Science project

[fitz73222]

Well fellas, I'm stumped on this one... This is an interesting corrosion discussion. My flats boat has something going with respect to a corrosion issue. Now some background. Several months ago, I decided to clean up some nicks and scratches and a little surface rust on my Motorguide great white bow mount trolling motor. The motor housing is steel and the nose cone and rear cap are aluminum and I'm assuming was painted with some sort of Ecoat or powder coat originally and has held up very well for 15 years in saltwater, so I do a light sanding on the steel midsection of the motor housing to remove some slight surface rust bubbling, prime with zinc cromate and top coat with some OMC white laquer I had in stock. Waited a couple of weeks to make sure the paint cured correctly and toss the boat in and go fish. So after about 5 hours on the flats I lift the trolling motor to stow it and see the stainless shaft is loaded with pits from what looks like galvanic activity and the shaft zinc is untouched. I also notice the motor housing I painted looks like it's bleeding rust from where the aluminum nose cone and prop end cap meet the steel housing.

So of course WTF comes to mind! I painted this thing with marine friendly, inert primer and paint made for exactly what I'm doing. So when I pull the boat out another discovery; the scratches on the outboard gearcase, my stainless prop and jackplate are covered with raised iron deposits! Never seen alumimum with rust on it before. So I get home, wash all this rust off and start reading my resources about galvanic corrosion, I definatley have some electron exchange going on between the trolling motor and the outboard/jackplate. So I check all the ground paths, perfect continuity, diassemble the trolling motor control head, looking for a positive touching the stainless trolling motor shaft to see if I'm lighting up the shaft and causing a current path, nothing! Check all the outboard grounds, nothing. The only observation is about 300 millivolts being generated between the trolling motor steel housing and the stainless shaft with the trolling motor running, which I assume would be normal with any electro magnetic fields. But with all this going on, my zincs on the trolling motor and the outboard are untouched. They are clean and bright. What's weird is that it seems that the cathode and anode relationships have now reversed and the alumimum and stainless are being attacked and the zincs are untouched. I can only assume at this point that I have some kind of mild short in the trolling motor housing that's causing this, but all I did was touch this thing up and paint it. Maybe Motorguide uses some kind of insulating paint or primer to block electromagnetic activity when dipped in saltwater; not sure. So short of dropping $600 on a new trolling motor I would like to figure this out since I sure don't want my boat turning into plop plop fizz fizz everytime I fishing. Watcha think?

[RickK]

Interesting  
Maybe time to contact Motorguide?

[seabob4]

Obviously a toughie, Farley.  While the evidence is apparent on the TM and the O/B and JP, it entirely possible the issue is elsewhere in the boat, and chances are is coincidental with the paint touchup you performed.  Remember, everything on your boat shares a common ground, from the VHF to the NAV lights to the O/B to the TM...and thus there is always a path for ground back to the batt, from whatever appliance is the affected one.  A "bad" ground from any appliance can sometimes take the positive side of the 12V circle to find it's way back to the batt.  It's weird, but it happens.  

What I would start doing is checking continuity between all the 12V+ leads on your boat and the known good ground.  Chances are you'll find continuity somewhere between hot and ground...which could be the path that electricity is flowing through that is attacking the more noble elements in the system, instead of the other way around.

[fitz73222]

Obviously a toughie, Farley.  While the evidence is apparent on the TM and the O/B and JP, it entirely possible the issue is elsewhere in the boat, and chances are is coincidental with the paint touchup you performed.  Remember, everything on your boat shares a common ground, from the VHF to the NAV lights to the O/B to the TM...and thus there is always a path for ground back to the batt, from whatever appliance is the affected one.  A "bad" ground from any appliance can sometimes take the positive side of the 12V circle to find it's way back to the batt.  It's weird, but it happens.  

What I would start doing is checking continuity between all the 12V+ leads on your boat and the known good ground.  Chances are you'll find continuity somewhere between hot and ground...which could be the path that electricity is flowing through that is attacking the more noble elements in the system, instead of the other way around.

Thanks for the insight Bob, any relevance with me reading 300 millivolts between the TM case and shaft when it's on; but the shaft is not grounded that I can tell? Would this be considered enough stray current to cause this kind of accelerated electrolysis? Why aren't the zincs taking a pounding from this? All systems are working normally with one exception, my 757 Humminbird is having a hard time finding bottom while underway at idle or on plane. I cleaned the transducer face with alcohol and the transducer alignment is good, there are no visible breaks in the transducer cable but it is the third element that is underwater.

[seabob4]

Farley, remember, voltage is the measurement of potential energy.  In other words, the presence of voltage, albeit small, means that you have basically and anode and a cathode set up where one wasn't intended to be.  It would be my guess that the shaft and the housing either should be continuous, electrically, such that the potential is 0, or conversely, completely isolated such that there will never be continuity, thus again, the potential is 0..  I read a post where it said voltage in the 10s of millivolts wasn't a concern, however, in the hundreds of mV?  Definitely a concern.

[fitz73222]

Farley, remember, voltage is the measurement of potential energy.  In other words, the presence of voltage, albeit small, means that you have basically and anode and a cathode set up where one wasn't intended to be.  It would be my guess that the shaft and the housing either should be continuous, electrically, such that the potential is 0, or conversely, completely isolated such that there will never be continuity, thus again, the potential is 0..  I read a post where it said voltage in the 10s of millivolts wasn't a concern, however, in the hundreds of mV?  Definitely a concern.

