A bonding question I can't find the answer to

[JamesW]

Many things are bonded and grounded, so the bonding and grounding are connected and you cannot avoid this.

Electrical issues are incredibly difficult to diagnose, even for experienced electricians.

The aluminum is a sacrificial anode to everything else on the bonding grid and the grounding grid.

Oxidation is the loss of electrons.

When an oxidizer, like oxygen, chlorine, hydrogen chloride gas etc. comes in contact with a metal like steel, the iron loses electrons because the oxygen pulls the electrons loose.

This creates iron oxide (aka rust).

If you have a less noble metal like aluminum, zinc or magnesium connected by a wire, then the oxygen pulls the electrons from the less noble metal even though the oxygen is making contact with the steel.

All metal connected to the bonding grid and the grounding system is protected from oxidation by the least noble metal and the least noble metal takes all of the corrosion that would have happened to all more noble metal.

The total amount of corrosion is equal to what it would be if all metal was made from the least noble metal and this is all concentrated on the least noble metal.

The least noble metal acts as a reducing agent to oxidizers like oxygen.

The aluminum casing has a net loss of electrons due to an electrical and/or a chemical problem in the local environment.

Aluminum can form a protective layer of aluminum oxide, which helps prevent surface attack by oxidizers like oxygen because the aluminum in aluminum oxide is already oxidized, however, an electrical problem can reach below the surface and pull out electrons due to the conductivity of the metal.

An electrical problem cannot easily be found and it might be intermittent and only active during certain conditions.

Since everything is bonded and grounded, it’s difficult to isolate any stray current or voltage.

 

[JamesW]

I'm not sure what's behind it.

Can you look?

I did test for an electrical current, and there is none.

Tested how?

 

[mas985]

I have solar panels, so maybe an additional connection to ground. I'm not sure what's behind it. I did test for an electrical current, and there is none.

It might be more helpful to measure the AC/DC voltage between the bonding wire, disconnected from all equipment except the pool, and the house ground. This will tell you if there is any NEV on the house ground/neutral. But to do the measurement properly, you need to isolate the bonding wire from house ground which is why it needs to be disconnected from all the equipment and house ground.

Would it help to cover that connection with dielectic grease, so it won't get wet?

That might help but I would also cover the pump motor to keep water off it.

When looking at the pictures of the corrosion, it is happening pretty much all over the motor so I am not sure it is just due to the bonding point. There is clearly a defect in the design of this motor that is making it more susceptible to corrosion. That is why trying to keep all water off the motor might be the best approach.

 

[JamesW]

Galvanic corrosion only occurs in the presence of an electrolyte solution.

In my opinion, the whole electrolyte requirement is mostly exaggerated or misunderstood.

The aluminum cup anchor is corroded, where is the electrolyte?

The aluminum motor is corroded, where is the electrolyte?

Both metals do not need to be immersed in a salt solution for galvanic corrosion to occur.

 

[mas985]

In my opinion, the whole electrolyte requirement is mostly exaggerated or misunderstood.

You need ions to transfer current and that can only occur in a solution that contains ions. Basically, any water that is not distilled will contain ions.

The aluminum cup anchor is corroded, where is the electrolyte?

Pool water contains salt and pool water can easily get on an anchor since it is usually next to a pool. Rain just keeps the process going as the salt remains after evaporation.

The aluminum motor is corroded, where is the electrolyte?

Again, any water that hits the motor surface that has a salt residue. We know that pool water has salt even without a SWG. This can easily get on the pump motor when cleaning the pump basket. While this may happen once per week, rain can then keep the area moist so the corrosion can continue.

Both metals do not need to be immersed in a salt solution for galvanic corrosion to occur.0

I never said immersed. Both metals just need to have contact with the salt solution for corrosion to occur. A thin film is sufficient. Even heavy humidity might be enough if the surface is damp. Basically, a circuit needs to be formed between the two metals with low resistance direct contact, metal to metal, plus contact through the electrolyte.

 

[JamesW]

The aluminum sends electrons through the wire to other metals; that is the current.

The aluminum metal atoms become positively charged ions and these can build up and stop the flow of electrons.

All you need is pure water to neutralize the aluminum ions.

So, rain or even humidity can provide the necessary water.

