Cathodic (least corroded) Anodic (most corroded) Figure 1

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1 Corrosion If you've ever wondered why prospectors don't find nuggets of aluminium or iron in the wild it's because these metals (among many) can not exist for very long in their refined state. Like most metals, they react naturally with the environment to form less refined, but more chemically stable compounds such as oxides. For example, aluminium is commonly refined from its oxide form, known as Alumina, which is found in the brown earthy mixture called Bauxite. Precious metals like Gold can be naturally found in their refined form, because they do not readily react with the environment, or to put that another way, they do not corrode easily. The point is that corrosion of metals is a very natural process. Refined metals are inherently unstable and they try to revert to their more natural states through contact with the environment. When this process happens to something we'd prefer it didn't, we call that corrosion. Preventing or controlling it requires understanding the different types and exactly how it happens. While there are many types, by far and away the most common type of corrosion in marine applications (and usually with cars too) is Galvanic corrosion. This is a naturally occurring reaction that happens whenever two different metals are electrically connected together and immersed in a liquid capable of conducting an electrical current (an electrolyte). Galvanic Corrosion It's called Galvanic Corrosion after the 18 th century Italian physiologist Galvani who discovered how our muscles react to small electrical currents. A sensitive instrument for measuring small DC currents is still sometimes called a Galvanometer. Thus a corrosion reaction that is caused by naturally occurring small electrical currents came to be called galvanic corrosion. The Galvanic Series in Seawater Gold Titanium Stainless steel (passive) Silver Nickel Copper Tin Lead Stainless Steel (active) Iron Steel Cadmium Aluminium Zinc Magnesium Cathodic (least corroded) Anodic (most corroded) Figure 1 Any time two different metals are immersed in an electrolyte; the potential for galvanic corrosion is present. The size of the reaction (amount of corrosion) you get depends upon how close the two metals are chemically to each other, and how conductive is the electrolyte. The scale of metals showing how they react to each other is called the galvanic series, see figure 1. The distance apart two metals are on this scale determines how much reaction occurs, further apart means more corrosion. The electrolyte also determines how much of a reaction will occur, the more conductive the liquid, the greater the reaction. The reaction between, say silver and aluminium would be severe and rapidly cause IAME36_Galvanic Corrosion.doc 8Jun01 page 1 of 6

2 problems, whereas copper and lead together only have small reaction that would easily go unnoticed for a long time. Figure 2 shows typical corrosion rates for mild steel when exposed to different environments. Typical Corrosion Rates for Steel Marine (Surf) Typical Environment Marine (Calm) Mild industrial Non-polluted Urban Dry indoors Microns in First Year Figure 2 The main difference in each environment is the amount water available to act as the electrolyte, and the amount of minerals in the water that determine its conductivity. A dry indoor environment has only a small amount of reasonably clean water vapor in the air to act as an electrolyte, so the reaction is very slow. At the other end of the scale is seawater mixed with air (spray from the surf) which can be more than 100 times more corrosive. That's the basic theory involved, however to be able to recognise and deal with galvanic corrosion we really need to understand just what happens and what are the telltale signs, but without the need for degree in chemistry. The following explanations deliberately leave out the correct chemical symbols, in order to simplify the explanation for those of us without the chemistry degree. Figure 3 shows a Galvanic cell, containing the basic requirements, two different metals one of which naturally becomes the anode, the other being the cathode. The wires and voltmeter show an electrical path between these two metals. Both anode and cathode are immersed in an electrolyte (water in this example). What actually happens At the surface of the anode atoms of the metal are stripped of some of their electrons (e-) by the electrical force of the galvanic cell. The electrons travel through the metal and our voltmeter to the cathode where they will be eventually reunited with the metal atoms (metal+), plus other ingredients from the water. The metal atoms (metal+) travel through the electrolyte towards the cathode. At the surface of the cathode the electrons (from the anode) emerge and are combined with oxygen (O) from the water, some of the water (H2O) and the metal atoms (metal+) from the anode to form what's known as a hydroxyl solution (metal (OH)). IAME36_Galvanic Corrosion.doc 8Jun01 page 2 of 6

