Guide Guide to Galvanic Corrosion/Cathodic Protection

Intro
I am relatively new to Bit-Tech and am still reading up on a lot of the project logs and other topics on here, but one theme I have noticed in a few threads is the mixing (or desire to mix) of components in water cooled loops that are made from different metals. I am starting this thread to discuss some of the basic concepts behind galvanic corrosion and the idea of cathodic protection to help people prolong the lifespans of their water-cooled systems.

Disclaimer
I am not an expert, but I do have some work experience in the field of cathodic protection of oil pipelines and water mains. The same basic principles can be applied to any mixed-metal system. That being said, if you see incorrect or incomplete information, tell me! I'll be happy to fix it and admit my mistake

Galvanic Corrosion
The basic concept is that every metal has an eletrical potential (I won't get into the specifics about what eletrical potential is unless requested) measured in volts. When two different metals come in contact, either directly or indirectly (ie a water-cooled system), the balance of electrical potential in the system is uneven and will essentially try to balance itself out. This process manifests itself in what we observe as corrosion of one of the metals.

This chart shows which metals have a low potential (top of the chart) and which ones have a high potential (bottom of the char). To help understand the chart, let's define two very important terms:

Anode - in a galvanic system, this is the material that is dissolving and where the metal ions are flowing from​

Cathode - in a galvanic system, this is the material that is collecting the dissolved material and where the metal ions are flowing to​

Looking at the chart, the further away from each other that the two metals are, the more likely you will have corrosion. Also, when looking at two metals, the higher metal on the chart will always be the anode and the lower one will always be the cathode.

When utilizing two different metals in a system, it is a good idea to figure out the difference in electrical potential between the two. Anything more than a 0.50 volt difference will result in corrosion, with anything over a 1.00 volt difference resulting in significant corrosion. Other factors will affect the potential difference as well, including how far apart the two metals are, what coolants and additives are used, etc., but to keep the calculations simple and given the attributes common to most cooling systems we can ignore these for our rough reference here.

Example #1 (Galvanic Corrosion)
I have a system with a copper cpu block and an aluminum block on my gpu. There are two ways to figure out the voltage difference (or drop) between the two metals. The first method is to find a galvanic electric potential chart. However, it's hard to find two of these that have the same voltages so I try to stay away from them if possible. The other method is to create a circuit between your two blocks (either by stacking them on top of each other or by using copper wires to attach them) and using a multimeter to measure the actual drop between the two. This will give you a better idea of how much corrosion will take place, but requires a that you have a multimeter. For the most accurate reading, attach the negative probe to the metal with the lower potential and the positive probe to the metal with the higher potential. The difference between aluminum and copper should be anywhere from 1.5 volts to 2.0 volts, indicating that corrosion will take place in this system.

Cathodic Protection
One of the more prevalant methods of protecting against galvanic corrosion is cathodic protection. It is a crude method, but is used in everything from home water heaters to boats to oil pipelines to water mains. Essentially, you add a sacrificial anode that has a lower (more negative) electrical potential than the existing metals in your system. The most common material used for anodes in cathodic protection systems is zinc, and a quick search of any marine parts store will result in many variations of zinc anodes. Adding this anode to the system causes this metal to dissolve before the ones you care about. These sacrificial anodes have to be monitored and eventually replaced to maintain proper cathodic protection.

Example #2 (Cathodic Protection)
For this example, we will use the same setup and information from the first example. The sacrificial anode we will be using is made of zinc. As mentioned, zinc is the most commonly used anode for cathodic protection and is readily available. Wrap the zinc in cheesecloth, drop it in your reservoir and you're set.

Example #3 (Metallic Coating)
Again, we will use the same setup. What if I don't want to put a bag of zinc in my nice water-cooling system? Easy. There are a few different things you can do here:

1. Coat the aluminum with zinc. This is the same process used to create galvanized steel. The zinc will corrode first, protecting your aluminum block.
2. Coat the aluminum with nickel. Nickel has a potential that is about 0.05 volts difference than copper. The close potential will mean very negligible corrosion.
3. Don't drop the zinc into your system. Instead, create a circuit connecting the zinc to the aluminum block and to the copper block using copper wires. This circuit will be external from the water cooling system, but will still provide the cathodic protection.

Conclusion
I hope that this information helps everyone understand that corrosion in water cooled systems can be avoided with a little extra work. Eventually, any metal will corrode, but we can always prolong the process

 

Really good read

 

Nicely set out but there is a mistake I believe.

