Zinc location 1987 170oc

[chrissy t]

Hi everyone. I am having trouble keeping an aluminum gas tank in my 1987 aquasport 170 for more than 7 years.Thinking now it might be an electrolysis problem. After some looking I noticed there is no zinc on the boat. Does anyone have one installed on the transom? If so where is it located,near the drain plug? Thanks for any responses, Rob

[Capt. Bob]

Sacrificial zincs are used on outboards but I never seen one on the transom to protect the tank. Are you keeping the tank dry? Water trapped against it is the biggest cause of corrosion we've seen over the years.

[boatnamesue]

A bit confused....what do you mean "keeping" an aluminum gas tank?  Your below deck fuel tank?  If an aluminum tank is installed properly it will last 20+ years and does not require zinc. 

[chrissy t]

It is a built in tank that is mounted under the deck below the console. This is the 2nd tank I have had to replace in 12 years. Both tanks had pin holes in the top of the tank. The welder who made the tank said I have an electrolysis problem because there are no zincs on the boat and the aluminum tank is the weak point.I don't know what is properly installed,the tank was banded with stainless straps in the cradle and properly grounded to the filler and the ground system.Please let me know if something is missing.Thanks,Rob

[mshugg]

You gotta keep salt water away from the tank.  does your 170 still have the tank hatch and rigging trough cover.  These are both points of entry for water.  Make sure that these areas are well sealed and the screws too.

What are you using to isolate the ss straps from the aluminum? Stainless and black rubber (containing carbon)   can both cause galvanic corrosion when in contact with aluminum.  You might consider using the Pascoe method for installing your next tank.  https://www.yachtsurvey.com/fueltank.htm  It eliminates areas that water can remain in contact with the tank.  The only downside is that 5200 will make it a bear to replace the next tank.  I did that on my rebuild, and installed pie inspection ports under my console and leaning post, so I can periodically rinse the tank with fresh water.

Another option would be to coat your tank with coal tar epoxy or prime and paint. 

Another option

[Capt. Bob]

Both tanks had pin holes in the top of the tank.

Interesting.
Is it possible for you to post a photo or two of these pinhole locations?

Thanks.

[chrissy t]

The tank had rubber straps about 3/16 in thick and 2 in wide between the tank and stainless. Unfortunately the tank is at the welders getting the replacement made. The holes seem to be where the inspection plate is on the deck above the tank where the filler neck ,vent tube  and sender are located on the tank.Another thought is the battery sits in the console not to far from the inspection plate and maybe battery acid could be dripping on the tank even though it is in a battery box.Good article about proper installation which I will follow on the install. Thanks again for all the suggestions and help, Rob

[Capt. Bob]

Keeping it dry is IMHO the most important thing for aluminum tank longevity. This isn't meant to imply it can't get wet. If you live in an area with high humidity coupled with hot days and cool nights, you will get tank sweat from condensation. The key is to allow the tank surface to dry. Aluminum, when exposed to air forms an oxide on the surface that is fairly protective from corrosion. This natural coating only works if the surface is allowed to dry. We usually see corrosion forming under the tank straps or supports where water has become trapped against the tank and does not dry over time. Salt or fresh, if it remains in contact, you can expect pinhole corrosion.

Keep the tank as dry as possible and allow air to circulate whenever not in use. I keep my boat under a tarp and open the inspection plates to allow air to enter the coffin. The tank surface has the oxidation that you can rub off (to a degree) but has no visible or felt pitting/corrosion. I have no doubt that some (very little really) might exist underneath after 9 years but nothing to worry about because I keep the coffin as dry as possible.

Good luck.  

[boatnamesue]

Read this thread: https://classicaquasport.com/smf/index.php?topic=14273.0

Details my installation of 170 fuel tank using modified pascoe method.  The principal of the method is to elevate the tank off the coffin to allow water to free flow thru coffin and into bilge.  Pin holes in your tank is caused by crevice corrosion, which is caused by stagnant water contacting tank.  Using foam to stabilize the tank is a sure thing for corrosion.  There is NO way to keep water out of the coffin, despite what you may or may not read.  Condensation is absolute, especially if you're located in FL.  All saltwater outboards have an anode, usually mounted on lower bracket to deter electrolysis.   

[chrissy t]

the problem with my corrosion was on the top of the tank. No problem with bottom or sides. I looked at my motor again and there are zincs on the lower unit which had no indication of electrolysis.I think in the future I will open the inspection plate to allow fresh air in there more often.Thanks for all the replies and hope I can help someone in the future, Rob