A PREDICTIVE ELECTROCHEMICAL MODEL FOR WELD METAL HYDROGEN PICKUP IN UNDERWATER WET WELDS

Weld metal hydrogen pickup in underwater wet welding is severe due to the presence and dissociation of water surrounding the welding are. Th is undesirable behavior can be minimized, however, with the use of oxi dizing-type electrodes. The purpose of this investigation has been pla ced on the fundamental understanding of the effect of hydrogen pickup by the slag on the weld metal diffusible hydrogen content in direct cu rrent, shielded metal are welding (SMAW)for both electrode-positive po larity (DCEP), and electrode-negative polarity (DCEN). To accomplish t his purpose, 20 experimental oxidizing electrodes containing systemati c ferric oxide (Fe2O3) additions, ranging from 0 to 70 wt. percent, to the pur coating were investigated. The mole fraction ratio of CaO/SiO 2 in the fluxes ranged from 0.05 to 0.35, independent of the ferric ox ide additions. Underwater bead-on-plate welds were deposited on ASTM A 36 steel coupons at 0.27 m (city) water depth using a gravity feed sys tem. Welding parameters were held constant throughout the experiments. Weld metal diffusible hydrogen content was determined using the mercu ry displacement method according to current AWS standard. To correlate weld metal hydrogen content with slag chemistry, the slag hydrogen co ntents were also determined. The measured diffusible hydrogen contents showed that Fe2O3 was effective in reducing weld metal hydrogen conte nt. Higher hydrogen values were always related to lower Fe2O3 contents initially present in the flux, for instance, 71 mL/.100g (DCEP - 0 wt . percent Fe2O3) as compared to 31 mL/100g (DCEP - 36 wt. percent Fe2O 3). Amazingly, diffusible hydrogen as low as 13 mL/100g was obtained w ith the use of DCEN polarity along with 53 wt. percent Fe2O3 in the fl ux coating. X-ray diffraction (XRD) conducted on different slags showe d that the lower diffusible hydrogen values were always associated wit h the presence of fayalite (2FeO . SiO2). Complementing XRD analysis, Mossbauer spectroscopy analyses carried out on different slags showed that all ferric (Fe3+) oxide initially present in the slags had transf ormed to ferrous oxide (FeO), free or combined. Chemical analyses show ed that weld metal hydrogen pickup was strongly dependent on the solub ility of water in the slag systems. The total and diffusible hydrogen content in the weld metal increased monotonically with increasing slag hydrogen content. Finally, variations in weld metal hydrogen as well as slag hydrogen content with both polarity and iron oxide content in the slag were successfully predicted using an electrochemical model th at describes the slag/metal interface equilibrium In this investigatio n, the slag/metal interface has been identified as responsible in cont rolling the weld metal hydrogen pickup. The model assumed that hydroge n was present in the slag as (OH) - ions and that FeO displayed ideal solution behavior.