We are requesting your assistance in resolving a rust problem that we are experiencing with one of our stainless steel fabricated products.

Our wet electrostatic precipitator systems, typically fabricated from type 304l or 316l stainless steel, are used on fiberglass forming and curing ovens, wood dryers, wood-fired boilers, and steel scarfing processes.

Our problem is that when carbon steel stiffeners are welded to the stainless steel shell, the angle, or channel, rusts and stains the stainless steel shell. We have tried painting the stiffeners first, but the paint is burned off during the welding process. We clean and paint the welded areas, but the material continues to rust.

We have been told that we are experiencing a bimetalic corrosion problem, not just normal atmospheric corrosion.

Is it true that carbon steel should not be welded to stainless steel? Or, can they be welded without corrosion problems if we protect the metals from water with insulation and siding?
It is true that a wet surface of stainless in contact with carbon steel will rust at an accelerated rate.

This is because there is an electrochemical potential difference between the two metals, and the water serves as an electrolyte. Once the rust settles on the stainless, the stainless also will rust because the rust deposited on the stainless steels catalyzes more rusting.

If you make the stiffeners of stainless also, that should take care of the problem.

If you continue to use carbon steel stiffeners, you might investigate the application of a zinc spray (a thermal spray) onto the carbon steel after welding.

We have a job that we have to weld for some engineering changes. It is for an Asian automobile manufacturer, and the steel we are welding is a cast iron referred to as FC250. I am not having any luck at all welding this material. We have tried TIG welding with no pre-heat and with a pre-heat of 400 degrees F. We have tried several specialty stainless filler metals, but we are still not getting a good weld. The weld we are making is multi-pass. Can you help?
The FC250 specification can be found in JIS G 5501 standard for Grey Iron Castings. There is no specified composition, but the fact that it really is a grey cast iron is very helpful. The right electrode can be either a 55 percent nickel filler if you are making a multi-layer repair and are not machining the weld, or our a 99 percent nickel filler if you have to machine the first layer of weld metal. You will need preheat and interpass temperatures of about 400 degrees F. You should preheat the entire casting, not just the weld area, and you should cover the entire piece with insulation after welding and let it cool very slowly.

This column is sponsored by Penton and the Lincoln Electric Co., Cleveland. Dave Barton is a senior welding engineer in the Application Engineering Group of The Lincoln Electric Co. He oversees welding procedure development for both new technology and existing products, performs failure analyses for customers, and serves as a consultant on welding application problems. Barton has been with Lincoln Electric for 21 years. Send your questions for Mr. Barton in care of WDF by e-mail to: askdav@penton.com.