question for the corrosion experts

[danners]

Hi guys,

apologies if this is asked & answered i couldn't quite learn what i needed with the powers of google.

Was inspecting the underside of my pressed tinny and i've noticed some marks which kind of look like corrosion. It's a 445 4 seasons sea wasp with 3mm bottom painted white inside, the marks are fairly wide spread (where i can see them is mainly underside of the bow) and it's not just 1 or 2 which is concerning me a bit.

there are no leaks the boat is watertight, never have any water in the bilge or draining from the bungs on a normal trip that doesnt involve waves comin over ::)

I had a look in the hatch under the casting deck, removed the anchor tray (plastic) battery box etc and found no swivels, lead, hooks or other metal contaminants and the paint seemed fine though a little dirty/scummy.

there does not appear to be any corrosion on the top side of the plate

I've attached some photos to show you what the marks i'm seeing look like, does anyone know what could be causing this and what I need to do to fix it?

The hull was mfgrd in 2006

thanks
Dan

http://img220.imageshack.us/img220/3927/corro3.jpg (http://img220.imageshack.us/i/corro3.jpg/)
http://img704.imageshack.us/img704/4235/corro2.jpg (http://img704.imageshack.us/i/corro2.jpg/)
http://img801.imageshack.us/img801/4198/corro1.jpg (http://img801.imageshack.us/i/corro1.jpg/)
http://img507.imageshack.us/img507/6954/corro4.jpg (http://img507.imageshack.us/i/corro4.jpg/)

[danners]

as a further note, the only thing I can think of that I have been doing differently (assuming this is a recent development, I'm not entirely sure when the problem started) is bringing an extension lead from the wall socket in the garage to charge my deep cycle while it's still in the boat. i've been leaving the terminals from the electric motor plug on the battery while charging, but disengaging the kill switch so the current shouldn't be flowing from the terminals to the electric motor socket (i hope?)

if this sounds like a terrible idea, and likely to be causing electrolytic corrosion on the hull i'd very much appreciate any heads up and will gladly start removing the terminals from the battery before charging (possibly revert back to dragging the battery up the driveway each time lol)



thanks again
Dan

[shano]

yep looks like corrosion / electrolosis! how have you got the electrics earthed like lights etc? not using the boat as a earth?

[TheRealAndy]

Can you get a closer pic of that corrosioin cell. Try using macro mode (the flower symbol) on your camera.

Whist it looks like electrolosis it mya just be an isolated bit of pit corrosion turned nasty. Do you wash the salt water off the bottom of your boat properly?

[danners]

that's as close as the camera will focus on macro mode, camera is about 3 inches away there and the corrosion is about 3-4mm wide max

there's a fair bit of it around the bow area and can't see it elsewhere, i hose the saltwater off each time

RE the lights, i'm not entirely sure; it is set up the way it was when I bought it, any tips on what to look for?

thanks for replies

[shano]

main thing make sure all electrics are earthed back to the battery, and like andy said do you wash the salt off the bottom also?

[danners]

just had a look (in the rain lol) and it appears everything is earthed to the battery.

one thing I haven't been doing is being vigilant about isolating the batteries when not in use, perhaps with all this recent rain and live batteries some problems have started?

I do wash the salt off the hull and it does only appear to be on the bottom of the hull, not inside/under the hatches etc.

One thing i've noticed is the sacrificial anode on the outboard looks a little cleaner than it used to, with less corrosion build up on it than i remember seeing last i looked. could i have a bad earth lead somewhere perhaps?

cheers for the help
Dan

[TheRealAndy]

If the battery is isolated, the only ground will be via the outboard, which is standard. I would hazard a guess and say its just isolated pitting. Pitting is usually a result of salt water. Only way to prevent it going further is to remove the pit, usually steel wool will do this. Just rub it back until its clean and keep an eye on it.

[danners]

thanks Andy,

I will steel wool or sand any spots i can see back and will spend more time hosing it down each trip

cheers again
Dan

[oldboot]

The other thing you might find is that the pitting started from some sort of contamination.
Grinder spray, splash of something alkaline or acidic.

the important thing is to get the corroded material off as it harbours moisture and other corrosive products.
I'd steer away from STEEL wool.....scotchbrite or fine sand paper would be better.

The smoother aluminium is the better it stands corrosion.

cheers

[TheRealAndy]

The other thing you might find is that the pitting started from some sort of contamination.
Grinder spray, splash of something alkaline or acidic.

the important thing is to get the corroded material off as it harbours moisture and other corrosive products.
I'd steer away from STEEL wool.....scotchbrite or fine sand paper would be better.

