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Author Topic: electrolitic corrosion  (Read 6949 times)

Doug

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electrolitic corrosion
« on: October 10, 2006, 06:36:54 PM »
Hi everyone....

I noticed this picture:

http://listerengine.com/coppermine/displayimage.php?album=lastup&cat=0&pos=7

Those nuts and washers look nice and new and shiny but you can see a "6" on the flat washer, and I think this means thats 316 SS hardware.

Stainless steel is more noble than cast so cast iron will corode faster to protect the stainless. This is the opposit reaction of putting zinc anodes on ships ( since zink is less noble than iron it corrodes faster to protect the hull )

Copper is also more noble that iron and is somethign to be aware of in exposed areas where rust may form in the presence of water coolant ect....

Doug

Quinnf

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Re: electrolitic corrosion
« Reply #1 on: October 10, 2006, 07:42:06 PM »
Doug,

I think you mean galvanic corrosion.  Electrolytic involves an electrical current impressed on the system from outside.   ;)

In order for galvanic corrosion to occur you need three things:

1.  Dissimilar metals.  Check
2.  In electrical contact.  Check
3.  An electrolytic medium in contact with the above.  No got.

Unless all three conditions are met, you're safe.

That said, it's always a good idea to avoid dissimilar metals anywhere they can get wet because hot coolant from a weeping gasket can provide element #3 and eat up that stud which is already too short).


Quinn
« Last Edit: October 10, 2006, 07:54:30 PM by Quinnf »
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aqmxv

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Re: electrolitic corrosion
« Reply #2 on: October 10, 2006, 08:48:54 PM »
Quinn has it summarized nicely.  The only things I'd add are:

1) keep your listeroid out of the rain to avoid adding the electrolyte
2) There can be thread galling problems between SS and hard ferrous alloys.  Pick your anti-seize carefully.

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Doug

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Re: electrolitic corrosion
« Reply #3 on: October 10, 2006, 11:48:54 PM »
DOH !!!

Thanks for the correction Quin, yes I meant to say galvanic. I have some real issues with SS bolts and hard ware on steel. My deparment buys a lot of SS brass and copper for some jobs but sooner or later they find there way holding enclosers to racks of parts on mobile equipment. In an acid, moist, high heat enviroment like where I work parts can rust out so fast it scary and dissimilar metals cause this to accelerate to a point where things can be fine on one PM inspection and falling off a few months later....

Doug

phaedrus

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Re: electrolitic corrosion
« Reply #4 on: October 11, 2006, 03:56:10 AM »

I believe that you’re somewhat mistaken about stainless steel being more noble than iron, yes, it is, but it's not that simple…

Cast iron is not quite the same as chemical “iron” listed in some series, rather it is an alloy of carbon and iron – and can present in several different structures. The corrosion dynamics of cast iron are not, I believe, fully understood, and they are very complex. For example, in water cast iron can corrode to an oxide allowing monatomic hydrogen (a metal, look at the periodic table) to dissolve in the remaining iron-carbon matrix. If this takes place under high pressure and the iron is then removed to an area of low pressure the hydrogen tends to outgas and reform as molecular hydrogen, which takes up much more room – and bang! Chunks of rusty cast are blown off the surface. Dissolved hydrogen tends to make ferrous metals brittle too.

If you’re interested in a good handbook on the matter I can recommend Nigel Warren’s “Metal Corrosion in Boats” (http://www.amazon.com/Metal-Corrosion-Boats-Nigel-Warren/dp/0877421331)  Sounds like your boss needs a copy.

But about stainless…….  Actually in practice stainless steel occupies two places in the galvanic series – it is very active in the absence of oxygen, and much less so with oxygen present. The potential for the active form is something like –0.5 v while cast irons run about –0.6v or less.  What is going on there is the oxide film is less active than the bare alloy metal….but the oxide is more or less transparent and quite thin…and it can go away if there's no oxygen present to keep it in place.  And yes indeed, galling is almost universal with stainless and anti seize, the right one, is very useful.

It is, of course, very irritating to find a sloppy mix of dissimilar metals in machine practice, I agree. I share your displeasure. This poor practice can be dangerous and costly.

I myself am pretty fussy about using the right fasteners and slathering them up with bedding compound where they may be wetted and corrode. Sikaflex or 3M-5300 can be messy, but makes a superb bedding compound when it is essential to use dissimilar metals and then keep them dry.

A practical galvanic series (for salt water – not too far from your situation, I expect)  is at http://www.ocean.udel.edu/seagrant/publications/corrosion.html
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Doug

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Re: electrolitic corrosion
« Reply #5 on: October 11, 2006, 09:00:09 PM »
I guess that about covers it....

