When does stainless steel surface corrosion/pitting become a structural issue?

[Corrosion-Eng]

You are seeing just the tip of the ice burg. Replace the shaft.


It is caused by your vessel and is electrical current flowing to the Earth taking your vessel with it. It is correctable by measuring current flow to the Earth and disconnecting equipment until current cannot cause marine electrolysis.

Do a Internet search for Marine electrolysis and click Conquer marine corrosion.

[stormalong]

When you replace the shaft use Aquamet 22 propeller shaft material. I have seen a few boats with severe rudder shaft corrosion while my now 34 Y.O. solid 2 inch aquamet 22 rudder post looks like new. Another thing that is common with rudder shaft is that it is pipe instead of solid. Lighter yes but much more likely to fail from corrosion.

My rudder weighs a ton. It take two people and a lot of leverage to get it in and out. But I don't worry about rudder failure.

[thinwater]

Originally Posted by boatpoker

Absolutely not one single Certified Marine Corrosion Analyst will use the word "electrolysis" which describes the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an induced current being passed through an electrolyte. . You need to get your information from more reliable sources.

https://www.nace.org/resources/gener...anic-corrosion

file:///C:/Users/Wallace/Documents/SURVEY%20NOTES/MOISTURE%20CONTENT/WEB%20PAGE%202/electrolysis.htm

People educated in these matters will not use that word. As I said. if it involves electricity in a boat you are strictly limited to galvanic or stray current corrosion. When I wrote my certification exam I was told use of that word was an automatic failure My instructors were both consultants to the USCG on corrosion.

I also get a lot of free instruction in these matters from my buddy, dock neigbour and consultant to the US Navy on their electric warships, he retired year as President of IEEE although he still consults with the New Panama Canal on their electrical systems.

Lots of big words (I too am a licensed inspector, NACE and API). Yes, the terminology is incorrect, but he is not wrong that pitting is often cause by concentration cells and differences in oxide coatings, through mechanisms closely related to classic galvanic corrosion. But no reason to scold over an explanation that is close enough, when the details are often unknowable.



What matters is that the depth of the corrosion is really, really difficult to estimate from surface appearance. I've seen mere "discolorations" that went through 1/2-inch plate. This leaves you with a number of imaging alternatives, none of which will be cheap. Even then, you will need to grind and fill with weld (not epoxy), which will leave the shaft slightly weaker, depending on the details.



It is hard to say "OK" for such a critical, high stress item. I'd bite the bullet.

[NPCampbell]

Originally Posted by boatpoker

I have no idea what someone means when they use the word "electrolysis". That term does not apply to corrosion. Corrosion involving electricity in marine metals is limited to only two types, galvanic or stray current. Neither are indicated by the OP's photo. This is clearly crevice corrosion.

The positive anode in an electrolysis reaction is absolutely associated with corrosion.

I've used (reverse) electrolysis many a time to de-rust iron items that I've found with a metal detector. The anode gets torn up in a hurry.

The term "electrolysis" simply means using an electric current to drive a chemical reaction. The "stray currents" you mention cause electrolysis if the part in question is grounded or part of a circuit.

I would expect corrosion due to electrolysis to be orders of magnitude higher than the relatively weak galvanic corrosion because the corrosion of the anode is driven by the amount of current flowing between the anode and cathode. Galvanic corrosion is limited to <2 volts generated by the potential differences generated by two dissimilar metals in an electrolyte. Electrolysis is bounded only by the voltage of the external source, i.e. an incorrectly wired 220V shore line.

[RaymondR]

"Absolutely not one single Certified Marine Corrosion Analyst will use the word "electrolysis" which describes the decomposition of water (H2O) into oxygen (O2) and hydrogen gas (H2) due to an induced current being passed through an electrolyte. . You need to get your information from more reliable sources."

The dissociation of the water molecule into hydrogen and oxygen under the influence of an electrical current is an effect experienced by many substances and is commonly used in extractive chemistry, sodium from molten sodium chloride for example. A similar process works with metals in an electrolyte.

Galvanic is what's going on in a battery, dissimilar metals in an electrolyte.

Electrolysis an induced current flow ie. EMF driven current flow, through two metals in an electrolyte. Don't need no galvanism if you impose the current from an external source.

Some stainless, like 316L is quiet amenable to welding and since stainless is very commonly used in industrial applications I'm fairly sure there will be a safe procedure.

I just don't trust the damned stuff for general use and on a rudder stock which oscillates backward and forward at a slow surface velocity a few gouges working in a plain bearing are not going to cause a problem and because of the easier entry of cooling and lubricating fluids it may even be an improvement.

[teneicm]

We ended up replacing both our rudder posts. Here's one of the old posts sliced through at one of the corroded areas so you can see how deep it goes. That's not to say that a different cross-section wouldn't tell a different story.