[dspear99ca]

Hello.  I have searched far and wide on the internet for this info, forgive me if I've missed it someplace else.

I would like to know what the formula is for the corrosion of stainless steel in salt water in an anoxic environment.  I have a boat with a fiberglass rudder and stainless steel rudder post encased in the fiberglass.  The rudder has some cracks and leaks some brownish liquid when I take it out of the water (ocean = salt water).  I've read all about how stainless can corrode without oxygen present to renew the passive protective surface layer, but haven't been able to find any formula which might show what the products of corrosion/decomposition are.  I did find one diagram which indicated Fe³⁺ ions combining with OH^- to form ferric hydroxide Fe³⁺(OH^-)₃, is this correct?

What I want to do is figure out what the corrosion products are, so that I can test for them to determine whether or not my stainless rudder post is corroding inside the rudder, or if possibly it's just some brown organic growth in the solution.  Any suggestions as to how to do this would be welcome - so far none of the 3 local testing labs (I am in Vancouver Canada) have been able to do testing for specific iron oxides.

Thanks in advance for any help.

[Borek]

I doubt you are dealing with anoxic environment. Surface waters are saturated with oxygen.

If there is oxygen present Fe³⁺ will be always present in the steel corrosion products (no matter if it is stainless or not).

Stainless steel contains addition of chromium which is responsible for passivation. It reacts with the oxygen and yielding insoluble chromium oxides.

This is tough, can't think of any obvious way of checking what you are dealing with, plenty of fine prints here.

[DrCMS]

The rudder has some cracks and leaks some brownish liquid when I take it out of the water

Assume your rudder is corroding and has been for some time.  Fix it or replace it before it fails on you.

[dspear99ca]

Assume your rudder is corroding and has been for some time.  Fix it or replace it before it fails on you.

Therein lies the rub.  The cost of the rudder is ~$7,000 + shipping from Florida, by the time I'm done with paying the boatyard to haul it and the guys who do the installation I'd be looking at upwards of $10,000.  It would be a surefire way to ensure that any potential problem is fixed, but the most expensive one.  Hence my post about testing for Fe3+ in the solution. 

The rudder post is cathodically protected by a sacrificial zinc anode - all metal fittings which touch the water are wired together as well as the zinc anode.  BUT.... the connection to the rudder post is just friction contact with a copper strip so as to allow the post to rotate.  The copper corrodes in salt air so it's possible that the contact between the two metals is (and has been for some time) less than perfect.

Throw in the fact that the cracks and liquid are from one localized area which happens to be a cosmetic cap covering mounting bolts which contains an air-filled void which may be compromised... the bond between the two dissimilar materials (fiberglass and plastic) is where the leaking is occuring, so I'm really not sure if the rudder is leaking at all and don't want to spend a fortune replacing a part that's not corroded.

[Enthalpy]

Corrosion is much, much more complicated than one reaction formula.

However, 316L shall NOT corrode in seawater. It's meant for that use, and experimentally 316L excels at this job.

The flaw lies very probably in the epoxy. Upon hardening, most epoxies release hydrochloric acid and more noxious products that do corrode most metals, including stainless steel. This is a design error.

Some epoxies are designed to release no corrosive products. They cost a fortune. The chip industry has one fab in Japan to provide the epoxy that encapsulates integrated circuits without corroding the ultrathin metal pads. To my opinion, the solution in a boat is to use an other material than epoxy+fiberglass, or to the very least, harden the epoxy away from the metal, then flush it for long.

Sacrificial zinc won't help against that, nor is it needed with 316L used properly in seawater. I wouldn't focus on metal contacts and corrosion couples, as in my experience they aren't essential. They do matter with ultrapure metals, for instance in the self-discharge of batteries, but not with alloys, as these contain already corrosion couples at every crystal.

[Aldebaran]

As already indicated by some of the previous comments corrosion can be a complex problem and it’s not specifically my area of expertise but if you are looking for a relatively simple test for Fe³⁺ ions then adding thiocyanate ions will produce a characteristic intense deep red colour. Your original post indicates local labs have not been able to help you identify Fe ions, not clear why they can’t but the thiocyanate test is pretty straightforward and you could do it yourself if you can obtain a suitable thiocyanate reagent. Having said that if you did get a positive reaction from the test I’m not sure that really solves your problem as to how serious the corrosion may be . It’s a pretty sensitive test and I would think you’re almost certain to have Fe³⁺ present to some degree as previous replies have indicated.

[dspear99ca]

However, 316L shall NOT corrode in seawater. It's meant for that use, and experimentally 316L excels at this job.

I like your answer, but I am still a bit unsure.  What I've read is (and, I say this with many grains of salt as it was all found on the magical interweb) that it is not uncommon for corrosion to happen when seawater seeps into cracks and small spaces where there is no oxygenated water flow to ensure a steady supply of oxygen, leading to contact of the stainless with stagnant, oxygen-depleted salt water.  The lack of oxygen means no regeneration of the protective passive layer, and the Cl^- ions naturally present in sea water form hydrochloric acid which causes the corrosion.

One question I guess I would have is does there need to be physical scoring of the existing oxidized surface layer for corrosion to start, or can it be spontaneous?  I mean, when it went into the rudder the stainless was protected by this layer, and there's no way anything is going to scrape that coating with fiberglass surrounding it.  Informationally the boat is 42 years old and has always been in salt water.

I will talk to the labs again, I perhaps wasn't asking for the right assay.  They typically test for iron as a contaminant in drinking water, I just didn't think this was specific enough a test but maybe it is with a seawater control sample to measure against.

I guess a 3rd option would me for to just cut it open and have a look.

[Enthalpy]

Oxygenated water: I've read it too, but have no opinion as I didn't observe such conditions.

The Cl^- ions corrode usual stainless steel directly, without forming the acid. They make seawater more corrosive than freshwater. Usual stainless steel like the 304 doesn't survive. 316L does thanks to lower carbon ("L"), and specifically against Cl^-, added Mo. It also contains more Ni, which doesn't hurt.

Corrosion needs no mechanical action.

I have a strong intuition that the epoxy corrodes your 316L, especially if it was hardened around the metal. That's a common situation, one could say: a design error. I'd prefer to modify the rudder to use only metal, or glassfiber parts already hardened separately, and 316L parts welded on the shaft that can be separated from the fiberglass for inspection.

If the steel took 42 years to corrode, you won't see much Fe ions in a sample taken in a minute. The aspect is a more reliable indication for corrosion.