Influence factors and protective measures of galvanic corrosion

Date：2022-09-07 16:42:35Views：747

Galvanic corrosion, also known as dissimilar metal corrosion or contact corrosion, refers to the phenomenon that when two materials with different electrochemical properties form a circuit with the surrounding media, the corrosion rate of the metal with positive potential slows down, while the corrosion of the metal with negative potential accelerates. The evaluation methods of galvanic corrosion of materials or components mainly include exposure evaluation experiments and electrochemical measurements. This article has collected and sorted out some materials, hoping to have greater reference value for all readers.

The potential difference is the driving force for the formation of galvanic corrosion. When two different metals are in a conductive aqueous solution, there is usually a potential difference between the two metals. The premise of galvanic corrosion is that there must be a certain potential difference between the anode and cathode. For example, "stainless steel pipe" with positive potential is coupled with "carbon steel pipe" with negative potential, "stainless steel pipe" is cathode and "carbon steel pipe" is anode. The larger the potential difference, the greater the galvanic corrosion tendency.

There are three necessary conditions for galvanic corrosion:

1、 Potential difference

The potential difference is the driving force for the formation of galvanic corrosion. When two different metals are in a conductive aqueous solution, there is usually a potential difference between the two metals. The premise of galvanic corrosion is that there must be a certain potential difference between the anode and cathode. For example, "stainless steel pipe" with positive potential is coupled with "carbon steel pipe" with negative potential, "stainless steel pipe" is cathode and "carbon steel pipe" is anode. The larger the potential difference, the greater the galvanic corrosion tendency.

2、 Electronic channel

Electron channel is an electron channel formed by galvanic corrosion after wire connection or direct contact. The electrons lost by the iron in the "carbon steel pipe" reach the surface of the "stainless steel pipe" and are absorbed by the corrosive agent.

3、 Electrolyte

Galvanic corrosion requires electrolyte covering or immersion in the contact area between two metals. The electrons lost by the iron in the "carbon steel pipe" form ions, and the electrons on the surface of the "stainless steel pipe" are taken away by the corrosive agent in the electrolyte (such as oxygen in the air). The electrolyte becomes an ion channel.

According to the characteristics of galvanic corrosion, only one of the three necessary conditions is changed, and the bimetallic corrosion is terminated. Galvanic corrosion can be prevented by electroplating, painting and adding corrosion inhibitors.

(1) In the design, try to avoid the contact of different metals or alloys. If it is unavoidable, the material combination with similar potential sequence shall be selected as far as possible, that is, the metal with the electrode potential as close as possible (preferably not more than 50mV) in the working environment shall be used as the contact couple, and the area combination of large cathode and small anode shall be avoided as far as possible.

(2) Galvanic corrosion is related to the contact area of bimetal. The larger the contact area, the lighter the corrosion. Reduce the area of the metal with positive electrode potential and try to increase the surface area of the metal with negative electrode potential.

(3) If allowed, add corrosion inhibitor into the medium to slow down the corrosion of the medium.

(4) During construction, insulation can be considered at the joints of different metal materials to electrically insulate the metal in contact and increase the dielectric resistance. For example, all contact surfaces at the flange connection shall be protected with insulating protective materials as gaskets or coatings.

(5) For some small parts that must be assembled together, surface treatment must be adopted. For example, galvanized steel flange surface and anodized aluminum alloy surface. These facial mask have high resistance in the atmosphere and can be used to reduce the corrosion of the galvanic couple.

(6) A third metal material is inserted between two different metals to reduce the potential difference.

(7) It should be considered that the anode parts that are easy to be corroded should be replaced or repaired during maintenance. The parts that are easy to be replaced should be used as the anode and the thickness should be increased to prolong the service life.

(8) Use paint coating. When using the coating, the coating must be coated on the cathode metal, which can significantly reduce the cathode area. If it is coated on the anode surface, due to the porosity of the coating, part of the anode surface may be exposed to the medium, which will cause the area combination of large cathode and small anode to accelerate corrosion. For example, when magnesium alloy is in contact with other structural materials in neutral medium, its corrosion is hydrogen depolarized galvanic corrosion. This requires that the coating has good adhesion to magnesium alloy and can block the penetration of external media. At the same time, it is required that once the medium penetrates to the surface of magnesium alloy, the surface of magnesium alloy can be passivated, which puts forward higher requirements for the primer.