Both metals need to be in contact with an electrolyte in order for galvanic corrosion to take place. The electrolyte serves to conduct electrons and ions from the anode to the cathode. If you simply wire a piece of zinc to your waterblock there is no path for the ions to move from the anode to the cathode and there will be no protection. The sacrificial anode needs to be immersed in the electrolyte (loop fluid) in order to be effective.

Moriquendi

 

Thanks for the feedback and for putting it in the water cooling thread

You are correct about the ions needing a path from the anode to the cathode for the protection to work. This path can either be through the electrolyte, or through a normal circuit. So I clarified it from this:

To this:

Often, the electrolyte is just regular soil as in this diagram:

 

Another mistake: both metals also need to be in electrical contact with each other. So as long as, say, your aluminium block is electrically isolated from your copper radiator, no (or at least very little) corrosion can take place.

Think of a non-rechargable battery. A battery basically consists of a cathode and anode and an elecrolyte. Nothing happens (much) to the battery until a circuit is completed across both poles of the battery; once that happens galvanic corrosion takes place as a current flows through the circuit from anode to cathode. the battery is spent when the corrosive reaction has finished.

 

From what I remember (and it could be fuzzy or incorrect since it's been a few years since I worked in the field) but any conductive substrate can complete the circuit. The main one we ran into was soil. It always amazed me that (like in the diagram) metals that were in soil, but hundreds of meters away from each other, would still corrode. However, in the case of PC Cooling, the conductive substrate would be the coolant itself. Since the coolant is conductive, ions would flow from the aluminum to the copper. No external electric current is needed. An external electrical current would greatly accelerate the corrosion if it were present.

Of course, this is all assuming that you are using a coolant that is conductive, such as distilled water or anything that is water-based. I did find these coolants that claim to be non-conductive. Assuming they work, then you can ignore this whole thread

And thanks for all of the feedback so far. The more accurate we can get this information, the more it will help everyone.

 

I don't think that the electrolyte would also be the conductive substrate --else batteries would rapidly self-discharge.

In any case I use Fluorinert so I can't say I have a lot of personal experience in the matter.

 

I think batteries work slightly different

How do alkaline batteries work?

I think there is a slight insulator that keeps it from discharging. Honestly though, they seem like black magic to me lol.

 

You definitely need a electrical connection between the two metals in addition to the electrolyte. The metal ions flow through the electrolyte, and the electrons stripped from those ions flow through the electrical connection. Without the electrical connection, those electrons would go nowhere, and a massive charge would build up with no power input (overunity = bad).
Most of the time, this electrical connection is the groundplane: The CPU heat-spreader (and the surrounding metal shield and mounting holes) and the case itself are both connected to ground (and the radiator is generally mounted to the case with metal screws), so there is an electrical connection between the two. If you mounted the water block to an older CPU with no heatspreader using a plastic clip, or mounted the radiator to an external nonconductive object (or isolated it with some sort of internal plastic mount with separate offset screw holes), you should experience no galvanic corrosion.

 

galvanic corrosion and cathodic protection

Hi
i think that there is a little confusion with the mechanism of galvanic corrosion and cathodic protection. I'm a corrosionist and, even if the article is well written, it contains some errors:
-Galvanic corrosion is a type of corrosion mechanism that occur when two metals are joined and there is an electrical path. (for example water) is not necessary to have an external circuit or electrical connection, the imporant point is that where there is the contact between the metals there is an electrolyte
- is true that a great difference in potential of the two metals, see a galvanic series of metals of a galvanic chart, mean an higher likelihood to have corrosion but there are other important factors to be considered:
1) the area ratio of the two metals; if the area of the less noble material is much bigger than the area of the most noble metal than the effect of galvanic corrosion in minimized.
2) is necessary a good elctrolyte to carry the current between the two metals, if the elctrolyte is not a good electrical conductor than galvanic corrosion will be neglible.
for example distilled water is not a good electrolyte...
3) the potential of the metals is different in different elctrolytes ,in seawater is different from fresh water

cathodic protection is a tecnique to control corrosion but is not correct to say "Don't drop the zinc into your system. Instead, create a circuit connecting the zinc to the aluminum block and to the copper block using copper wires. This circuit will be external from the water cooling system, but will still provide the cathodic protection. "

If you want to protect the external surface of something like a pipeline immersed in water or buried than it coulb be correct but if you want protect for example the internal surface of a water heater or a water tank than you've to immerse the anodes, zinc, into the water to protect the internal surface.

Vict

 

Is there such a profession?

In any case, useful to get input from an expert. Thanks!