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Ian B
07-17-2006, 07:53 AM
Here offshore, we use small rupture disks to protect equipment against overpressure. They're screw-in brass bodies that hold thin nickel domed disks (about 5 thou thick). If pressure goes too high, in this case 300psi, the disks pop.

We've been having troubles with the disks failing without being overpressured (I tested a batch - the new disks popped at the right pressure). Under a microscope, it was obvious that external corrosion had pitted the disks, weakened them and they were failing.

Differential metals, salt water humid environment, electrolytic corrosion; simple!

So - rather than changing materials (we have a good stock of the bodies & disks), we tried packing the outside of the disk & housing with grease to keep the salty water away; no electrolyte, no corrosion, right?

Pop!

Same problem. I pulled a disk that had been grease packed from new and in service for 2 months; the outside of the nickel disk is black and pitting has occurred, especially where the disk is in contact with the brass. There's no sign of water having penetrated the grease.

Questions:
- Could the grease (it's a normal lithium-based workshop grease) be causing the corrosion & acting as an electrolyte?
- Any suggestions what we can do to prevent the corrosion? Different coatings, for instance? I'm thinking of using Boeshield...

tia,

Ian

speedy
07-17-2006, 08:12 AM
Ian, have you tried Chesterton nickel compound? I use it as a quick fix corrosion preventative on my vehicles or machinery. I also used it on my bikes and in areas on these machines that were subjected to grit,grime, heat and water wash at speed. No problems there either. Not the same environment but it works for me.
I first came upon this product while working at Marsden Point where it was used on high pressure/heat vessels. Brilliant stuff.
I will have to purchase some more as my tin is almost empty. trouble is that all I have been able to find here is the aerosol version of the product which is IMO a wastefull messy type of application.
http://www.chesterton.com/products/tp/product/?product=725
My 2c worth.

Ian B
07-17-2006, 09:02 AM
Thanks Ken,

We have something similar in stock; sort of copperslip, but then nickel-based. I'm thinking of trying a few different ideas and tracking the results for 2 months - I'll certainly try one with the nickel compound.

Ian

A.K. Boomer
07-17-2006, 09:27 AM
Lithium is good lubrication but i dont know how it stacks up for a corrosion inhibitor,,, I know this sounds simple but to keep water and grit out i use boat trailer wheel bearing grease,,, it has a "stick to your ribs" quality --- I used to use the highest quality grease i could find for everything but what good is it if it works out of the way and leaves the parts exposed....

Evan
07-17-2006, 10:47 AM
Read here about corrosion of yellow metals by extreme pressure lubricants and how to test for it (easy). Most extreme pressure lubricant additives are corrosive to copper.

http://www.schaefferoil.com/pdf/GearDriveLubricants.pdf

Magic9r
07-17-2006, 06:23 PM
Grease is a suspension of oil in a soap base, your results don't seem that surprising.
Try a thin coating of molten wax instead of grease.
How about silicone grease? I don't think theyhave the same composition as petroleum greases,
Or maybe about a brush painted etch primer followed by brushed 2-pack epoxy paint?
Can you get a non-metallic rupture disk for your application? I know some are available in a ceramic for some pressure ranges.
Good Luck,
Nick

Ian B
07-19-2006, 06:02 AM
Thanks for the suggestions guys; today, we installed 3 disks with different protective coatings;

- Silicone grease
- Copper based antiseize
- Nickel based antiseize

I'll let you know how they stand up...

Ian

nheng
07-19-2006, 12:16 PM
Instead of a grease based approach, how about conformal coating the disks prior to installation (similar to what's used to seal printed circuit boards in harsh environments), or perhaps specifying them with a thin laminate of (appropriate) plastic on one or both sides. Either of these should have minimal effect on the rupture characteristics, but of course would need to be tested. Den

Evan
07-19-2006, 01:04 PM
I would try coating with polydimethylsiloxane. It's an inert silicone oil used in among other things, Pamela Anderson's front bumpers. Once something is coated with that stuff it is almost impossible to remove and leaves a permanent layer. It should make no difference to the rupture properties and doesn't react with much of anything. You may have some around in the form of fuser oil in a cleaning wick assembly for a laser printer.

