Store Links Mobile - Shop Now


1 of 2 < >


The iBoats Forums will be down for maintenance starting at 1 PM MST on 12/16/2017. The estimated amount of downtime is 3 hours. Thanks for being a part of the iBoats community!
2 of 2 < >

Help Tip: If you have a question that has not been answered to your satisfaction in the archives, it is always best to start a new thread of your own. By starting your own thread, you will receive the maximum number of views by forum members.

Below are some additional forum policies in hopes of all iboats members will follow, Thank you.

1. Please do not reply to old topics or hijack existing topics. Old topics of a technical nature are like a library book, Please do not write in them.

2. Old topics should be considered archives and used for reference only. Please do not reply to them.

3. Do not take over someone elseís topic (aka hijack) with your own question, even if it is similar. If you have a question that has not been covered to your satisfaction in the archives, it is always best to start a new topic of your own.

4. If you have a question for the original poster (OP) and the topic is over 30 days old, send the OP a PM, he may not even visit the forums any longer, or may not notice your question in the old topic.

5. By starting your own topic, you will receive the maximum number of views by forum helpers that may not even notice your question when itís posted at the end of someone elseís topic. And those answers will be specific to your particular issue.

6. Please do not post to topics that have been inactive for more than 3 months unless you are the original poster. We have very active forums and any topic that remains inactive for that long should be considered "dead". It is especially confusing when there is an entirely new question posted to an old topic.

7. Posting at the end of any topic is considered to be hijacking the original posters topic which in turn subjects the topic to be closed if it continues to happen thus not making it fair to the original poster in the future had for some reason he/she needed to return for additional information or provide an update of the problem solved which is always welcomed within a reasonable amount of time frame.

8. Please note that you should see a red banner pop up near the bottom of each inactive topic asking you not to reply to old topics. The Red banner will read: Please note this topic has been inactive for 90 days. For the best results, please start a new topic.

Thank you all in advance for doing your part in helping iboats run a smooth ship.

Additional forum rules linked below.
See more
See less

Dealing with corrosion on underwater metallic fittings, by Stillfishing

This topic is closed.
  • Filter
  • Time
  • Show
Clear All
new posts

  • Dealing with corrosion on underwater metallic fittings, by Stillfishing

    From various posts on different threads, there seems to be a lot of misunderstanding when it comes to metallic corrosion and its protection on small boats, so I’ve taken the liberty of posting the comments below. If there are any experts out there who can add to this information, I’m sure it will be welcome. These comments are an over-simplification, and serve to hit the bones of the corrosion problem only, but sufficient information is here to help identify and fight most small boat corrosion problems.

    Water. All water is an electrolyte. Corrosion slows in fresh, still, cold water, and is faster in warm, clear, moving salt water. That’s why a cold battery has less power to crank your car than a warm one. So fittings on a boat used in Northern lakes will last longer than in a Californian bay. This also means that corrosion can take place while your boat is on its trailer, if it has bilge water, sodden flotation foam, damp sludge in the leg, or even if it stands in the rain.

    Galvanic Corrosion. This is the correct term, although we commonly call it electrolytic corrosion. In the days of sailing ships, it was found that screwing a lump of scrap iron to the copper-bottomed hull preserved the copper from corrosion. Investigative science followed from there, and today we can table a whole range of metals in order of their susceptibility to galvanic corrosion. At the top (most noble) are platinum, gold, titanium and 316L stainless. At the bottom (least noble) are metals like cadmium, zinc, and magnesium. When a galvanic couple forms, one of the metals in the couple becomes the anode and corrodes faster than it would all by itself, while the other becomes the cathode and corrodes slower than it would alone. The driving force for corrosion is a potential difference between the different materials, and water - any water - completes this circuit.

    The metals that concern us are listed here, in order of their resistance to electrolytic corrosion:

    LEAST ANODIC (Most Noble) hard to corrode
    Stainless steel - high chrome (i.e. grade 316L)
    Nickel (passive)
    Stainless Steel - low chrome
    Cast iron
    Wrought iron
    Mild Steel
    2024 aluminum
    Zinc Magnesium and its alloys
    MOST ANODIC (least noble) easy to corrode

    Compatibility. Metals closest to each other in this Table will have the least tendency for the ‘weakest’ to corrode. Metals farthest apart will have the greatest tendency. This is good news and bad news. Use widely dissimilar metals on your underwater fittings - aluminum and stainless, for example - and you have a large corrosion potential. Sacrificial anodes, on the other hand, are purposely made from metals that easily corrode (that's why we loosely call them 'zincs'). Correctly sizing, placing and maintaining these anodes utilizes this incompatibility to good effect.

    Proximity. Bad news - the closer dissimilar metals are to each other, the greater the tendency for the least noble to corrode. This means that if you have a stainless prop on an outboard leg, the leg could corrode faster than it would if you had an aluminum prop, because aluminum is ‘weaker’ in the galvanic table. The same principle applies to the proximity of any other underwater fitments. Even stainless steels, depending on their content (low chrome or high chrome) can have widely diverse corrosion resistance, and putting two types of stainless in close proximity will speed up the corrosion of one of them. The good news is that fitting anodes either in close proximity or in physical contact with ‘weak’ metals will prolong their life. So a zinc screwed tight to your out drive leg will protect both the leg and the aluminum prop. Contact. Dissimilar metals in physical contact will accelerate the corrosion of the ‘weakest’. Screw a stainless bolt into an aluminum hull and corrosion will take place on the aluminum around the bolt head. No easy cure for this one, unless you also use zinc washers under the bolt head, but these will cause leaks as they corrode away. Best not to do it at all. That’s why aluminum boats use aluminum rivets.

    Cure? There is only one complete cure - keep your boat warm and dry, and never take it anywhere near water!

    Precautions. Reduce the number of dissimilar underwater metals where possible. Keep most noble and least noble fittings as far apart as possible. Fit correct anodes (differing waters require different anode materials) not just to your leg, but also to other major fittings around your boat. If anodes are not fitted to stainless swim ladders or trim tabs, these items can speed the corrosion of less noble metals elsewhere on your hull - like your expensive outboard leg! Notice that most metallic fittings are on the transom - all in proximity to each other! Keep all anodes clean, regularly remove the whitish oxide that forms on the anodes with a file if necessary, and replace them when they are half gone.

    As I said - this is an over-simplification, and there are a lot of variables. But the bottom line always is - neglect spells future expensive trouble.

    Last edited by JB; October 3rd, 2007, 09:27 AM. Reason: reformatted by JustMrWill. Thanks.

  • #2
    Sign up today
    Re: Dealing with corrosion on underwater metallic fittings, by Stillfishing


    "Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, chocolate in one hand, martini in the other, body thoroughly used up, totally worn out and screaming "WOO HOO what a ride!"