CORROSION OF PIPE WELDS MADE WITH METAL-CORE WIRE

Finally, the higher corrosion rate of the metal-core weld might also be explained by its relatively larger level of reinforcement compared to the E6010 deposit. This abrupt change in geometry of the pipe wall would almost certainly cause an increased level of turbulence in the flow of water through the pipe. Local disruptions and eddy currents would result where the weld bead intersects with the base material. This theory was substantiated by the fact that the weld clearly did not corrode uniformly, but rather preferentially at one weld toe. It was ultimately confirmed that the direction of flow through the pipe was indeed from the highly corroded weld toe to the one that was far less corroded.

Enhancing this effect is the presence of sand in the water. Any grains of sand that are flowing along the pipe wall suddenly come into contact with the reinforcement of the metal-core weld bead. This abrasive wear accelerates the corrosion rate of the weld bead and eventually cavitates the weld. Conversely, an E6010 weld with its lower level of reinforcement will not act as an impediment to the flow of sand through the pipe.

USE OF STAINLESS STEEL FILLER METAL
The fabricator also inquired about the viability of using of a stainless steel electrode for this application. As mentioned above, the weld metal can act as either the anode or the cathode in galvanic corrosion. Because austenitic stainless steel filler metal is more “noble” than mild steel, it will act as the cathode when used as the filler material on carbon steel pipe. While it is true that the weld would likely not corrode, it is highly probable that the heat affected zone – acting as the anode – would deteriorate. This would be especially true if the weld exhibited the same large back-bead or level of reinforcement.

RECOMMENDATIONS
Now that it was shown to the fabricator why E6010 was the preferred electrode over metal-core wire, the focus shifted to determining the ideal welding process for this application. Was it E6010 stick electrode or something else entirely? After all, the stick electrode welding process is considerably slower than the GMAW process, and productivity would suffer if they were confined to using E6010 electrode.

Regardless of the process and electrode selected, it is imperative that the reinforcement level on the inside of the pipe be kept to a minimum. This serves to keep the fluid flow within the pipe more laminar, and reduce the tendency for localized corrosion at the toe of the weld. Also, keeping the reinforcement small will prevent the abrasive wear of the sand at the toe of the weld.

For these reasons, the use of the STT^® GMAW process with SuperArc^® L-50 solid wire from Lincoln Electric (Cleveland, OH) was offered as an improvement over the metal-core wire currently being used. The reinforcement would be minimal as shown in Figure 2, thereby improving fluid flow. Furthermore, an additional benefit of the STTprocess is that more passes will be required compared to metal-core. This would have the effect of improving grain refinement, and thus improve resistance to chemical attack.

Although the silicon and manganese content of SuperArcL-50 is higher than that of E6010 stick electrode, it is still lower than what has been measured in metal-core wire deposits. Despite SuperArcL-50 possessing this one slight disadvantage compared to E6010 electrode, this is far outweighed by the productivity gains that can be realized with SuperArcL-50 and the STTprocess compared to SMAW. This is because the travel speeds utilized with pipe fabrication with the STT process have been shown to be at least twice as fast as those used in stick welding.

The takeaway message is that galvanic corrosion is almost always present between welds and the low-carbon steel base metal in an aqueous environment. However, its effects can be mitigated by the maintaining the appropriate bead shape and selecting the proper welding electrode and process. Thankfully, both of these factors can be achieved while not sacrificing productivity.

References
1.  Queen, D.; Gulbrandsen, E.  “Guidelines for the Prevention, Control, and Monitoring of Preferential Weld Corrosion.” Society of Petroleum Engineers. 2004, SPE 87552. 2.  David, J.R. “Corrosion of Weldments.”  ASM International.  2006.