Millman
07-19-2006, 01:12 PM
{{polydimethylsiloxane.}} Having 500 lbs. of that stuff in my tool boxes was required by most companies I worked for. They would come out on a regular basis and check your boxes. You could be fired for having less than 476 lbs. in your posession. Evan, what were you thinking????

Evan
07-19-2006, 01:19 PM
I have a couple of jugs of it in my shop, if you will pardon the expression. :D

You can buy a product at any auto parts store called Rain-X for coating windshields so the rain beads up and runs off. It's polydimethylsiloxane.

J Tiers
07-19-2006, 02:01 PM
Grease is a suspension of oil in a soap base, your results don't seem that surprising.


Ah, but there are greases, and then there are greases. They differ in the "soap base", among other things.

And the term "soap" refers to a classification of compound. We are not talking about "Dial" here....

The soap may be any of several types. The types vary in temperature response (melting point), in resistance to water, and other properties. A part slushed in a water resistant grease (if it doesn't react with the grease composition) should stay good for a considerable time if it doesn't get hot enough for the grease to run off.

But, I don't know how resistant a lithium grease is. It would presumably be a lithium soap base, and whatever the properties of that base are, that's what you would have.

The lithium types are made for a particular purpose, and I don't think slushing parts is one of them.

pcarpenter
07-19-2006, 02:01 PM
Here we go again.....Give Evan a break Millman. I knew just what he was talking about when he mentioned it, from a previous post that I am sure you would have given him crap about too. It is used as fuser oil and most folks could come up with a bit if it was useful...maybe a lot more as the stuff is used in lots of laser printers and copiers as a consumable. The amount left in an empty bottle would go a long way.

Thanks to a post from Evan about the same material before, I was able to get some and put it to a very good use on the plastic sheave bushings in the VS head on my Bridgeport. There are plastic bushings glued in place that slide up and down both on the motor shaft and the spindle end. Petroleum is questionable on plastic and you don't want to use anything that will sling off and get on the belt. polyhexawhatyoumacallit is ideal. Get any on anything and wipe it off and it will be really slick for a really long time. A thin film works wonders and it is OK on plastic.

Paul

ttok
07-19-2006, 05:18 PM
How about coating the outside of the frangible disk with paint? A.T.

J Tiers
07-19-2006, 05:37 PM
or even plain old vaseline?

Mcostello
07-20-2006, 12:04 AM
How do you get it out of Pam Andersons front bumpers IS the question I like to see answered.

Rich Carlstedt
07-20-2006, 12:29 AM
Ian
I am somewhat familiar with Rupture disc's
we used them in Plastic extruders, only at 6,000 to 12,000 PSI.
First of all, you are working with Nickel, one of the best corrosion proof
metals around. I suspect that either you are not getting Nickel discs, or
the disc's have a flux product residue. or you are getting spikes in the pressure line, or you are running too close to failure during operations
So
Can you confirm the base metal and is it an import?
Have you washed the disc's before using them? or litmus test them
I have seen salt spray tests on Nickel and it sure seems your experience
is exceptional. If you want to protect the disk coat it with paraffin wax.
What confirms that the pressure is OK, We had the problem with several customers, and it was the pressure gages that were off. (ie. the disc's worked)
Lastly, does the failure occur at the same time of day ? or point of operation?
( customer had an operator close off the recording gauge , and increase pressure to get more outpoint )

hope i have given you food for thought.
Rich

wierdscience
07-20-2006, 12:32 AM
What about PTFE pipe sealant? Or a thin wipe of silicone gasket glue?

darryl
07-20-2006, 03:41 AM
Or polish them up with carnuba wax.

Evan
07-20-2006, 09:32 AM
Should chrome plate very nicely.