- Thread starter bert
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This looks like pitting arising from inclusions or impurities in the material. The holes can be quite deep and result ultimately in stress fracture, if the part is under high stress. One solution might be to grind the pits back to clean steel, and use TIG with stainless filler to fill the depressions, then grind or machine to suit.
Brad93
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You need to mechanically remove what is causing the pitting which is probably ferrous material. This can be done with a wire brush grinder.
Then passivate With something like this:
Then passivate With something like this:
Antox 71E Plus Pickling paste
<font color="#ff0000"><strong><u>Pickling paste is only used for the removal of weld burn on stainless steel.</u></strong> <br><br></font>Available in 2kg tubs with acid resistant brush. <br><br><strong><em><u>Discounts available on 6 x 2kg case purchase.(see discount page)...
www.elmerwallace.co.uk
Brad93
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That will bend the shaft tigging little spots. Would not recommend that.
Bullet2012
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Are thes out of the boat ?
bert
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Yes, They sit in shaft tubes but the water doesn't naturally change much.
bert
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So clean out the pits as best I can and the pickling paste?You need to mechanically remove what is causing the pitting which is probably ferrous material. This can be done with a wire brush grinder.
Then passivate With something like this:
Antox 71E Plus Pickling paste
<font color="#ff0000"><strong><u>Pickling paste is only used for the removal of weld burn on stainless steel.</u></strong> <br><br></font>Available in 2kg tubs with acid resistant brush. <br><br><strong><em><u>Discounts available on 6 x 2kg case purchase.(see discount page)...www.elmerwallace.co.uk
bert
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I would but they are 12 feet long
if you can avoid welding shafts do so.
I may be wrong (and I’m hesitating to disagree with @voipio here because his comment re inclusions makes a lot of sense), but my immediate thought is that it is crevice corrosion, and caused by a lack of oxygen. It doesn’t usually occur in stainless that is exposed underwater, but common inside shaft tubes where there is little or no flow, and also common in keel bolts, especially in wooden boats. I’ve seen much worse, in prop shafts that are much newer! I’m assuming it is 316 ??
Ross365
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Pickling sounds like a good idea, but if you do, I think you really need to passivate afterwards. Think of the pickle as removing "unprotective material" and passivation as restoring the protective chromium oxide film. Doing one without the other is not so good. Unfortunately, I don't know where you can buy small quantities of suitable materials.
I'd be inclined to say that doing nothing is a bad idea.
I'd be inclined to say that doing nothing is a bad idea.
I have seen pits like that often enough on shafts, keelbolts & other fasteners on boats. Often it is on parts where suppliers have cut corners & used 304 (A2) grade instead of 316 (A4). However i have seen it on A4 as well, Last keelbolt job i did was on a UFO 31 yacht that a fellow club member was taking on the Arc (Atlantic cruise rally) He noticed some movement around the keel area & called me in. A week of grinding fibreglass ensued & another week of reinforcing it all properly with new glass. Half the 30mm bolts were loose & the 2 ton lead keel must have been flapping about all over the place. The keelbolts & nuts were pitted very similar to that & it was classic crevice corrosion. I wanted to pull the worst bolts & replace them, the owner wouldnt afford it & i torqued them up. When the rally started from the Azores I followed the fleet on the GPS tracker & every day the boat was still there. When she finished i was mighty glad. She made it back as well.
I have seen a set of 12mm 316 keel bolts bolts fail completely from crevice corrosion in just three years.
I have seen a set of 12mm 316 keel bolts bolts fail completely from crevice corrosion in just three years.
Ross365
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Can anyone say whether crevice corrosion is more common on 304/316 in recent years than it was 50 years ago (probably not). But if these problems relate to inclusions, does that raise questions more about the original quality of the steel rather than the grade or the service conditions?
Most of my experience is from over ten years ago going back to the late 70's but its always been there, often when a well meaning owner or yard had replaced old wought iron or galv iron bolts with stainless, put any grade stainless through a salty wet oak keel (low oxygen area) & you have a recipe for corrosion. Other aspect is where was the boat moored? was it in a marina with shore power currents floating about? Electrolysis can have strange effects & that is a black art!
I dont think the quality of the stainless has dropped but put it this way if 304 stock is a third less price & you can charge the same for it as 316 which one are you going to use if you can get away with it?
I knew two firms who made marine parts not a million miles from me that did this.
I dont think the quality of the stainless has dropped but put it this way if 304 stock is a third less price & you can charge the same for it as 316 which one are you going to use if you can get away with it?
I knew two firms who made marine parts not a million miles from me that did this.
Brad93
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Those pits are crevice corrosion then. Didn't realise it was in a tube. Over time You basically get an oxygen deprived media so the passive layer gets depleted with local cathode/anode galvanic corrosion. As this occurs the media turns acidic and then the corrosion becomes even more aggressive.
Your stainless becomes about as much use as mild steel. Even 316 is sensitive to this.
The PRE numbers give you an idea of how well an alloy will stand up to this type of corrosion.
Best bet is to either stop any liquid getting into the tube by smothering it in grease or something or make sure the liquid can be exchanged with fresh when in use so it isn't sealed.
You've probably caught that just in time.
Your stainless becomes about as much use as mild steel. Even 316 is sensitive to this.
The PRE numbers give you an idea of how well an alloy will stand up to this type of corrosion.
Best bet is to either stop any liquid getting into the tube by smothering it in grease or something or make sure the liquid can be exchanged with fresh when in use so it isn't sealed.
You've probably caught that just in time.
bert
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So the shaft tubes are designed to be in water as they run on 3 cutlass bearings. Not sure if it's possible to modify the tubes to flow more water than they do.
She sat for 2 years not running and the anodes had gone and plugged Into a 240v power supply so she was electrolysis ing herself.
Are these shafts usable or am I going to need to have new ones made up?
She sat for 2 years not running and the anodes had gone and plugged Into a 240v power supply so she was electrolysis ing herself.
Are these shafts usable or am I going to need to have new ones made up?
