two different metals

Electricman15

Can anyone help me, I'm doing my first DIY install of baseboard heating. Well I have been noticing on some photos of installs black pipe is being joined with cooper or brass fittings. I was told you are not to join two different metals. Is that true?

ChrisJ

Opinions

You're going to get many different opinions on this subject.



My understanding is when you connect dissimilar metals together and fill them with a liquid you essentially make a battery. One side being the anode and the other being the cathode. basically when you connect steel and copper together over time the iron will slowly be pulled over to the copper side, accumulating on the copper surface while the iron will slowly disappear and rust until you have a leak.



Now that said, I have 80+ year old refrigerators which have copper and steel brazed together as well as dissimilar aluminum screws screwed into steel nuts. They lasted over 80 years with zero problems.



I used some copper around my steam boiler, mainly below the water line because I feel the positives of it out weight the negatives in those locations. Will this cause the cast iron fitting I have the copper screwed into to rott out? I believe there is no doubt that it will but due to the thickness of the cast iron fitting there is a good chance it will not be in my life time. It is my understanding the reaction takes place very close to the joint so something a foot or two away shouldn't be effected.



It's typically a good idea to transition from steel to copper using a brass connector of sorts. If you can use a brass nipple or fitting they say it helps because brass is in between copper and steel so it's not as dissimilar.



Here is a good Wiki page on the subject.

http://en.wikipedia.org/wiki/Galvanic_corrosion

Roger_at_SorbOx

Galvanic corrosion

As Chris said, connecting two different metals in the presence of a liquid forms an electrochemical cell (same idea behind a battery). It allows one metal to function as the anode--giving up electrons, and corroding at an accelerated rate--and the other to function as a cathode, receiving electrons and being protected from corroding.



Here is a presentation from the RPA archives about corrosion causes, damage and prevention. Discussion of galvanic corrosion begins at 34:08, and there is a chart of the electrode potential of various metals. (the less noble metals will function as an anode, corroding more quickly. The more noble ones will function as a cathode).



https://www.youtube.com/watch?v=03uoOsgk_M8&feature=youtu.be



But galvanic corrosion (like any form of corrosion) depends on conductivity of system water. More conductive water will cause faster corrosion; less conductive water will slow the process. That helps to explain why sometimes people won't have big problems with galvanic corrosion.

icesailor

Dis-similar:

Historically and in practice, it is far safer in a heating system than compared to jumping out of airplanes with a parachute.

KC_Jones

Boats

I grew up around boats and my grandfathers had bronze, stainless, brass all under water.  There was corrosion, but on boats Zinc is added as a sacrificial anode so the others don't corrode.  When you see before and after of those zincs it amazing what happens.  In that situation the salt water is the biggest factor.  It conducts very well way more than any water in a heating system.  It's extreme, but a good illustration of what can happen.I think this concept is similar to what is done with water heaters, direct the corrosion to something easily replaceable to save the expensive parts.  It is really fascinating stuff.  I am a designer and work with aluminum, galvanized and Stainless primarily.  We are always conscious of what metals are in contact in the "wet area" of any unit and make all efforts to isolate them.  Since our units aren't submerged we can get away with simple isolation.  Temperature can be a factor as well.  In very cold temperatures the effect is minimized.  On our evaporators we don't even do isolation.  Chris this could be why it holds up so well on those refrigerators?!

Gordy

Hydronic safe

Yeah Ice mines been there for 62 years black pipe to copper with no issues.hard to get away from some kind of dissimilar piping/component connection. Unless it's an all iron hot water radiator, or steam system.



Not to say this topic is not important to be aware of. There is always a chance of stray current in the water pipes through improper bonding, and or grounding of electrical systems which would accelerate corrosion.

Techman

two metals

What is the purpose of di-electric fittings on DHW units? Part of my job function onboard a WW2 submarine was to inspect/replace all sacrificial zink elements on a routine inspection. Electrolis does take place!

icesailor

Aluminum Boats:

For interesting examples of electrolysis and corrosion, trot on down to your local salt water Marina and look for any aluminum sail boats. You will notice at some place on the boat, a wire running from a fitting on the deck, looped over the safety lines, and the wire in the water. With a big zinc plate on it. Salt water is a wonderful battery solution. If you are looking for a nice lightweight aluminum skiff for your salt water adventures, always check to be sure that someone didn't paint the bottom with copper anti-fouling paint. It will dissolve the aluminum boat and it will leak like the "Leaki-Tikki on that old TV series with Gardner McKay, Adventures In Paradise.

