Corrosion in Ally boats.

[Paikea]

Some time ago I wrote the following article on minimising corrosion in Aluminium boats. It was not published however I thought that it might be of interest to some raiders.

Combatting Corrosion in Aluminium Boats

By; Paikea

Aluminium boats, particularly those fitted with electrical systems often show signs of corrosion after only a few seasons use. Typical signs are paintwork “bubbling” around fittings such as stainless steel rod holders, and any fittings fastened to the boat using stainless steel bolts or screws. Occasionally we see welded on fittings such as bollards welded directly to the aluminium sheet or plate gunwale showing similar paint “bubbling”. In time the paint flakes off around the affected fittings and the aluminium exposed by this process shows clear signs of advancing corrosion. This normally presents as a dirty white, powdery surface accompanied by “pitting” of the surface. Not a pretty sight to anyone who cares for the appearance of their boats.

Experts tell us that the culprit for this type of corrosion is electrolysis caused by dissimilar metals (typically the boats aluminium and fixtures or fitments made from a different metal) creating a mild electrical current when in the presence of an electrolyte (electricity conducting medium) such as salt water. The electrical current then causes the decomposition (corrosion) of the least “noble” of the two metals which in the case of aluminium boats is the hull of the boat at the point where the more “noble” metals come into contact with it.

The generation of electricity via chemical reaction (galvanic action) is a complex subject and even experts in the subject admit to difficulty in predicting the extent of it. However most appear to agree that the problem can be minimised if not completely prevented by insulating the dissimilar materials from each other and where possible sealing the area against contact with salt water. Aluminium boat manufacturers try to do this by using a marine sealant such as Sikaflex to generously coat the engine mounting bolts where they pass through the boat’s aluminium transom coupled with large aluminium washers under the nuts on the inside of the transom also generously coated with Sikaflex or similar product. (Not silicone based sealants which can cause corrosion from the medium used to cure the product). Unfortunately Sikaflex is messy to use and as such it is not really practical to apply it to every fitting or fixture. Instead some manufacturers use a brush on insulating medium to liberally coat screws and bolts holding fixtures in place as they are being installed. All good as long as the product is carefully applied in the first place and remains intact over the life of the boat, an unlikely scenario. There is also a limit to what manufacturers can do to completely insulate/seal all fixtures and fittings and still remain competitive in the marketplace.

Of course there is nothing to stop boat owner’s themselves insulating/sealing their boats fixtures so as to minimise corrosion from electrolysis. I did that for my last ally boat, a 5.10 M Trailcraft sportfish Centre Console that I bought in 2009. (since sold and bought a Tournament 1800)

Via the internet I located a supplier of insulating washers made from hard nylon. (see Pic below) These washers come in a wide range of sizes in terms of their internal diameter and length of shoulder. A shoulder depth of 3mm was ideal for my boat which (apart from the bottom) is made from 3 mm thick plate. I figured that fitting these washers to every fixture requiring fastening using stainless steel screws or bolts would be a good start to preventing corrosion. The supplier’s catalogue also listed a range of hard nylon bolts and nuts that could readily be used to fasten fixtures that do not need the strength of metal bolts or screws. In my boat these included the Blue Seas fuse board that I fitted (see Pic), flush mounted VHF radio, recessed EPIRB holder, switch panel, instrument panel, microphone clip, compass base, navigation lights, cable clamps etc. Using nylon bolts for these fittings removed the source of electrolysis at that point.

I started by removing the plastic rod holders which were fastened to the gunwale using 3/16 inch stainless steel metal thread screws tapped into the gunwale plate. From my observations the area around rod holders (both plastic and stainless holders) often shows corrosion after only a few seasons use. There were no gaskets fitted between the rod holders and the gunwales and the area under the flanges of the rod holders held a lot of dust which proved to me that salt water could readily find its way under the flange of the rod holder and the aluminium gunwale and from there between the stainless screws and the aluminium plate starting electrolysis and subsequent corrosion. I enlarged the screw holes in the gunwale to accommodate the shoulder of the insulating washers, made a gasket from sheet rubber purchased from Clark Rubber and reinstalled the rod holders. I then continued and installed insulating washers to all fixtures that required stainless screws or bolts for their structural strength and where practical applied Sikaflex or rubber gaskets between metal parts such as the brackets holding the bait board and the transom.

Did I totally prevent corrosion in the Trailcraft? There was no sign of any after three years however only time and the future owners of my boat will know for sure. I am pretty confident that insulating and sealing the dissimilar materials will prevent corrosion for the life of the boat.

Stepped nylon washer insulating the Stainless from the aluminium. Nylon washers can be fitted under round headed screws.

Selection of nylon screws, nuts, washers etc. Cheap as chips and do the job.

Blue Seas fuse panel attached using nylon screws.

I hope the above might be of interest/help to some owners of ally boats

Cheers

Paikea

Edited July 30, 2013 by Paikea

[Catchin Jack]

What a top read mate, thanks for putting that up. I'm sure it'll help others on here.

[leonardgid]

thank you for all the advice.

[Guest Force3]

Hi Paikea,

Great article.

Could you advise the supplier for the nylon washers please.

Cheers

Adrian

[fisherman666]

This is a very good post. It proves wrong the very famous saying "she'll be right mate"

[squidboy]

Excellent post, if Raiders can't find those plugs I have my boats bolts all heat shrunk so no stainless touches the aluminum.

[Paikea]

Thank you for your kind words re this post. I am glad that the topic has created some interest.

Sorry that I have taken so long to respond to the request for details of the nylon parts supplier. I have been helping daughter and son in law to add a new laundry to their home near Byron, a tough place to spend a couple of weeks.

The company is Supreme Fasteners located at Belrose. Their web site is www.supfast.com.au/

Ask them to send you a catalogue (or you may be able to download a copy) and then use the catalogue to determine the part numbers of whatever you are after. As they normally supply bulk quantities of their products they are not very keen to either post or over the counter sales of small quantities. I found the best way to deal with them was to visit their warehouse in Belrose, be nice to the owner/manager and pay cash for whatever you need.

The nylon parts are A1 and I cannot recommend them highly enough as a means of preventing corrosion in ally boats.

Cheers

Paikea‎