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In virtually all situations, metal corrosion can be managed, slowed or even stopped by using the proper techniques.

Corrosion prevention can take a number of forms depending on the circumstances of the metal being corroded.

Corrosion prevention techniques can be generally classified into 6 groups:

1. Environmental Modifications
2. Metal Selection and Surface Conditions
3. Cathodic Protection
4. Corrosion Inhibitors
5. Coating
6. Plating

Environmental Modification:
Corrosion is caused through chemical interactions between metal and gases in the surrounding environment. By removing the metal from, or changing, the type of environment, metal deterioration can be immediately reduced.

This may be as simple as limiting contact with rain or seawater by storing metal materials indoors, or could be in the form of direct manipulation of the environmental affecting the metal.

Methods to reduce the sulfur, chloride or oxygen content in the surrounding environment can limit the speed of metal corrosion.

For example, feed water for water boilers can be treated with softeners or other chemical media to adjust the hardness, alkalinity or oxygen content in order to reduce corrosion on the interior of the unit.

Metal Selection and Surface Conditions:
No metal is immune to corrosion in all environments, but through monitoring and understanding the environmental conditions that are the cause of corrosion, changes to the type of metal being used can also lead to significant reductions in corrosion.

Metal corrosion resistance data can be used in combination with information on the environmental conditions to make decisions regarding the suitability of each metal.

The development of new alloys, designed to protect against corrosion in specific environments are constantly under production. Hastelloy® alloys, Nirosta® and Timetal® alloys are all examples of alloys designed for corrosion prevention.

Monitoring of surface conditions is also critical in protecting against metal deterioration from corrosion. Cracks, crevices or asperous surfaces, whether a result of operational requirements, wear and tear or manufacturing flaws, all can result in greater rates of corrosion.

Proper monitoring and the elimination of unnecessarily vulnerable surface conditions, along with taking steps to ensure that systems are designed to avoid reactive metal combinations and that corrosive agents are not used in the cleaning or maintenance of metal parts are all also part of effective corrosion reduction program.

Cathodic Protection:
Galvanic corrosion occurs when two different metals are situated together in a corrosive.

This a common problem for metals submerged together in seawater, but can also occur when two dissimilar metals are immersed in close proximity in moist soils. For these reasons, galvanic corrosion often attacks ship hulls, offshore rigs and oil and gas pipelines.

Cathodic protection works by converting unwanted anodic (active) sites on a metal's surface to cathodic (passive) sites through the application of an opposing current. This opposing current supplies free electrons and forces local anodes to be polarized to the potential of the local cathodes.

Cathodic protection can take two forms. The first is the introduction of galvanic anodes.

This method, known as a sacrificial system, uses metal anodes, introduced to the electrolytic environment, to sacrifice themselves (corrode) in order to protect the cathode.

While the metal needing protection can vary, sacrificial anodes are generally made of,or from, metals that have the most negative electro-potential.

The galvanic series provides a comparison of the different electro-potential - or nobility - of metals and alloys.

In a sacrificial system, metallic ions move from the anode to the cathode, which leads the anode to corrode more quickly than it otherwise would. As a result, the anode must regularly be replaced.

A second method of cathodic protection is referred to as impressed current protection.

This method, which is often used to protect buried pipelines and ship hulls, requires an alternative source of direct electrical current to be supplied to the electrolyte.

The negative terminal of the current source is connected to the metal, while the positive terminal is attached to an auxiliary anode, which is added to complete the electrical circuit.

Unlike a galvanic (sacrificial) anode system, in an impressed current protection system, the auxiliary anode is not sacrificed.

Corrosion Inhibitors:
Corrosion inhibitors are chemicals that react with the metal's surface or the environmental gases causing corrosion, thereby, interrupting the chemical reaction that causes corrosion.

Inhibitors can work by adsorbing themselves on the metal's surface and forming a protective film. These chemicals can be applied as a solution or as a protective coating via dispersion techniques.

The inhibitors process of slowing corrosion depends upon:

• Changing the anodic or cathodic polarization behavior
• Decreasing the diffusion of ions to the metal's surface
• Increasing the electrical resistance of the metal's surface

Major end-use industries for corrosion inhibitors are petroleum refining, oil and gas exploration, chemical production and water treatment facilities.

The benefit of corrosion inhibitors is that they can be applied in-situ to metals as a corrective action to counter unexpected corrosion.

Coatings:
Paints and other organic coatings are used to protect metals from the degradative effect of environmental gases.

Coatings are grouped by the type of polymer employed. Common organic coatings include:

• Alkyd and epoxy ester coatings that, when air dried, promote cross-link oxidation
• Two-part urethane coatings
• Both acrylic and epoxy polymer radiation curable coatings
• Vinyl, acrylic or styrene polymer combination latex coatings
• Water soluble coatings
• High-solid coatings
• Powder coatings

Plating:
Metallic coatings, or plating, can be applied to inhibit corrosion as well as provide aesthetic, decorative finishes.