No salt of any type is necessary.

The aluminum combines with the hydroxide in water to form a neutral aluminum hydroxide.

There is enough water in the air to neutralize the aluminum ions.

You would have to keep the aluminum perfectly dry including zero humidity to keep it from corroding.

 

[JamesW]

Most references say that both metals need to be immersed in the same electrolyte to create a conductive path, but this is not accurate in my opinion.

The conductive path is the copper bond wire.

The cup anchor is not immersed in the same electrolyte as the other metal.

All you need to do is to dissolve and/or neutralize the positively charged ions at the less noble metal.

If the less noble metal gets wet, the ions will dissolve into the water and/or get neutralized by combining with hydroxide or oxide to form a neutral molecule.

 

[JoyfulNoise]

I tend to agree with Mark on this. James, you are greatly oversimplifying how corrosion occurs. It doesn’t have to be galvanic in nature. It can also be chemically induced corrosion where surface defects retain moisture. Pitting corrosion and crevice corrosion are examples of corrosion processes that don’t require contact with any other dissimilar metals. There is charge transfer in those cases but it occurs in the form of localized redox reactions where the underlying metal acts as a conductive pathway and the pit or crevice is where the electrochemical reactions occur. Chloride and sulfates enhance these types of corrosion and they can occur even when a material has an impressed cathodic current or no galvanic current exists at all.

I don’t disagree that some of this can be galvanic when wet but that probably just starts the reaction and then localized corrosion occurs in parallel and probably at a faster rate than any galvanic part of it.

 

[JamesW]

Certain types of aluminum are more reactive than even zinc.

As the anode gets oxidized, it becomes positively charged and the ions need to migrate away from the anode or they need to be neutralized by negatively charged ions like oxide, hydroxide, sulfide, chloride, sulfate, nitrate etc.

If the aluminum gets wet, the aluminum ions can dissolve into the water.

No electrolyte is needed.

 

[JamesW]

my pump motor has been replaced for the third time in 5 years.

I told him the corrosion occurs within 3 months of the motor being replaced.

you are greatly oversimplifying how corrosion occurs.

Something in the local environment is causing accelerated corrosion of all metals.

The aluminum is the least noble metal on the bonding grid, so it will act as a sacrificial anode to everything else on the grid.

The aluminum will corrode preferentially to all other more noble metal.

So, there is definitely galvanic corrosion.

Maybe corrosive chemicals like acid or chlorine are being stored nearby and causing corrosion.

If it is not chemical, then there is an electrical problem causing a direct current flowing electrons from the aluminum to all other metal and to whatever is causing the DC voltage.

Unless you find and fix the local problem or implement an effective mitigation solution, the corrosion of all metal will continue to be excessive.

Going from a pump motor with an aluminum case to a motor with a steel case will slow the motor corrosion, but it will probably not stop the corrosion.

So, you need to solve the local issue or use some sort of mitigation like an anode, which can be powered or unpowered.

Anodes, including impressed current cathodic protection, can be very effective if done correctly.

 

[mas985]

Most references say that both metals need to be immersed in the same electrolyte to create a conductive path, but this is not accurate in my opinion.

Hmm. That has not been my experience. Most just show the presence of an electrolyte. But the point here is that you need ions in order to dissolve the metals. Pure de-ionized water does not have ions so no reaction can occur. However, that water will not remain de-ionised very long in the open environment.

The conductive path is the copper bond wire.

The cup anchor is not immersed in the same electrolyte as the other metal.

I believe it is. It is the bond wire in the cup usually connected at the bottom of the cup which is copper and both would be in the ion solution at the same time.

But in each of these pictures, it show an electrolyte as being necessary for the reaction to occur so I think this proves my point.

Also, the previous picture with "moist soil" is also an electrolyte because it contains ions.

 

[JamesW]

The anode simply needs to get wet to dissolve the positively charged aluminum ions into solution.

Even perfectly distilled deionized water will work.

It does not require that both the anode and cathode be immersed in the same electrolyte for corrosion to occur even though this is what the drawings imply.

All water from any source like rain or humidity also has negatively charged ions, which can neutralize the positively charged aluminum ions.

