Transmission Electron Microscopy Study on the Precipitation Behaviors of Laser-Welded Ferritic Stainless Steels and Their Implications on Intergranular Corrosion Resistance

The intergranular corrosion susceptibility of ferritic stainless-steel weldments is strongly dependent on chromium carbide precipitation phenomena. Hence, stabilization is widely used to mitigate the aforementioned precipitation. In contrast, stabilization has proved ineffective to fully prevent intergranular corrosion due to segregation of unreacted chromium during solid-state heat-treatments. To analyze the precipitation behavior of 17 wt.-% chromium ferritic stainless steels during laser welding, sheets of unstabilized and titanium-stabilized ferritic stainless steels were welded in a butt joint configuration and characterized with special consideration of precipitation behavior by means of transmission electron microscopy. While unstabilized ferritic stainless steels exhibit pronounced chromium precipitate formation at grain boundaries, titanium-stabilization leads to titanium precipitates without adjacent chromium segregation. However, corrosion tests reveal three distinctive corrosion mechanisms within the investigated ferritic stainless steels based on their inherent precipitation behaviors. In light of the precipitation formation, it is evident that immersion in sulfuric acid media leads to the dissolution of either grain boundaries or the grain boundary vicinity. As a result, the residual mechanical strength of the joint is substantially degraded.

Investigation on the Correlation between Inclusions and High Temperature Urea Corrosion Behavior in Ferritic Stainless Steel

The influence of inclusion size and number density on high-temperature urea corrosion (HTUC) behavior of ferritic stainless steels was investigated in a simulated working environment of selective catalytic reduction (SCR) system in commercial vehicles. There is a positive correlation between the control level of inclusions and the resistance of HTUC. By slightly increasing the content of Nb in ferritic stainless steels, the inclusions, especially TiN, were significantly refined, and thus displayed an improvement in HTUC resistance. The interface between inclusions and the matrix becomes a fast channel for chromium precipitation during high-temperature nitriding induced by the decomposition of urea. Chromium nitrides will precipitate around the inclusions and wrap the inclusions, which will decrease the chromium equivalent of the matrix and reduce the resistance of ferritic stainless steels to HTUC. In addition, the high-temperature oxidation accompanied with thermal fatigue also makes the inclusions more likely to become the crack nucleation source, which can accelerate the material thinning and reduce its service life.

Corrosion of Chromia and Alumina Forming Ferritic Stainless Steels under Dual Atmosphere Exposure Conditions

Surface morphology and chemistry of oxide scales formed on select chromia and alumina forming ferritic steels have been studied after exposure to a dual atmosphere of hydrogen and air. Localized Fe-rich oxide nodules with surface whiskers/platelets form at the onset of corrosion. The initiation and growth of localized nodules and breakdown of the passivation are attributed to the presence of hydrogen, inclusion of iron oxide in the passivating scale, and subsequent growth of iron-rich oxide due to the establishment of redox (H2-H2O) atmosphere and modification of oxide defect chemistry.