There are four common types of metallic coatings:

1. Electroplating: A thin layer of metal - often nickel, tin or chromium - is deposited on the substrate metal (generally steel) in an electrolytic bath. The electrolyte usually consists of a water solution containing salts of the metal to be deposited.
2. Mechanical plating: Metal powder can be cold welded to a substrate metal by tumbling the part, along with the powder and glass beads, in a treated aqueous solution. Mechanical plating is often used to apply zinc or cadmium to small metal parts.
3. Electroless: A coating metal, such as cobalt or nickel, is deposited on the substrate metal using a chemical reaction in this non-electric plating method.
4. Hot dipping: When immersed in a molten bath of the protective, coating metal a thin layer adheres to the substrate metal. Corrosion Control Methods. A Guide to Corrosion Protection. Auto/Steel Partnership. 1999. Source: More About Corrosion:

Types of Corrosion

What is Corrosion? Calculating Rates of Corrosion Metals by Type:

Platinum Group Metals (PGMs) Base Metals Metalloid An Overview of Heat Transfer Fluids Introduction:

Heat transfer fluids have thousands of applications involving the removal, the addition and the movement of heat energy from one location to another. In the fields of physics and engineering all fluid measurements were historically based on the behavior of water. Therefore, this discussion will begin with water as a reference heat transfer fluid against which all other fluids and their characteristics can be compared.

Water As A Heat Transfer Fluid: While being commonly available and the least expensive of all possible options, water has features and idiosyncrasies that make it ideal for some applications and totally useless in others. Water has three fundamental disadvantages.

1. First, for low temperature applications, water provides no freezing protection, becoming a solid at 32F. If that were not enough, this transition to a solid is accompanied by expansion with enormous potential force.
2. Second, water has a limited upper level temperature range governed by its sea level boiling point of 212F.
3. And finally, the third disadvantage is an inherent lack of corrosion protection which is often further exacerbated by the presence of dissimilar metals in the fluid circuit creating an electrochemical cell promoting galvanic corrosion potentials.

Historically, these issues have been addressed by the addition of a hydrocarbon solvent called ethylene glycol along with various corrosion inhibiting additives. The resulting mixture with water will be recognized as ordinary automotive antifreeze. The most common mixture ratio of Ethylene Glycol and water is the ubiquitous 50/50 mix in which the freezing point is lowered to –34F and the boiling point is increased from the 212F for pure water to 228F (at atmospheric pressure, of course). For maximum freezing point protection the ratio is changed to 67% ethylene glycol and the balance (33%) water. The resulting freezing point is suppressed to –84F. Additionally, it is important to note that the boiling point will be raised somewhat also. It is often observed that if the addition of ethylene glycol is so beneficial why don’t we just run it at 100%? Since pure ethylene glycol freezes at about 8 F F and boils at 330 F, it appears that the freezing level has been improved and the upper end boiling point has been extended by 50%. However, the use of straight ethylene glycol will result in a 25% reduction in heat capacity, or heat carrying, capability compared to water. In an automobile the cooling system (the radiator) would have to be increased in size, therefore, by roughly 25% to provide the same cooling capacity.

Heat Capacity—The First Requirement: Since the primary purpose of heat transfer fluids is to carry heat, it is reasonable to talk about a given fluid’s ability to absorb and hold a quantity of heat energy. As energy is absorbed into a fluid, its temperature increases. In the case of water, the definition of the “BTU” (British Thermal Unit) is the amount of energy required to increase the temperature of one pound of water by one degree ˚F. In the metric system of measurements, this energy is measured in terms of the “Joule” which is defined by the ability to raise one kilogram of liquid by one degree ˚C. These values are called “Specific Heat Capacity. As we mentioned earlier, all characteristics of fluids are compared to the performance of pure water. The heat capacities of various fluids are all compared to the heat capacity of water as a percentage or a fractional factor called “Specific Heat Ratio.” Think of it as a correction factor. For example; The specific heat capacity of water = 1.0 BTU/lb-˚F The specific heat capacity of pure ethylene glycol = 0.57 BTU/lb-˚F Thus, The ratio of specific heats is 0.57 divided by 1.0 which is 0.57 In the metric system (SI units); The specific heat capacity of water = 4.19 kJ/kg-˚C The specific heat capacity of pure ethylene glycol = 2.38 kJ/kg-˚C Thus, The ratio of specific heats is 2.38/4.19 0.57 Specific heat capacity values will also vary slightly with temperature. Therefore, specific heat values will typically be defined for a given temperature or temperature range.