3 If the anode in our galvanic cell was aluminium, then the result at the cathode is aluminium hydroxide, a white powder that forms inside blisters in the paintwork or grips screws like a vice inside aluminium housings. If the Figure 3 anode was iron, then iron hydroxide occurs which is one of those brown flaky deposits we know collectively as rust. The whole process is driven the fact that two metals of different potential are immersed in an electrolyte and electrically connected together. What if there's only one piece of metal? Doesn't steel rust and aluminium corrode sometimes apparently all by themselves, can this also galvanic corrosion? Yes, it is possible for galvanic corrosion to occur in a single piece of metal, when something affects the surface so that some of it becomes an anode. The most common cause of this phenomenon is any coating or disturbance that prevents or restricts oxygen getting to the surface. Then that area with less oxygen starts to act as an anode to the other surrounding areas that still have plentiful oxygen and a galvanic cell is setup in a single piece of metal. Figure 4 shows how this occurs, using the same symbols as above. Barnacles and other living growths are a common way this problem can occur on metal boats. They keep their contact area wet, but their intimate contact can also prevent oxygen getting freely to the surface. Anything soft that retains water (like wood) clamped to a metal hull can also do the same. If you were thinking this sounds a good case for leaving all coatings off the metal, unfortunately it's not that simple. In some cases that's OK, for example small aluminium dinghies are rarely painted because they are open to plenty of fresh air and there is usually little chance of a more noble metal being in close contact to form a cathode. However, if someone bolts something made of copper or lead to one, then the bare surface has no protection and it could be in real trouble. The anodising of aluminium is an example of a deliberate oxide coating that is hard and adheres tightly to the metal forming an effective barrier to all but the most aggressive environments. Some special steels will rust only a little until they get a coating of rust, then hardly corrode at all from then on. However, in most cases rust is a soft, loose powder like build up that actually traps pollutants (like salt from sea spray) and then when a little rain water comes by, creates a powerful electrolyte. This is often a problem with seaside steel structures. The Importance of Oxygen Oxygen plays a large part in the corrosion story. We already noted above how having oxygen present at the surface of the metal is important in ensuring random anode sites aren't created. It also plays another very important role with many everyday metals that is quite invisible to us. This is because many metals have a naturally occurring extremely thin and invisible oxide coating that serves to protect them from potential reactions with neighboring metals. IAME36_Galvanic Corrosion.doc 8Jun01 page 3 of 6

4 Random Anode Site Figure 4 Aluminium, Nickel and Stainless Steel, to name just three, are all metals that have generally good resistance to corrosion because contact with oxygen in the atmosphere provides them with a very thin, but effective oxide coating. It's there while ever there is oxygen around and automatically regenerates whenever the metal surface is disturbed. Note how in the galvanic series of metals shown in Figure 1, stainless steel is shown twice? Passive stainless steel is the usual condition of this metal when there oxygen around. However, if oxygen is prevented from getting to the surface it becomes active and moves down the list considerably. Note also from figure 1 how Aluminium is low down on the list, but is generally considered to be a corrosion resistant metal, commonly used around our houses, boats and cars. This is because the invisible oxide coating is very tightly adhering and protects the metal against reaction with all but the most aggressive environments. In really aggressive environments, special aluminium alloys and surface coatings are developed to increase resistance to corrosion. A good example would be the aluminium alloys used commonly in aircraft that are very strong and lightweight. To get the strength required copper is used as an alloying component here, but in an aggressive environment like seawater these alloys corrode badly. So for marine applications aluminium alloys using silicon and manganese are more common. Lower strength, but much better corrosion resistance. A good example is the many unpainted small aluminium hulls you see commonly around our waterways. If they are kept relatively clean and well maintained, these boats can happily survive in a seawater environment without any special coatings. Prevention Now that we've seen how it happens, what can we do to reduce or prevent corrosion? From the above you'll see that we need 4 criteria for Galvanic corrosion - 1. An Anode 2. A Cathode 3. An electrical connection between anode and cathode 4. An electrolyte Take any one of these away and the reaction stops. Simple, but not always easy. Actually the simplest fix is to take away the electrolyte, or in our case take the boat out of the water (and drain the engine cooling system). This is exactly what happens with most trailered recreational craft, and is one of the main reasons why they generally have few corrosion problems. Not everything can be made from the most corrosion resistant alloys, or kept separate from other metals, so surface coatings become important for corrosion prevention. Anodising, for example, is used a lot on decorative aluminium items and works well in mild environments, but not so good near the sea. In marine applications special alloys in conjunction with surface conversion treatments with names like Allodine or Lyfanite serve to IAME36_Galvanic Corrosion.doc 8Jun01 page 4 of 6

