The corrosion is mainly related to the contact interface and the surface treatment of the connector.
There are two main mechanisms for the increase of the connector resistance caused by corrosion:
1) The metal surface coating of the military spec connector is formed by the chemical reaction between the contact interface and the air;
2) Corrosive substances penetrate into the contact interface and reduce the contact area.
Three common corrosion types that must be considered:
A. Surface corrosion
It refers to the corrosion film covering the surface of connector, such as the tin oxide, palladium or palladium alloy oxide;
B. Corrosion migration
It refers to the migration of corrosive substances to the surface of electrical power connectors and to the contact area. The application environment is sensitive to corrosion migration, such as the environment where sulfur and chlorine exist.
C. Pitting corrosion
If the corrosion migration happens in a small hole, small discontinuous holes will occur on the electroplating surface, and this corrosion mechanism is called pitting corrosion. The contact resistance is not affected by the small hole itself, but only when the small hole becomes the source of corrosion will the contact resistance decrease.
Because of the abrasion, the sensitivity of the contact interface to corrosion increases. The base layer is protected and the coating surface is optimized through the surface treatment of the base material. However, the abrasion will lead to the loss of surface treatment function.
Factors affecting wear: V=(KFnI)/H
V is the abrasion loss per cycle, K is the friction coefficient, Fn is the positive force, I is the sliding length, and H is the material hardness of the contact surface.
The friction coefficient K is determined by the geometry, the positive force, the surface hardness, the lubrication condition and the material.
With the increase of positive force Fn, the abrasion of the adhesion and the corresponding grinding also increases, thus increasing the abrasion.
L is the sliding length. It is obvious that with the increase of L, the abrasion will increase, so the insertion depth should be limited.
H, the surface treatment hardness, affects the area of contact area, and the matching of hard and soft surface treatments will transfer the soft wear material to the hard surface. Therefore, the electroplating materials of the two matching parts in military grade electrical connectors are usually the same.
The abrasion can be minimized by carefully selecting the right material (surface hardness), controlling the positive force and using lubricants.
For the failure of the connector, the loss of positive force will reduce the mechanical stability of the contact interface, and the decrease of the mechanical stability will increase the sensitivity of the contact interface to the mechanical or thermal induced strain disturbance, thus increasing the contact resistance.
There are two main aspects of positive force loss:
A. Permanent deformation
Permanent deformation refers to that the connector deviates from the original position due to the plastic deformation, resulting in the reduced offset. Therefore, the reduction of positive force is resulted from the overstress in the process of inserting and extracting, which is usually caused by incorrect or rude inserting and extracting. It should be solved through the structural design of the shell and/or the connector, such as adding the guide structure to prevent over insertion.
B. Stress relaxation
The result of stress relaxation is the decrease of stress s and the decrease of positive stress.
Stress relaxation is inevitable and can only be controlled. The speed of stress relaxation is related to the material selected for connector design, the applied stress and the temperature of application environment. The stress relaxation depends on time and temperature.
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