Raw water pump corrosion

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AusCan

AusCan

Guru
Joined
Jul 15, 2012
Messages
3,218
Location
Australia
Vessel Name
Kokanee
Vessel Make
Cuddles 30 Pilot House Motor Sailer
I was making use of the extra time on hand, and checking the raw water impeller yesterday. The impeller didn't seem to be coming out as easily as usual. With only access from above, I couldn't see what the problem might be. Instead I pulled off the complete pump to have a better look. I'm glad I did.

The end of the impeller drive shaft had corroded away leaving just enough to keep the impeller turning.

My initial thought was that it's a stray current problem.
Now I'm thinking it might be cavitation issue due to lack of water flow. I always thought my thruhull valve on the intake was small at only 3/4" but when I double checked it today it turns out it is only heavy walled 1/2" valve. It may have been big enough for the original Volvo (or not), but it may have problems supplying the water flow for a 4 cylinder diesel.

Any thoughts?
 

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I dont think cavitation caused your issue. With the impeller in place the shaft is basically protected from the water flow and any sort of cavitation. Also cavitation would cause wear in which I would think the surfaces would be bright.

I assume that shaft is stainless. It looks rusty which would point towards crevice corrosion. Also it just looks like crevice corrosion to me.
 
If it was a stray current or galvanic corrosion issue I believe it would be bright.
 
Old Dan,
No the engine isn't bonded.

I would have thought if this was the issue, that the brass would be disappearing rather than the stainless. But there is so much about bonding a corrosion that I don't understand. I read an article that suggests complete bonding and it makes sense, then I read another that says not to bond and that makes sense to me too.

You may be right, Duvie. It could be crevice corrosion.

I sent the pic to Vetus to see what their technical guys have to say. So far their sales guy is offering a 30% discount on a new pump.
 
Stainless corrodes in the absence of air. Out of the water, stainless develops a protective layer. In the water it creates a biofilm that causes corrosion. Usually in crevices or localized areas. It also dependent on chlorides in the water and water flow.
Stagnant salt water, like inside a pump that doesn't run often, encourages biofouling that can lead to crevice corrosion. Even 316. Lots of info on the web.
Best way to stop it is to pump a few gallons of fresh water thru the raw water system before storage.
 
Yes Lepke, that could be the answer.

I used to do that with my old raw water cooled Volvo. It may be prudent to do the same with this engine if I'm not going to use it for a while. Perhaps install a tee & valve tied into the fresh water system so I can close my seacock valve and purge the system with fresh water before shutting down.
 
I suspect that you have checked to see if a magnet will stick to the shaft ?
 
Amazing how bad things can actually get and keep working just fine, until....

Like one of our members tag line says, "Everything on your boat is broken, you just don't know it yet"

pete
 
I don't agree yet as it looks to me that the remaining tab is bent and there is a decent crack at it's base. It looks a little twisted to my eye as well....I don't see how that can be caused by any sort of corrosion. I'd go looking for the broken off tab. Too new to the forum to figure out how to post an enlarged picture but you'll see what I mean if you enlarge the photo.
 
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Greetings,
Mr. B. I see what your saying but one tab (top one in picture) has been gone a long time. The disappearance of said, missing tab is most probably due to "crevice corrosion". https://en.wikipedia.org/wiki/Crevice_corrosion


Somehow the pump was still working to some effect but the remaining tab was doing all the work and cracked as a result. Just a WAG on that part.
 
Do you see the crack at the base of the other tab and the slight counterclockwise twist to the remaining tab? Had similar issue with sailboat friend- impeller fused to housing as it sat a very long time. When he cranked it over impeller blew up. We didn't discover the twist to the tabs until trying to get the new impeller in. I do agree the crack and twist could be from running awhile with only one tab. I'm curious when was the last time the OP checked that impeller- I would think years ago.
 
Lepke said:
Stainless corrodes in the absence of air. Out of the water, stainless develops a protective layer. In the water it creates a biofilm that causes corrosion. Usually in crevices or localized areas. It also dependent on chlorides in the water and water flow.
Stagnant salt water, like inside a pump that doesn't run often, encourages biofouling that can lead to crevice corrosion. Even 316. Lots of info on the web.
Best way to stop it is to pump a few gallons of fresh water thru the raw water system before storage.
Click to expand...

As a Certified Marine Corrosion Analyst. I agree.

I've seen similar on boats that sat in water for years without moving.

EDIT : Oooops .... my certification expired yesterday, Now a retired corrosion analyst :)
 
SS comes in different "flavors". Some alloys are better than other.
I had a problem with my Northern Lights exhaust casting. Had it replaced. NL was called and their response was basically, 'yea we know so we changed the alloy.' No recall, no discount for the new type.
 
I checked the impeller several times and its always looked like new. This is the first time I've unbolted the pump to allow me to see the shaft.

A magnet test is indicating a high level of iron. The shaft is certainly not 316 stainless. I'm waiting to hear back from the supplier.
 
I've done some checking with Jabsco/Xylem regarding on the shaft material. They finally got back to me and confirmed the shaft is made from 431 stainless steel.

