Accelerated corrosion of the more active metal, acting as the anode, when metals of different potential are in contact in an electrolyte. Also called dissimilar-metal corrosion.
Japanese name: ガルバニック腐食
Galvanic corrosion appears in 2 practice questions across 2 topics, including Corrosion and surface treatment, Composite materials.
When different metals are electrically connected and immersed in the same electrolyte, the metal with the lower (more active) corrosion potential becomes the anode and the one with the higher (more noble) potential the cathode. Corrosion of the active metal speeds up compared with when it is alone, while corrosion of the noble metal slows down. To judge which metal will dissolve, the ranking of corrosion potentials measured in real environments such as seawater (the galvanic series) is used rather than standard electrode potentials. In seawater, the metals become more noble roughly in the order magnesium, zinc, aluminum alloys, steel, copper alloys, passive stainless steel, titanium and graphite.
The corrosion rate changes greatly not only with the potential difference but also with the area ratio of anode to cathode. The larger the cathode and the smaller the anode, the more the large dissolution current, matched to the oxygen reduced at the cathode, is concentrated on the small anode. Painting only the anode can therefore backfire, since a small flaw in the coating becomes a small anode and is dug out deeply; if painting, the cathode side is painted too. In fresh water or thin moisture films in air, which have high electrical resistance, corrosion concentrates near the contact, while in well-conducting environments such as seawater the effect extends over a wider area.
The surest method is to combine metals that are close together in the galvanic series. When different metals must be used, measures include insulating them electrically with plastic washers or sleeves, designing joints so that water does not collect, and cutting off the electrolyte path with sealants. Fasteners are made of a material more noble than the parts they join, so that no small anode is created. For combinations of aluminum alloys and CFRP, a glass fiber layer is inserted between them for insulation, or titanium alloy fasteners are used.
Used well, galvanic action becomes a means of protection. In hot-dip galvanized steel sheet, even if a scratch exposes the steel, the surrounding zinc dissolves first and protects it (sacrificial protection). The sacrificial anodes attached to ship hulls and harbor structures work on the same principle. A well-known failure is the Statue of Liberty: the insulation between its copper skin and iron framework broke down in the damp, the iron framework corroded, and in the 1980s restoration the iron bars supporting the skin were replaced with stainless steel. It is important to determine at the design stage which way a potential difference will act.
Answer: B. A large copper plate fastened with small steel rivets
When dissimilar metals are in contact, the active steel becomes the anode and the noble copper the cathode, accelerating corrosion of the steel (galvanic corrosion). The amount of oxygen reduced at the cathode is roughly proportional to the cathode area, and the anodic dissolution current equals it, so the corrosion current is set mainly by the cathode area. With a large copper cathode plate and small steel anode rivets, a large current concentrates on a small area, and the rivets corrode severely in a short time. Conversely, with small copper rivets in a large steel plate, the corrosion current is spread over a wide steel surface, so the corrosion rate (per unit area) is small. The choice saying the area ratio has no effect is therefore also wrong. When joining dissimilar metals, the rule is to make the fasteners the more noble metal or to insulate the joint.
Answer: C. CFRP excels in specific stiffness but conducts electricity, so it readily causes galvanic corrosion in contact with aluminum alloys
Carbon fiber has a density of about 1.8 g/cm³, lower than glass fiber (about 2.5 g/cm³), and a Young's modulus of about 230 GPa or more, over three times that of glass fiber (about 70 GPa), so CFRP excels in specific stiffness and specific strength and is used in aircraft and sporting goods. However, it is expensive, and because carbon fiber conducts electricity and has a noble electrode potential, contact with aluminum alloys readily causes galvanic corrosion of the aluminum; countermeasures include placing a layer of glass fiber between them. It is CFRP that conducts electricity, not GFRP. Transparency to radio waves and high insulation are characteristics of GFRP, which is used for radomes, electrical insulating parts, and inexpensive boat hulls and bathtubs.
Yes, they are the same phenomenon. It is called galvanic corrosion because it works like a galvanic cell, and bimetallic or dissimilar-metal corrosion when the emphasis is on its cause. It should be distinguished from stray-current corrosion, which is driven by external currents.
In environments exposed to rain or seawater, the carbon steel side corrodes more easily. It is especially dangerous when the carbon steel area is small, as when carbon steel bolts fasten stainless steel plates; insulate the joint or make the fasteners stainless steel as well.
Without an electrolyte no current flows, so it hardly occurs. However, it does occur when condensation or salt deposits form a thin film of water, so care is needed outdoors and near the sea.
When dissimilar metals are in contact, the active steel becomes the anode and the noble copper the cathode, accelerating corrosion of the steel (galvanic corrosion).
Steel and copper are used in electrical contact in seawater…However, it is expensive, and because carbon fiber conducts electricity and has a noble electrode potential, contact with aluminum alloys readily causes galvanic corrosion of the aluminum; countermeasures include placing a layer of glass fiber between them.
Which correctly compares carbon-fiber-reinforced plastic (C…CFRP (carbon-fiber-reinforced plastic) GFRP (glass-fiber-reinforced plastic) Young's modulus Specific stiffness Faraday's law of electrolysis Local cell Passivity Pitting corrosion Sacrificial anode Standard electrode potential