Electric prevention for corrosion

This refers to passing electricity through metal objects such as steel underwater or underground to prevent their corrosion.

Corrosion of metals occurs as if one tiny part of the metal were the positive pole of a battery and another part the negative pole, and the two were short-circuited. This is called local cell action, and the metal at the negative pole of the local cell is ionized and corroded, and the electrons generated in this process move to the positive pole, causing oxygen reduction and hydrogen generation. If zinc or magnesium, which have a more negative potential than the negative pole of the local cell and a greater tendency to ionize, are connected to a steel body, these metals corrode preferentially and the steel body is protected from corrosion. This method is called the sacrificial anode method or galvanic anode method. In contrast, if the object to be protected from corrosion and a stable electrode material such as graphite are connected to the negative and positive ends of a DC power source, respectively, the electrons flow from the positive pole to the negative pole in the opposite direction to the current, so the target object is not ionized, and oxygen reduction occurs on its surface, preventing corrosion. This is called the external power source method. All of these methods protect the metal by injecting electrons into it, and are therefore called cathodic protection methods, but in the case of structural objects such as titanium, it is also possible to protect them from corrosion by connecting them to the positive pole of an external power source and steadily forming a corrosion-resistant oxide film over their entire surface, which is called anodic protection. Cathodic protection is used to protect a wide range of objects, including ship hulls and buried underground pipes.

[Hiroshi Yoneyama]

"Electrodeposition and Corrosion" by J.M. West, translated by Tatsuo Ishikawa and Toshio Shibata (1966, Sangyo Tosho) " An Introduction to Corrosion and Protection of Metals" by C. Lundgren, translated by Shiro Yoshizawa et al. (1978, Kagaku Dojin)

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

水中や土中にある鉄鋼などの金属物体に電気を流して、その腐食を防ぐことをいう。

　金属の腐食は、あたかもその微細なある箇所が電池の正極、別の箇所が負極となり、これらが短絡されたものであるかのようにしておこる。これを局部電池作用といい、局部電池の負極の箇所の金属はイオン化して腐食され、その際に生じる電子が正極の箇所に移動して酸素の還元や水素の発生をおこす。鉄鋼体にその局部電池の負極よりも負の電位をもつイオン化傾向の大きい亜鉛やマグネシウムを接続すると、これら金属が優先的に腐食され鉄鋼体は腐食を免れる。このような方法を犠牲アノード法、流電陽極法などという。これとは異なって、防食すべき物体と黒鉛などの安定な電極材とを直流電源の負と正端にそれぞれ接続すると、電子は電流とは逆向きに、正極から負極へ向かって流れるので、対象物体はイオン化されず、その表面で酸素の還元などがおこり腐食を免れる。これを外部電源法という。これらの方法は、いずれも金属に電子を流し込み防食するのでカソード防食法（陰極防食法）といわれるが、これに対して、チタンなどの構造物体では、これを外部電源の正極に接続してその表面全面に耐食性の酸化膜を定常的に形成させて防食することも可能であり、これをアノード防食法（陽極防食法）という。電気防食は、船体、地下埋敷管など広範囲の物体の防食に用いられている。

［米山　宏］

『J・M・ウエスト著、石川達雄・柴田俊夫訳『電析と腐食』（1966・産業図書）』▽『C・ラングレン著、吉沢四郎他訳『金属の腐食防食序論』（1978・化学同人）』

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