Specific Gravity: We just said that Specific Heat defines the ability of a certain weight of fluid to hold heat energy. It should be readily apparent that for any given volume capacity of a cooling system, there is going to be a fixed weight of fluid circulating within it. Therefore what goes hand in hand with Specific Heat is Specific Gravity—the density of a given fluid compared to the density of water. The higher the Specific Gravity, the more dense is the fluid and therefore the heavier a given volume of fluid is. And the heavier the fluid circulating in a given volume, the more heat energy it can hold. Thermal Conductivity: As with metals and other solids, in addition to their abilities to hold heat energy, fluids have the ability to conduct heat energy. Obviously, this is an important factor since it governs the ability to put energy into a fluid and then get it back out again. The units for thermal conductivity are defined as a conversion factor that relates the ability to move one BTU of energy through a square foot of material one foot thick for each 1˚F of temperature differential from one side of it to the other. This value is typically defined for a specific temperature or temperature range.

Viscosity—The Resistance To Movement: Once we have a fluid that can carry the heat volume desired in the temperature ranges of expected operation, and we understand its thermal conductivity—the ability to move energy into and out of it, the next question that arises is just how easy the fluid will be to move around. While certain fluids will have the ability to absorb and hold enormous amounts of heat energy, they are so viscous that the pump horsepower required to move them around would be prohibitive. Additionally, their viscosity might change with temperature making them ideal as long as they are hot but they become solids at ambient temperature (e.g. liquid sodium used in some nuclear reactors). Additionally, viscosity affects cavitation tendencies within water pumps. In pumps where there is a low pressure area that drops fluid pressure below its vapor pressure there is a tendency to create cavitation with the ensuing erosion effects. In this case a fluid with a higher viscosity will have less of a tendency to cavitate.

Boiling & Freezing Points: Of critical importance also, is to understand under what temperature circumstances the fluid in question becomes unusable.

Boiling Point: This is defined as the temperature at which a fluid boils. Boiling is the transition of a fluid from liquid to gas. The boiling point of any given fluid will vary with the pressure under which it is operating and the degree to which it is diluted with other fluids.

Freezing Point: Conversely, the freezing point is defined as the temperature at which a fluid solidifies. Freezing is the transition of a fluid from liquid to solid. The freezing point of any given fluid will vary with the degree to which it is diluted with other fluids.

Surface Tension: Let’s return again to water as the base reference. We’ve all seen how water beads up when it is poured onto a smooth surface. This tendency to create beads instead of flowing out uniformly over the surface is due to what is called surface tension. It is the measure of the ability of any given fluid to wet the surface from which it is to conduct heat energy. Surface tension is measured in units of dynes per centimeter and the lower this value, the more “wettability” of a surface a fluid has. There are products available containing special surfactants (e.g., Red-Line’s “Water-Wetter”) to cut surface tension of water for the purpose of improving the ability of water to transfer heat energy.

Corrosion: It should come as no surprise that one of the chief disadvantages of water as a heat transfer fluid is its proclivity to induce corrosion of metals. Most heat transfer systems are comprised of metals, and dissimilar metals at that, serving to exacerbate the corrosion potential. It’s The Water: Water, in its chemically pure state (aka, deionized water), is nearly a perfect electrical insulator. It is however, highly polarized, meaning it will easily dissolve other substances. And when it dissolves substances it can make very strong electrolytic solutions conductive enough to carry significant electrical currents. Alternatively, tap water is usually laced with both dissolved minerals and chlorine. The chlorine ions can be highly reactive and corrosive toward aluminum and even the stainless steels. Alternatively, molecules that can’t be dissolved by water are considered to be non-electrolytes and include most of the organic substances (e.g. sugar, alcohol, glycerin, benzene). Note the difference between miscibility and conductivity. Alcohol, for example, is highly miscible in water but still non-conductive. Most practical heat transfer systems are fabricated from various metals which are selected based on their weight, their conductivity, and their ease of manufacture. Given the inherent corrosiveness of water as an operating fluid, we are faced, therefore, with the need to inhibit the corrosion rate of the materials. The most common metals found in heat transfer systems are cast iron, steel, aluminum, copper, brass and the constituents of solder (tin, zinc and lead).

Corrosion Inhibitors: Now that we understand the nature of the causes of corrosion, we are ready to consider methods for its inhibition. Corrosion inhibition can be accomplished through three fundamental approaches: Anodic Inhibitors These are chemical compounds intended to prevent the production of ions at the anode (the metal’s surface) by forming an insoluble barrier layer. Typical examples of anodic inhibitors are the various silicate compounds—sodium silicate, potassium silicate used to protect aluminum. These compounds react with corrosion products and form thick surface films to protect against further corrosion. Cathodic Inhibitors These are compounds that capture any ions given up by metal atoms at the anode thus preventing their migration to the cathode—effectively stopping the flow of galvanic current. Adsorption Inhibitors Here the surface acquires atoms, ions or molecules from liquid contact. The ions or atoms thus adsorbed are called adsorbents. These ions form strong bonds with the surface through electron sharing. They are insoluble and adhere strongly. Certain organic compounds thus adsorbed into a given metal’s surface become bonded irreversibly.

