Alumimium corrosion - tinnie

[LWB123]

Hello from Brisbane,

I have a Savage Ranger runabout which has until this past couple of years mainly been used in freshwater situations. It is now largely used on the Brisbane River for water skiing with the kids. This part of the river is quite salty and the boat get a thorough wash down and WD40'ing etc after it gets home each time.

I have noticed a few small corrosion blisters starting to form under the paint around the tonneau studs and ladder attachment points, and also in the odd place - mostly near the gunnels - where there is nothing actually attached to the hull. I am assuming that the former is likely to be due to the unlike metals coming together (aluminium, brass, stainless steel), but not quite sure about the latter.

For sure, while no expert in metalurgy, I knew that aluminium does corrode and especially when antagonistic materials come together - we own Land Rovers! My issue is what to do about it from hereon? What I have been doing for preventative maintenance does not seem to be doing the trick - assuming there is a trick, of course.

I guess there will be no shortage of tinnie owners out there with some useful advice?

While on the topic of Savage - genuinely a great boat! However, seeking out service from Savage (or Mercury) is not so great - I have been trying to get some bright white touch up paint to repair the odd scratch and ping mark on the painted hull. Try as I might to get the right stuff through the Savage dealer network (including ringing the company itself), I keep getting a dull cream colour passed off to me as the genuine article -`Savage White'. Well my eyesight might not be too great, but Blind Pugh can see that dull cream aint bright white - and it stands out like the proverbial canine jewels. Does not seem that offering up the model number, hull serial number or year of manufacturer has any impression on the Savage parts crew (they supplied the wrong colour back seats despite being sent photos of the front ones). One dealer did suggest a Wattyl code - but unfortunately the minimum quantity of purchase would have covered a couple of Oberons with paint to spare - also was at general odds with my wish to keep the cost down. I also tried a couple of Quintrex dealers to see if they could help with the small touch up tubes you used to get when you buy a new boat - they all look about the same bright white colour with any variation likely to be be closer than Savage `Cream' - not much luck there either. Any suggestions?

Cheers,

[Rayngie]

Ask a boat shop where to get sacrifical anodes, should do the trick, they attatch to the boat and corrode themselves rather than the boat...no idea how, they just do.

[LWB123]

Thanks for that advice - it had sort of crossed my mind to explore that option, but never got past that stage. I know they do that on the motors, lobster pots etc.

Was cleaning to boat up again yesterday afternoon after a stint on the river and it was on my mind I suppose. The spots individually are still quite smal, and none have ruptured through the paint yet.

Anyway, I will check it up with the local dealer when he opens again after New Year.

Somehow, I had managed to get it in my mind that stainless steel and aluminium are less of a problem than aluminium and lead or brass. However, there is a small spot starting to form in the aluminium panel around one of the ski rope attachments that I had understood to be stainless steel at the time that I fitted them - ex BCF. I was sure that ithey were not plated brass?

Cheers,

[lro11]

When Northside Marine set up my Sportfish they used Duralac on all the stainless screws this stopped the electrolysis. make sure when you buy new zinc anodes for the outboard you buy the purest zinc you can get, people complain that they get eaten away too quickly, better that than the outboard. Also check the logbook in case there are some inside the powerhead and replace them when they are two thirds gone, don't wait until they are all gone.

[LWB123]

Thanks for that advice - genuinely appreciated.

I will follow up on the Duralac option (also the anodes) - most of the problem spots are around tonneau buttons held in place with small stainless steel screws.

These would have been fitted as an aftermarket option by the previous owner and possibly were not treated at the time - maybe 7-8 years ago.

Not too late to nip the problem in the bud.

Cheers,

[Roverray]

You need to make sure the studs and fittings are high quality 316 stainless , if you wire brush use a stainless brush only. if you sand or grind use proper discs for alloy, any contamination will cause corrosion.
some fitting also contain metal elements not compatable.

To stop bimetalic corrosion you must exclude the electrolyte (moisture) from the joint. If it is in a solution it needs to be connected electricaly to a lower order metal in the solution (zinc) as an annode.

Do not use the hull as a conductor for lights etc, run two conductors. Any bleed of oxides of higher order metals on to bare alloy will lead to corrosion. Not sure how Flouride and chlorine in the water adds to corrosion but any moisture dispersal spray must assist.

Watch your paints - you must use etching primers and paint for alloy without other metals or use a rubber based undercoat. Not to do so will also cause corrosion.

[crump]

rub it back, apply brown vinegar and seal with "Penetrol".

[LWB123]

Thanks for the suggestions - genuinely appreciate the time and insights provided.

Cheers,


You need to make sure the studs and fittings are high quality 316 stainless , if you wire brush use a stainless brush only. if you sand or grind use proper discs for alloy, any contamination will cause corrosion.
some fitting also contain metal elements not compatable.

To stop bimetalic corrosion you must exclude the electrolyte (moisture) from the joint. If it is in a solution it needs to be connected electricaly to a lower order metal in the solution (zinc) as an annode.

Do not use the hull as a conductor for lights etc, run two conductors. Any bleed of oxides of higher order metals on to bare alloy will lead to corrosion. Not sure how Flouride and chlorine in the water adds to corrosion but any moisture dispersal spray must assist.

Watch your paints - you must use etching primers and paint for alloy without other metals or use a rubber based undercoat. Not to do so will also cause corrosion.

[LWB123]

Thanks for taking the time to offer some useful advice - genuinely appreciated.

Cheers,


rub it back, apply brown vinegar and seal with "Penetrol".

[clean32]

Not sure how Flouride and chlorine in the water adds to corrosion but any moisture dispersal spray must assist.
.

They both increase Alkali ( not to be confused with alkalinity) or rise the PH. any alkali will return aluminum to its natural state " aluminum oxide"

Both Chlorine and fluorine are oxidizers in there own wrights.

[LWB123]

The response is genuinely appreciated.

Cheers,


They both increase Alkali ( not to be confused with alkalinity) or rise the PH. any alkali will return aluminum to its natural state " aluminum oxide"

Both Chlorine and fluorine are oxidizers in there own wrights.