Flux covering for underwater welding electrodes should have some special characteristics in addition to the usual characteristics required in air welding. Because of arc constriction effect, the current density of underwater arc column is more and therefore deeper penetration is obtained in underwater welding. The arc should therefore have soft behaviour. Purely cellulosic electrodes are unsuitable for underwater welding as their arc is harsh and has digging ten­dency. Arc should have high stability to counter the extinguishing effect of water. Because of poor visibility conditions the coating should give easily removable slag to assist in multipass welding. Coating should be made non-conducting and non-hygroscopic by applying suitable insulating and water-proof paint on flux covering.

Soft arc behaviour

Rutile and iron powder coatings give soft-arc. These rods have therefore been used quite successfully in underwater welding. Iron-oxide coated electrodes give better strength and duc­tility than plain rutile ones in flat and horizontal position. For multipass, all position welding these rods fail because the solidified flux on the bead surface is difficult to remove for subse­quent pass to be made. Rutile electrodes are therefore preferred. Hibshman and Jensen have however found welds stronger in tension than base plate when they used cellulosic electrodes. Their results have not been confirmed by other investigators. Rutile electrodes are therefore preferred by most of the underwater welders these days.

High arc stability

Because of the extinguishing effect of cold water surrounding the arc, the problem of arc stability in water deserves special attention. Compounds having low ionization potential (e. g. salts of potassium and cessium etc.) or compounds that promote electron emission tend to stabilize arc in shielded metal arc welding. By manipulating electrode coating composition an arc with better stability can be obtained. With a very stable arc, weaving of the weld bead may also be possible. This will permit larger heat inputs to the weld per unit length, larger bead size (mm2) and lesser hardening. This will further improve the strength properties. Non-conducting and non-hygroscopic coating

Ordinary coatings, which are invariably porous, absorb water when immersed in water. The moist coating gives porous welds and permits current leakage (through electrolysis). To protect the electrode from these two effects waterproofing non-conducting paints are used. Hrenoff in 1934 used shellac, Peillon process recommended paraffin wax, underwater cutting and welding manual of US Navy recommends Shellac, Ucilon or Celluloid dissolved in acetone for this purpose. Waugh and Eberlein 1954 recommended shellac as good coating. Avilov in 1955 used Kuzbass Varnish and bitumin dissolved in petroleum spirit, Karmabon and Berthet in 1962 settled for Vinyl lacquer on the basis of their experience. Because of varied opinion on this issue, this aspect has also been thoroughly studied by Khan in 1979.

15.6.1 Special Electrodes

Iron powder additions are sometimes made to the flux covering to increase the electrode depo­sition rate

Hrenoff et al. in 1934 used special flux covering coating (chalk and water glass: first layer; iron oxide and water glass: subsequent layers. They used Shellac as a water proofing coating. They found that the electrode was successful in fresh water but sea water required water proofing. Tensile strength was lower than that found by Hibshman and Jensen when they used cellulosic and oxide electrodes. This may be due to poor visibility in his experimental set-up.

Van der Willingen in 1946 used “self made Iron Powder heavy coated electrodes.” He found these electrodes to be easy to use in low visibility conditions, have high deposition rate, and excellent drag or contact welding characteristics.

15.3 POLARITY

Electrode negative polarity produces less undercuts and spatter, better, bead shape, more regular welds and minimum corrosion damage to the electrode holder. Polarity made little difference to weld appearance or visibility. Barrel length was however more with electrode positive. Electrode positive or negative polarity and alternating current could all be used for underwater welding.

15.4 SALINITY OF SEA WATER

Electrical conductivity of water was found to increase with salinity. For bottom sea water it was approximately 0.03 mohs per cubic centimeter. It was easy to initiate and maintain the arc in saline water, penetration was increased. In MIG welding, salinity was reported to increase droplet size, reduce the number of drops per unit time and power consumption. Madatov in his work of 1962 concludes that salinity improves bead shape of underwater welds.