Sarathlal
New Member
The anode looks good.....,thats why im doubting its a material defect...feverly shore power is used/ i can say not using shore power for this boat.....
Pitting on Stainless steel duoprops
- Thread starter Sarathlal
- Start date
GrahamHR
Well-Known Member
The anodes are not connected electrically to the stainless steel parts of the props. So, the anodes do not protect them. The condition of the anodes has no relevence to what is happening to the stainless steel.
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DAKA
Well-Known Member
my money's with you
Its not cavitation, cavitation would clean the rust off and leave a nice shine !
My first thoughts were foreign body in the SS , however the area of rust is too large so I thought maybe a paint drip had caused the issue, seeing as the props were fitted brand new Im going to guess that someone fitted them and left stickers on ,which destroyed the corrosion resistance of the ss by starving the oxygen.
NB
Photos of this sort need to be taken on macro setting, most cameras over £60 have the setting and allow a very close close up, look for the 'flowers' icon.
GrahamHR
Well-Known Member
my money's with you
My first thoughts were foreign body in the SS , however the area of rust is too large so I thought maybe a paint drip had caused the issue, seeing as the props were fitted brand new Im going to guess that someone fitted them and left stickers on ,which destroyed the corrosion resistance of the ss by starving the oxygen.
No paint or stickers on my first set; corrosion occured ultimately forming voids/ pits below almost completely intact thin skins of uncorroded stainless steel
No paint or stickers on my second set; exactly the same type of corrosion as on the first set.
There are a lot of small pits near the plastic bush, suggesting the corrosion originates from that region; any porosity exposed on machining the bore of the prop hub would form pretty nice crevices when the plastic bush is pressed up against them.
Incidentally, the drive itself shows no sign of corrosion, but it is of course protected by anodes and isolated from the stainless steel parts of the props.
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vyv_cox
Well-Known Member
Unfortunately there is a great deal of misinformation in this thread, and some total nonsense. I have struggled to find the composition of the Volvo props to this specification but I have found substitute or equivalent ones that are made from 15/5 PH. I assume from the description of this alloy that the Volvo ones are the same, or another precipitation-hardening steel, such as 17/4 PH. Stronger materials are required for high power props for fast boats, which is why nickel aluminium bronze is not specified for these duties.
Austenitic stainless steels (300 series) have good corrosion resistance but quite low strength, which cannot be increased by heat treatment. Martensitic stainless steels, 400 series, are heat-treatable but their resistance to corrosion in seawater is not great. Somewhere between these two are the precipitation-hardening stainless steels, which can achieve quite high strength and good corrosion resistance. However, their resistance to crevice corrosion, and to pitting, which occurs by the same mechanism, is not very good. There is a useful paper on the subject, which although old is very relevant, at http://www.dtic.mil/dtic/tr/fulltext/u2/a010468.pdf.
The paper describes crevice corrosion that was particularly severe at weld heat-affected-zones. I do not know the construction method of the Volvo props but your photos suggest that they may be fabricated. Even if cast it would seem that crevice or pitting corrosion might be a problem. I know of cases where pitting was initiated at relatively minor scratches or other surface damage in 400 series stainless steels and there are examples on my website of crevice corrosion beneath fouling or perhaps blobs of antifouling paint.
DAKA
Well-Known Member
vyv cox is on the ball so I will leave him to it
just as a thought seeing as you dismiss my paint / sticker theory........
on lift out was there the usual black gunk of grease/unburnt diesel/exhaust soot oozing out of the prop hub ? that would be enough to starve the stainless of oxygen to create the problem under the prop locking nut.
GrahamHR
Well-Known Member
vyv cox is on the ball so I will leave him to it
on lift out was there the usual black gunk of grease/unburnt diesel/exhaust soot oozing out of the prop hub ? that would be enough to starve the stainless of oxygen to create the problem under the prop locking nut.
My boat is petrol engined; no black gunk on the prop, just a light coating of thin brown slime ( plus a few barnacles and tube worms, but those were in the hubs).
The prop locking nuts on these drives only contact the aluminium hubs (and the propshafts of course !); no contact at all with the outer stainless prob hub and blades
The props seem not to have been fabricated; they are probably cast, On both the sets I have here is some witnessing of where there is a very slight mismatch where the tooling cores for the blades butted up against the cores for the hub
edit; I just had a closer look at them. It looks like there were 3 outer tooling cores, each with a cavity for one prop blade and a 1/3 part of the hub outer surface. Presumably with a single inner core for the hub bore, which was machined post casting to match the required size for plastic sleeve.
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