5 make the surface more passive and receptive to paint coatings. Then chromate based primers and paint coatings are added. The chromate's are electrically attracted to the aluminium surface because they have an overall negative charge (that is too many electrons) where the anode site on the aluminium surface is overall positive (not enough electrons). To even things up, most chromate type primers also contain a metal (zinc is common) which is slightly positive in charge, so that at the cathode site where there is a lot of electrons available, they combine with the primer rather than contribute to the galvanic corrosion process. This electrical attraction is what makes these coatings tightly adhere to the metal and serve as a good base for a barrier or top coating that further reduces corrosion by keeping the electrolyte away from the metal. Its worth bearing in mind that paint coatings on most metallic marine devices are not just for cosmetic purposes. Another other area we need to be aware of to prevent corrosion is what is known as the "area ratio effect". This states that the size of the anode and cathode, in relation to each other, also determines how severe any reaction might be. A large cathode area connected to a small anode will see a big reaction and very rapid corrosion of the anode. For example if you had an aluminium screw in a copper hull, it would be in severe trouble with a massive attach on the screw. On the other hand, a copper screw in a aluminium hull would have a relatively small reaction confined to a tiny area of the hull around the screw hole. Still you'd be wise to avoid it just the same by using metals closer together on the Galvanic Series table, or if you can't do that, separate them electrically (insulate) and/or from the electrolyte (surface coatings). The last and most common method to reduce corrosion is by Cathodic Protection. This means making sure the thing you want protected is the cathode in any galvanic cell that may be created around it. There are two common ways. One is to use an external DC current source to apply an equal and opposite current to that created by the Galvanic cell, so that they cancel each other out. Requires an on-board electronic system with electrodes to measure any corrosion currents and uses a separate current source (usually the 12-volt battery) to counteract them. These electronic corrosion prevention systems can work very well, but tend to be expensive for recreational boaters, so they generally rely on the second method - sacrificial anodes. Here a piece (or several pieces) of metal are electrically connected to the items we want protected, and immersed in the same electrolyte. The metal is chosen to ensure it will be an anode to any part of the items we want protected. So for most boats where the aluminium in the hull or engine/drive system is what we want protected, then the sacrificial anodes will need to be made of Zinc or Magnesium. Of course the area ratio affect works here too ensuring a rapid reaction, so most manufacturers will carefully select their sacrificial anode materials to ensure they remain an anode, but are not too far apart on the Galvanic Series that they will corrode away too rapidly. For marine service technicians the most important things to know are - 1. How corrosion occurs so they can analyse corrosion related service problems, and anticipate potential problems when assembling new craft, engines or accessories. 2. Be aware of the correct use of surface coatings to prevent corrosion, including gasket and screw sealers. This area also includes lubricants like grease (for example - using a grease containing copper in an aluminium underwater housing could be embarrassing). 3. Sacrificial anodes must be the correct material and electrically connected to the thing they are intended to protect. Checking anodes with an ohmmeter is part of most outboard motor services. IAME36_Galvanic Corrosion.doc 8Jun01 page 5 of 6

6 Port Stern Bracket Starboard Stern Bracket Anode Lower Hydraulic System pivot pin Here's a good example of how NOT to install a sacrificial anode. The picture shows the stern brackets and hydraulic trim system of a large outboard motor. The lower end of each stern bracket has corroded to the extent that the lower pivot pin for the hydraulic system is now hanging in the air! This rig lives in the water full time, and in one of the worlds high salt and warm water regions, the Gulf of Mexico. Some corrosion is inevitable in these conditions without deliberate steps to prevent it. In this case someone has attached a sacrificial anode, of generous size, to the port side stern bracket. But they did not get the anode ELECTRICALLY connected to the outboard, the very thing it's trying to protect. The anode is attached by a self-tapping screw that passes through a slot in the stern bracket and into the boat's fibreglass transom. The stern bracket still has its original coats of protective paint, plus the anti-fouling paints added after the outboard was installed. Thus the anode is insulated from the stern brackets where it touches by several coats of paint, and the screw holding it on makes no contact at all with the engine. With no electrical contact to the engine the anode was useless in preventing the corrosion reaction. IAME36_Galvanic Corrosion.doc 8Jun01 page 6 of 6