Pitting corrosion inside Bukh dv20 exhaust elbow

[europe172]

Question .
I have a four year old stainless steel exhaust elbow, noticed a slight mark on the outside, took it off to discover deep internal pitting. A quick google brings up articles about sulphur corrosion, how can I prevent this happening, would a post engine run flush with clean water help or is it related to exhaust temperature?

 

[Malish]

A metallurgist will give you a better answer (Vyv... where are you..?). but my hunch is that a fresh water flush will have little effect. Stainless as a metal isnt indestructable and the combination of salt water and acidic combusion fumes which are oxygen depeleted probably nibbles away at it over time to cause the pitting. The original Bukh fitting is a thick lump of cast metal to encourage longevity for much the same reason.

 

[Catalina36]

I fresh water flush mine after every run, which benefits the elbow and the heat exchanger. It may do no good but if the thing fails at least I'll know I did my best.

 

[sgr143]

Possibly more due to chloride ions in the seawater, rather than sulphur:
" Corrosion mechanisms in stainless steel: Pitting corrosion
Pitting is a localised form of corrosion which can occur as a result of exposure to specific environments, most notably those containing chlorides. In most structural applications, the extent of pitting is likely to be superficial and the reduction in section of a component is negligible. However, corrosion products can stain architectural features. A less tolerant view of pitting should be adopted for services such as ducts, piping and containment structures. If there is a known pitting hazard, then a molybdenum bearing stainless steel will be required. "
Article: Corrosion mechanisms in stainless steel

 

[europe172]

Thanks to those who answered.
the pitting is opposite where the sea water is injected so maybe some reaction with steam, I will try an elbow with claimed improved cooling design to see if this reduces the problem.

 

[vyv_cox]

Thanks to those who answered.
the pitting is opposite where the sea water is injected so maybe some reaction with steam, I will try an elbow with claimed improved cooling design to see if this reduces the problem.

Pitting corrosion in chlorides is one of the unwelcome properties of many stainless steels. Do you know the grade of the elbow? Many have been made in 304, which has rather poorer pitting resistance than those with a little molybdenum, i.e. 316. I doubt that the design will have as much influence as the material.

Some engine manufacturers are now making elbows in aluminium, although I do not know which grade. This could be the solution but I have yet to see any feedback.

 

[europe172]

Thanks, the last one I brought was listed as 316, this one is supposed to have a more even temperature

 

[Malish]

I bet there is a price differnece between the grades so a queston for Vyv - is there an 'ordinary sailor' test that could distinguish a 'good' stanless like 316 from a 'poor' one like 304 to prevent a naughty fabricator from making a little extra profit?

 

[vyv_cox]

I bet there is a price differnece between the grades so a queston for Vyv - is there an 'ordinary sailor' test that could distinguish a 'good' stanless like 316 from a 'poor' one like 304 to prevent a naughty fabricator from making a little extra profit?

Sorry, no there is not. Same for various copper alloys. When I was still working in labs it was a simple matter but no longer. The kit prices start at £1000 and go nearly as far as you like.

 

[Malish]

Thanks, I guessed as much...

 

[europe172]

I would like to think that as the material costs are only a small part of the production costs and the difference between 304 and 316 are small , it is most likely that those listed as 316 are, the issue is 316 is also vulnerable to pitting, I just don’t know why mine was very localised.

 

[Malish]

so would I, but if you want to make a little more profit....

 

[vyv_cox]

I would like to think that as the material costs are only a small part of the production costs and the difference between 304 and 316 are small , it is most likely that those listed as 316 are, the issue is 316 is also vulnerable to pitting, I just don’t know why mine was very localised.

Molybdenum additions improve pitting resistance but do not eliminate it. There are alloys with even more molybdenum (at high price) but even then pitting corrosion is not completely prevented.

 

[blackfish]

View attachment 92220Thanks, the last one I brought was listed as 316, this one is supposed to have a more even temperature

I'm thinking of getting one of these to replace my defunct Bukh components. What has been your experience (pitting excluded.) Did it fit without any modifications? Thanks, Tom.

 

[Chris_Robb]

I fresh water flush mine after every run, which benefits the elbow and the heat exchanger. It may do no good but if the thing fails at least I'll know I did my best.

Glad to see someone else does that! I have a simple change over cock to my fresh water (with a non return valve) and flush regularly if leaving for a week or more

 

[europe172]

I'm thinking of getting one of these to replace my defunct Bukh components. What has been your experience (pitting excluded.) Did it fit without any modifications? Thanks, Tom.

It fitted straight only, my only comment is that it is 304 and not 316. , I reread the description and that is how it is described. But as the 316 one was pitted I am hoping that the design of this one will prevent it. I believe that the direct contact of water on to the hot exhaust produces corrosive compounds, this improved design should prevent this .

 

[Carib]

It fitted straight only, my only comment is that it is 304 and not 316. , I reread the description and that is how it is described. But as the 316 one was pitted I am hoping that the design of this one will prevent it. I believe that the direct contact of water on to the hot exhaust produces corrosive compounds, this improved design should prevent this .

I bought one of these (Crazyjane173 on Ebay) - the seller was very helpful. I realised the standard one wouldn't fit because of my heat exchanger above (freshwater cooled version) so he modified it at very modest cost to move the inlet attachment through 45 degrees. It was a 42mm exhaust hose fitting as standard. My existing set up had 50mm ID hose, but I think that's because I had a non-standard elbow to start off with - but worth double checking yours. Time will tell but it seems very well made.

 

[blackfish]

It fitted straight only, my only comment is that it is 304 and not 316. , I reread the description and that is how it is described. But as the 316 one was pitted I am hoping that the design of this one will prevent it. I believe that the direct contact of water on to the hot exhaust produces corrosive compounds, this improved design should prevent this .

Thanks. I've ordered one and it's on its way.