Damage from Metal/Aluminum Contact

[Jerry_NJ]

Attached are three pictures of one of two handles on the rear of my 1995 Discovery 14' Aluminum fishing boat. These were on the boat when I purchased it about a year ago.

The metal of the handle looks to be a cheap base metal core with a platting that is eroding away. It is clear also that some damage to the aluminum skin has resulted, but I think it is cosmetic, not pictured, I have scraped the area and don't find any deep penetration of the aluminum.

I plan to replace the handles with stainless steel, or perhaps on I use a quality brass (or similar) mounted on stainless steel washers to buffer between the handle and the aluminum - anyone try this before?

Is there any chemical treatment I should apply to the aluminum to assure the effects of the previous handle isn't still causing erosion? If I repaint, I will appreciate any recommendations on primer and paint... I prefer oil base paints on metal but suppose there are good latex paints designed specifically for aluminum.

 

[Willyclay]

Re: Damage from Metal/Aluminum Contact

My 1966 Lone Star had nylon washers/backing under the heads of all the SS bolts and deck hardware. By 2006, there was lots of corrosion except where the factory had provided this protection for the aluminum. Maybe worth considering. Good luck!

 

[Grandad]

Re: Damage from Metal/Aluminum Contact

I think you would be wise to replace the handles with aluminum, not stainless or brass. Many aluminum boats do have stainless steel fittings (as mine does), but as long as the parts are not continuously wet, there's normally not a problem. You have already experienced corrosion that looks like it may have been accelerated by dissimilar metal contact, so I'd replace with aluminum handles.
See example Mighty Grip Transom Handles - Springfield 1840054 - iboats
When you mount the handles, use a bedding compound between the handles and the transom, not to try to keep the parts separated, but to ensure that no water gets trapped between the parts. Trapped water will turn corrosive, even between similar metals. I like butyl tape for bedding parts, but it's not available everywhere. I don't see an Iboats listing, but Sailrite has it.
Trimmable Butyl Tape White 3/4" - Bedding Deck Hardware Sealant
I'm not aware of a chemical treatment. I'd sand with aluminum oxide sandpaper and paint with Rustoleum. - Grandad

 

[Frank Acampora]

Re: Damage from Metal/Aluminum Contact

Those handles are cheap chrome plated white metal. White metal is an alloy of zinc and several other base metals. It is easy to cast parts from this and relatively inexpensive, even after chrome plating. This type hardware is used on many lower priced boats. The problem is simply that it doesn't last. The chrome plating is thin and it starts to pit relatively quickly. Then, the base metal starts to go. This is a very common problem.

When you change to stainless steel, there will be some galvanic reaction between the stainless and the aluminum. To help prevent this, isolate the two metals with a plastic washer or bet the stainless in 5200 or silicone RTV. Be certain the holes for the bolts are slightly oversized ( to minimize aluminum contact) and be certain the bolts are coated well with a barrier compound. Your boat should outlive you. Much better would be to find some good, strong, aluminum handles as mentioned above, held with high quality aluminum bolts..

 

[minuteman62-64]

Re: Damage from Metal/Aluminum Contact

Cast aluminum handle - plenty strong enough. Aluminum bolts? I'd go with SS. Most of the aluminum boats I see in San Diego Bay (including mine) use SS for fastenings.

 

[minuteman62-64]

Re: Damage from Metal/Aluminum Contact

Have to hedge my comment a bit. A couple of searches indicate high strength aluminum fasteners approach the strength of mild steel. I'd still be more comfortable with SS based on what I see on other aluminum boats.

 

[dingbat]

Re: Damage from Metal/Aluminum Contact

First off, that handle is not aluminum. It's chrome plated Zamak. Zamak is primarily zinc alloyed with aluminum, magnesium and copper. Zinc is highly reactive material. Commonly used as sacrificial anodes to protect lower units. Exposed to an electrolyte, the interaction of the zinc, aluminum, magnesium and copper alone is enough to cause your problem. You can insulate, isolate, paint, plate to your hearts content. You can not eliminate/ mitigate the problem until you remove the Zamak from the picture.

Do yourself a favor and buy a 316 stainless steel replacement. Attach it to your boat with 316 SS hardware and "Bed" the handle w/ silicon or something similar to prevent moisture from getting trapped between the handle the hull. Once done, you'll be good to go. Don't use big box store hardware. The 303/304 hardware they sell has a high carbon content and will "stain" when exposed to salt water.

FWIW: Galvanic corrosion is an interaction between elements. Steel, aluminum and stainless steels are not elements, they are generic terms for large groups of alloyed metals. To determine a particular alloy's suitably for an application you have to look at chemistry (elements) that make up the alloy.

Some stainless plays well with aluminum. Some don't because the alloy contains elements that are highly reactive to aluminum. Simply saying use stainless steel is meaningless w/o identifying what alloy to use

 

[Thalasso]

Re: Damage from Metal/Aluminum Contact

As Frank Acampora said. You have dissimilar metals. You need to isolate one from the other. It can be anything from a nylon washer to a gasket as long as the don't come in contact with each other.If you boat in saltwater the water becomes the electrolyte. Make sure you rinse off the boat when done.

 

[magster65]

Re: Damage from Metal/Aluminum Contact

I didn't see any stainless ones in the iboats parts section so hopefully I'm not doing anything wrong by mentioning "Marine Part Depot". I've dealt with them a few times now and am 100% happy.

 

[dingbat]

Re: Damage from Metal/Aluminum Contact

.....

 

[dingbat]

Re: Damage from Metal/Aluminum Contact

double post