Tecnoseal Answers - Materials Part 3: ZN, AL e MG
With today's article we close the conversation started two weeks ago on materials. As mentioned previously, sacrificial anodes are used for cathodic protection. They can be composed of some types of alloys that make them suitable for multiple uses in different environments.
Significant, as seen, is the galvanic or nobility scale of the materials that tells us which metal can be used to protect another simply by looking at the position that the two cover on this scale. This is because the intrinsic electrochemical characteristics in each type of alloy make an anode more or less appropriate than another. For example, in fresh water the alloy that would be more appropriate to use is the magnesium one. At the end of the article you can find the alloys certified internationally used by Tecnoseal for the production of sacrificial anodes and their characteristics.
The first type of alloy is the most common: the zinc one. There are different types available according to the U.S.MIL., such as type 18001-H (JK), characterized by the addition of cadmium and aluminum. These are ideal for use in sea water, but also used in lakes and rivers. A good anodic behavior is also obtained from high purity zinc (99.995%) without additions of other elements.
It is good practice to change them regularly, even if they has been little corrosion. Use with temperatures above 55°C is not recommended.
On the market there are numerous alloys of aluminum selected by the manufacturers and also directly by the users. These are all characterized by the addition of zinc and indium. The exact contents of these impurities, in particular iron and copper, which must not exceed 0.15% and 0.005% respectively, are very important.
Characteristic of the aluminum anode is the high nominal electrical capacity which is triple compared to the same mass of Zn. This makes it possible to use aluminum in structures in which if Zn anodes are used, they should be used in huge quantities with installation and replacement problems.
They are often used in the merchant naval sector (transport, cruise ships or cisterns), where there are long dry docks planned or in any case on boats whose hull remains permanently in the water (barges, house boats, etc). Recently, many manufacturers of outboard engines have switched to the massive production of Al anodes also due to the increasing objection to the use of Zn anodes in some areas of the world due to cadmium, material always present in the alloy of Zn, but considered harmful to the sea.
The magnesium alloys, characterized by potentials above 1500 mV (ref Cu/CuSO4), are mainly used in the presence of high resistivity electrolytes (fresh water, soil, etc.). In all of them there is the addition of manganese 0.2-1.2% and for the most common versions also 2-4% zinc and 5-6% aluminum. These last types are also used in case of high temperatures, such as inside a boiler, and they are particularly indicated in the case of interventions on structures that require rapid effects or temporary cathodic protection.
The anodes in Mg have a performance of around 50% and while They have a fast-acting use in a marine environment, They also deliver a high degree of protection. However, due to the speed of self-corrosion, They are not recommended for navigating in brackish water. Another limitation to their use is the fact that magnesium is a metal that is difficult to store, because it starts to corrode when it comes into contact with moisture causing oxidation on the surface, and difficult to process, due to its dangerous ability to ignite.