Another place to notice fine things happening, is an unskilled welder who backs his welding truck up to a steel boat, docked next to a wharf with steel sheathing. Connect the ground clamp to the top rail next to the dock, and go into the engine room or some other far away place to commence welding. If the stars are aligned and all the moon is in the Seventh House, and the ground cable is on the ground, the closest way to complete the circuit (if the ground clamp isn't a good connection) is for the current (dc) to go back to the welder through the easiest way. Which might be along the water line, across the water, through the steel sheathing, through the ground, inducted through the ground cable on the ground, and back into the welder. The problem is often noted when they notice the boat is sinking because the current going back slices a line along the water line of the hull, allowing water in. Happens regularly. Always put the ground clamp as close to the source you are welding for the best and safest connection.

The only good ground is a better ground.

ChrisJ

Not exactly

KC,

It all depends. The refrigerant lines which are copper and brazed into steel in places do not seem to have problems for some reason. However the aluminum bolts which directly touch the stainless evaporator and then are threaded into steel bolts which sweat constantly do have some issues.



However, my point was even with issues after 80 years none of failed. We do replace them though just because how can you put it back together like that. I use 316 stainless hardware on my restorations. I've been tempted to use nylon to stop the sweating by everyone seems to think they will be too weak.



The pictures are of two different units so you will notice differences, but the before photo is still accurate of what you find when you tear them apart. The second picture is of course after the work is done. The evaporator is isolated from the top plate via strips of cork. The large fender washers are isolated with thick rubber washers which cannot be seen other than maybe the depth of the two bottom ones in the after picture.

icesailor

Submarines: (SIlver Brazing)

Submarines are a different animal altogether.

I define Submarines as a huge enclosed can that sinks that is a platform for a huge 3 phase generator(s) that can fire projectiles while under water, with a huge storage battery inside. A potential source of all kinds of problems, None of them good.

If you don't keep that 3-phase well balanced, you can sink the boat.

We had a discussion a while back on soldering techniques. About the importance of heating from the back to front of fittings so you are sure that the whole fitting is hot for the rod material to properly flow. The Submarine Thresher that sank off Cape Cod in 1963, was blamed on improperly brazed sea water lines. Not properly filled.

icesailor

Rusting Steel Bolts:

CJ, The rusting of the steel bolts are because of exposure to the air and is caused by the moisture in the air and the dew point. If you create a condition where moisture laden air can not come in contact with the fasteners, the rusting should go away. Does the plate get condensation? Cover the entire plate with Armaflex sheet with holes cut out for the nuts and additional pieces to cover the nuts.

Steel nuts are much harder than stainless steel nuts and can be torqued to a far higher number than Stainless Steel without Galling. Especially if the studs are stainless steel. In MY experience. They make special thread lubricating compounds for Stainless Steel and Aluminum. Aluminum without copper. Which standard Never-Seize  is a primary ingredient.

ChrisJ

Bolts

Ice,

The original bolts were aluminum into steel washers.





As far as torquing, this is only holding the evaporator. This is why I've considered nylon.

SWEI

DHW contains oxygen

and usually chlorine as well, both of which feed corrosion.

hot_rod

Bingo

It's the O2 that is needed to support corrosion. In a closed loop piping system the goal is to eliminate or control the O2. That is why inhibited glycol and products like Fernox and Rhomar are so important, they control the O2.



This is the main difference between hydronic and plumbing systems. Plumbing, open loop, systems see fresh oxygenated water, so it is important to address dis-similar metals.



Once you eliminate the O2 from hydronic systems, assuming no additional air (o2) is allowed in, very little potential for corrosion.



Remember learning this lesson with non-barrier plastic and rubber radiant tube?





Some corrosion is good, it provides a protective layer to protect the metal. Anodized aluminum is really "rusted"



Hydronic inhibitors and inhibited glycols contain film providers as part of the blend. A very thin protective coating is on all the components to prevent corrosion.



Some good reading here, plenty of experience in the UK with corrosion as they still use open, expansion vessels in many heating systems.



http://www.midlandcorrosion.co.uk/pdf/Corrosion-Its-Causes-and-Prevention-in-Heating-and-Cooling-Systems.pdf

ChrisJ

Oxygen

hot rod,



I'm not entirely sure oxygen is required. As far as I am aware all that is needed is an electrolyte for galvanic corrosion.



For example lead acid batteries do not require oxygen to work

Gordy

There are many Factors

O2,PH, Dirt, and many other things play arole in how aggressive the corrosion can be.

icesailor

Fastenings:

Nuts? Or Bolts?

They still sweat from Condensation. And you can torque a SS nut or bolt a lot more than an aluminum one.

icesailor

Water:

All water has Oxygen in it. If it doesn't, it isn't water.

hot_rod

o2

from that UK site mentioned

ChrisJ

corrosion.

Interesting.

Perhaps I'm mistaken.

Gordy

Chris

Your not mistaken in what 3 things need to take place for Galvanic corrosion to happen. As in mentioned in my last post there are other variables that can make it more aggressive.



As the hydronisist knows O2 is an enemy anyways even in a perfect world.



