Another important factor in galvanic corrosion is the area effect or the ratio of cathodic to anodic area. The larger the cathode compared with the anode, the more oxygen reduction, or other cathodic reaction, can occur and, hence, the greater the galvanic current. From the standpoint of practical corrosion resistance, the least favorable ratio is a very large cathode connected to a very small anode. This effect is illustrated in the following series of pictures. The galvanic Table indicates that iron is anodic with respect to copper and therefore is more rapidly corroded when placed in contact with it. This effect is greatly accelerated if the area of the iron is small in comparison to the area of the copper, as shown below.
Steel rivets on a copper bar submerged in 3% sodium chloride solution at the start of the experiment
Steel rivets on a copper bar submerged in 3% sodium chloride solution after six months
Steel rivets on a copper bar submerged in 3% sodium chloride solution after ten months
However, under the reverse conditions when the area of the iron is very large compared to the copper, the corrosion of the iron is only slightly accelerated.
Copper rivets on a steel bar submerged in 3% sodium chloride solution at the start of the experiment
Copper rivets on a steel bar submerged in 3% sodium chloride solution after six months
Copper rivets on a steel bar submerged in 3% sodium chloride solution after ten months
If the electrical conductance of the electrolyte bridging the galvanic contact is low, either because the bulk conductivity is low or because the electrolyte is present only as a thin film, the effective areas taking part in galvanic cell reactions are small and the total amount of corrosion is relatively small, although it may be severe immediately adjacent to the metal junction. Under immersed conditions in many supply waters, which generally have a relatively low electrical conductivity, adverse effects are uncommon if the contacting metals are of similar area. Thus galvanized steel pipes can be used with brass or gunmetal connectors, but serious corrosion to the pipe end is likely to result if the contact is directly to a large area of copper, such as a tank or cylinder. Similarly, stainless steel and copper tubes can usually be joined without problems resulting, but accelerated corrosion of the copper tube is likely to occur if it is attached to a stainless steel tank.
Under immersed conditions in a highly conducting electrolyte, such as seawater, effective areas will be greater and severe corrosion may be encountered on small anodic areas of many metals. Extremely small anodic areas exist at discontinuities, such as cracks or pinholes, in cathodic coatings such as magnetite (millscale on iron) and copper plating on steel.
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