Pipe Supports - Material Issue

[CharlieH]

I will be installing a horizontal section of 304H SS pipe in an outdoor, refinery environment. The plan is to weld a pipe shoe to the pipe and rest it on existing galvanized steel. However, I am concerned with galvanic corrosion.

What is the best approach?

1. Make the pipe shoe of 304H and rest it directly on the existing galvanized steel?

2. Make the pipe show out of another material that will not react with the galvanized steel? If so, this will require me to weld dissimilar metals together. If this is not a problem, do you have any suggestions on the material?

3. Proceed with 304H pipe shoe, but put some type of insulating material between the two metals.

4. Some other approach I have not thought of. Perhaps some of you have suggestions for this application.

Thanks in advance for taking the time to read and respond to this post.

Regards,
Charlie

 

[RossABQ]

A number of manufacturers (incl. Pipe Shields) make a polymer-faced slide plate you could bolt to the existing steel. They also make it as a guid or with limit tabs.

 

[0707]

If I were you I should go for your point-3 solution as RossABQ also mentioned.

The only concern will be the temperature you have to choose a sliding plate material, which withstands your temperature.


luis

 

[NickiConway]

I have a similar situation and I suspect I will go with solution No. 3 as well. But I though I would share the following:

304SS is considered a cathode (noble metal)
Carbon/Gal Steel is considered an anode (active metal)

The relative area of each of the exposed metal surfaces is also a consideration. If the area of the cathode (304SS) is very large, and the anode (Gal)is very small, the current produced is likely to be very high and the anode will corrode quickly.

For example, consider a large stainless structure with carbon screws (in an electrolyte of course), the carbon screws will corrode, but a large carbon structure with stainless screws will corrode very little if any.

Hope this helps !

Nic

 

[MalcolmA]

I work at a chemical plant, and we have stainless steel shoes resting directly on galvanized carbon steel supports with no corrosion problems.

 

[NickiConway]

In order for galvanic corrosion to occur, three elements are required.

1) Dissimilar metals
2) Metal-to-metal contact
3) Metals in the same conduction solution (usually called an electrolyte)

If any of these elements is missing, galvanic corrosion cannot occur. So if there is no chance of an electrolyte, then no corrosion will occur.

 

[RossABQ]

Rain would clearly provide the electrolyte, if it's outdoors

 

[BouzidLazhar]

MalcolmA's suggestion is also a good approach as Zinc (for galvanized CS) or cadmium are considered as a sacrificial anodes.

 

[NozzleTwister]

'Zinc embrittlement' is the phenomena I'm familiar with concerning SS pipe and galvanized supports or support steel. I'm not sure of the exact chemical mechanism but basically the zinc contaminates and weakens the grain boundaries of the stainless with the risk increasing at higher temperatures and higher stress situations.

The concern is normally for pressure containing parts coming into contact with galvanized parts and not the pipe shoes sitting on galvanized steel.

I do have other comments though. Since you are using 304H grade stainless, I assume your pipe must be very hot, possibly 1000 - 1400 deg. F.

You need to take a close look at the temperature of your shoe where it contacts the galvanized steel. That temp. could easily exceed what the steel and or galvanizing can withstand.

If that's the case you can extend your shoe height or use pre-insulated pipe supports (Hot Shoes) such as Pipe Shields, mentioned above, now owned by Piping Technology & Products or High Temp Slide Plates. The PT&P website is

http://www.pipingtech.com/

and you can see these products.

Good luck,


NozzleTwister
Houston, Texas

 

[rconner]

I have nothing against galvanizing nor zinc coatings (as I am aware they are widely used and it appears in at least some applications areas with some success). However, as it would appear some applications may involve rather hot piping or conditions in the back of my mind I seem to recall additionally that zinc type coatings at least in some corrosive environments may exhibit entirely different behavior in hotter conditions than they do at room temperature (protective behavior commonly advertised/discussed). Additionally, it might not take much hotter temperatures to bring about reversals of behavior (that might not be explained by pure Arhennius effects? -- if you are interested in such complex phenomena and same can't be explained in this forum, you may wish to try the "Corrosion" forum.]

 

[CharlieH]

Thank you all for your input.

NozzleTwister and most recently rconner have hit the nail on the head with their input. The high temperature will melt the zinc coating around 775 deg F.

So, I managed to find a hot shoe that I think will suit our needs.

Thank you all very much.

Until next time....

Charlie

 

[bucs2047]

Might be late in game, but with 304 in the past I've used teflon type support plates.