Corrosion on Flange Bolt

[aries27]

Does corrosion will result in a carbon steel bolt
fastened to a mating flange of different material,
one flange is carbon steel and the other is stainless?

Please advise.

 

[MatAI]

If you check the Galvanic Series Chart, carbon steel is anodic to stainless steel, and it, the carbon steel, will experience accelerated corrosion dependent on how the surrounding environment supports corrosion. To stop the corrosion, electrically isolate the carbon steel from the stainless steel with an isolation flange, bolts inclusive, or use similar stainless steel. To stop the corrosion, eliminate the electrical conductivity between the two metals.

 

[aries27]

Appreciate your quick response.

How about if I changed the bolt and nut material to alloy steel(e.g. A193-B7 and A194-2H), fastened to a carbon steel and stainless steel mating flange,
Is there any occurrence of corrosion on the bolt and nut?

Please reply.

 

[MatAI]

If you have electrical continuity between two pieces of metal and they are in a corrosive environment (an electrolyte is present), the metals will corrode in accordance with the difference in their potentials. Believe it or not if you take a new carbon steel bolt and attached it to an older piece of steel and an electrolyte allows electrical flow between the two metals, the new steel can corrode due to the difference in potential. A metal corrodes in reagrd to its position in the potential series from the most corrosive (magnesium, zinc) to the least corrosive (Platinum, gold). The best thing to do is get a copy of the Galvanic Series from any corrosion handbook, and if one metal is above another in the series, the lower metal will corrode to protect the higher metal if electrical continuity exists. As I said before, if you want to stop corrosion, electrically isolate the two metals. Good luck.

 

[mshimko]

One caution to previous advice from MATAI: while the galvanic series can be used to predict which metal will be subject to corrosion in a galvanic couple (a thermodynamics question), it should not be used to predict the rate of corrosion (a kinetics question). Rate prediction would involve use of polarization curves (typically "E" versis "Log(i)).

As to methods to eliminate galvanic corrosion; electrical isolation is one method. Another is to find more "galvanically compatable" materials.

 

[JStephen]

Aries, I've never seen any real good information on the topic. Of course, one of them will corrode, as the response above says. But how fast? Will it corrode in your lifetime? How conductive does the electrolyte have to be? How dissimilar do the metals have to be?

For example, if that bolt is just exposed to the atmosphere, then that's not an electrolyte. So does that mean this type of corrosion is never a problem when only exposed to the atmosphere? That the objects have to be submerged or dampened for there to be a problem?

Are cast iron and steel dissimilar? Two different grades of carbon steel? Practically every bolted or riveted structural joint ever made has fasteners that are dissimilar to some extent from the base metal. Even weld metal is dissimilar from the base metal. Yet you really don't see this electrical isolation very often, nor do you see the corrosion that you might expect from the lack of it.

It's not too uncommon to attach carbon steel items with stainless bolts, although less common to go the other way.

We have customers who insist that galvanized items be electrically isolated from epoxy-coated carbon steel, and I have never quite understood the logic in that.

 

[unclesyd]

Following up n the post by JStephen I have never seen a problem with accelerated corrosion on fasteners used in situations such as yours, no rates higher than a piece of CS would corrode in the atmosphere.

As we have a pretty good atmospheric corrosion rate on CS at our site we use Teflon or Epoxy coated CS or alloy fasteners for flanges exposed to the atmosphere. We use uncoated fasteners, except a little copper based anti-seize, on insulated lines that are well above the dewpoint.

We have a tremendous number of CS/SS connections inside a building where both the temperature and humidity are high so we coat all Alloy fasteners with DAG, colloidal graphite. This is to assist in the removal of same.

 

[aries27]

Many thanks for accommodating reply to my inquiry.

 

[49078]

try posting this question in the corrosion forum. There are a lot of people in that forum that would be able to give you the best possible answer.

good luck