Functions of Electrode Coatings

Welding Operations, I
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WELDING OPERATIONS I - OD1651 - LESSON 1/TASK 1

(a) The cellulose-coated types are composed of soluble cotton

or other forms of cellulose with a small amount of potassium,

sodium, or titanium and, in some cases, other minerals.

This

coating provides protection to the molten or solidifying metal by

developing a gaseous zone around the arc and a slag deposit over

the weld.

(b) The mineral coatings consist of sodium silicate, metallic

oxides, clay, and other inorganic substances or combinations

thereof.

With the mineral-coated electrode, protection to the

molten or solidifying metal is provided only by a slag deposit.

composed of various quantities of the substances previously

described for each of these coatings.

These coatings provide

various protection effects to the arc, and to the molten or

solidifying metal, depending on the type of base metal being

welded.

e. Functions of Electrode Coatings. Some of the more important

functions of the coatings on the shielded-arc or heavy-coated arc

electrodes are described in the following subparagraphs.

(1) The coatings produce a reducing or nonoxidizing atmosphere

around the arc; thus, preventing the contamination of the metal

in the arc by oxygen and nitrogen from the air.

Without this

coating, the oxygen would readily combine with the molten metal,

remove alloying elements from the metal, and cause porosity and

oxidation of the weld. The nitrogen would cause brittleness, low

ductility and, in some cases, low strength and poor resistance to

corrosion.

(2) The coatings reduce impurities such as oxides, sulfur, and

phosphorous so that these impurities will not impair the weld

deposit.

(3) They provide substance to the arc, which tend to increase

its stability, so that the arc can be maintained without

excessive spattering.

(4) Coatings reduce the attractive force between the molten

metal and the end of the electrode, and reduce the surface

tension of the molten metal. Vaporized and melted coatings cause

the molten