Performance of different coating types

The composition and thickness of the coating have a great influence on the welding properties. This affects both the stability of the arc, the material transfer during welding, and the viscosity of the slag and the weld pool. In particular, the particle size of the droplets transferred during the arc is affected.
This diagram shows the droplet transfer of four basic types of coatings: cellulose (a), titanium oxide (b), acidic (c), and basic (d).
Cellulose coatings are mainly composed of organic substances, which burn in the arc and form a shielding gas to protect the welding position. In addition to cellulose and other organic substances, the coating contains only a small amount of substances that stabilize the arc, so that almost no slag is generated. Cellulose-type electrodes are particularly suitable for vertical downward welding, because there is no need to worry about slag flowing down.

Acidic coatings (A) are mainly composed of iron and manganese minerals, which generate a lot of oxygen in the arc. This oxygen is also absorbed by the liquid metal being welded, thereby reducing its surface tension. The result is a very fine, jet-shaped droplet transfer and a shallow weld pool. This type of electrode is therefore not suitable for welding in a forced position. The arc is very "hot" and allows for rapid welding, but it can easily cause undercutting. Because of these disadvantages, pure acid electrodes are now almost no longer used in Germany.

Nowadays, the titanium oxide acid type (RA) is used, which is an electrode composed of a mixture of acid and titanium oxide, which has welding properties corresponding to the coating. The titanium oxide coating (R/RR) mainly consists of mineral titanium oxide (TiO2) and ilmenite (TiO2.FeO) or artificial titanium oxide. The welding characteristics of this type of electrode are fine to medium droplet transfer, smooth melting with little spatter, a very fine weld surface, easy removal of surface slag and good re-ignition properties. This last feature can only be manifested if the electrode coating has a high TiO2 content. If this type of electrode has been used for a certain period of time, it can be re-ignited without cleaning the melted arc crater on the electrode. When the TiO2 content is high enough, the slag layer on the arc crater is conductive like a semiconductor. When the edge of the arc crater of the electrode touches the workpiece, sufficient current will flow to re-ignite the arc, so there is no need for the core of the electrode to directly contact the workpiece. This spontaneous re-ignition performance is very useful for welding processes that are often interrupted, such as short welds.

In addition to pure titanium oxide types, this type of electrode also has some mixed types. Here we mention the titanium oxide-cellulose type (RC), in which part of the titanium oxide is replaced by cellulose in the coating. The cellulose burns during welding, so less slag is formed on the weld surface. Therefore, this type of electrode can also be used for vertical downward welding (welding position PG), and it also has good welding performance if used in other common welding positions.

Another mixed coating is titanium oxide/basic type (RB). The coating thickness of this type is thinner than that of RR type. The slag characteristics of this coating make it particularly suitable for vertical upward welding (PF). There is also a basic coating (B), which is mainly composed of basic calcium oxide (CaO) and magnesium oxide (MgO), and some calcium fluoride (CaF2) is added as a slag diluent. Too much calcium fluoride will make the weldability of AC current worse. Therefore, pure basic electrodes cannot be welded with sinusoidal AC current. Of course, there are some mixed coatings that contain less calcium fluoride and can be welded with AC current. The particle size of the droplet transfer of basic electrodes is medium to large particles, and the molten pool is very sticky. This electrode can be used for welding in all welding positions. Due to the high viscosity of the molten pool, the weld formed is obviously convex and coarse feathery. The welded joint has good toughness.

Basic coatings easily absorb moisture. Therefore, special attention should be paid to storing the electrodes in a dry place. Wet electrodes must be dried before use. This dry electrode can produce welds with very low hydrogen content. The deposition rate of the electrode is usually less than 105%. Some electrodes add iron powder to their coating to make them have a higher deposition rate. Most of these electrodes have a deposition rate higher than 160%. They are also called iron powder type or high-efficiency electrodes. This type of electrode has a high deposition rate and is more economical than ordinary electrodes in many applications, but its application is usually limited to welding in flat welding positions (PA) and (PB).