O2 can be at varying levels in water. Obviously as low as you can go before its no longer water is the ultimate goal Ice.

icesailor

Atmospheric Pollution:

If you didn't know this, Oxygen was the first atmospheric pollutant. The Atmosphere was CO2. I guess there was O2 in water, Hydrogen and Oxygen, but on the first Continent, Gonwandaland, Now Australia, green slime and Stromtalites appeared and started converting CO2 through Photosynthesis and the by-product was Oxygen. The other by-product was calcium grains that turned into (over time) Coral Reefs and mountains. Limestone is a compressed byproduct. If you want to know where all the CO2 is locked up from the Atmosphere, take some limestone, put it into a heavy plastic bad, and pour acid into the bag to mix with the limestone. Put a lit candle on the floor and open the bag up. Don't pour out the liquid but hold the open bag near the open flame. Pour the heavier than air out of the bag. The candle flame will go out. No O2 left.



Somewhere recently while wasting time watching Youse Two's Videos, there was one with this scientific woman discussing water and how a single molecule changes water from one thing to another. Bicarbonates of one thing or another.

KC_Jones

clarification

On the evaporators I was referring to the entire unit is in a cold room and with the lower temperatures this effect is lessened dramatically.  I honestly do not know the science behind it, but I have seen some older units that still look brand new and have galvanized steel, aluminum and stainless steel all in contact and there has been zero corrosion.  These units get wet especially the "warmer" rooms, but the cool temps seem to keep the corrosion at bay.  I will have to pose the question to our engineer this week and get the details on why this is.  The condensers and coolers we build that are outdoors in ambient temperatures AND being "heated up" rejecting room heat are a different story.  We have to be very aware if dissimilar metals are in contact and isolate as required.  Even with this sometimes there can be issues if a part has poor galvanizing or if a customer didn't passivate the unit properly (ugh).  The more I learn about all this the more fascinated I am.  The world is a wonderful place filled with learning I just wish life wasn't so short so I could learn it all.  On a side note I wish Ice gave classes just for fun because his posts are some of the most interesting I read on this site.  As always thanks all for the education!

Gordy

delete

Gordy

Thresher

Silver brazed salt water pipe was a theory, but may not have been the cause. especially after a short period of service.



http://www.navytimes.com/article/20130404/NEWS/304040021/50-years-later-look-what-really-sank-Thresher

KC_Jones

Thresher

I may be thinking of something else, but I thought one of the theories behind that sinking was freezing due to expansion of compressed air.  Supposedly there was some kind of screen on the compressed air output into the ballast tanks.  When they tried to do an emergency blow the screen iced up effectively blocking the air flow and preventing them from going into positive buoyancy.  This was all after their loss of power.  Of course I could be remember the wrong accident?!  Without actually eye witnesses who can really say for sure what happened.  On a side note my wife and I visited Arlington this year and just randomly walked around and came across a headstone marked as having died in the Thresher...such a sad story all around.  Brought a tear to my eye when I was explaining it to my wife.

Gordy

Theories

Yeah who knows. The link to the article was based on deductions of radio communications during the incident. Deducing what should have been communicated from previous theories to what was communicated, and theorizing based on that what may have caused the incident. What a way to go never knew what hit them. The article said that when the hull breached it was less than 1/10th of a second. faster than the crew could even comprehend what was happening. But I think they knew what was going to happen that would have been worse....the waiting for it to happen. Total respect for those submariners especially testing a new design like that.

icesailor

Nasty RO Water:

Like how RO/Reverse Osmosis water is so pure, that it attacks everything. So, you need to add something to it to give it something to do. Like add a neutralizing filter to it to add hardness to it.

Water isn't what some think it is. If it comes out of a tap and it is good, you are lucky. If it isn't, look out.

Roger_at_SorbOx

Oxygen v. Oxygen

Just to clarify, there is an important difference between Oxygen the element (O), and Oxygen gas molecules (O2). Unfortunately they both have the same name.



So yes, all water contains the element Oxygen, since it is H2O. But that does not mean that water necessarily contains dissolved O2 gasses. Normal tap water contains 8-11 mg/l (or ppm) of dissolved oxygen (O2). But according to the Society of German Engineers, dissolved oxygen should be below 0.1 mg/l if you have low conductivity water (that's 100 times less), and 0.02 mg/l if you have high conductivity water (500 times less). And it is definitely possible to have H2O that is free of dissolved O2 gas (just not if it's exposed to the atmosphere).



In general corrosion requires oxygen, but with an acidic pH, it can occur even without oxygen.



I posted this document on another thread, but I think it's more applicable here. It gives a pretty detailed run-down of causes of corrosion, how it affects different metals, and treatment methods.

There's lots of misinformation out there about this stuff...

icesailor

Technacalities:

That's way too technical. You'll confuse the Climate Change deniers and they might have a melt down. At a minimum, a case of the O2 Vapors.

Roger_at_SorbOx

spark notes

Here's the spark notes version