Electrolyisis in SS exhaust?

[Streetcar]

The stainless steel exhaust elbow on my genset engine has corroded. Is it possible that this is due to electrolysis?

A year ago I replaced the ss exhaust elbow on a Ferryman engine in my genset. There is now beading and rusting on the elbow just before the water jacket. The supplier claims that this is caused by electrolysis rather than a poor quality part.





Is it possible that stray currents are causing this?

[transmitterdan]

Doesn't look like galvanic corrosion.

Is the elbow connected to a rubber hose? Is the elbow grounded by a wire?

[mike d.]

That looks like it is ready to tear if there is any side ways tension on it or put on it! What is the crust on the left back side of the pic? that doesn't look good either. But all those little bubbles or droplets are rust that has come through to the outside from within!!!!

[Neptune's Gear]

Stainless is not the best material for an exhaust elbow. It corrodes fairly quickly if it is where the hot salt water is injected into the exhaust. It's not so bad further down the exhaust where it's cooler, but it really does not like the heat and salt together....
Unlikely it's galvanic corrosion, but not impossible. Like Transmitter Dan said, whats at the other end - a rubber hose?

[Streetcar]

The exhaust elbow has 4 points of attachement:

1. Rubber exhaust hose
2. Bolted to exhaust manifold (with Gasket)
3. Rubber saltwater injection hose
4. SS support bracket bolted to engine block

The crust on the left side of pic is where the elbow is in contact with the sound insulation on the genset casing. It is just deposited material and cleans off easily.

[Neptune's Gear]

Is the water injection point above or below the photo?
Is the insulation the alloy faced silver one? - if so, it is a possible source of stray current....

[Streetcar]

Water injection is below the pic.
Insulation is non metallic.
Surely the ss elbow would have to be in contact with a 'higher up metal' for there to be electrolysis? If so what would that be?

[Andrew Troup]

Originally Posted by Streetcar

Water injection is below the pic.
Insulation is non metallic.
Surely the ss elbow would have to be in contact with a 'higher up metal' for there to be electrolysis? If so what would that be?

Electrolysis is the term usually reserved for corrosion arising purely from stray currents from an external voltage source, and does not require a 'higher up metal'. In fact, if the voltage is high enough in the correct direction, a 'lower' metal can consume a 'higher' metal.

I'm guessing you're thinking of what's usually termed "galvanic corrosion", where the voltage source is provided by metals with higher and lower galvanic potential.

Carbon, rather surprisingly is (for the purposes of galvanic corrosion) a "higher up metal" than almost anything else, certainly higher than both active and passive conditions of stainless steels. It is often overlooked.

Carbon attaches to the interior of the elbow at the point where the cooling water causes the exhaust gases to condense. I have seen instances where this has caused a stainless elbow to pit from the inside outwards at this location.

Not sure if that's what you're seeing, though: it doesn't look like random pitting.

If I had to hazard a guess from the photo I'd probably try to establish whether a crack (perhaps arising from incorrect weld filler rod, or procedures) has allowed the passage of moist gases from the inside to the outside, where they have crystallised.

[Neptune's Gear]

1st a proviso - I am not a corrosion specialist! You may find one on here....
My thoughts are;

Being that the water injection is below this point, and to have electrolysis, you have to have a conductive fluid (seawater) you don't. It's not electrolysis.

Being where it is - just above the weld, I think it's crevice corrosion, I'd take the fitting off, and look at the inside. If it is at the edge of the weld, or under it, it's poor quality welding... Can happen in an exhaust, low O2 environment etc.

Whatever, that fitting is stuffed. Replace it.

Good Luck!

[Streetcar]

Thanks. I agree it looks ready to shear off.

At nearly 400 USD and less than a year old I am trying to get a warranty exchange but I am being told that my boat must have a serious galvanic problem.

I do not know enough about these problems and I am struggling to understand how this SS exhaust corrosion could be caused by stray currents.

[Neptune's Gear]

Agreed Andrew.
Take it off and have a look on the inside. Clean the connection/Weld inside and out. If the problem is restricted to the weld, then the weld quality is suspect. If the problem is covering an area larger than the general weld area, or gets worse towards the water injection point, come back to us....with pics!
The difference between the terms galvanic corrosion, crevice corrosion, and electrolysis is above most people's heads. All Streetcar wants to know is if it's the fitting itself, or an external force due to the installation. At this stage.... We can get into more detail later if required.
Cheers
Matt

[Streetcar]

Thanks guys. I'll come back with the info.

[Cheechako]

Pretty much par for the course for SS. Hot salty water, enclosed ... perfect for corrosion. I once replaced a corroded SS one with iron pipe fittings... actually lasted longer.
If welded assembly, they could improve it's life by solution heat treating after selding. But most manufacturers would pay for that. Are those droplets at the weld line?

[Cheechako]

Originally Posted by Cheechako

Pretty much par for the course for SS. Hot salty water, enclosed ... perfect for corrosion. I once replaced a corroded SS one with iron pipe fittings... actually lasted longer.
If welded assembly, they could improve it's life by solution heat treating after selding. But most manufacturers would pay for that. Are those droplets at the weld line?

Sorry about the nonsense... having trouble with my keyboard. Should read:

Pretty much par for the course for SS. Hot salty water, enclosed ... perfect for corrosion. I once replaced a corroded SS one with iron pipe fittings... actually lasted longer.
If welded assembly, they could improve it's life by solution heat treating after welding. But most manufacturers wouldnt pay for that. Are those droplets at the weld line?

[boatpoker]

It is not electrolysis ... which is the change occuring in an electrolyte due to the passage of current. Do not take advice from anyone who uses this term.

It is extremely unlikely (near impossible) that it is galvanic corrosion due to the metals involved.
it is extremely unlikely that stray current is involved again other metals in the chain would have corroded first.

Most likely crevice corrosion from poor welds and improper grade of stainless which is a questionable material for this application to begin with... or stress corrosion of which many grades of stainless are susceptible.