Comparison of Rutile and Cellulosic Electrodes

“Varieties of Electrodes and Their Constituents”

There is a wide array of manual metal arc (MMA) electrodes available, grouped into three categories based on their primary flux component: cellulosic, rutile, and basic. These electrodes typically consist of a core wire, usually 2.5–6mm in diameter, coated with flux. The core wire is commonly crafted from low-quality riming steel, while the flux contains several elements that aid in refining the weld microstructure.

The composition of the flux significantly influences the behavior of these electrodes.

Table 1 outlines the main constituents of different electrode types and the corresponding shielding gas produced upon combustion.

The following summary outlines the characteristics of general-purpose electrodes, specifically rutile and cellulosic types.

Rutile Electrode:

The difference between the E6012 and E6013 electrodes is that the E6012 covering contains sodium, while the covering of E6013 contains potassium. Both can operate under direct current (DC+), with only E6013 suitable for alternative current (AC). Maintaining a constant current is advisable to counterbalance welder hand unsteadiness.

With a high titania (titanium dioxide) content, rutile electrodes yield a smooth bead surface, easy slag removal, and a smooth arc. Their flux mainly generates carbon dioxide upon combustion.

Although containing cellulose in smaller amounts compared to dedicated cellulosic electrodes (up to 10% ), rutile electrodes produce relatively higher levels of hydrogen due to cellulose presence and moisture. This limits their use to mild steels below 25mm thickness and thin-section low-alloy steels of C/Mo and 1Cr1/2Mo types.

Rutile electrodes are suitable for welding in all positions except vertical down. Deposition can be enhanced by incorporating iron powder, leading to increased metal deposition at the same current, albeit limited to flat positions only.

They offer medium penetration, a quiet arc, and minimal spatter. These electrodes produce a substantial amount of self-releasing slag, requiring minimal cleaning post-welding.

While widely used as general-purpose electrodes, they are not recommended for structures requiring high toughness. Table 2 summarizes their mechanical properties.

Table 2 Typical mechanical properties obtained with E6012 and E6013 AWS A5.1/A5.1M, 2012

Cellulosic Electrode:

Similarly to rutile electrodes, the differences between E6010 and E6011 cellulosic electrodes are the electrical parameters used during welding and their type of covering. The covering of E6010 contains sodium; E6011 contains potassium. They can both run under direct current (DC+) but only the latter is suitable for running under alternative current (AC). The MMA process can be used in DCEN, DCEP or AC but again a constant current is recommended to counterbalance the unsteadiness of the welder’s hand.

The gas shield from cellulosic combustion comprises hydrogen, carbon monoxide, and carbon dioxide, with approximately 30-45ml of hydrogen/100gm in the weld. This high hydrogen content offers good weld pool protection but increases hydrogen levels in the weld metal and heat-affected zone (HAZ).

This hydrogen-rich shielding gas necessitates higher voltage (around 70V) during welding. However, it also poses a risk of hydrogen-induced cold cracking if not managed properly.

Cellulosic electrodes allow for deep penetration, high deposition rates, and the ability to weld in the vertical down position (also known as stove piping). However, they generate significant spatter and fumes while producing minimal slag, requiring skilled welding expertise.

Table 3 Typical mechanical properties obtained with E6010 and E6011, AWS A5.1/A5.1M, 2012

Due to their characteristics, cellulosic electrodes are primarily used in cross-country pipelining but have limited usage in welding storage tanks. They necessitate skilled welders and careful consideration of steel composition and preheat requirements.

Comparison of Electrode Characteristics:

Table 4 Comparison of electrode characteristics

Guidelines to Avoid Hydrogen Cracking with Cellulosic Electrodes:

Hydrogen cracking can occur at near-ambient temperatures due to three conditions: presence of diffusible hydrogen in the weld, tensile stresses, and susceptible microstructure. While tensile stresses and microstructure can be controlled to an extent, managing diffusible hydrogen content is crucial.

The amount of diffusible hydrogen in the weld metal relies on the cooling rate from welding temperature. Cooling conditions significantly impact hydrogen content; slower cooling rates reduce hydrogen content.

Recommendations for Cellulosic Electrode Use:

Only welders with recent qualifications for using cellulosic electrodes should conduct welding activities with them.

Similar preheat measures to rutile electrodes should be applied before welding to manage cooling rates and hydrogen release.

Cellulosic electrodes should primarily be used for the root run and followed by a hot pass using rutile electrodes within a specific time frame to reduce hydrogen content and refine the weld.

Single-pass fillet welds with cellulosic electrodes should be avoided to minimize sensitivity to hydrogen cracking.

Cellulosic electrodes should not be dried as they rely on atmospheric hydrogen for shielding. If damp, they can be dried in an oven at 120°C; otherwise, soaked electrodes should be discarded.

Post-heating techniques, such as maintaining interpass temperature or raising temperature immediately after welding, can aid in hydrogen removal.

Conclusion:

Cellulosic electrodes demand skilled welders and meticulous handling due to their inherent hydrogen content and welding characteristics. Their use is best limited to specific applications, primarily for root runs and in situations where vertical down welding is required. Precautions are necessary to manage hydrogen content and reduce the risk of hydrogen-induced cracking. Qualified welders following stringent guidelines can effectively utilize cellulosic electrodes for specific welding needs.