DC vs. AC Polarity for SMAW


Q: I am hoping you could shed some light on a topic that seems popular on forums. Why are 6011 stick electrodes not specified or not used for root passes and pipe welding in general (other than AC pipe welding)?  Do 6010 rods have better mechanical or operating properties than 6011?  I know for higher strength pipe, there are higher strength XX10 rods.  However, for 60,000 tensile, what is the advantage of 6010 over 6011?

DC Welding Output Graph
Figure 1: Graph of Welding Output on DC+ or DC- Polarity

      
A: E6010 and E6011 are two American Welding Society (AWS) classifications for Shielded Metal Arc Welding (SMAW) electrodes (stick electrodes).  These two types are very similar.  Both are mild steel (60 ksi minimum tensile strength) electrodes with a cellulosic based coating for all-position, multiple applications welding.  They have similar arc or operating characteristics and mechanical properties.  The main difference between the two is with the recommended welding polarity.  E6010 electrodes are intended for direct current (DC) only.  While E6011 electrodes can be used on alternating current (AC), as well as DC.   More specifically, an E6010 electrode has a high cellulose sodium type coating and an E6011 electrode has a high cellulose potassium type coating.   The latter helps keep the arc ignited as the welding output alternates from positive to negative.  In perhaps over simplified terms, an E6011 electrode is like an E6010 electrode designed to operate on AC polarity.

Your question leads to a more general discussion of DC vs. AC welding output.  In most cases, DC is the preferred welding polarity.  Whether it is DC+ (electrode positive or “reverse”) polarity or DC- (electrode negative or “straight”) polarity, DC produces smoother welding output than AC.  Figure 1 is a graph of DC welding output vs. time.  The output is at a consistent current level all the time.  All electrodes can be operated on DC polarity.  For more critical welding applications, such as pipe welding and / or welding on higher strength, low alloy steels, DC polarity is used almost exclusively. 

Therefore, referring back to your question, it is logical that only E6010 electrodes would be specified for a more critical application such as pipe welding, and not E6011 electrodes.  Note that for stick welding in general, DC+ polarity is most commonly used.  It produces a good bead profile with a higher level of penetration.  DC- polarity results in less penetration and a higher electrode melt-off rate.  It is sometimes used, for example, on thin sheet metal in an attempt to prevent burn-through.   


On the other hand, with AC output, the welding current alternates from positive flow to negative flow and back again.  In North America, electricity alternates at a rate of sixty times per second or 60 hertz (while most other global regions produce electricity at 50 Hz).  Figure 2 is a graph of AC output, often referred to as an AC sine wave graph.  Note that 120 times per second the welding output crosses the centerline, representing zero amperage or no output.  While this state of no output occurs for only a split second, the result is that with many electrodes the arc tends to frequently “pop out” or extinguish on AC polarity.  To overcome this problem, some electrodes are designed specifically to operate on AC.  They have certain elements in their coating which help keep the arc ignited as the output goes through periods of low and no output (loosely represented by the red zone on the figure 2 graph). 

However, the resulting arc still tends to have more fluctuation or flutter than it does on DC polarity.   Figure 3 lists the various types of coatings and currents, per the AWS A5.1 Filler Metal Specification for mild steel covered electrodes.  Note the electrodes that are intended for DC only and those that can be used on both DC and AC.  Note also that the polarities are listed alphabetically rather than by primary and secondary recommendation. 

      

AC Welding Output Graph
Figure 2: Graph of Welding Output on AC Polarity

 Carbon Steel Electrodes for SMAW
Table of Various Stick Electrode Coating Types and Currents

          

 

In general (and in at least North America), DC is the preferred polarity with all electrodes.  However, there are a few situations where AC polarity is used.  The first, most common situation is when you have no choice.  This is because you are using a power source with AC output only.  This is typical with low cost, entry-level type welding machines, often referred to as a “buzz box” welder.  A few common electrodes used with these small welders include E6011, E6013 and special “E7018 AC” types. 

A second situation in which to use AC polarity is to remedy arc blow problems.  This is a phenomenon in which the arc wanders or blows out of the joint, and is more common when using large diameter electrodes at higher current levels.  While there are other remedies to arc blow problems that can be used with DC polarity, switching to AC is often an effective fix.  Common electrodes used on AC at high current levels include E6027 and E7024 types.