The oxidation is caused by the oxygen or other chemicals that are contacting the metals anywhere in the grid or some sort of electrical voltage causing the bonding grid to lose electrons.

The iron in the steel rebar in the concrete might be exposed to oxidation, but the electrons come from the aluminum through the bond wire causing the aluminum to corrode and protect the steel.

This is why aluminum is used as a sacrificial anode.

 

[mas985]

I agree, there are definitely are other forms of corrosion. All metals will corrode to some extent without contact to other metals. That was not my point. My only point was that galvanic corrosion requires an electrolyte and that electrolyte can be a thin film of water with a salt.

 

[JamesW]

My only point was that galvanic corrosion requires an electrolyte and that electrolyte can be a thin film of water with a salt.

Why is an electrolyte required?

 

[mas985]

Galvanic corrosion (also called bimetallic corrosion or dissimilar metal corrosion) is an electrochemical process in which one metal corrodes preferentially when it is in electrical contact with another, in the presence of an electrolyte.

It is definitional. But practically, it is necessary for the flow of current. It completes the circuit. And it is the ions which react with the metal causing it to degrade much like what happens in a SWG after the coatings wear off.

Galvanic corrosion - Wikipedia

en.wikipedia.org

 

[JamesW]

How is the motor immersed in an electrolyte?

Even if it gets wet, it is not immersed in the same electrolyte as the rest of the metals on the bonding grid.

 

[mas985]

I thought we covered this already. The motor does not need to be totally "immersed" for galvanic corrosion to occur. Water just needs to be in simultaneous contact with dissimilar metals to bridge the connection and then current will flow. Again, think of this picture:

The electrolyte can be a very thin coating. Just sufficient for ion current flow. It doesn't take much for it to become conductive. There can also be a different metal between the anode and cathode as long as it is conductive for electron flow. It is the ions in the electrolyte that causes the damage to the metal.

This is also possible in your cup example. Usually the cup will be bonded to a copper wire at the bottom of the cup and to a point that is most likely still exposed to the environment through the opening of the cup so it may be possible for both the cup and the copper wire to be wet at the same time and in the same general location. That is all that is needed for galvanic corrosion to occur. It doesn't matter what the other end of the bond is connected to. In fact, you could just have a very short piece of copper connected to the cup and galvanic corrosion could still occur.

Also, to be very clear, I am not saying that motor failure is definitely due to galvanic corrosion, I am just saying it cannot be ruled out. As I pointed out earlier, the corrosion appears to be occurring over the portion of the motor connected to the wet end which could indicate the aluminum is simply oxidizing from getting wet although the paint should have prevent it. However, if the paint coating was not applied correctly, the aluminum might still oxidize.

But presence of water is necessary for both of these mechanisms to occur so keeping the motor dry would help in both cases.

 

[JamesW]

I suppose that the electrolyte make sense to balance the charge otherwise you would get a buildup of positive charge on or around the anode and a buildup of negative charge around the cathode and this would prevent further movement of electrons.

In any case, there seems to be excessive corrosion due to environmental factors.

Many people have Intelliflo pumps and, while there are many reports of corrosion, most of those seems to be due to a seal leak.

The fact that the motor is corroding so fast points to a local issue.

The excessive corrosion on other metals is also evident in the pictures.

Unless the local cause can be found and remedied, then the problem will likely continue.

Maybe anodes would help slow the corrosion.

An impressed current cathodic protection system would probably help, but that needs to be professionally done to make sure that it is correct and safe.

You can get “overprotection” which can cause problems.

Changing to a steel casing would help, but I would be concerned about all of the metal that is corroding at an accelerated rate.

The heater and galvanized gas pipe will fail prematurely as will all other metal like rebar etc.

If you can’t find the cause of the problem, I would look to mitigation strategies like ICCP or a bunch of magnesium, zinc or aluminum anodes.

 

[mas985]

Could it be simple paint failure. This looks familiar:

Update: Major types of automotive aluminum corrosion

As the aluminum F-150 generation enters its fourth year on the market and its two top truck competitors Silverado and 1500 join it with increased aluminum content, repairers should familiarize them…

www.repairerdrivennews.com

 

[JamesW]

All metal is corroding and the motor has failed 3 times.

In my opinion, there is an electrical problem.