Discussion in 'Technical Discussion' started by gooddeal, Apr 16, 2004.
Any recomendations on reducing or eliminating corrosion blisters on aluminum hulls?
If you're refering to paint blisters then this is the result of poor paint prep or a scratch which wasn't touched up. If you're talking about the small corrosion pitting on bare aluminum, this is usually the sulphur in diesel fuel coming from the exhaust. I used to have a partially painted Eagle Craft which suffered from both, it was 13 years old mind you. There isn't much you can do about either, apart from the paint touch ups. We already use low sulphur fuels and paint prep on a large surface is quite difficult to get perfect. A good painter can repair these, with proper blending techniques, without costing too much.
unpainted aluminium oxidizes naturally and the surface "corrosion" is actually protection from serious damage. As for painted aluminium blisters are in one word - unavoidable.
I dunno you guys...sheesh...sometimes I worry about you crew!
A blister under the paint with white powder in it is a sign of galvanic corrosion.
Google search it! - eliminate the source & problem solved - fail to do so & kiss ya alloy hulled boat goodbyee, in double quick time - just sit & watch it turn to white powder and blow away in your slip in front of your eyes!
Galvanic corrosion occurs when two exposed dissimillar metals are together in a solution. That means this will happen below the water line only and if the metal is EXPOSED. If you have a blister under your paint and there is no scratch or chip exposing the metal then the painter did not etch the paint which cleans off the natural coating that JHA was talking about. If you have any sort of blister above the water line is it chemically imposible for galvanic action to happen because the metal is not exposed in a solution. If you do see a white powder or gel growing from exposed aluminum UNDER THE WATER LINE, then yes, that is dangerous levels of galvanic action.
OK, here is a website address of a couple who has the answers to the blistering topcoating problem:
trouty are you the same guy that post on BoatDesign.net?
Some pretty radical thinking. Just fell upon it on a google search. My dad is a marine consultant, concentration in project management, but also design, so it sort of sparked my interest.
I'm glad to hear, your not one of these lads who shows up in a loud V8 panel Van with wide white lettering tyres, painted fire engine red with yellow and orange flames painted down the bonnet, past the hole where the supercharger sticks up into it's scoop - with a mattress in the back and a picture / mural of Conan the Barbarian painted down the side - and a sign that says "If you see this van a rockin don't come a knockling, don't laugh - your daughter might well be inside!" - and proudly announces "he's here to collect my daughter for a date"....
Sadly and as much as I hate to admit it, it was probably exactly that which attraced my daughters mother to me all those 25 something plus years ago, (much to her parents chagrin at the time) but all the same I KNOW what a ratbag I was at that age and what evil intentions were always foremost on my mind, so these young whipper snappers don't fool me for even a second!
OK - to your questions then...
One question I had about aluminum is:
From the start of galvanic corrosion how long does it take to compromise the yacht structurally? If not properly addressed at the start?
How long for compromise is a variable – how long is a piece of string type question. And of course the answer is it depends.
It depends on a lot of things.
1.Where on/in the hull the galvanic corrosion occurs
2. What electrolyte the hull is immersed in
3. What currents are involved?
4. What grade Alloy is used
would be a few of the major issues that could influence the correct answer to your question.
To expand upon them a little.
1. What part of the hull is eaten away might influence structural integrity mor4eson than a different part. If it’s a critical part from an engineering viewpoint that takes a lot of load bearing of compression / tension or torque stresses then earlier hull failure might be expected than if some attached deck hardware is the victim that gets corroded away…
2. Depending upon the level of salt in the water (Eg Freshwater lake/river, or Estuarine Tidal Inlet, or open ocean may dictate the efficacy of the galvanic cell created and hence the rate at which the metal is corroded away.
3. Where on the elemental table of elements the various metals involved in creating the galvanic cell are found, in other words their relative nobility (willingness to give up/lose electrons to other less noble metals) will help determine the rate at which the most or least noble metal in the galvanic cell is eaten away. Addition of things like stray current from an adjacent boat in a dock or poor/leaking earth shore power installations on the dock or adjacent vessel, will also have an effect on the rate at which galvanic corrosion occurs.
4. Again the Grade of alloy (percentages of other more or less noble metals in the alloy mix) will also determine the rate at which Galvanic corrosion occurs. Marine grade Alluminium alloys (AMS Alloys) (Alluminium, Magnesium & Silicate) vary in percentages of Magnesium and Silicate in the alloy. More makes it more ductile and malleable which is great for pressing and stretch forming complex compound shapes/curves and planing strakes but bad in terms of accelerated rate of galvanic corrosion due to the presence of greater quantity of lesser nobility metals in the parent alloy., Conversely the higher marine grade alloys have less of these higher / lower nobility metals – but don’t handle bending / folding etc any where near as well – and require special press arrangements of large radius press bars into V break blocks to achieve a bend without cracking / breaking the plate.
That’s roughly how it works off the top of my head.
I have heard some arguments about the benefits of steel vs. aluminum, but most of it was relevant to cost, not long term performance.
Both have their advantages and disadvantages like any boat building material.
It’s interesting that a lot of the professional fishers etc that are moving away from GRP / FRP hulls toward metal are building commercial fishing boat hulls from plate alloy. This seems to apply up to say the 70 ft – 125 ft range roughly…
But, those with large working trawlers etc all seem to go steel hull.
No doubt they all have their reasons – it may well be to do with weight for trawlers with extended booms (and sometimes full nets weight as well) – that steels heavier and gives a better GZ stability curves line (the boats more stable and less likely to roll over!).
Steel when you build with it you have to allow a lot more thickness for a relatively long life while allowing for the termites (rust) to eat maybe half or more of it away before it becomes structurally unsafe.
Alloy on the other hand pretty much corrodes and forms a protective layer and self protects (if you don’t allow galvanic corrosion / electrolysis to eat it away first). So you don’t have to allow that extra thickness for longevity that you do with steel.
Steel due to it’s superior strength could actually build an equivalent strength vessel to alloy with about 1/6th the thickness – BUT it would rust away in weeks or months so we see much thicker steel being used to allow for the high attrition rate in marine use of steel.
They both have their individual properties that suit them to certain uses and skippers pick which one best suits their needs when selecting a hull material to build from.
P.s. Yes M = Δ T rules, coz trouty says so!!!!
Tom beardens one of natures gentlemen QOcean and a very very clever physicist.
Time is the answer (and you if you live thru whats comming) will very likely get to see all of it revelead within your lifetime - the dawning of the new age..."the time of the end", or more correctly, "the end of time!" as we know it and the end of it's dominion of our lives.
I hope you make it to become a time lord along with the rest of us! You havent that long to wait, Dec 12th 2012 isn't that far away. (Specially NOT for a time lord!)
Use the Power QOcean - the hidden 3 space energy trapped (compressed) within Time....
P.p.s He -who hasn't switchd on html scripting in my posting profiles? Carl???
Mass Equalls Change in Time, was what was sposed to show M = Delta T. Durn Puters!
The "Best" Boat Hull
All materials have their strengths & weaknesses.
For a good compendium of articles comparing aluminum, steel and fiberglass - click here:
For other, more specific problems with aluminum - there's always TEF-GEL: