Use of AC Voltage Provides Corrosion Resistance

Quote of the Day

I will either find a way or make one.

— Hannibal. I have worked with people that were going accomplish a task no matter what. I refer to these folks as a force of nature.

Figure 1: Corroded F-Connectors (Source).

Some of the most vexing problems I have dealt with in my career are related to connector corrosion problems. While corrosion can create a hard failure (easy to find),  more often it creates an intermittent failure (hard to find). Intermittent failures can be very expensive to find. For example, the automotive industry spent years dealing with intermittent connector problems that were caused by fretting.

There are a number of ways to deal with corrosion:

• Seal the connector to keep moisture out.

  This is hard to do. We frequently required customers to use caulking to seal cable entry points, but overtime the caulking fails. It is also very difficult to ensure that you have adequately sealed the connection.

• Use gold coatings to ensure the connection does not corrode.

  This is commonly done, but it must be done properly. Many folks try to get by using a gold flash costing, i.e. 2 μinch to 10 μinch thickness. I have found gold flash to be useless in an outdoor telecommunications product. I only approve connectors for outdoor use with 30+ μinches of gold.

• Use anti-oxidation lubricants/pastes to ensure keep the connection from corroding.

  These certainly can help. For example, you frequently see pastes in home electrical wiring where aluminum and copper wires meet. I have repaired numerous electrical "failures" in my neighborhood by applying anti-oxidant pastes to my neighbors' aluminum service entry cable. For example, half of one neighbor's house would occasionally lose power. It turns out the service entry contact for that phase had corroded.

I had a discussion with a coaxial cable expert last week who mentioned that the cable industry used AC power voltages to prevent corrosion on coaxial cables. I had not thought about this before, but the cable folks have reported that the copper core and aluminum shield in the cable undergo galvanic corrosion in moist environment (Figure 1). Using an AC voltage will eliminate the ion exchange that occurs from having a fixed bias voltage (link). In the case of the cable industry, they pass power over coaxial cable using 60 V or 90 V AC signals.

Corrosion reactions are typically reversible based on the applied voltage. When you apply a DC voltage, the ions (usually metals and salts) continually go in the same direction, which means their effects accumulate. This is why we plate metal with DC voltage – never AC. With respect to corrosion, we do not want any reaction products to accumulate. With AC voltage, the corrosion reaction spends an equal amount of time going one direction as the other. Thus, if we do something bad on one half-cycle of voltage, we will undue the damage on the next half-cycle.

Back when I worked on US Navy systems, I would often see AC voltages being used on connectors where DC would have been more convenient for the designers. I asked an old-timer why and he told me that using AC reduced the effects of saltwater corrosion.

I must admit that the effects of saltwater corrosion were amazing. In one case, we had an undersea sensor that was anchored to the sea bottom. The anchor chain we used was made of  thick steel (~1 inch diameter) coated with a protective paint that was used on sea-based oil rigs to stop corrosion. During the deployment of the sensor, the anchor chain was nicked and the paint was removed from this tiny location. When we came back six months later, the chain was severed by corrosion at the nick.