There are no easy answers to stop the formation of silicate islands
Q: I am an engineer for a company that manufacturers industrial components that are painted to give corrosion resistance. We currently use the Gas Metal Arc Welding process and are having issues after painting, with the paint popping off along the weld seam. We have tracked the problem to the little slag islands that are on the surface of the weld. How can we eliminate or control the islands without blasting the parts prior to painting?
A: What you are seeing on the surface of the weld is referred to as silicate islands.
These “silicate” islands are more complex than just silicon and oxygen, and really are made up of manganese, silicon, iron and oxygen along with other trace alloys.
The best way to manage “silicate” formations is to reduce the oxidation potential of the shielding gas and reduce arc length. By reducing the oxidation potential of the shielding gas you will permit the use of lower arc voltages — shorter arc length — which does give you the benefits of two factors.
Realize that the molten drops, when exposed during free flight can form preferential oxides with silicon and manganese. Longer arc lengths allow greater exposure for oxidation, so they increase the formation of “silicates.”
Other factors that lead to greater arc length include the desire to reduce levels of spatter. That is based on arc stability, and will increase “silicate” formation.
In addition, surface contaminants such as oil or mill scale, however lightly adhering to the surface, will result in increases in arc length (voltage) to achieve acceptable arc stability and therefore increase silicate formation.
Blasted parts allow the minimum arc length (voltage) to be used and therefore produce the lowest silicate formation.
Shortening the arc length also is better for travel speed — a shorter arc length will increase travel speed. You also will lower exposure to oxidation, and therefore reduce “silicate” formation.
The use of a good quality welding consumable wire will allow the use of lower arc lengths to achieve acceptable stability, particularly with the shielding gases that have lower oxidation potential and that reduce the formation of silicate. This is an important concept and an essential component in the control of silicate island formation.
There are new gas metal arc welding (MIG welding) wire products available that have an interesting way of managing the silicate formation. These new wires alter surface tension through alloy balancing.
Keep in mind that this does not mean that silicate island formation is totally eliminated. Rather, it is displaced from normal locations, such as tight areas like the toe of the weld, and are coalesced, often deposited in the center of the weld for easy removal.
The same fundamentals listed above have to be observed regardless of wire alloy to minimize the “silicate” formation.
Choose shielding gases that produce minimum oxidation potential such as; 95 percent argon/5 percent CO
Operating the welding arc at the lowest voltage to achieve acceptable arc stability is essential to achieving reduced of silicate island formation.
To facilitate your goals, you should consider some of the newer specialized pulse MIG welding waveforms that are designed to short to the weld pool every pulse cycle. That will drive reductions in nominal arc length which, in turn, can lower silicate formation as well.
There is no “sound bite” answer, but there are two overwhelming fundamentals that have to be managed: Oxidation potential of the shielding gas and low arc length stability.
Anything that goes against these principles will produce more “silicate” regardless of alloy.
This column is sponsored by Penton Media Inc. and the Lincoln Electric Co., Cleveland. Dave Barton askdav@penton.com. 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:
To my readers: I want to express my deepest appreciation for your interest in the column. I will be moving on to a position with another company shortly, making this my final Q&A Column. Additionally, I would like to thank both Penton Media Inc. and The Lincoln Electric Co. for allowing me the opportunity to produce this monthly column, and to help the many readers with welding-related issues.
Best wishes, Dave Barton
