A welded specimen may corrode uniformly over its entire surface (Fig. 9.8a). The weld metal may corrode less than the base metal (Fig. 9.86) or more than the base metal (Fig. 9.8c) depending upon the composition of weld metal during solidification. In addition the base metal may corrode adjacent to weld metal in the HAZ. During high-temperature welding stresses will develop just adjacent to weld metal and corrosion occurs in HAZ just touching the weldmetal (Fig. 9.8d). At low temperature welding the corrosion may be intergranular away from weld-metal in HAZ touching the base metal (Fig. 9.8e).
9.9.1 Factors Affecting Corrosion Resistance of Welded Joints
1. Metallurgical structure composition of base-metal and weld-metal.
2. Thermal and mechanical treatment history before welding.
3. Welding process.
4. Welding procedure (manual, automatic, number of passes, welding speed, current and voltage.
5. Shielding gas composition and flow rate.
6. Size and geometry of weld deposit.
While reporting corrosion data for a welded joint, the items in the above list should also be reported.
The most common corrosion resistance evaluation method is to measure the weight lost during exposure to corrodent and convert it to an average corrosion rate using the formula
R = KW ADT
where R = corrosion rate in depth of attack per unit time K = constant (value depends on units used)
W = the weight lost by the specimen during the test A = total surface area of the specimen D = specimen material density T = duration of the test.
The above formula suits well to the conditions shown in Figs. 9.8a, 9.8b, 9.8c. For Figs. 9.8d and 9.8e, the selective corrosion may be significantly large without resulting in a large amount of weight loss. This may cause error in finding average corrosion rate.