Clarifying the effect of Cr on the corrosion resistance of low-alloy steel in a brackish water

• The corrosion rate of steel decreases with increasing immersion time and Cr content. • Cr is more prone to be dissolved than Fe and it promotes α -FeOOH nucleation. • Cr 2 O 3 , FeCr 2 O 4 and α -FeOOH co-deposit to form the inner rust layer. • Cr-rich rust layer has the property of blocking the migration of chloride ions. • Cr-rich layer continuity is enhanced with higher Cr content and prolonged immersion. As is well known, Cr-containing low-alloy steels possess better corrosion resistance than low-carbon steels in various service environments, which is closely related to protective rust layer formed on their surfaces. In this paper, corrosion mass loss, polarization curves, electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and transmission electron microscopy (TEM) were employed to study the effects of Cr and its content variation on the formation process, composition, structure of the rust layer and the corrosion resistance of low-alloy steels in a brackish seawater. Results show that most of the dissolved iron is transformed into rust, with a small amount entering the solution. Cr substituted in the crystal lattice of the steel is more likely to be transformed into corrosion products than Fe, and it is directly enriched in the inner rust layer in the form of FeCr 2 O 4 and Cr 2 O 3 , which effectively inhibits the dissolution of Fe. With the increase of Cr content and the extension of immersion corrosion time, the contents of FeCr 2 O 4 and Cr 2 O 3 distributed in the inner rust layer increase, and the continuity of Cr-rich layer is enhanced. They significantly promote the formation of α -FeOOH through the nucleation effect, resulting in a decrease in the corrosion rate of low-alloy steel.