The electrode consists of core wire and coating.  That is to say,  painting the skin (coatings)  on metallic cores and forming outer uniform makes an electrode.  The materials of core  wire  vary  from  different  types of electrode.  The core wire is the inner.  In order to  ensure the quality and performance of the weld, there  are  strict  requirements  on the contents of  metal elements in the core,  especially on the harmful impurities  (such as sulfur, phosphorus)  content,which should be limited strictly. And the material of the weld should be superior to base metal.

     Generally,the core is a kind of wire with a certain length and diameter.When welding,welding core has two functions: firstly,conduct the welding current and form an electronic arc,thusconvert the it into heat energy; secondly,the core itself is a melting metal as the formation of the weld  filler metal.The skin on the surface of the electrode  is the coating, which plays an extremely important role in the welding process.Without coating, the oxygen and nitrogen in  the air will intrude in the molten bare metal while welding,thus will oxidize or nitride iron element and other useful elements,such as carbon,silicon and manganese and left in  the weld,causing  the  weld  slag  or crack.Meanwhile,the gas that has melted into the welding  pool may  produce  a  large number of  pores  in the weld.All  the  above  factors can  reduce  the mechanical propeties (strength,shock value) of the weld greatly,and also may brittle the weld. The function of  coating  is  to isolate oxygen outside  from the weld,  thus forming a protection cover. The  bare electrode without coating  is not suitable for arc welding. This  is because the arc welding has a poor stability,more splash and bad  weld.Through longterm practice, the welders gradually  found  out  that paiting some mineral material (namely the coatings)  in core wire,the performance of electrodes can be improved greatly.
 
   According to the different condition, elding electrodes can be divided into three categories: classifying by the purposes of  welding rod, by the main chemical composition property of coatings, and by the slag of the melting coatings.

(1)In accordance with the uses of electrodes,  the electrodes  can be divided  into structural steel electrode,heat resistant steel electrode,  stainless steelelectrode,  surfacing electrode, low temperature steel electrode, cast iron electrode, nickel and nickel alloy electrode,copper and copper alloy electrode,aluminum and aluminum alloy electrode and electrodes with special purposes.
(2) In accordance with the main chemical composition property of coatings,welding electrodes can be divided into titanium oxide type electrode,titanium calcium type electrode, ilmenite electrode, iron oxide type electrode,cellulose type electrode, low hydrogen electrode, graphite electrode and base type electrode.
(3) In accordance with slag of melting coatings,welding electrodes can be divided  into acidic electrode and basic electrode. The main component of coating of acidic electrode  is acidic oxide, such  as silica, titanium dioxide, ferric oxide etc.On the other side, the main component of base electrode is basic oxide, such as marble, fluorite etc.
 
How to test the quality of welding electrodes?
(1) From the appearance of electrode. The surface of coating should  be smooth  without porosity or mechanical damage, and the coating is uneccentric and core wire should be without corrosion.
(2) From the strength of coating. Raise a single electrode into about a metre high, then  loose your  hand and  let it free fall on the smooth thick steel plate.If the coating is not peeling,it proves that the strength of coating is up to quality requirements.
(3) From the process of actual welding.That's to say verifying the quality of welding electrodes through the actual operation. The electrodes with good quality  have stable arc while welding, and both core and coating will  be melted evenly with rare splash, good weld and easy removal of slag.