The smoother aluminium is the better it stands corrosion.

cheers

Pitting is usually a result of chloride ions, so not so likely with grinding or contaminates unless it sticks to the aluminum and prevents good cleaning.

However, anything that gets rid of the pit will work. Fine wet and dry sandpaper or scotch brite or anything abrasive. You need to get it all, so if its already worked its way deep into the aluminium then you need to get it all out. Its exactly the same as steel.

I used to use caustic soda (draino) to clean pitting, and as a bonus it oxidises the aluminium and prevents further corrosion.

[oldboot]

What can and does happen is that some sort of contaminant gets on the aluminium and does not get cleaned off for some time.....this starts the process......once the pitting starts, its off and going.

One thing that can realy accelertae corrosion in aluminium is graphite...common pencil......I once made some rack angles up for a cassette deck...I scribed the marking out, and because it was hard to see, I ran over the lines with pencil..... not a facy job so i did not clean up the marking out.
I was surprised how fast the lines corroded, and this item was kept mostly clean & dry..certainy no salt water.

Think of what would happen if some steel grinder sparks stuck the the aluminium.......they would not come off easily and would make an excelent corrosion starter.

cheers

[danners]

the boat was fairly recently (month or so ago) in a welding shop getting some gussets put in for a false flooring, so it's definately concievable that some sparks mighta hit it- also would make sense in that I can't really see any pitting on the bottom of the hull, just on the front of the bow (bottom sheet but where it's angled up)

another thing, it's always had a few scratches in the ally which might have gotten things started, but anyway earlier this arvo I sanded around some of the worst looking bits and it seemed to come off quite easily so I think i've caught the problem early. will have to do plenty more sanding as it looks like there's a fair bit of shallow pitting

thanks again for the help
Dan

[TheRealAndy]

What can and does happen is that some sort of contaminant gets on the aluminium and does not get cleaned off for some time.....this starts the process......once the pitting starts, its off and going.

One thing that can realy accelertae corrosion in aluminium is graphite...common pencil......I once made some rack angles up for a cassette deck...I scribed the marking out, and because it was hard to see, I ran over the lines with pencil..... not a facy job so i did not clean up the marking out.
I was surprised how fast the lines corroded, and this item was kept mostly clean & dry..certainy no salt water.

Think of what would happen if some steel grinder sparks stuck the the aluminium.......they would not come off easily and would make an excelent corrosion starter.

cheers

Sorry, yeah forgot you are the corrosion expert. My bad.

[Camhawk88]

I would certainly stay away from the steel wool. Any unlike metals will enhance the corrosion. I wont use steel wool on anything metal- take a look at knives that are scrubed with it. The stainless steel will usually end up with little brown spots all over it. As old boot said, once a contaminant gets into metal the corrosion is underway.
Any work been done one the trailer perhaps? Grinding or welding would throw some sparks onto the bottom of the hull where the corrosion apears to be. It may take a year or 2 so may have occured before you took ownership?

[rayken1938]

How about sending the photos to the manufacturer and asking their opinion of.
a. The cause.
b. Remedy.
Cheers
Ray

[danners]

I've just crawled right under with a torch and had a look,

I reckon the not washing the salt off enough is the answer here. minor corrosion patches like the ones pictured can actually be seen all over the bottom of the hull, whereas previously I thought it was just over one small area at the bow.

the patches are very easily removed with a few wipes of fine grit sandpaper. for the most part i cant even feel the patches of corrosion by wiping my finger across it and i haven't come across any bad pits that need any substancial amount of metal sanded away to get rid of,

so i guess i'm up for a lot of awkward under the trailer sandpapering over the next couple of weeks to get the hull shining clean now lol. in the past i've always given it a quick hose off inside and out but from now on i'll be getting out the karcher and blasting the **** out of it, as i could also see some salt crust remaining on the bottom of the hull

i'm glad i asked because i'd hate to come back in a couple of years and complain about 1000 holes in the bottom of the hull :o

thanks again
Dan

[suzygs1000]

Sorry to disillusion you, danners, but salt water does not corrode aluminium, unless there are extreme ph levels, which can stop the oxide from reforming quickly if it is damaged. It can, however, facilitate galvanic or dissimilar metal corrosion.

In other words, under normal circumstances, hosing your hull down will not make much difference to whether or not the aluminium is going to corrode.

There must be factors other than the salt at work.

Dave.

[tigermullet]

From the photographs, there is too much corrosion for it to be anything but electrolysis. Somewhere there will be a wire or a metal backing of an instrument or switch in direct contact with the hull.

[TheRealAndy]

For the record, salt water does corrode aluminuim.

