Chemistry Forums for Students > Physical Chemistry Forum

316L stainless steel corrosion product components

(1/2) > >>

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:

--- Quote from: dspear99ca on February 28, 2023, 05:44:23 PM --- The rudder has some cracks and leaks some brownish liquid when I take it out of the water

--- End quote ---

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

dspear99ca:

--- Quote from: DrCMS on March 01, 2023, 07:02:33 AM ---Assume your rudder is corroding and has been for some time.  Fix it or replace it before it fails on you.

--- End quote ---

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.

Navigation

[0] Message Index

[#] Next page