Bimetal corrosion
Bimetal corrosion (also contact corrosion , galvanic corrosion ) according to DIN EN ISO 8044 is corrosion caused by the electrochemical reaction of two different metallic materials or other electron-conducting solids. The prerequisite is a different position in the electrochemical series , different corrosion resistance of the materials, their direct contact and common wetting by an aqueous corrosion medium . This creates a galvanic corrosion element , comparable to a short-circuited battery : As a result of the electricity generated, the less noble (i.e. less corrosion-resistant) material is destroyed by electrochemical removal .
This type of corrosion is noticeable in everyday life, for example
- when using screws, nuts, nails or rivets made of less noble metallic materials on components made of stainless steel , brass or copper
- when copper wires were clamped together with aluminum wires to conduct electrical current. Also, intermetallic compounds , resulting from the contact of the metals have an influence on the corrosion.
- When different metal materials are installed in a water pipe or heating system (for example in the case of heat exchanger pipes in the boiler, supply lines, radiators and solar collectors).
- Copper sheet metal on roofs is therefore always attached with copper nails.
Although the causes have been known since the beginning of the 19th century, bimetal corrosion has repeatedly led to damage and accidents.
reaction
Two different metals in electrical contact are in a liquid medium as electrolyte can act as normally water with dissolved salts , to electrodes . Due to different electrode potentials , an electrical voltage builds up between them, so that electrons from the atoms of the less noble metal migrate into the more noble metal and from there lead to a reaction with the electrolyte. In return, the ions of the less noble metal react with the electrolyte. The short circuit between the metals and the ion current in the electrolyte creates a closed circuit that determines the extent of the redox reaction : the anode is oxidized and gradually destroyed, while the cathode is reduced and thus protected.
The electrical voltage between the materials depends above all on the difference in their standard potentials, which can be seen from the electrochemical series , and also on the concentrations of the components of the solution ( Nernst equation ). Corrosion depends on the current actually flowing, i.e. the current density in relation to the surfaces . A direct current from an external source can amplify this current, which is important in microelectronics .
For bimetal corrosion, three prerequisites must be met: an electrical contact, an electrolytic connection and a potential difference between the two metals. Large-area anodes are less prone to corrosion than small-area cathodes than vice versa. A greater potential difference, an increasing area ratio of noble to base material, higher temperatures and increasing aggressiveness of the electrolyte (see pH value ) increase the corrosion effect.
Further manifestations
Bimetallic corrosion can also occur between the components of an alloy if precipitates at the grain boundaries of the crystals form a local element or micro- element on the surface . An example is graphite and iron in cast iron : compared to iron, graphite acts like a noble metal and can lead to spongiosis .
If cutting tools are used alternately for noble and base materials, the base materials can be corroded by remaining or pressed-in particles such as grinding dust .
A "carryover" of material is also the basis of the corrosion, which is to be prevented with the so-called flow rule according to DIN 1988-7 for pipes: If pipes made of different materials are connected and there is enough oxygen in the pipe, as is the case with drinking water , for example deposit dissolved copper ions on galvanized steel and thereby cause bimetal corrosion. To prevent this, the less noble metals should be arranged in front of the more noble metals in the direction of flow.
protection
There are two strategies for protecting against bimetal corrosion: Either an attempt is made to prevent electrolysis (passive corrosion protection ), or it is used in a controlled manner (active corrosion protection).
Prevention of electrolysis
Electrolysis cannot occur if an electrically conductive connection between the materials is prevented by insulation or a contact between the metal and electrolyte is prevented by a seal .
Direct contact between pipelines made of different materials is prevented, for example, by so-called separation or insulation fittings. Similarly, there are aluminum-copper connectors for electrical installations. They prevent the aluminum from decomposing, which leads to an increase in electrical resistance .
Protection against electrically conductive liquids ( electrolytes ) can be provided by a coating . Alternatively, chemicals ( corrosion inhibitors ) can reduce the effectiveness of the electrolytes .
If the electrolyte is formed by the action of air humidity , measures can be taken to dehumidify the air or to prevent condensation .
By selecting the appropriate material, the existing potential differences can be reduced from the outset.
The coating with a noble metal layer is only effective as long as this layer remains intact; If the damage is minor, the corrosion sets in more intensely. This is a problem, for example, with the tinplate of food containers or with gold plating in dental technology . With the active protection of steel by the less noble zinc (see following section), on the other hand, the bare steel is protected from corrosion even if the zinc layer is slightly damaged.
Active corrosion protection
A controlled use of the electrolysis takes place with "active" protective measures:
- By using a sacrificial anode , for example made of zinc or magnesium , the bimetal corrosion is limited to this. The material to be protected is thus made a cathode ( cathodic protection ). Sacrificial anodes are typically used on the outer skin of ships as well as on the inner walls of pipelines and liquid containers.
