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Home > Ordnance Documents and other related manuals > > Functions of Electrode Coatings
Figure 2. Arc Action and Electric Welding Polarity
Polarity of Welding Current (figure 2, View B).

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.
(c) The  combination  of  a  mineral  and  cellulose  coating  is
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
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