Recently i did repeated repair welding of 304-mild steel weldment using SMAW. While analyzing the corrosion behavior of the HAZ of mid steel side of the weldment, i have observed a lowering of corrosion resistance as the number of repeated repair increases. Widmanstatten ferrite and pearlite are observed in the micro structure of HAZ (mild steel). Moreover, the pearlite content increases as the number of repairs increases. What may be the reason for deterioration of corrosion resistance?
The main driving force for the corrosion is the electrochemical potential difference between two locations which creates transfer of electric charges (electrons and ionic chemical species. One should not forget that the weld region has not only the different microstructure but also it has different chemical composition, that means it is impossible to avoid the formation of the micro electrochemical cells. Dependending upon the hierarchy on the standard electrode potential (SEP) scale,which varies from the lowest negative (anodic) values (i.e., Mg -2.363) up to the extreme positive figures (cathodic) such as gold (+1.498), the region having lowest level becomes anodic, and preferentially corrodes compared to the higher ordered region, which becomes cathodic. For the present case since stainless steel contains Cr, which has greater negative ( -0.744) value than the iron ( - 0.440) rich (mild steel), it will be subjected to more galvanic corrosion .
the main reason for corrosion in weldments is the formation of galvanic cells in HAZ between the base metal and consumable weld. Can you identify by 5D ultrasonic test. Please, see the attached file.
u said, Moreover, the pearlite content increases as the number of repairs increases.
(welding is the heat treatment), if after any heat treatment process you got pearlite more than ferrite face, this situation will increase the galvanic couples cells and the corrosion rate will be increased if the area ratio of cathodic become bigger than anodic, usually ferrite is anode and peralite is cathode.
Thank You Dr. Rana Afif Majed Anaee and Mr. Mohamed Abdelgawad Gebril...
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