Acidity & Alkalinity: The pH scale is a measure of whether a liquid is acidic (low pH, below 7.0) or alkaline (high pH, above 7.0). Knowing that most metals are adversely affected by acid and not alkalinity, one is tempted to simply assure high alkalinity in a heat transfer fluid. But things are never this simple. It turns out that different metals behave differently regarding corrosion in different and often overlapping ranges of pH. Returning to the ubiquitous water/ethylene glycol mixture, it should be noted that over time, this mixture will become acidic (low pH). To prevent this, a pH buffer such as sodium borate is added to the fluid to maintain the pH within a certain alkaline range.

Other Considerations: Material Compatibility: In addition to corrosion concerns, the selection of a heat transfer fluid must also consider its direct chemical effects on the materials of construction used in the cooling system. Generally, this is not an issue for the metals but rather for the polymers and elastomers—the plastics used for housings, for example, or the hoses used to carry the heat transfer fluids.

Toxicity: Water as a heat transfer fluid couldn’t be safer. But add to it another fluid like ethylene glycol and you have a compound that is toxic to man and animal. This toxicity has led some in recent years to substitute propylene glycol for the more common ethylene glycol. The lowest toxicity level is found with 3M’s Novec HFE-7100 series of engineered fluids. This fluid has many of the properties of water including inertness and lack of toxicity without water’s corrosivity. Its key drawback is its high cost.

Flammability: Obviously, a fluid with a flammability problem in the presence of hot surfaces is less than ideal. However, for some applications where high temperatures are not an issue, the flammability simply becomes one more characteristic to be traded off against some other benefit or set of ideal characteristics.

Disposal & Environmental Concerns: More and more attention is being paid to the environmental impacts of various chemicals and the implications of their disposal after use.

Characteristics of The Ideal Heat Transfer Fluid: As we have seen, water has a series of characteristics; some making it ideal as a heat transfer fluid and others that are distinct disadvantages. Since the ideal fluid does not exist, the search for a more ideal heat transfer fluid has become a compromise—a trade-off of features and benefits for specific applications. Additionally, there are any number of special additives that have been developed over the years to enhance the properties of a given fluid. Heat Transfer Fluid Families: Now that we understand the basic parametric measurements involved in heat transfer fluids, let’s look at the various chemical families of fluid systems in common use: Ethylene Glycol/Water Formulations: Most commonly found as antifreeze in your car. Propylene Glycol/Water

Formulations: A non-toxic alternative to the ethylene glycol in automotive antifreeze. Organic acid formulations, organic acid technology (“OAT”)(“Dexcool”): Marketed as a “long-life” or “extended-life” antifreeze by General Motors. Hydro Fluoroethylene (“HFE”) fluids (3M Novec HFE-7100): An engineered fluid with many of the characteristics of water (i.e., non-toxic) Will not corrode metals. Perfluorinated Polyethers (PFPE’s) (Galden Fluids): Generally used in high temperature applications for their inherent inertness. Silicone oil based fluids (Syltherm): For high temperature applications at lower cost than PFPEs. Diphenyl oxide/biphenyl fluids (Therminol): Mineral oil fluids: High heat capacity High operating temperatures Low cost Ethylene Glycol Base Material for nearly all OEM coolants Different dyes different colors Different additives to address engine needs and water quality Inorganic Additive Technology (IAT & HD-IAT) Organic Additive Technology (OAT) Silicate OAT (HOAT) Phosphate OAT (POAT) Propylene Glycol Low Toxicity Limited HD diesel applications Can be use as a waterless coolant or as a mixture with water Silicates Phosphates Borates Nitrates Molybdates Organic Acids 2EH OAT OAT HOAT Cavitation protection

Thermal Conductivity

Copper 401k (W/m-K)

Brass 120k (W/m-K)

