Abstract
Stainless steels have good corrosion resistance in many environments but welding or aging can decrease their resistance. This work focused on the effect of aging time and ultrasonic nano-crystal surface modification on the passivation behavior of 316L stainless steel. In the case of slightly sensitized 316L stainless steel, increasing the aging time drastically decreased the pitting potential, increased the passive current density, and decreased the resistance of the passive film, even though aging did not form chromium carbide and a chromium depletion zone. This behavior is due to the micro-galvanic corrosion between the matrix and carbon segregated area, and this shows the importance of carbon segregation in grain boundaries to the pitting corrosion resistance of stainless steel, in addition to the formation of the chromium depletion zone. UNSM (Ultrasonic Nano Crystal Surface Modification)-treatment to the slightly sensitized 316L stainless steel increased the pitting potential, decreased the passive current density, and increased the resistance of the passive film. However, in the case of heavily sensitized 316L stainless steel, UNSM-treatment decreased the pitting potential, increased the passive current density, and decreased the resistance of the passive film. This behavior is due to the dual effects of the UNSM-treatment. That is, the UNSM-treatment reduced the carbon segregation, regardless of whether the stainless steel 316L was slightly or heavily sensitized. However, since this treatment made mechanical flaws in the outer surface in the case of the heavily sensitized stainless steel, UNSM-treatment may eliminate chromium carbide, and this flaw can be a pitting initiation site, and therefore decrease the pitting corrosion resistance.
Highlights
Since austenitic stainless steels have good properties of strength and toughness, and high corrosion resistance, they have been widely used in the field of power plants, chemical plants, and other industries [1]
The high corrosion resistance of austenitic stainless steel is due to the chemical stability of the passive film formed on the surface, and the properties of the film depends upon the alloying elements of the steels
12%; the passive film is mainly composed of Cr2 O3, and the corrosion resistance can be improved by the addition of Ni, Mo, W, N, etc
Summary
Since austenitic stainless steels have good properties of strength and toughness, and high corrosion resistance, they have been widely used in the field of power plants, chemical plants, and other industries [1]. If the steel may be slowly cooled or exposed over the range of sensitization temperatures (500–850 ◦ C) for a long time after the solution heat treatment at over 1050 ◦ C, intergranular chromium carbide may be formed and may reduce the corrosion resistance [1]. The high mechanical energy imparted by UNSM to the steel surface may induce the diffusion of the alloying elements [30,31,33], and can affect the intergranular corrosion resistance of the stainless steels. Our group reported that even though chromium carbide was not precipitated, aging time drastically increased the intergranular corrosion rate of 316L stainless steel, and we confirmed that the increased intergranular corrosion rate of slightly sensitized (not carbide formed) 316L stainless steel was due to the carbon segregation along the grain boundaries. Treatment on the anodic polarization behavior of aged 316L stainless steel and proposed its mechanism
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