According to the Australian Stainless Steel Development Association, grade 431 is a martensitic stainless steel which means it has been thermally hardened. This makes it suitable for pump shafts. The problem in this case is that 431 is less corrosion resistant than grade 304 stainless. ASSDA states that "Grade 431 is sometimes used for boat shafting and works well in fresh water but is usually not adequate for sea water."

https://www.assda.asn.au/blog/40-grade-431

Hmm - not a good choice for a raw water pump. :nonono:
I passed on the info to Vetus, but was not happy with their response.

So it sounds like I've got a lemon for a raw water pump. For now, I'll just flush the raw water circuit with fresh water at the end of each use. I'm currently preparing to install a system which will make this a simple task. I'll also be pulling the pump regularly to check the condition of the shaft. I'll also look to see if another type of pump is available.
 
I wonder if hardened stainless has to have a higher iron content than others that are less susceptible to corrosion. The more corrosion proof the softer the metal and it could twist easier? Maybe that's why they use it as those tabs are pretty thin on my pump. Next question for corrosion experts- wouldn't the bronze corrode before any kind of stainless?
 
Rich mans game to mix iron and seawater!
How much is a new pump? I might consider having a shop make the shaft out of a duplex SS alloy. But, a few questions come up. Does the shaft act as a bearing surface? Turn an old prop shaft (aquamet 22) down to the correct dimensions. Lots of options come to mind.
 
Cavitation damage will show up on the housing or end plates on the discharge side of the pump. Take a close look at these areas. If you see erosion then your through hull is too small.
John
 
Its a small pump on my little 42 hp engine. A new one was $AUD 350 (about USD$250) less a 30% discount I was given by the local Vetus distributor. It's not so much the cost as the concern about reliability. Although this one made is to 500 hours, I wouldn't want to push my luck with it lasting that long again.
I looked into the material used on other Jabsco pumps, and any that state the shaft material all say that it is 316 stainless. For whatever reason this one uses 431 stainless, and I didn't get a good reason why.
 
AusCan said:
Its a small pump on my little 42 hp engine. A new one was $AUD 350 (about USD$250) less a 30% discount I was given by the local Vetus distributor. It's not so much the cost as the concern about reliability. Although this one made is to 500 hours, I wouldn't want to push my luck with it lasting that long again.
I looked into the material used on other Jabsco pumps, and any that state the shaft material all say that it is 316 stainless. For whatever reason this one uses 431 stainless, and I didn't get a good reason why.
Click to expand...

If you find a metallurgist, you can ask him or research the internet.
 
Johnma,
There is a bit of wear on the housing, but it looks like it may be due to bits of the corroded shaft that dropped off may have caused the damage.
Regardless, I'm installing a larger through hull next time I slip the boat, just to be sure. A new Groco seacock, throughhull and strainer just arrived in the mail.
 
Began said:
I wonder if hardened stainless has to have a higher iron content than others that are less susceptible to corrosion. The more corrosion proof the softer the metal and it could twist easier? Maybe that's why they use it as those tabs are pretty thin on my pump. Next question for corrosion experts- wouldn't the bronze corrode before any kind of stainless?
Click to expand...

Good bronzes will do better in seawater than SS.

Much of SS corrosion resistance is due to the Chromium forming a highly resistant skin .The catch is to form that skin there must be oxygen present. Cut off or reduce oxygen free flow to the SS surface and that skin will fail. The iron is no longer protected so rusting can occur.
Weld joints done improperly , reducing Chromium, can suffer corrosion even though the parent metal of the part is fine.
Worse is SS can develop crevice corrosion which show on a surface as a tiny crater but what you can,t see without cutting or other testing it has produced a cavity seriously compromising the part.
There are SS alloys that are far more resistant but regardless it is possible for SS to be damaged this way if oxygen is cut off long enough.
SS must be chosen properly to perform. The wrong alloy choice can be trouble. Oxygen must be present for protection.

There are some books specifically dealing with metals use in marine applications, problems
,Choosing and so on.

Remind me and I will post two I have read over the years. I,m not home and the names escape me right now.
 
In my experience, xylem has ruined every company they have bought by ruining their products. I know of a certain Jabsco bilge pump where an internal part changed from stainless to steel, and lasted a very short time. Pump turned on and did nothing. Not what you want when your bilge pump is called for. My experience with Rule pumps has been similar post ITT/Xylem purchase. Haven’t met a rule pump since that didn’t fail.

At this point I won’t let a Xylem product on the boat, and have switched to Johnson and Whale for the most part.

For your raw water pump, I think there is a good chance that you can get a Johnson or Sherwood pump that will fit, and might be a good long term solution. You could contact Depco in the US to see. They are quite good.
 
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The books:
Metal Corrosion in Boats -- Nigel Warren ISBN 0 540 07397 0 [1980]
The Boatowners Guide To Corrosion -- Everett Collier ISBN 0-07-155019-4 ]2001]

They are good guides to metals, corrosion and seawater.

They are getting old but may still be available or look for them in Ebay, especially the first.
 
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