Pitting corrosion of 316 stainless steel 6

[ramsay1]

I have a 316 grade stainless steel muffler box on the water cooled exhaust system of my marine engine. The muffler box circulates hot salt water and exhaust gases from the engine to reduce combustion noise (rather like a silencer on a car). It is isolated at each end by non-metallic connectors to the exhaust system. The box has classic pitting corrosion at the bottom, particularly near the welds at each end. I have had a plate welded over the pitted perforations where sea water is leaking from the box but need to take preventative measures. Is a sacrificial anode a possibility?

 

[chief]

Insulated/isolated at either end by non metallic connectors. Would appear you have a electrolytic/galvanic action problem in your stainless section.
Advise the use of a ground from the stainless section to the hull. Back up with a sacrificial anode and possibly use a sacrificial mild steel spool at the first connector

Offshore Engineering&Design

 

[SJones]

The application is pretty aggressive for 316. If the box remains full of water when the boat is at rest, then an anode should be fitted at least. If the boat is lifted from the water and the muffler dries out that is even worse. Another problem is that the exhaust gases will create an acidic environment that will consume the anode more rapidly than in native seawater. You may be better off looking at a high nickel alloy: at least Inconel 625 and possibly higher dependent upon the temperature of the water circulation. Bigger bucks but no pitting and no anodes.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

 

[EdStainless]

An anode will probably not help much. You just can't expect 316 to take this service, especially the welds, and especially in the bottom where a bit of water will stand all of the time.
You might get away with using 2205. You would need an even higher alloy like AL-6XN to be real safe, but 2205 might work and not cost a fortune.
You need to make sure that there are as few welds in the bottom as possible. Make sure that the way things are shaped that NO water will stand in the bottom when the boat is off.


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Plymouth Tube

 

[HEC]

Pitting sounds more like chloride attack. The welds are more susceptable to this too hence the localised attack around the welds. EdStainless I would support your assesment for 2205, if you needed to stay with metal box. There are plastic exhaust boxes available, Nanni Diesel is one source, there may be others.

Mark Hutton

 

[fdac]

I doubt that it's an electrolysis / galvanic corrosion problem. Grounding it out is unlikely to help. Continuous hot seawater is too aggressive for 316 and as suggested a more highly alloyed stainles or cupro-nickel would be better.

 

[strider6]

it seems like a classic pitting corrosion in seawater. the pren of SS316 is not so high and pitting will start at approx 20°C /30°C as you can see in the link:

http://www.assda.asn.au/index.php?option=com_content&task=view&id=170&Itemid=152

galvanic corrosion can increse the problem..
there are system of cathodic protection for stainless steel in seawater like the RCP system (

http://www.forcetechnology.com/NR/rdonlyres/32E33B94-8500-4CD8-8B53-190569524EF1/1407/20644en.pdf)

but i don't think it's applicable in you case.
S


Corrosion Prevention & Corrosion Control