CORROSION OF PIPE WELDS MADE WITH METAL-CORE WIRE

Here is the analysis of a situation that recently occurred at a fabrication site, plus some of the remedies that were recommended.

Recently I was asked to investigate the cause of corrosion of welds joining sections of low-carbon, small-diameter steel pipe, and to recommend a possible alternative welding process and electrode. The welds in question were made with an E70C-6M gas-shielded metal-core wire. The pipe – used at a drilling site – carried highly chlorinated water that contains significant amounts of carbon dioxide and sand. Adding to the mystery were the comments of the fabricator, who indicated that when these same pipe joints were welded with E6010 stick electrode, the weld did not corrode. I will now outline my analysis of the corrosion as well as some suggested remedies.

FACTORS LEADING TO CORROSION
The Venn diagram shown in Figure 1 illustrates the circumstances comprising the recipe for corrosion. The region on the lower left of the diagram lists conditions relative to the weld and base material. These conditions include differences in base metal composition leading to a galvanic potential, susceptible large-grain microstructures, and the presence (or lack thereof) of a passive oxide layer (mill scale) on the surface of the weld and/or base material.

The circle on the lower right highlights a few environmental factors that can contribute to corrosion. These factors incorporate environment composition (in this case, acidic chlorinated water with abrasive sand) combined with the temperature of the fluid flowing through the pipe, the concentration of (chloride) ions in the solution that facilitate galvanic potential, and the flow rate of the fluid.

To a lesser extent, we may see some of the stresses noted in the upper circle exert an influence on the corrosion of the welds. Based on the circumstances, it is the author’s belief that the only significant stresses present in this case would be residual stresses. These residual stresses would be caused by the relatively small diameter of the pipe, as well as the inherently high travel speed (low heat input) of the GMAW welding process.

ANALYSIS
Galvanic potential is considered to always be present between the weld metal and the base material due to differences in the metal chemistry. Essentially what we have are two dissimilar metals in close proximity to one another. In some cases, the weld metal acts as an anode (which loses electrons to the base material), thereby corroding faster than the base material. And in some situations the opposite is true, and the weld acts as a cathode and causes greater corrosion of the base material in the heat affected zone. Regardless of the direction of current flow, galvanic corrosion is always present – albeit most often at an extremely small level.

In this case, however, the conditions present in the pipe that have caused the weld metal to act as an anode have been accelerated. The water flowing through the pipe – with its high levels of dissolved CO₂ and chlorine – has created a low-pH electrolytic environment conducive to galvanic corrosion. ¹

Furthermore, the weld metal – deposited via an E70C-6M metal-core wire – was shown to have higher levels of silicon and manganese than the base material. These same two alloying elements have been shown to make steel more anodic.² Supporting this theory were the comments of the fabricator, who stated that when E6010 electrodes were used to weld the root pass, the weld corroded far less quickly. Not coincidentally, the E6010 electrodes contained significantly lower levels of silicon and manganese.

Next, the surface condition of the inside of the pipe was examined. It was observed that the surface area of the metal-core weld exposed on the inside of the pipe had almost no passive oxide layer or slag coating. This was clearly visible on the fabricated samples that had not yet been put into service. On the other hand, an E6010 weld will have a considerable amount of slag after welding. As mentioned above, a passive oxide layer helps to slow down the corrosion rate by reducing the available surface area exposed to the electrolytic media. This was also considered to be a factor in the higher corrosion rate of a metal-core weld compared to an E6010 weld.

Another consideration is the grain structure of the weld. A larger grain structure has been shown to be more susceptible to galvanic attack. Although no microscopic analysis of the grain structure of the welds was conducted, it is conceivable that the grain size of the metal-core welds would be larger than that of the E6010 welds. This is because a greater number of weld passes were required to fill the joint with an E6010 electrode, leading to a higher level of grain refinement.