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Thread: Simple views about corrosion/electrolysis and rust

  1. #1

    Simple views about corrosion/electrolysis and rust

    Hello all.

    Yet some more words about aluminum boats, saltwater, electrolysis and plain old rust. Complementary to what sounds like some good advice by a couple of guys already.

    Hopefully easy to understand in a few pdf documents.

    Attachment 121404

  2. #2

    Re: Simple views about corrosion/electrolysis and rust

    Ron electrolysis and aluminium but rust? I thought electrolysis eats aluminium but removes rust from steel?

    Sent from my SM-G900I using Ausfish mobile app

  3. #3

    Re: Simple views about corrosion/electrolysis and rust

    Steel can most definitely suffer from galvanic corrosion or electrolysis. We regularly find heavy pitting in the bilges of steel vessels with little to no rust present. Can be as simple as a tool or bolt being left in the bilge causing a galvanic reaction - much like a sinker in a tinnie.

  4. #4

    Re: Simple views about corrosion/electrolysis and rust

    Quote Originally Posted by gazza2006au View Post
    Ron electrolysis and aluminium but rust? I thought electrolysis eats aluminium but removes rust from steel?

    Sent from my SM-G900I using Ausfish mobile app
    Gazz

    Lets first concentrate on electrolysis and steel (the trailer) as its easier to understand being big.

    Electrolysis is the electrical term for what happens when an anode (+ve pole) and a cathode (-ve) pole are placed together in an electrolyte bath (salt water).

    In the case of a boat trailer, the problem is simply chemical corrosion between the ferrous metal of the trailer and saltwater in presence of oxygen. The end corrosion result is called ferrous oxidisation which is more commonly known as rust. Pretty straight forward so far.

    So how do we protect something from the chemical rusting process involving salt water?

    Don't put it in saltwater? Don't use ferrous material (steel)? Don't let oxygen or saltwater reach the steel?

    The more commonly known process is simply to put a physical barrier between the steel, the saltwater and the oxygen in the air.

    Paint the trailer completely with fishoilene? That'll help but you'll end up with all the neighbourhood cats at your place. Fishoilene is simply a physical barrier that sticks to the steel surface.

    Paint the trailer with paint? That's ok as a barrier too UNTIL it chips or cracks letting saltwater and oxygen onto the steel and you get.....ferrous oxide..... bloody rust again. Grind it off and repaint.

    What if we put something on the trailer that gets into every nook and cranny (better and less labor intensive than painting).

    Immerse the whole trailer in it so it shuts out the saltwater and the oxygen as it cools. Will that work?

    Sure will.

    Something that has a low melting point (to keep electrical heating costs down), sticks like ...t to a blanket and is relatively cheap and that has little or no galvanic problems with steel. Like zinc.

    Well, there is a galvanic problem with a zinc and steel combination. In fact zinc has a galvanic problem with most other metals. But its cheap and has a low melting point.

    It takes a while for the problem to become evident but the zinc coating will gradually be corroded away by the galvanic action particularly at the edges and sharp joints of the steel where the zinc coating is thinnest.

    As the zinc gradually corrodes away, those steel sharp edges etc will become more and more exposed to the saltwater and oxygen combination. We again get.......rust forming at those vulnerable points. Back to the zinc bath after a rust cleanup.

    What's actually happening is that the zinc is acting as a physical barrier but is being eaten away gradually by galvanic action with the steel until its no longer an effective barrier. That gradual eating away by galvanic action is called electrolysis.

    In this case we're actually using that dreaded electrolytic reaction as a way to prevent more serious problems like those than occur in aluminium boats.

    So we can see that there are good uses for electrolysis and there are bad cases of electrolysis.

    Follow that so far?

    Steel suffers from 2 infections. One is a chemical reaction (rust) and the other is an electrochemical reaction (electrolysis caused by galvanic incompatibility between metals).

    Clear as mud?

    I've got some more to come that will make things a bit clearer but there'll have to be a bit of a rethink about some of the myths relating to corrosion and electrolysis and there are plenty. Use of monel metal and stainless steel are two of those myths.

    But I digress.

    The zinc has been "sacrificed" over time to combat rust forming on the steel bits of the trailer. There's the first clue about combating electrolysis induced by galvanic action of metals.

  5. #5

    Re: Simple views about corrosion/electrolysis and rust

    Chapter 2.

    Take careful note of the colourful galvanic chart.

    Corrosion caused by electrolysis is the biggest threat that you'll face to your trailer, hull and fittings and you can't eliminate it unless you use the same material right through (and that's simply not possible).

    Best you can get on an aluminium hull is to use aluminum screws or rivets (not with a steel mandril though or you're no further advanced).

    Maybe use an aluminium alloy trailer? Plenty of them around these days and you don't see rust on them. However, be careful In respect of the fittings ensure that similar (or as close as possible) colour matching (for galvanic voltage difference) is used. As an added bit of insurance resort to some barrier protections.

    Look at the position of monel on the chart. Its very close to stainless steel in terms of galvanic voltages so care needs to be taken with it too (just like ALL the other metals). Being at the end of the chart (like stainless/monel in one direction and zinc at the other end) means that extra care needs to be exercised in thinking about which fasteners/screws/bolts/rivets to use on your aluminium hull.

    Galvanic action will occur between metal fittings on any hull for that matter (timber, fibreglass or poly) but its restricted to fittings and not the hull material itself.

    Attachment 121442

  6. #6

    Re: Simple views about corrosion/electrolysis and rust

    Quote Originally Posted by Ronje1 View Post
    Chapter 2.

    Take careful note of the colourful galvanic chart.

    Corrosion caused by electrolysis is the biggest threat that you'll face to your trailer, hull and fittings and you can't eliminate it unless you use the same material right through (and that's simply not possible).

    Best you can get on an aluminium hull is to use aluminum screws or rivets (not with a steel mandril though or you're no further advanced).

    Maybe use an aluminium alloy trailer? Plenty of them around these days and you don't see rust on them. However, be careful In respect of the fittings ensure that similar (or as close as possible) colour matching (for galvanic voltage difference) is used. As an added bit of insurance resort to some barrier protections.

    Look at the position of monel on the chart. Its very close to stainless steel in terms of galvanic voltages so care needs to be taken with it too (just like ALL the other metals). Being at the end of the chart (like stainless/monel in one direction and zinc at the other end) means that extra care needs to be exercised in thinking about which fasteners/screws/bolts/rivets to use on your aluminium hull.

    Galvanic action will occur between metal fittings on any hull for that matter (timber, fibreglass or poly) but its restricted to fittings and not the hull material itself.

    Attachment 121442
    Great explanation thank you , Matt
    A bad days fishing has got to be better than any day at work......


  7. #7

    Re: Simple views about corrosion/electrolysis and rust

    Chapter 3

    Barriers and sacrificial anodes simplified

    Attachment 121518

  8. #8

    Re: Simple views about corrosion/electrolysis and rust

    Have a look at the attached chart for compatibility data.

    Maybe its a bit easier to see what's compatible with what.

    But probably more important is what's NOT compatible BEFORE the damage is done 'cos then its too late.

    Remember that galvanic corrosion can eat away screws/fasteners too (not just the aluminium hull).

    When you see 1,4,5 and 1,5 on the chart, 1 means that the base metal (the aluminium hull) is ok but 5 means that the screw/fastener itself is in danger of getting eaten away.

    Attachment 121519

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