Thanks for the input guys especialy the links.


Doug

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Re: electrolitic corrosion
« Reply #6 on: October 12, 2006, 05:34:51 AM »
So, what's the bottom line?

Is galvanized pipe suitable to run coolant to a cast iron radiator from a cast iron engine with bronze fittings? 

I know not to run galvanized pipe in the fuel system, but how about cooling?  Exhaust?  If copper is bad why'd they make car radiators out of it?

BTW--- Plans are for a foot of rubber radiator hose between the engine and the radiator on both lines.  Does that mitigate the effect?

Frankly, I'd heard of the sacrificial plates on boats, but it never crossed my mind in stationary engines.
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Currently running PS-Kit 6-1/5Kw...and some MPs and Chanfas and diesel snowplows and trucks and stuff.

phaedrus

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Re: electrolitic corrosion
« Reply #7 on: October 12, 2006, 07:25:05 PM »
bottom line? It's not so simple. Best to learn the chemistry. In a nutshell metal is metal and not something else because it has extra electrons in the outer shell of the atom. These electrons can move to other atoms...that's "conduction". Different metals and different allony have different "electron pressure" and "share" or "lose" electrons more or less freely. Some metals conduct better n others... Gold is a very good conductor, for example, and iron less so, and so forth - a full list of this makes up the "galvanic series". Obviously if dissimilar metals are in contact one has made an electric cell. If, then one places the cell in a liquid that can conduct, like jacket cooling water, the cell expresses a voltage and a current. The more "noble" metal recieves material, electro-plates, with the less noble metal.
If the metals don't contact then they can't conduct. The copper radiator is isolated from the iron motor - eg no problemo. Of course that's not always practical. Best to use one metal in entire system. Brass, in particular, being very active due to the zinc that makes it brass, can lose the zinc, "dezincify", leaving a fragile ghost of the fitting that will fall apart. (Many boats sink that way.) I hope you read up on the subject though, it's really neat to understand. Best. P
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Quinnf

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Re: electrolitic corrosion
« Reply #8 on: October 12, 2006, 07:57:20 PM »
Jack,

You don't have to learn chemistry.  You can use a rule of thumb.  You're not an old carpenter, so I know you probably still have thumbs! 

Galvanized steel pipe goes fine with cast iron with or without bronze in between.  Old plumber's trick:  bronze nipples make a good transition between steel pipe and copper, but connect steel to copper directly and the copper loses. 

However if you connect copper to steel through a brass nipple the brass will eventually lose in domestic water applications.  When in doubt use bronze between dissimilar metals.  And Phaedrus is right about boats zinking (!) because someone didn't know the difference between a brass valve and a bronze valve in an application below the waterline.  Folks balk at paying $30 for a gate valve at the marine supply joint, thinking they can get the same thing at Ho Depo for $5.  So they get one, and a year later or so, their protective zinc anode falls off the prop shaft in the middle of winter and shortly thereafter that valve starts leaking around the valve stem . . . if they're lucky.  Lots of boats have zunk (past tense of zinc)  like that.

Just as in a car.  You've got a copper (or aluminum!) radiator bolted to the frame.  The engine's bolted to the same frame.  And it may have an iron block and an aluminum head.  Dissimilar metals, connected together, immersed in an electrolytic solution.  Recipe for disaster.  Coolant with anti-corrosion stuff in it will keep that from happening.

Quinn
« Last Edit: October 12, 2006, 08:13:46 PM by Quinnf »
Ashwamegh 6/1, PowerSolutions 6/1 "Kit" engine, and a Changfa R175a that looks like a Yanmar I once knew

Doug

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Re: electrolitic corrosion
« Reply #9 on: October 12, 2006, 11:04:19 PM »
You learn stuff here all the time....

I just knew that metals in contact would corrode and the more noble material would sacrifice the less to protect itself. Now I have a little clearer understand how things work and what not to do!

Doug

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Re: electrolitic corrosion
« Reply #10 on: October 13, 2006, 03:47:53 AM »
Somehow I knew it was more complicated than looking good.   ;)

So, is engine oil an active electrolite?  Grease?  Fuel?

I see 'rally' engines and pumps with brass and copper and bronze parts on them, are there some places or circumstances where certain metals 'fit'?

I have some 1/2"pipe to 1/2"copper tube fittings here....can they be used as the vent for the steel fuel tanks?  How about fuel return lines?