Aluminum 237k (W/m-K) SESCO Cathodic Protection Tutorial What is Corrosion? What is Cathodic Protection?Corrosion is defined in Webster's dictionary as ;…the action or process of corrosive chemical change…a gradual wearing away or alteration by a chemical or electrochemical, essentially oxidizing process. In the performance of our daily work and job responsibilities we usually relate corrosion to the rusting and wasting away of a pipe which is buried in the ground. In this sense, corrosion can be considered as a process of natural forces working to restore the refined works of man to their original state of complete and uniform equilibrium. Thus, in the case of the buried pipe, corrosion is the process of natural forces working to restore the iron in the steel pipe, through rusting, to its original stable form of iron oxide, or native iron ore. The type of corrosion with which we are most familiar, and the type which causes extensive damage to buried pipe, is electrochemical corrosion. This form is also widely known as galvanic corrosion, and is sometimes loosely referred to as electrolysis. Electrochemical Corrosion (fig. 1) ansparent takes place when two different metals come into contact with a conductive liquid -- usually impure water or soil moisture -- resulting in a flow of direct current electricity. The current always flows away from the anodic metal (anode), and the anode is corroded. The current flows through the electrolyte to the cathodic metal (cathode), but the cathode is not corroded. The potential that causes the current to flow is always due to some kind of difference between the anode and the cathode, such as a difference in the two metals, concentration of the conductive liquid, a difference in temperatures, a difference in the amount of oxygen present, or some other difference in conditions. , four conditions must always be present to create a galvanic cell and for corrosion to occur. There must be two different metals, one acting as the anode, and the other acting as the cathode. There must be an electrolyte to provide a path for current to flow from the one metal to the other. And there must be a direct electrical contact between the two metals to complete the electrical circuit. The flow of current through the electrolyte is always from the anode to the cathode. Wherever electrical current leaves the anode to enter the electrolyte, small particles of iron are dissolved into solution, causing pitting at the anode. Wherever the current enters the cathode, molecular hydrogen gas is formed on the surface and the cathode is preserved and protected from corrosion. If one of the four conditions of a galvanic cell is removed, corrosion cannot continue. It is the removal of one of the four conditions, to reduce or interrupt the flow of galvanic current, which is the basis for cathodic protection and all other forms of corrosion control. Probably one of the most common galvanic cells that we can consider as an example is the simple flashlight battery (fig. 2)The metal, zinc, is used as the case of the battery and is the anode. The carbon rod in the center of the battery is the cathode. And the space in between the two is filled with an acid (or alkaline) substance, which is the current conducting material, the electrolyte. Three of the four conditions of a galvanic cell are present, so there is yet no reaction. But when the battery is connected to an external circuit, and electric current is then caused to flow from the zinc (anode), through the electrolyte to the carbon rod (cathode). Oxygen is evolved at the face of the anode, particles of zinc are dissolved into the solution, and hydrogen gas is deposited on the carbon rod. If the flow of current is not stopped, the zinc case will corrode to penetration, and the electrolyte will leak from the battery case.

Different corrosion cells can occur within the same inch on the same joint of pipe, or they could even be miles apart on a well-coated pipeline. Some of the corrosion cells on a pipe may be caused by the various elements of iron, manganese, carbon and trace elements which all occur within the typical composition of carbon steel.

Some cells can be created by the different iron compounds found in mill scale (fig. 4), or in atmospheric rust.

Tool marks (fig. 5), scratches, new surfaces exposed by pipe threads are almost always anodic to other pipe surfaces and are subject to active corrosion.

When a piece of pipe is removed from an existing pipeline and replaced with a piece of new pipe (fig. 6), or with a tee, or with some other new structure, the new material is almost always anodic to the old material. The old material is not necessarily made of any better quality material to resist corrosion, except that it is already polarized and protected by various protective films created by the prior action of the electrolytic process.

These are relative values only, compared to hydrogen, and the specific voltages will vary under the different conditions of different electrolytes, different temperatures, concentration, etc. Any group of metals such as this, arranged in order of the magnitude of their solution potentials, is commonly called by several names, a galvanic cell, or and electrochemical series. If any two metals are selected, joined together electrically and buried underground, they will establish a galvanic cell. The metal that is higher in the list, the one having the higher solution potential, will be anodic to the other. The anodic metal will corrode, and the metal that is cathodic will be protected from corrosion.

This is the basis in principle of cathodic protection. Cathodic Protection can be defined as…the control of electrolytic corrosion…by the application of direct current in such a way that the structure to be protected is made to act as the cathode of an electrolyte cell.

The greater the separation of the two metals in the series, the more rapidly will the galvanic corrosion proceed. While those metals shown higher in the list will create higher potentials, they are relatively unstable and expensive to produce in commercial quantity. As we come down in the list, magnesium is the first metal which is economical to produce in commercial volume. It is for these reasons: that magnesium is relatively high in the galvanic series, and relatively low in cost, that it is used in the manufacture of sacrificial soil anodes for the cathodic protection of steel pipe buried in the ground.

When a sacrificial magnesium anode (fig. 11) is buried in the ground and is connected to a steel pipeline through a copper wire, a strong galvanic cell is established (in the soil electrolyte) having a potential of approximately 1.5 volts. The potential thus created by the formation of this galvanic cell is then usually sufficient to overcome all other naturally existing galvanic cells on the pipeline in the immediate vicinity of the anode. That is to say, all of the anodes of the small naturally existing cells on the pipe have been made to be cathodes to the new magnesium anode which has been attached to the pipe. The magnesium anode will corrode instead of the pipe; the magnesium alloy metal will be sacrificed to save the steel pipe. This is the purpose and the effect of cathodic protection.

While magnesium anodes can be beneficial in slowing the rate of corrosion on a buried pipeline under ideal conditions, they do have their effective limitations.

