STAINLESS STEEL FLANGES MATED WITH GALVANISED CARBON STEEL

[stuarte]

Please can someone tell me what the effect wiil be if these flanges are mated .Do they require an insulating bolt set and if so what spec to use and why.Please alaborate on what happens between the two metals if not insulated.

 

[mcguire]

The galvanized coating will protect everything until the zinc coating is gone. Then you have a problem with the carbon steel.
This is not a good combination. Use stainless all over or use galvanized. Unless there's a very finite lifetime to your design, don't mix a long-life stainless with galvanized. What's the point?

 

It all dependens on the atmospheric conditions where this joint is, what the fluid temp. is and the life expectancy of the piping system is.
Corrosion will eventually occur when the galvanising breaks down. Once the galvanising breaks downthe corrosion rate is increased as the stainless is the more noble material.
A simple insulating flange set (which is not expensive to buy)will solve this problem.

 

[MJCronin]

To all,

I disagree with MCGUIRE and AMIST.....

What has been said is fundamentally true.... however, at the point where the galvanizing (zinc) has been depleted, you must wonder if a galvanized system is appropriate for the ambient conditions at all!!

The corrosion between carbon and stainless steel is based on the potential differences in the electromotive scale. There is very little potential difference between the two and the corrosion rate is very low. If the carbon steel flange corrodes, it was not meant to be in the ambient conditions anyway....

This corrosion topic was discussed in a 1977 book by the godfather of piping and pressure vessels, Helmut Thielsch. the book is titled:

Defects and Failures in Pressure Vessels and Piping
By Helmut Thielsch
Published in 1977, 464 pages

In this book, he states that it is not necessary to use insulating flanges on these joints....

Having said all of that, I am also aware that many of these joints have insulating flanges......this is simply the result of a "slap-dash" effort to get projects completed and satisfy the psychotic demands of butthole MBAs that run them.

My thoughts, recollections only....


MJC
ISBN: 0-88275-308-8

 

[mcguire]

MJcronin cites some good reasoning, however to imply that stainless and carbon steel have no potentially harmful galvanic interaction is very wrong and must be addressed.
Not only will carbon steel attack ( and attack on the zinc coating )be accelerated by coupling to stainless, the rust will lead to pitting attack on the stainless since iron ions destabilize passive films much as chlorides do.
I agree with you and Helmut about MBA's and flanges. Keep them dry and simple.

 

[Flareman]

Just to throw in some additional thoughts, look also at the temperatures anticipated during the expected lifetime of the equipment. There is work (although I can't remember the reference right now) which indicates that galvanizing in proximity with SS is bad when the temperatures run toward red heat because ionic zinc migrates into the SS and generates cracking.
I realize that if you're really looking at galvanized flanges the chances of the high temps are low, but then, why are you using SS on the other side?

 

[grandpre]

is it bad practise to gavanize a system that utilizes socket weld fittings.

 

[TerryOsser]

Stuarte, galvanic action only occurs if electrolytes are present. Are you concerned with internal or external corrosion. If internal figure out if your fluid is electrolytic. If its external the only area that will get corroded is around the bolts. The gasket will / should insulate, and the bolts are normally coated anyway. If the service is over 50 deg C the stainless steel should be painted too! The trend in oil & gas now is to not use insulation sets at all (as MJCronin orates. Insulating spools are used in CuNi/Titanium seawater service interfaces.
A problem shared is a problem doubled!