- Also, as a result of an active coating with a less noble metal (such as galvanizing steel), the coated metal is only attacked when the coating is completely corroded.
- Impressed current anodes are supplied with a permanently applied electrical voltage in order to compensate for the material-related potential differences. They are often used in larger liquid containers.
Passivation
Passivation is a mixture of both strategies : Corrosion is permitted or promoted if it causes a protective oxide layer to form on the less noble metal. The material to be protected is made into the anode and forms an oxide layer that can be more corrosion-resistant than many more noble metals. The Pilling-Bedworth ratio is used to calculate this protective effect . Chromium steel is protected by the fact that its alloy component, chromium, forms an oxide layer. Passivation can also be created in a targeted manner: anodizing is a common protection against corrosion, especially with light metals such as aluminum and its alloys . Due to its more stable oxide layer, titanium is used when it is exposed to high loads .
Suitable material combinations
Fasteners in construction
If a metallic component is less noble and at the same time has a smaller surface area than an adjacent component, it is usually at risk of corrosion.
In construction , more than in other areas of technology, rules of thumb can be assumed. The following recommendations apply under the following conditions:
- The first-mentioned material in each case relates to screws and other fasteners and fittings that have a surface area that is at least 10 to 40 times smaller than the receiving component
- A moderately aggressive corrosive medium (condensation water, rainwater, etc.). Additional protective measures may be required in the vicinity of sea water .
- Moderate ambient and material temperatures
Stainless steel can be combined well with larger components made of copper , brass and steel , as well as very well with larger components made of aluminum .
Aluminum can be combined well with larger components made of stainless steel, copper and brass. This generally also applies to larger components made of bare steel.
Copper can be combined well with larger components made of brass. This also generally applies to larger components made of stainless steel, aluminum and steel.
Brass can be combined well with larger components made of copper. This also generally applies to larger components made of stainless steel, aluminum and steel.
Galvanized and then black passivated steel can be combined well with blue and yellow passivated , galvanized steel. This generally also applies to larger components made of bare steel. The combination with larger components made of copper, brass, aluminum and stainless steel is not recommended.
Galvanized and then yellow passivated steel can be combined well with larger components made of blue passivated, galvanized steel. This generally also applies to larger components made of bare steel and black passivated, galvanized steel. The combination with larger components made of copper, brass, aluminum and stainless steel is not recommended.
Galvanized and then blue passivated steel can generally be combined with larger components made of bare steel, as well as with black and yellow passivated, galvanized steel. The combination with larger components made of copper, brass, aluminum and stainless steel is not recommended.
The combination of bare steel with larger components made of galvanized steel is not recommended and the combination with larger components made of copper, brass, aluminum and stainless steel should be avoided.
Plumbing installation
Flow rule for drinking water installations
In pipelines that carry oxygen-rich water such as drinking water , for example, copper ions dissolved from copper pipes can be transported by the water and then deposit on galvanized steel and lead to pitting corrosion .
The so-called flow rule according to DIN 1988-7 states that ever more noble metals must be used in the direction of flow of the medium.
In particular, make sure that no galvanized pipes are installed behind copper pipes in the direction of flow. Copper lines behind galvanized pipes, on the other hand, are generally not a problem. If backflows cannot be ruled out due to pressure fluctuations, the two materials should not be connected directly, but rather separated by an adapter made of red brass or brass .
Heating circuits
In closed heating circuits , the oxygen saturation in the water is low. With the use of suitable materials, there is hardly any electrochemical corrosion when the oxygen contained in the water is used up. In contrast, in the open heating systems previously used and through non-oxygen-tight plastic pipes, oxygen could repeatedly get into the circuit.
The use of internally galvanized steel pipes should not be used in heating circuits, as the resulting reaction products are deposited as sludge in the heating system. In contrast, the combination of tubes made of bare steel and copper has proven itself. The use of components made of brass , gunmetal , stainless steel and aluminum is usually possible without any problems in the heating circuit. However, the use of aluminum requires control and control of the pH .
history
Bimetal corrosion is as old as metal processing , but could only be systematically combated once the mechanism of its formation was understood. Roman wooden ships were from around 500 BC. BC (see ships of antiquity ) covered with a lead skin, which was fastened with copper nails. As a result, bimetal corrosion developed between the semi- precious copper and the base lead in the strongly electrolytic seawater . There are findings of lead-coated copper nails that appear to alleviate this problem.