The driving potential of sacrificial anodes are completely limited to the difference in potential between the anode and the cathodes. As has already been illustrated, the maximum potential that might be obtained between magnesium anodes and steel pipe is slightly more than 1.5 volts. However, to overcome the stronger potentials of external interference as one example. Such interference often is produced as a result of the operation of a cathodic rectifier on a foreign structure in close proximity to the pipeline receiving the interference. Such interference could also come from DC electric railroads or some other source of stray DC current in the soil. But DC interference problems are usually corrected by mutual cooperation and compensation between the two structure owners, which procedures are too involved for explanation within this discussion.

The current output of a single galvanic anode is quite limited and can be affected by any one, or even all of the following conditions:

• The amount of bare steel to be protected, as related to the effectiveness of the coating on the pipe (if any).

• The resistivity (conductivity) of the soil electrolyte environment between the anode and the pipe structure.

• The size and physical shape of the anode.

• The metallurgical composition of the anode.

• The kind and amount of backfill material around the anode and the pipe.

• The physical distance between the anode and the pipe structure.

• The depth at which the anode is buried.

• The number of anodes attached to the pipe, and their spacing.

• The pipe-to-soil potential of the pipe structure.

While a single sacrificial anode could provide adequate cathodic protection to miles of a very well coated pipeline with few breaks in the coating, it can afford adequate cathodic protection to only a relatively few feet of large diameter uncoated pipe under severely corrosive conditions. Under these severe conditions it would be necessary to install many anodes (fig. 12) at very close intervals in order to provide sufficient potential to all surface areas of the pipe.

A more effective and positive method of cathodic protection is through the installation of a cathodic rectifier and impressed current ground bed system (fig. 13) instead of with multiple magnesium anodes. By this method an electrolytic cell is developed artificially, where the entire structure to be protected is made to be the cathode of the cell, and the installed groundbed is the anode. The current is made to flow from the ground bed to the structure by converting alternating current power to direct current and impressing the direct current into the earth through the ground bed.

A cathodic ground bed consists of a designed number of carbon, graphite, cast iron or junk steel anodes buried in the ground at various depths and configurations. Commonly, the ground bed will consist of approximately 20 cast iron anodes, installed in augured holes 15 feet apart, and backfilled with carbon dust to lower the anode-to-earth resistance. These anodes will all be connected together in parallel, with the header cable attached to the positive terminal of the rectifier.

The rectifier instrument consists of two basic devices: a transformer to convert purchased AC power from 115, 230 or other supply voltage, to the much lower DC voltage needed for cathodic protection; and the rectifying device to convert the low voltage AC to DC. A second cable is attached to the buried pipeline and connected to the negative terminal of the rectifier instrument to complete the return circuit.

Voltage adjusting linkage is provided on the rectifier so that the DC current output can be adjusted to any value as may be required to provide an adequate protective potential on the pipe structure. When the AC supply is turned on to the rectifier, the transformer reduces the AC voltage to the desired level, converting it to direct current through the rectifying stacks and is impressed into the earth through the ground bed. As with other galvanic cells, the impressed current collects on the bare steel surfaces, or at the voids in the coating, and the pipe is used as a return to the negative terminal of the rectifier to complete the circuit.

The rectifier-ground bed system has many advantages in the application of cathodic protection to a buried or submerged structure:

1. It allows for any reasonable driving voltage that may be desired for effective control of corrosion.

2. It allows for any reasonable current output that may be desired for effective control of corrosion.

3. It can be used with almost any resistivity soil environment.

4. The system can be used on bare or coated pipeline systems.

5. Structures of any size can be made to be cathodic and be protected.

The pipe-to-soil potential test (fig. 14) has been established by corrosion engineers as a standard measurement technique in the evaluation of corrosion control and the degree of cathodic protection applied to buried metallic structures. The copper sulphate half cell reference electrode is most commonly used to contact the soil. Normally, the natural static potentials of unprotected buried steel will vary from -0.30 to -0.80 volts, with reference to the copper sulphate electrode. Such differences in the pipe-to-soil potentials observed at various intervals along a pipeline indicate the voltage drops in the soil between the test points. This means that galvanic currents will flow through the soil between the anodic and the cathodic points, the magnitude of current flow depending on the resistivity of the soil (electrolyte) and the voltage drop in the galvanic cells.

If the static pipe-to-soil potentials along a pipeline were of equal values, galvanic currents could not flow, and there would be no corrosion.

When enough external counter-current is provided to a corroding section of a pipeline to exactly cancel out the galvanic currents, the pipe-to-soil potentials at the anodic points will be equal to the pipe-to-soil potentials of the cathodic points, and the voltage drop between the points would be zero. That point of theoretical potential equalization is usually at or near the open circuit potential of the anodic point.

For example, if the open circuit pipe-to-soil potential of an anodic point is -0.65 volt, then corrosion will be stopped if the potential of the cathodic point(s) is made more negative and equal to this value. This is a basic criterion for the cathodic protection of a buried structure. However, it would be impractical and almost impossible to determine the open circuit potential values and points of potential equalization along a pipeline, so corrosion engineers have established a second and more practical criterion for adequate cathodic protection.

