Galvanic Corrosion in a Stainless - Mild Steel Duct 2

[Sheeny]

We have a mild steel duct (1.5m dia) which returns outlet air from the second stage of our flash drier to the first stage. The material being dried in the drier is wood fibre (part of an MDF plant).

Typically the air through the duct is saturated or very close to it, between 35 and 50 degrees C.

The duct as it is is badly corroded, it is about 8 years old, and we are planning serious repair replacement work.

One of the options I have been asked to consider is progressive replacement with stainless (probably 304) rather than mild steel. My concern is the accelerated corrosion in the mild steel section attached to the stainless steel due to galvanic corrosion.

We need to maintain an electrical contact between all sections of the duct in order to avoid the potential for a static electrical spark which could cause a fire or explosion if the conditions are right with trace fibre and dust.

I am toying with a few ideas for the permanent SS/mild connection at each end:
1. A short section of MS ductwork that we sacrifice each couple of years.
2. An insulated joint and rely on earthing to ground to minimise the static electrical risk.
3. A fabric sealed overlapping joint.

Any comments about these options?

Where we temporarily connect to the old MS duct as we progressively replace it, am I being overly concerned about the accelerated corrosion rate?

Thanks in advance.

Al.

 

[unclesyd]

We have numerous combinations of SS/CS in all sizes of duct and I've never seen any measurable damage from galvanic action at the connections. Our ducts are connected either by bolting or a lock seam depending on the size. As we build our own duct there is a program to change to all SS on a piecemeal basis.

One major problem we see with CS duct is on the larger fabrications is during forming operation there is deliberate process to made secondary bends to stiffen the panel thick interns make moisture trap and we lose the duct from general corrosion.

 

[rmw]

For galvanic action to occur, there must be two metals with galvanic potential (which you will have) in the presence of a liquid that will conduct electricity. While you have moisture in the duct, I doubt that you have enough free liquid to worry about galvanic action. And only you know the answer to this, is the liquid that you have due to the saturation conductive?

Now general corrosion, that is another matter.

rmw

 

[jjdoodah]

A SS duct liner w/ a carbon steel outer framework is a very standard situation. The carbon stiffners can even be Epoxy coated to further resist corrosion. The carbon welds easily to the SS, either 304 or 316. Hope this helps.