Structure and Protectiveness of Rust Layers Grown on Low-Alloyed Steels Under Cyclic Wet/Dry Corrosion Tests

Abstract

Carbon steels are widely used in various infrastructures due to their superior mechanical properties in addition to low cost, but the mechanical properties can be deteriorated when steels are corroded. It is well-known that the weathering steels where small amounts of alloying elements such as Cu, Cr, Ni, Si and Mn are included form compact protective rust layers during the long-term exposure to atmospheric environment, resulting in superior corrosion resistance. However, as some of the alloying elements such as P that are effective to improve the atmospheric corrosion resistance of carbon steel cause segregation in the solidification process during welding, the amounts of such alloying elements have been strictly limited. The segregation of additive elements can be suppressed in stir friction welding, so-called FSW, in which metallic materials are joined in a solid state, leading to the assumption that the restrictions on the types and amounts of alloying elements can be reduced when FSW is applied instead of conventional welding. In the present work, the structure and corrosion resistance of rust layers formed on various low-alloyed steels designed for FWS are evaluated. Ingots of various low-alloyed steels including Al and P, both of which have been proved to be effective to improve the atmospheric corrosion resistance of carbon steel, were prepared using a high-frequency vacuum induction furnace. The ingots were hot-rolled at 1273 K into a plate with the thickness of approximately 2 mm and then cut into specimens with the dimension of 20 mm × 30 mm. All specimens were subjected to a laboratory cyclic corrosion test according to SAE J2334 which simulates a severe coastal atmosphere environment. In the SAE J2334 test, the specimens were kept for 6 h in a tightly sealed box where the temperature and relative humidity (R. H.) were controlled at 323 K and 100%, respectively (humid stage). Then the specimens were immersed in an aqueous solution containing 0.5 mass% NaCl, 0.1 mass% CaCl2 and 0.075 mass% NaHCO3 for 15 min. (immersion stage), followed by holding for 17h 45 min. in the sealed box at 333 K and 50%R. H. (dry stage). These three stages were repeated up to 30 cycles. After the cyclic corrosion test, rust layers on the specimens were characterized with X-ray diffraction and elemental distributions in the rust layers were evaluated with energy-dispersive spectroscopy. The corrosion current density was estimated from extrapolating Tafel slopes in polarization curves measured for the specimens subjected to the cyclic corrosion test. In XRD measurements for the specimens after 30 cycles of the cyclic corrosion test, peaks derived from α-FeOOH, β-FeOOH, γ-FeOOH and Fe3O4 were found. The fraction of the components in rust layers grown on the specimens was estimated, providing the fractions: 30-50% of α-FeOOH, 20-40% of β-FeOOH, 4% of γ-FeOOH and 30-50% of Fe3O4. Among the components, the fraction of α-FeOOH which is well-known as protective rust increased with increasing the addition of Al and P in specimen. Furthermore, it was found from EDS analysis on the cross-sections of the specimens that Al and P were distributed in the rust layers formed on the specimens in which Al and/or P were included. This indicates that Al and P can be related to the imcreased fraction of α-FeOOH in rust layers. The corrosion current density decreased with increasing the amounts of Al and P in the specimens, implying that Al and P improved the protectiveness of rust layers, that is, the increased corrosion resistance of low-alloyed steels. Similar trend was found in the corrosion rate estimated by the weight-loss method. These beneficial effects of Al and P are attributed to the increased fraction of protective α-FeOOH in rust layers.