 :o There seems to be a can of metal-eating, boat sinking, electrically charged worms loose!!

I can't wrap my mind around electrical theory for some reason.  I  did pretty well teaching welding until it came to carbon precip in SS....and I go 'blue screen' and non operative until I find some traction in the mechanics of it.   :P
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mobile_bob

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Re: electrolitic corrosion
« Reply #11 on: October 13, 2006, 03:57:04 AM »
just go down to your friendly neighborhood hd truck dealer and get a bottle of DCA additive, it is a suppliment to keep
all the corrosion, electrolysis, and other buggaboos at bay.

pretty cheap stuff, for a lister i would expect 2 to 4 units would be enough

they also sell test strips to test the protection level

if it is good enough for trucks, with their cast iron, steel, copper oil cooler cores, aluminum radiators and heater cores, it should be fine for a lister

bob g
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phaedrus

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Re: electrolitic corrosion
« Reply #12 on: October 13, 2006, 04:35:34 PM »
I agree DCA is great stuff.

Oils and fuels do not conduct electricity is a material sense, eg  there's little galvanic ation
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Quinnf

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Re: electrolitic corrosion
« Reply #13 on: October 13, 2006, 09:10:53 PM »
I think Bob's talking about an additive to coolant, not fuel; I'm not familiar with DCA.

Jack,

There CAN be water traped in fuel/WVO/biodiesel/hydraulic fluid/whatever you burn.  Bacteria can grow at the oil/water interface.  Some have the ability to digest oil and form more water as a metabolic byproduct which gives more bugs a place to grow, which  . . . you get the idea where the horror stories come from about diesel tanks growing "algae."  Others may be sulfur reducers that can feed on sulfur in the fuel, forming stinky hydrogen sulfide in the process.  Though they're sometimes referred to as algae, I've always read that they're bacteria, but the distinction between the two has become somewhat cloudy since I was in college.  Some of my favorite blue-green algae are now classified as photosynthetic bacteria.  'Course the same kind of people now say Pluto isn't a planet, so who gives a hang?

Fuel or oil or whatever you're burning by itself won't normally be corrosive, however if there is any water in it, the water will eventually settle in crevices and under the right conditions it can promote crevice corrosion or can become acidic and THAT can cause problems.  I suspect the sulfur level in the fuel is a factor there.  If you're sure there's no water in your fuel, such as after a fuel filter/water separator, copper/brass/whatever should be OK to use.  However anywhere there could be any water in your fuel/oil/whatever, it's not a great idea to use copper.

Yes, brass and bronze fittings are common on the exterior of old engines.  Even marine engines used to have a lot of brass/bronze.  I don't see a problem if the brass is kept out of contact with water.  In a working situation everything would have been covered with a thick coat of paint, so if the duffers want to strip the paint and shine up the brass, more power to them!

As for copper vent on a steel tank, it's OK to use as long as you go through a bronze fitting between the two.  At the very least, really goop up the threads with anti-seize grease.  All steel would be better, though.  Use whatever you have handy but keep an eye on it.  If you see a problem developing, then change the fitting or vent line.

Quinn

Ashwamegh 6/1, PowerSolutions 6/1 "Kit" engine, and a Changfa R175a that looks like a Yanmar I once knew

phaedrus

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Re: electrolitic corrosion
« Reply #14 on: October 13, 2006, 09:46:43 PM »
“DCA” is jargon, sorry. Stands for “diesel cooling additive”. Diesels tend to create cavitation on the water side of their liners at right angles to the crank axis, and mostly on one side, depending on rotation. It’s from a combination of piston slap and combustion pressure rise characteristics, so I’ve been taught. Anyway this cavitation can ruin a liner is a short time, and make a hole right through the wall.  I have no idea if this is a problem with Lister types, but if it is I would be surprised. When I got my 7.3 Ford the first thing I did was buy a can of DCA or whatever Ford calls it.

Still, the additive is cheap and, so far as I can see, can’t hurt. Google “DCA” or “nalcool”

From the web: “Supplemental Coolant Additives (SCA's) are important to ensure long diesel engine life. Their primary function is to protect the cylinder wall from cavitation erosion (pitting). Secondarily, they bolster the anti-corrosion additives in antifreeze. The purpose of SCA is to prevent cavitation erosion of the cylinder lines. The additive concentration should be checked every 4 months to assure that it is properly maintained. If the coolant system develops a leak that requires additional coolant to be added, the SCA concentration level should be checked and DCA-4 added as required to maintain the 1.5 to 3.0 units per gallon level.”
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