一周年庆典－－CATHODIC PROTECTION

BMA004

CATHODIC PROTECTION
Sacrificial anodes
Electrically coupling two metals with differing potentials will lead to corrosion on the more ignoble of the two metals. This we can use for protecting metals against corrosion by sacrificing one metal to protect another metal. By coupling the metal we want to protect to a suitable anode we will protect the first metal as long as the anode is not consumed (dissolved). Such an anode is termed a sacrificial anode and the metal it is protecting then becomes the cathode. We are in other words protecting the cathode in the galvanic cell and we usually term this cathodic protection (or CP).
The ignoble metals used for making the anode are usually zinc, aluminum or magnesium, however, the zinc is dominating. Magnesium is normally used for, e.g. stripping of rust scale from interior surfaces in tanks, in other words as a stripping anode. Magnesium is not allowed used permanently installed in tanks where flammable liquids are transported due to the danger of an explosion from sparks generated if the anode should fall down.
The composition of the sacrificial anode is very important, as there must be no possibility of passivating layers forming on the anode’s surface. This is no problem as concerns magnesium anodes and we are adding zinc (approx. 3%) and aluminum (approx. 6%) to the magnesium alloy to increase the current density and not to prevent passivating.
Zinc anodes, to be effective, have to contain as little iron as possible and the upper limit has been set at 0.0014%. Typically zinc anodes are supplied with an iron content of as little as 0.0003%. The fastening strip (made from iron) is often galvanized in order not to interfere with the anode. Besides the iron, zinc anodes may contain 0.5-1.0% aluminum and 0.2-0.5% cadmium. Some zinc anodes may in addition contain approx. 0.1 mercury.
Significant work has been invested in producing effective aluminum anodes due to this metal’s tendency to passivating. Normally some 5.0-7.0% zinc and 0.2-0.5% tin is added. Addition of 0.1% mercury may occasionally take place. It is also very important that the grain size and the structure in the alloy are correct, and this is achieved through a very special treatment with heat.
Sacrificial anodes of zinc or aluminum are generally self-regulating. The over-protection seen is limited to the immediate vicinity of the anode and is manifested through blistering of the paint system and calcareous deposits. The blistering of the paint system may be counteracted through application of epoxy paints to a dry film thickness of minimum 500 µm to the affected area. This is sometimes called an anode shield.
Zinc and aluminum are about equally used and both may be used on ship’s underwater hulls and in tanks. Zinc anodes may be used without restrictions; however, aluminum anodes may not be installed higher in tanks than what will give a kinetic energy (from falling) greater than 30 kpm. This means that a fifteen-kilo aluminum anode may not be installed higher than two meters in the tank.
Using sacrificial anodes for CP it is important to ensure full metallic contact between the anode and the steel. Welding is preferable to other types of fastening both for the direct metal contact as for the security against the anode falling off.
Impressed currentCP through sacrificial anodes is achieved through a negatively charged electrical current passing from the anode through a conductor to the mother steel of the ship (the cathode). At the same time a positively charged current is passing through the sea water (the electrolyte) thus forming a closed directed electrical current. The protection is dependent on an electrical current and we may use batteries or another source of electrical current instead of an ignoble metal.
The ship’s hull may thus be coupled to the direct current’s negative pole, whereas an anode is coupled to the positive pole. As anodic material pig iron, graphite, silver or titanium coated with platinum are used. Such a system is termed an impressed current (or IP) system.
IP is used to protect steel in water and in moist soil, and typical examples are pipelines, piles, buried tanks, ship’s underwater hulls, floating docks and piers. When using IP on sailing ships, it is normal to embed the anodes in the hull. The anodes are insulated from the hull by plastic compounds and an anode shield is usually applied around the anode in a diameter of 3-7 meters (depending on size). Here the thickness of the anode shield needs to be above some 1500 µm so plastics are used frequently, replacing paints.
The anodes are evenly distributed along the sides of the ship and at suitable intervals; reference cells (Zn or Ag/AgCl) are placed. These are also insulated from the steel in the hull and they will monitor the protection levels and increase or reduce the current released according to need (in automatic installations)
The greatest problem with IP is the so-called stray currents. Electrical current will choose the course of least resistance and will enter metal from water if the metals are in the way of the electrical current. Where positively charged current enters the metal, we get a cathode and where the positive current exists the metal we get an anode and dissolution of the metal. Stray currents may occur during welding onboard a vessel or when the IP of a vessel interferes with the IP of a pier.
Measuring cathodic protectionA zinc cell affixed to an electrically conductive cable is lowered into the sea 1-2 m below the surface in close vicinity to the ship’s underwater hull. The cable is connected to the positive pole of a milli-volt meter, and the negative pole of the milli-volt meter is connected to the ship’s hull (earthed). Good earth contact must be ensured.
The salinity of the seawater must be > 1% to ensure correct measurements.
The ship’s underwater hull is sufficiently protected by the zinc or aluminum anodes if the meter reads between –250 mV and 0 mV. Readings ranging from +200 mV to 0 mV indicates that the cathodic protection is insufficient. Readings < -1100 mV indicates that the hull is slightly overprotected. The potentials may vary along the hull and at least three measurements on each side of the ship should be taken (one at the bow, one amidships and one at the stern).
Please note:
Potentials ranging between +500 mV and 0 mV will generate alkalinity (OH- groups) on the interface between the steel substrate and the coating system. This means that paint coatings containing oils may not be used on ships’ underwater hulls in conjunction with cathodic protection.
Potentials more negative that 0 mV will cause the formation of hydrogen gas on the interface between the steel substrate and the coating system. This gas formation will remove (blow off) corrosion products, contaminants and coating systems unless special precautions are taken. Only impressed current systems (and anodes made from magnesium) will develop potentials more negative than 0 mV. The gas formation is most pronounced near the anodes and thus the design of an underwater hull coating system must include the so-called anode shields.