raw Water – The sample can be taken from a makeup valve or any suitable location supplying fresh raw water provided the line is flushed.
- Condensate – The sample should be taken directly from the condenser shell or from a sample line off the recirculating pump.
- Chilled Water – The sample should be taken directly from the evaporator shell or from a sample line off the recirculating pump.