It is generally accepted by the corrosion engineers that a structure will be under complete cathodic protection if the pipe-to-soil potential at all points on that structure is maintained at a minimum level of -0.85 volt. This value represents over-protection in most instances, since the points of potential equalization, as pointed out above, is usually less negative than -0.80 volt. This is the most practical and economical criterion to consider in testing for the existence of corrosion on any buried and coated pipeline.

Structure-to-soil potentials should be observed using a potentiometer, which draws no current, or a high resistance voltmeter, which draws only a very small current. A copper-copper sulphate electrode is used for the reference contact with the electrolyte (soil), and there must be direct contact with the structure (pipe).

To make a pipe-to-soil test observation, the lead wire attached to the copper sulphate electrode is attached to the positive (+) post of the meter. A wire attached to the negative (-) post of the meter is attached solidly to the pipe at any convenient point. This contact can be made by clipping directly to an above-ground valve, fitting, riser, or even by attaching to a probe bar pushed into the ground to contact the pipe.

The plug end of the copper sulphate electrode is then placed firmly against the moist soil at a position relative to the top of the buried pipe. If the soil is dry, it will be necessary to spill just a little water onto the ground (1/2 cupful) in order to ensure good contact between the soil and the electrode. The pointer on the voltmeter will then indicate the pipe-to-soil potential at that particular point on the pipeline.

Continuing and using the same direct contact to the pipe, but then using a very long wire between the meter and the copper sulphate electrode, it will be possible to move the electrode about and take many pipe-to-soil test observations at any number of intervals for hundreds of feet along the length of a pipeline.

If all test observations over the entire structure are found to be -0.85 volt or greater, it can be concluded that the entire structure is cathodic with respect to the sacrificial anodes (or with respect to the rectifier groundbed) and that there is no active corrosion taking place. This value of -0.85 volt considers a "built-in" constant of -0.52 volt as the solution potential between copper and copper sulphate in the reference electrode.

Pipe-to-soil observations should be made whenever there is any question or any doubt that the structure may not be under full cathodic protection. It is desirable to practice the re-observation of pipe-to-soil potentials at regular, say, six-month intervals to have assurance that no physical changes had previously been made that would upset the balance of the cathodic protection circuit. This is for confirmation purposes, and to discover any changed condition which could result in corrosion damage to the structure. The period of retesting pipe-to-soil potentials should never exceed one year.

Whenever any work is performed directly on the structure which may affect the cathodic protection balance, it would then be prudent to retest the pipe-to-soil potential at the completion of that work. Such work would include any activity which might affect the insulation or the shorting of any portion of the structure to another structure, any work on or addition to insulating fittings, any addition or removal of pipe to the length of the system, any pollution of the soil which my lower the resistivity of the soil environment, any indirect contact to the structure by a different metallic structure, any nearby construction and subsequent operation of a cathodic rectifier by others, any other new source of stray DC into the earth, etc. Any changes which do occur to reduce the pipe-to-soil potential on the structure under cathodic protection below a level of -0.85 volt should be removed or corrected to restore the structure to a protective level.

It should be remembered that cathodic protection is only one tool used in an overall program of corrosion control, and is often used to supplement other efforts to arrest and control the process of corrosion. Some of the other methods used in corrosion control are:

• Insulating the internal or external surfaces of a structure from the electrolyte by the installation of paint, wax, coal, tar, asphalt, plastic tape, epoxy resin, or other coating or lining material.

• Installing the structure in a high resistant or well-drained environment, such as in or on sand, crushed rock, etc.

• Cladding, lining, dipping, electroplating, or metalizing to coat a metallic surface with a metal, alloy, or material of superior resistance.

• The addition of selected chemical inhibitors, passivators, or dessicants.

• The removal of oxygen, carbon dioxide, or other gasses from, or the addition of inert gasses, such as nitrogen, to the environment.

• The careful consideration and selection off metals, alloys, plastics, ceramics, or other materials, to be used in conjunction with a necessary metal.

• Control of the environment to lower temperatures.

• The reduction of velocities and/or throughput at the face of the corroding material.

• The installation of in-line insulating materials.

The tendency of oxygen-deprived locations to become anodic is the cause of many commonly-observed patterns of corrosion

What you need to know

Make sure you thoroughly understand the following essential ideas which have been presented above. It is especially important that you know the precise meanings of all the highlighted terms in the context of this topic.

• Electrochemical corrosion of metals occurs when electrons from atoms at the surface of the metal are transferred to a suitable electron acceptor or depolarizer. Water must be present to serve as a medium for the transport of ions.

• The most common depolarizers are oxygen, acids, and the cations of less active metals.

• Because the electrons flow through the metallic object itself, the anodic and cathodic regions (the two halves of the electrochemical cell) can be at widely separated locations.

• Anodic regions tend to develop at locations where the metal is stressed or is protected from oxygen.

