Performance of different types of electrode coatings

The composition and thickness of the electrode coating have a significant impact on welding performance. This affects both the stability of the arc and the manner in which the material is transferred during welding, as well as the viscosity of the slag and weld pool. In particular, the size of the molten droplets transferred in the arc is affected.
This diagram shows the droplet transfer states of four basic types of electrode coatings: cellulose (a), titanium oxide (b), acidic (c), and basic (d).
Cellulose coatings are primarily composed of organic materials, which burn in the arc and form a protective gas to shield the weld area. Besides cellulose and other organic materials, the coating contains only small amounts of substances to stabilize the arc, so almost no slag is produced. Cellulose-type electrodes are particularly suitable for vertical down welding because there is no concern about slag flowing downwards.

Acidic coatings (A) are mainly composed of iron and manganese minerals, which generate a large amount of oxygen in the arc. This oxygen is absorbed by the molten metal during welding, thereby reducing its surface tension. The result is a very fine-grained, spray-type droplet transfer and a shallow weld pool. Therefore, this type of electrode is not suitable for welding in forced positions. The arc is very "hot," allowing for fast welding, but it is prone to undercut. Because of these drawbacks, pure acidic welding electrodes are now rarely used in Germany.

Currently, titanium oxide acidic type (RA) electrodes are used, which are a mixture of acidic and titanium oxide coatings, exhibiting corresponding welding characteristics. Titanium oxide coatings (R/RR) are mainly composed of the mineral titanium dioxide (TiO2) and ilmenite (TiO2.FeO) or artificially produced titanium dioxide. The welding characteristics of this type of electrode are fine to medium-grained droplet transfer, smooth melting process with little spatter, a very fine weld surface, easy slag removal, and excellent re-ignition performance. This last characteristic is only evident when the TiO2 content in the electrode coating is high. With this type of electrode, re-ignition is possible even without cleaning the melted crater on the electrode after it has been used for a period of time. When the TiO2 content is sufficiently high, the slag layer on the arc crater becomes conductive like a semiconductor. When the edge of the electrode's arc crater contacts the workpiece, sufficient current flows to reignite the arc, eliminating the need for direct contact between the electrode core and the workpiece. This self-reigniting arc capability is very useful for welding processes that require frequent interruptions, such as short welds.

Besides the pure titanium dioxide type, there are also some mixed types of these welding electrodes. One example is the titanium dioxide-cellulose type (RC), where a portion of the titanium dioxide is replaced by cellulose in the coating. The cellulose burns during welding, resulting in less slag formation on the weld surface. Therefore, this type of electrode can be used for vertical-down welding (PG position), and it also exhibits good welding performance in other common welding positions.

Another mixed type of coating is the titanium dioxide/basic type (RB). The thickness of this coating is slightly thinner than that of the RR type. The slag characteristics of this coating make it particularly suitable for vertical-up welding (PF). There is also the basic coating type (B), whose main components are basic calcium oxide (CaO) and magnesium oxide (MgO), with some calcium fluoride (CaF2) added as a slag diluent. Excessive calcium fluoride can impair the weldability with alternating current. Therefore, pure basic electrodes cannot be welded with sinusoidal alternating current, although some mixed coatings containing smaller amounts of calcium fluoride can be welded with AC. Basic electrodes have medium to large droplet transfer sizes, and the weld pool is very viscous. This type of electrode can be used for welding in all positions. Due to the high viscosity of the weld pool, the resulting welds have a distinctly convex and coarse feathered appearance. The welded joints have excellent toughness.

Basic coatings are prone to absorbing moisture. Therefore, special attention must be paid to storing the electrodes in a dry place. Damp electrodes must be dried before use. Using these dried electrodes can produce welds with very low hydrogen content. The deposition rate of welding electrodes is usually less than 105%, although some electrodes have iron powder added to their flux coating to achieve a higher deposition rate. The deposition rate of these electrodes is mostly above 160%. They are also known as iron powder type or high-efficiency electrodes. These electrodes offer a high deposition speed and are more economical than ordinary electrodes in many applications, but their use is generally limited to flat (PA) and horizontal (PB) welding positions.