new picture

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Big Pete
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new picture

Postby Big Pete » Fri Sep 09, 2011 4:08 am

Another photo, showing how not to do it!!
This is one of the bolt holes for an inspection door on a crane pedestal.
What do you think happenned?

BP
CraneInspectionDoors.JPG
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It is always better to ask a stupid question than to do a stupid thing.

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JK
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Re: new picture

Postby JK » Wed Sep 14, 2011 2:17 am

you know, I can't help but think that the door was so coroded that the bolt head pulled right through.

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Big Pete
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Re: new picture

Postby Big Pete » Sat Oct 22, 2011 10:32 pm

Right again JK, the door pretty much fell off without removing the bolts.
It is another example of using the incorrect materials. The holes are rusted in perfect hexagons, exactly matching the bolt heads. Why? Because someone decided it would be better to use stainless steel bolts, than normal mild steel bolts. A splash of Sea water for electrolyte and a nice batterry was made. Nice example of Galvanic corrossion.
It reminds me of an oil rig standby vessel I sailed on over 20 years ago, the ship was fitted with a sprinkler system to keep the ship cool if it had to fight a platform fire. All the pipe work was made of galvanised steel, screw threads were cut on the ends of the pipes and stainless steel nozzles screwed on ! BP managed the ship and they couldnt understand why all their expensive corrossion resistant S/S fittings all dropped off and disappeared overboard after a couple of months.
BP
It is always better to ask a stupid question than to do a stupid thing.

Pengze
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Re: new picture

Postby Pengze » Sun Nov 11, 2012 3:53 am

Dear Sir,
All the while, usually when we encountered rusted & corroded mild steel bolts & nuts, we will replaced with stainless steel or galvanised bolts & nuts especially where the seawater is in contact / in the vicinity / exposed to the surrounding sea conditions.
Pls advise if this is wrong and why ?
You mean to say that we have to replace with mild steel bolts & nuts again ?
Appreciate yr kind comments.

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Big Pete
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Re: new picture

Postby Big Pete » Mon Nov 12, 2012 12:02 am

Hi Pengze,

If you have two dissimilar metal,s ie Stainless Steel and Mild Steel in contact, in the presence of an Electrolyte (Sea Water) you are creating an Electrical batterry and causing Galvanic Corrossion. The Metal that is more reactive will be destroyed by Galvanic Corossion. It is better to use Mild Steel Bolts when bolting mild steel components and Stainless Steel Bolts when Bolting up stainless steel components. You would have the same problem with Brass Bolts and Steel components.
If you cover the threads with grease when you bolt up and paint the Mild Steel bolt heads properly you will not have any problems with corossion.
BP
Last edited by Big Pete on Tue Nov 13, 2012 12:15 am, edited 1 time in total.
It is always better to ask a stupid question than to do a stupid thing.

Pengze
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Re: new picture

Postby Pengze » Mon Nov 12, 2012 7:35 am

Dear Sir,
If I may correct, you mean to say Stainless steel bolts & nuts for stainless steel components and not for mild steel components.
What if we used stainless steel bolts & nuts for mild steel components and painted up, would that cause corrosion ?
How about stainless steel bolts & nuts for cast iron components ?
Looking forward to yr valuable feedback.

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Big Pete
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Re: new picture

Postby Big Pete » Tue Nov 13, 2012 12:35 am

Hi Pengze,

as you saw I made a typo there and have corrected it now.

Stainless steel is much more expensive than mild steel, why would you buy stainless and then paint it?? There can also be a problem with crevice corrossion on stainless steal threads. If you use Stainless Steel exposed to Salt Water you have to use the correct grade, some grades of Stainless Steel (there are hundreds of different Alloys classed as Stainless Steel) many DO corrode in Sea Water.
If the paint coating breaks down there could be very intense local corossion.
There has been discussion here on Galvanic / Electrolytic corrossion before, basically if you want to avoid corosion you use materials that are close together in their Chemical reactivity.

There was an old English definition of an Engineer "Someone who can make something for 20 pence that any fool could make for a pound". Painting Stainless steel would be like Silver Plating solid Gold jewelery.

Same problem with Stainless bolts and cast iron. The Bolts are always cheaper than the fittings they are bolted to, so why would you use Stainless Steel bolts that will not corrode themselves but will destroy the expensive fittings they are used on??

Galvanised or BZP (Bright Zinc Plated) Bolts are good, the zinc will corrode before the mild steel of the bolt or the fitting, and they are far cheaper than stainless.

BP
It is always better to ask a stupid question than to do a stupid thing.


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