What is causing this ?

[GregF]

It sure sounds like a defect in the anodizing on the toons or just a poor choice of alloy.
Aluminum gets it corrosion resistance by controlled corrosion. An oxide layer forms and stops further corrosion. Something is not happening right here

[fwood]

Looks like electrolysis. Generator on the boat? Is the boat bonded to the electrical system of the motor? Good idea to install the anodes for the boat IMO. I would try magnesium anodes as well as zinc. I would also have the lake water checked as mentioned before. Are the pontoons anodized?

[fredeboy]

Hope it is not Zebra Mussel droppings!!

[GregF]

I am wondering if these are even anodized. If you really want a mirror finish, it won't be anodized. That is usually a dull finish like you have on the rails.

[Bamaman]

Large sheets of aluminum are not anodized, but the railings usually are.

When using muriatic acid based pontoon cleaners or "coil cleaner", you've got to be sure to get the product on the toons, but not the square anodized tubing. It'll ruin the anodized sections.

You'll often feel some roughness to the aluminum, which is most often tig welding spatters. It can be smoothed out with sanding/polishing--but probably not worth the effort on a new boat.

[woolznaz]

In an earlier post in this thread the OP clarified they are not pits, but spots. Does that rule out electrolysis? That makes it sound more like something getting on the toons rather than the metal being broken down. Am I off base?

[GregF]

It sure sounds like it is just something in the water reacting with the aluminum. It is above and below the water line.
Rinsing the toons off and drying them might help this.

[Ron Burgundy]

It looks like hard water/salt water spots to me, but he said that CLR and Lime Away didn't remove any of it.

[slingshot]

It's electrolysis and you have a short in your electrical system that is grounding out to your boat and it's toons. The electrolysis is only happening on the area of the pontoons that are in contact with the water as you can see in the photo and is occurring only when you are in the water. Your safe when on the trailer because it's not grounded due to rubber tires. You shouldn't have to use zincs in fresh water rather, you should go about the task of finding the short using a voltmeter. Use your voltmeter by touching the leads to known negative such as your batteries "negative" terminal and the other to your toon. I'm sure you'll get volts. Then start going through your system by disconnecting accessories and gauge panel one by one checking to see if you still have volts until you find the culprit. I've had to do this twice with other boats, once the culprit was worn insulation causing the alternator wire going to batteries to ground out to a rail. The other was a pinched wire in the trim switch. It's a pain but you can find it.

[Redneck_Randy]

slingshot,

It's my understanding that electrolysis only happens below the water line. What ever is going with our toon is happening above and below the water line.

[slingshot]

The "waterline" is only the line where the water come to at rest. While underway water will go higher (constant splashing) enabling electrolysis to occur just above the waterline. As you can see in the photo it tapers off the higher you go. You can be sure by simply testing with a voltmeter ($19 from home depot). With your battery on if you use a switch and your boat on the trailer, put the black lead of the volt meter to your negative battery terminal and put the positive lead of the voltmeter to your pontoon. If you get volts then it will confirm electrolysis.

[GregF]

If you do that with a digital meter you will always get "voltage" unless your boat is bonded to the negative battery terminal. You can hold digital meter leads up in the air and get a random number generator. It is the antenna effect (aka phantom voltage or any other number of names).
Bonding the boat can cause electrolysis if you connect to anything else. (shore tie, battery charger etc)
You need a current path to have electrolysis.

[slingshot]

Hi Greg: I've tested mine and I get "0". I got 8 to 10 volt until I found my problem which was the alternator wire grounding out on a rail due to a pinch that wore through the wires insulation. It's my understanding that unlike a car, Boats are not grounded out to the chassis or hull rather it's an "isolated system" or "closed system". That's due to boats being submerged in conductive material, water or worse, salt water. Cars are not due to rubber tires.

[Bamby]

Hi Greg: I've tested mine and I get "0". I got 8 to 10 volt until I found my problem which was the alternator wire grounding out on a rail due to a pinch that wore through the wires insulation. It's my understanding that unlike a car, Boats are not grounded out to the chassis or hull rather it's an "isolated system" or "closed system". That's due to boats being submerged in conductive material, water or worse, salt water. Cars are not due to rubber tires.

Mostly but not entirely "true". Yes the body of the boat is not utilized for ground returns in a boat, but their is one exception to this rule... The engine itself is grounded by a ground lug from the negative side of the battery. Since motor grounding is universal in application the motor can set up a path for any electrical leakage back ground, but this should trip or break a fuse in the circuit should a short occur.

I'm also curious as to what is creating Redneck_Randy's problem but I'm having a hard time accepting electrolysis as the issue in this case also. I'm thinking it would take some real serious power for water splashing etching his pontoons above the general waterline itself.

[STEVEBRENDA]

Are we to assume it happened to the other toon as well?