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CorrosionCorrosion is a constant threat to heat exchange equipment. In order to control it, we need to understand the corrosion process caused by the deadly combination of water, metals and oxygen. Heat exchangers are usually made of mild steel, admiralty brass, copper and sometimes stainless steel - all metals prone to corrosion in some degree. We are all familiar with rust or corrosion of steel objects: that is when the iron in the steel reverts to its natural oxide state. Metals tend to oxidize when exposed to
air, water or even other metals. Oxygen is a main ingredient in the corrosion
process and dissolved oxygen is abundant in open recirculating cooling systems. The reactions for corrosion are similar to a wet cell battery. When a battery makes an electrical current, the current flows from an anode to a cathode. A neutral electrolyte fluid in the battery completes the circuit. When this happens, metal at the anode - for various reasons - begins to dissolve, leaving behind negatively charged particles called electrons. The electrons are now attracted towards the cathode. There, depending on the water quality, they can combine with oxygen and water, forming negatively charged ions called hydroxides. These hydroxide ions in turn migrate though the electrolyte back to the anode, completing the circuit. So it goes on ... and the corrosion at the anode grows. There are two important points to be noted: the same process takes place in your cooling water system, using the water as an electrolyte and both anodes and cathodes can exist along the surface of a single piece of metal. If the surface of a metal were perfectly uniform, corrosion would be minimal. But rough spots and even microscopic manufacturing imperfections create differences in what is called "electrical potential". These differences in potential generate areas of high charge, cathodes, and areas of lower charge, anodes. So the scene is set for a continuous electrical process - and continuous metal loss at the anodes. In a single piece of metal, the negative electrons move from the anode area to the cathode are and can form hydroxide ions with the oxygen hi the water in a continuous process, dependent on the water quality. The electrical charge is, in fact, conducted along the dissolved minerals in the water. So the more mineral impurities, the higher the conductivity, and consequently more severe corrosion problems. The anode, on the other hand, determines the actual amount of metal destruction. The anode may exist over a fairly broad area of the metal, or it may occur in a small scratch or imperfection. At a large anodic site the metal loss is fairly even and generalised in nature. With a small anodic site, which may be losing metal at the same rate, the results are very different. Because a small anodic site can't provide electrons over a broad surface, destruction occurs deep in the metal. This type of corrosion is referred to as localized pitting attack and is often responsible for leaks and tube failures. The complex effects of corrosion on a cooling system can be disastrous. Leaks, blocked heat exchange tubes, production problems, and even total equipment failures. For you, it means increased maintenance and equipment repair and replacement.
So what causes corrosion and what measures are there to reduce corrosion to acceptable levels? Corrosion causes The rate of corrosion is measured in mils per year or "mpy". Since a mil is defined as one-thousandth of an inch, a corrosion rate of five mpy means that five thousandths of an inch of metal thickness are being lost each year. Alternatively it may be measured in millimetres per year.
Several factors affect this rate.
1. Temperature At higher temperatures chemical reactions speed up. Generally, a 10 °C rise in temperature doubles the rate of reactions in the corrosion cell. Even temperature variations within a single piece of metal can cause the warmer portions to become anodic, leading to severe metal loss. Therefore the potential for corrosion is greatest in heat exchangers where the temperatures are hottest. 2. Water velocity Water velocity is another factor affecting corrosion. Particles in fast moving water are likely to wear away any chemical coatings put on the metal to protect it. Solids in slow moving water are likely to settle-out on to metal surfaces, preventing chemical treatments from reaching the metal. Water flowing in exchangers at less then 0.6 metre per second can get very hot - and heat speeds the corrosive/electrical cycle, as we have seen.
Another factor to beware of is 3. Galvanic corrosion Galvanic corrosion is caused when two different metals, joined together in construction of the cooling system, are exposed to the water. In the galvanic series, which lists metals according to their tendency to corrode, the lower metals corrode first. Magnesium and galvanised iron are very active, gold and silver more stable. The further apart two metals are on the chart, the greater tendency for galvanic corrosion if they are joined. So the chart is useful for us, when joining metals, to choose those near each other in the list.
4. Deposits Deposits as well as preventing chemical treatment reaching metal surfaces, also lead to corrosion in a more threatening way. Anodes can form secretly beneath the deposits. Deep localised pitting results. Microbiological matter in a system is often responsible for this type of corrosion. 5. Ratio of cathode/anode size As explained earlier, the ratio of the size of the cathode to the anode affects how quickly electrons combine with oxygen and water and thus the rate of corrosion. Corrosion is a naturally occurring phenomenon but it can be controlled in a cooling water plant at acceptable levels. We have seen some of the physical corrosion factors: but there are also chemical factors. The first is the pH level of the water. pH is a measure of the acid-alkaline content. A pH of seven is considered neutral. Lower pH levels - acidic solutions - are more corrosive as they accelerate the reaction of electrons with oxygen and water. Low pH water also dissolves some protective oxides that may have formed at the anode, exposing new metal surfaces for further corrosion.
Secondly there is the level of oxygen in the water. More oxygen means more electrons can be used up, accelerating corrosion and oxygen is the lifeblood for corrosion. Finally, there are harmful solids and gases in the water. Dissolved solids increase the conductivity, speeding up the electrical corrosion process and they can also interfere with corrosion coatings. Dissolved gases may enter the water from process contamination for example ammonia and carbon dioxide or from atmospheric pollution such as sulfur dioxide. These will alter the pH of the cooling water.
Corrosion reducing measures We have gone into the physical and chemical factors affecting corrosion, now we need to look at the measures to reduce corrosion to acceptable levels. Sacrificial anodes "Sacrificial anodes" are one method of reducing corrosion. Knowing that metals lower in the galvanic series corrode first, such as zinc or magnesium (flash names), we can deliberately build these into the cooling system. They will form sacrificial anodes, drawing the corrosion away from other areas. But to be really useful, "sacrificial anodes" would need to be placed in each pass of each head of a condenser, and throughout the cooling system. Choice of material Corrosion-resistant and rust-proof material in the construction of the plant are other options, but usually not very practical or economical in a large system. Chemical corrosion inhibitors The most effective way is the use of chemical corrosion inhibitors. These are various film forming chemicals which prevent the oxygen getting to the metal surface, disrupting the corrosion cycle. By maintaining the correct dosage of corrosion inhibitors, the plant operators can ensure an adequate amount of treatment chemical is in the system at all times keeping the protective film intact. Corrosion "potential", we know, is greater if the pH factor - the amount of acid or alkalinity - is low. Acidic water also harms chemical films. On the other hand, very high pH levels - greater alkalinity - also interfere with some chemical films and can lead to the formation of scale or deposits. So pH must be monitored closely. If high, sometimes acid is dosed but clearly, as we have seen, too much acid has its own problems. Your plant will have a specific conductivity range. Conductivity is easily measured and can normally be regulated by adjusting bleed-off. The test and control procedures will be covered in more detail later. Your Local representative will give specific details and information regarding the water treatment program for corrosion control in your plant. more about corrosion |
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