Ok, first I'll re-check my millivolt reading to confirm. Next, I'll look for continuity between positive and ground (power off of course) in all the circuts. My thinking is that I'm emitting a positive current flow from the TM and the iron spew from the steel TM anode, or in this case is a cathode (because of positive voltage?) housing is collecting on the well grounded outboard and jackplate although non ferrous material? I don't understand how how electrons collect on something that they're not compatible with? Is it like meeting a smokin hot chick in a bar with a positive polarity and you have a positive polarity and you pour enough alcohol and whit into her to where she changes polarity and becomes passive, lowering resistance?

[gran398]

Farl, weird problem. I have to agree though, some type of partial short.

I can relate however to the path of least resistance

[fitz73222]

Farl, weird problem. I have to agree though, some type of partial short.

I can relate however to the path of least resistance

I knew you would get this Scotty.... Thanks for all you do.

[gran398]

Thanks my bro. I try nowadays to input where I have knowledge. Otherwise, I lurk.

This is interesting. Looking forward to what we find

[seabob4]

We have the presense of electricity, and we have the electrolyte (the conductor) through which it flows.  So we have the basics for a chemical reaction...which is basically what your seeing, Farley.  If you have materials in the "equation" that are un-balenced, with say a valence of +1, +2, and you introduce into said equation materials that have a valence of -1, -2, there is a very good chance they will combine, given the right electrolyte, typically a salt.  Think Chlorides, suifides, halides, etc...

[dburr]

Under the correct circumstances just about anything can be plated, copper on plastic is a good example in the correct "bath" as the Old Man used to put it.. Voltage is not as big a deal as the current flow, if nothing jumps at you Farley with a straight up ohmmeter, do you have access to a megger?  This one may be a PIA and the current flow may be low enough that you will not see it with a regular meter.. Where there any breaks in the paint before the touch up?  Unless there is some oddball reaction with the paint and there should not be, the timing of the touch up may be a red herring..

I don't mean to sound like a dink but better you then me     !

[Capt. Bob]

Obviously a toughie, Farley.  While the evidence is apparent on the TM and the O/B and JP, it entirely possible the issue is elsewhere in the boat, and chances are is coincidental with the paint touchup you performed.  Remember, everything on your boat shares a common ground, from the VHF to the NAV lights to the O/B to the TM...and thus there is always a path for ground back to the batt, from whatever appliance is the affected one.  A "bad" ground from any appliance can sometimes take the positive side of the 12V circle to find it's way back to the batt.  It's weird, but it happens.  

What I would start doing is checking continuity between all the 12V+ leads on your boat and the known good ground.  Chances are you'll find continuity somewhere between hot and ground...which could be the path that electricity is flowing through that is attacking the more noble elements in the system, instead of the other way around.

Thanks for the insight Bob, any relevance with me reading 300 millivolts between the TM case and shaft when it's on; but the shaft is not grounded that I can tell? Would this be considered enough stray current to cause this kind of accelerated electrolysis? Why aren't the zincs taking a pounding from this? All systems are working normally with one exception, my 757 Humminbird is having a hard time finding bottom while underway at idle or on plane. I cleaned the transducer face with alcohol and the transducer alignment is good, there are no visible breaks in the transducer cable but it is the third element that is underwater.

Coincidence???

Gleaned from a MotorGuide manual.

Establishing a Common Ground: (SB)

MotorGuide recommends isolating the trolling motor
battery/batteries from
the main engine battery.

Electrolysis Issues
– Using the engine starting battery as
a source of power for any trolling motor may cause
electrolysis on metallic parts.

If you have followed the battery wiring and
installation instructions in this manual and your
boat continues to have electrolysis issues, you
will need to separate the trolling motor from any
other boat electronics.

Remove the engine starting battery from the
wiring configuration of
the boat and isolate the
power circuit for your trolling motor.
•
Establish a Common Ground:
Common ground
means the ground for the main engine accessories
and your trolling motor are connected to the same
negative ground terminal.
•
Not having a common ground can cause severe
corrosion or electrolysis. If left unchecked, damage
may be caused to your trolling motor and boat.
•
Establishing a common ground connection will allow
increased sensitivity and improve detail on a sonar
display. (Could this be the culprit SB refers to?)

Seems you're both thinking on the same lines as the above.

Can this be related to each other in some strange way?

[fitz73222]

What's strange Bob is that this set up is 15 years old. All the grounds are tied together, and I've always used a combination battery for engine starting and the trolling motor. This is the third flats boat I've done this way. Everything is working normally, Scotty will tell you, this is a garaged, eat off of it clean boat that looks like new so I don't have a corrosion issue until this popped up. But that is some good reading and somewhere between what you posted and Bob C is the solution.

[Capt. Bob]

Fitz,
The thing I find "different" is how quickly it happened as opposed to what we normally experience with corrosion (5 hrs.)  

This is indeed interesting.

[fitz73222]

For sure, It's not going back in the water till I figure it out! I'll re-group and start more testing after work tonight.