The first study of bimetal corrosion was commissioned by the Royal Navy in 1763 because iron nails used to secure copper plates on the hull of the frigate HMS Alarm corroded unusually quickly. However, the causes could not be understood before the establishment of the theory of electricity by Alessandro Volta and the electrolysis by Alexander von Humboldt in 1795. Johann Wilhelm Ritter noticed in 1798 that the voltage series of metals is identical to the series of their corrosion resistance. Only the British chemist Humphry Davy was able to base his experiments on theoretical considerations and discovered in 1824 that copper on the hull of warships could be protected by attaching zinc and cast iron plates as sacrificial anodes. In 1833, his student Michael Faraday found a calculation basis for improving this corrosion protection by establishing that the mass of the corroded matter is proportional to the electrical charge (see Faraday's laws ). Impressed current anodes , which require an electricity supply , have only gradually gained acceptance since the beginning of the 20th century.
During the renovation of the Statue of Liberty for the 100th anniversary in the mid-1980s, it was discovered that a coating of shellac and asbestos , which was supposed to electrically isolate the iron framework from its copper cladding, had become porous and offered space for electrolytes, so that bimetal corrosion was well advanced .
In the case of the NATO helicopter NH90 , it was found that the graphite-like carbon fibers in carbon fiber-reinforced plastic behave like noble metal and lead to bimetal corrosion when they come into contact with metals.
literature
- DIN EN ISO 8044: 2015-12 Corrosion of metals and alloys - Basic terms and definitions .
- Helmut Kaesche: The corrosion of metals: physico-chemical principles and current problems . 3rd edition, Springer, Berlin 2011, ISBN 978-3642184284 .
- Elsbeth Wendler-Kalsch, Hubert Graefen: Corrosion damage theory . Springer, Berlin 2012, ISBN 978-3642304316 .
Web links
- Harald Schreckenberger: Risk of contact corrosion in CFRP components , Stuttgart 2013 , PDF accessed on June 4, 2016
Individual evidence
- ↑ Cf. R. Schneider et al .: Long-term behavior of aluminum-copper compounds in electrical energy technology , in: Metall , 63: 2009, no. 11, pp. 591–594, URL: https://www.kupferinstitut.de /fileadmin/user_upload/kupferinstitut.de/de/Documents/techUnterstuetzung/KS/Artikel/2009/591_Cu_Schneider.pdf , accessed on 2 Sep. 2018.
- ^ Friedrich Ostermann: Application technology aluminum , 3rd edition, Springer, Wiesbaden 2014, p. 233. ISBN 978-3-662-43806-0
- ↑ Klaus Mörbe, Wolfgang Morenz, Hans-Werner Pohlmann, Helmut Werner: Practical corrosion protection: Corrosion protection of water-bearing systems . Springer, Berlin 2013, p. 25. ISBN 978-3709188941
- ↑ Herbert Beneke: Lexicon of corrosion and corrosion protection . 2nd edition, Vulkan, Essen 2000, p. 251. ISBN 3-8027-2918-8
- ^ Franz-Josef Heinrichs, Bernd Rickmann: Technical rules for drinking water installations: Installation. Comment on DIN EN 806-4. Beuth, Berlin 2012, p. 64. ISBN 978-3410224891
- ^ Heinrich F. Kappert, Karl Eichner: Dental materials and their processing. 1. Basics and processing. Thieme, Stuttgart 2005, p. 168. ISBN 978-3131271488
- ^ Andreas Kalweit, Christof Paul, Sascha Peters, Reiner Wallbaum: Handbook for technical product design: Material and production . Springer, Berlin 2006, p. 532. ISBN 978-3540214168
- ↑ List of recommended material pairings from the building materials manufacturer Würth: https://wueko.wuerth.com/medien/produktinfo0000/pdfNEW/07570.pdf , accessed on April 14, 2018.
- ↑ Explanation of the flow rule on bosy-online.de [1] . Retrieved May 13, 2019.
- ↑ a b Walter of Baeckmann, W. Schwenk: Handbook of cathodic protection: theory and practice of electrochemical protection methods . 4th edition, Wiley, Weinheim 1999, pp. 2-4. ISBN 978-3527625734
- ^ KR Trethewey, J. Chamberlain: Corrosion: for Students of Science and Engineering . Longman, Harlow 1992, pp. 4-5. ISBN 978-0582450899
- ↑ Robert Baboian, E. Blaine Cliver, E. Lawrence Bellante: The Statue of Liberty Restoration: Proceedings of The Statue of Liberty, Today for Tomorrow Conference, October 20-22 1986 . National Association of Corrosion Engineers, New York 1990, p. 94. ISBN 978-1877914126
- ↑ http://augengeradeaus.net/2014/07/korrosion-beim-niederlaendischen-marine-nh90-der-bericht-zum-nachlesen/comment-page-1/ accessed on June 4, 2016