• Contact with a different kind of metal, either direct or indirect, can lead to corrosion of the more active one.

• Corrosion of steel can be inhibited by galvanizing, that is, by coating it with zinc, a more active metal whose dissolution leaves a negative charge on the metal which inhibits the further dissolution of Fe2+.

• Cathodic protection using an external voltage source is widely used to protect underground structures such as tanks, pipelines and piers. The source can be a sacrificial anode of zinc or aluminum, or a line-operated or photovoltaic power supply.

Sacrificial Anodes

The earliest experiments on cathodic protection were performed with zinc anodes that were electrically connected to copper plates immersed in seawater. As can be seen on the galvanic series, such an arrangement would produce a cathode (copper) and an anode (zinc). In the large galvanic cell so formed, the zinc cylinder corroded away in a manner to protect the copper substrate. This method of cathodic protection can be used with other combination of metals providing the necessary current to the metal to be protected, as Sir Humphry Davy and Michael Faraday illustrated almost two centuries ago.

When two metals are electrically connected to each other in a electrolyte e.g. seawater, electrons will flow from the more active metal to the other, due to the difference in the electrical potential, the so called 'driving force'. When the most active metal (anode) supplies current, it will gradually dissolve into ions in the electrolyte, and at the same time produce electrons, which the least active (cathode) will receive through the metallic connection with the anode. The result is that the cathode will be negatively polarized and hence be protected against corrosion. To calculate the rates at which these processes occur, one has to understand the electrochemical kinetics associated with the complex sets of reactions that can all happen simultaneously on these metals.

Magnesium Anodes

Magnesium has an equilibrium potential of -2.61V vs. SCE and therefore theoretically can provide a very large driving voltage . However, practical measurements indicate relatively more noble corrosion potentials probably due to the electrochemical inefficiency of the metal as a sacrificial anode. The low efficiency (50-60%) has been attributed to hydrogen evolution at local cathodes and complex surface chemistry at the anode surface. The theoretical current capacity for a magnesium anode is approximately 2200 Ah kg-1 whereas actual measured values are in the range of 1200 Ah kg-1.

Basic Facts in Metal Cleaning

Preparation for processing it for application 0f a non-metallic zinc phosphate for either corrosion protection or painting

In metal cleaning it is generally overlooked that water plays the most important part. Its condition (hard or soft), and degrees thereof determines the performance of the chemicals dissolved in it. The efficiency of any cleaner is directly proportional to how if performs in water. The chemical must do three important things. First, it must tie-up or inactivate hard water ions for better rinsing and cleaning. Second, it must lower the surface tension of the water— in effect, it makes water wetter. Third, it must ionize or dissociate when it is added to water.

An ion is an electrically charged chemical particle in solution. And in all acids and also in water the electrically charged particle is the Hydrogen ion. It is this ion that has such a pronounced effect on the activity or strength of an acid. The counterpart of this ion in an alkaline water solution is the Hydroxyl ion, the degree of presence which determines the alkalinity of the solution.

Oxygen is the one chemical ion which is present in all alkalies, acids, water, and in air, and it is needed to maintain the chemical reaction known as oxidation. Two examples of this are: the rusting of iron or steel, and the white corrosion products on zinc plated surfaces. Also, Oxygen is the cause of breakdown or decomposition of certain chemical products .

Although all acids and alkalies have an effect on all metals to varying degrees, some react so minutely that physical evidence is not observable. In the extreme cases. the reaction is violent, manifested in the form of boiling and gassing at the metal surface, which is a sign of a pronounced attack on the metal. In effect, the metal is being dissolved. Generally, strong alkalies have the most drastic effect on aluminum and zinc, while strong acids more particularly attack steel. Strong oxidizing acids such as Nitric and Chromic readily attack copper alloys.

Metal cleaning is decidedly affected by the type and degree of attack a particular chemical has on a given metal. Also of importance on the clean-ability of a chemical compound is the hardness of the water. That is, in the amount of calcium and magnesium dissolved, and the wetting ability.

Other factors having a significant and influencing effect on the cleaning ability of a chemical compound are, namely:

HEAT - The higher temperatures accelerate cleaning.

CONCENTRATION - of the chemical determines reaction time, speeding up removal of the soils and oils and greases from the surface by the simple reason that it brings fresh solution into contact with the contaminants, and the physical action itself helps to dislodge the materials clinging to the surface.

AGITATION

Remember, the above four factors are the most important ones to be learned, because all metal cleaning situations can be related back to one or all of them. If properly applied, these four factors will enable you to perform all your cleaning with a minimum of problems. 2

1 http://www.cheresources.com/content/articles/maintenance-repair/forms-of-corrosion?pg=2

2 Corrosion Drs, Stan Scislowski

Further In Depth Reading...

It's The Water | Reserve Alkalinity | Step up to the Plate | Noble Metals | Anode Data | Energy  |  Backwards Thinking | Hungry Water