I can show you all a boat that spends very little time in the water, that has no motor or electronics on it, that looks exactly the same. In fact, I shall go take a few photo's of it on the weekend as I need to get an idea of its value.

[tigermullet]

For the record, salt water does corrode aluminuim.

I can show you all a boat that spends very little time in the water, that has no motor or electronics on it, that looks exactly the same. In fact, I shall go take a few photo's of it on the weekend as I need to get an idea of its value.

Well, that's got me beat. Our boat, unpainted aluminium, has been on the water (swing mooring) for just over twelve years without any sign of corrosion. It has been inspected regularly and, a couple of months ago, I caved in and had it anti-fouled. So, maybe the correctly placed sacrificial anodes have been saving it from harm.

Weird stuff is galvanic action and electrolysis.

[oldboot]

Lots of people are obcessed with both electrolisis and glavanic corrosion in aluminium, and haplly blame all corrosion on it.

it may be true in an ideal situation that salt water does not corrode pure aluminium.

But I can guarantte you that aluminium corrodes quite happliy all by it self with no salt water exposure in the real world.

ADD an infrequent exposure to and incomplete exposure to salt water, particularly if there is some sort of contamination of or impurity in the aluminium to start things going...and it is off and running.

The sources of inpurities or contamination could be many...steel grinder spray was first to come to my mind.

pits from stone chips and minerals off the road or from PH active soils may be another.

Once there is something to retain moisture and residual salt, the problem perpetuates.

Perphaps some one can explain in detail the process where salt turns to acid,to corrode aluminium.

If you are interested google "Crevice corrosion"......what happens in a crevice will happen in a pit.

the solution remains the same..... scrub it till it shines and then keep it clean.

cheers

[danners]

from the looks of it the boat had a fairly hard time at the ramps before i took ownership (plenty scratches on the front areas and it was used at the kings beach ramp by prev. owner)

bit of the corrosion is near some of those scratches and others close by so could be little gouges

anyhow ive been slowly sanding back and getting the bad looking bits out so hopefully wont have any issues

needs a bit of sanding though some parts have lots of really tiny little spots of corrosion over largish areas and can be hard to reach under the trailer lol

[TheRealAndy]

from the looks of it the boat had a fairly hard time at the ramps before i took ownership (plenty scratches on the front areas and it was used at the kings beach ramp by prev. owner)

bit of the corrosion is near some of those scratches and others close by so could be little gouges

anyhow ive been slowly sanding back and getting the bad looking bits out so hopefully wont have any issues

needs a bit of sanding though some parts have lots of really tiny little spots of corrosion over largish areas and can be hard to reach under the trailer lol

Dont sand hard. Use the finist grit possible. Scotchbrite and elbow grease.

[PinHead]

I've just crawled right under with a torch and had a look,

I reckon the not washing the salt off enough is the answer here. minor corrosion patches like the ones pictured can actually be seen all over the bottom of the hull, whereas previously I thought it was just over one small area at the bow.

the patches are very easily removed with a few wipes of fine grit sandpaper. for the most part i cant even feel the patches of corrosion by wiping my finger across it and i haven't come across any bad pits that need any substancial amount of metal sanded away to get rid of,

so i guess i'm up for a lot of awkward under the trailer sandpapering over the next couple of weeks to get the hull shining clean now lol. in the past i've always given it a quick hose off inside and out but from now on i'll be getting out the karcher and blasting the **** out of it, as i could also see some salt crust remaining on the bottom of the hull

i'm glad i asked because i'd hate to come back in a couple of years and complain about 1000 holes in the bottom of the hull :o

thanks again
Dan

by the looks and what you say the feel of it is..my tip..Aluminium Oxide..air reacts with aluminium..it is actually a naturally occurring protection on the aluminium..if it is that then leave it there.

[rayken1938]

Have you checked with a voltmeter between the positive terminal of the battery and the hull?
Cheers
Ray
edit bad thought the motor is earthed to the hull so you will get a voltage reading.

[TheRealAndy]

For the doubting thomases, Get a piece of aluminium, dunk it in the salt water, then put it away for a few months, no electricity, no continuous submersion in salt. I had to do a similar experiment at uni, works a treat.

[danners]

good tip ray i had not thought of that, will try tomorrow. am i seeking a voltage of zero?

ive been using very fine grit sandpaper and basically just cleaning off the bad looking bits. im sure alot of the tiny patches are just the oxide layer but the bigger patches i've photographed and around some of the scratches appear to be more

cheers
Dan

[rayken1938]

Sorry for the missinformation, Dan bad thought the motor is earthed to the hull so you will get a voltage reading.
Cheers
Ray

[Spaniard_King]

You can disconnect the earth from the engine and try the voltage check..LOL Ray