Atmospheric Corrosion Behavior of Q235 Steel Exposed to the Subtropical Marine Environment in the East China Sea for Two Years

TL;DR

This study examines how Q235 steel corrodes over two years in a marine environment in the East China Sea, focusing on rust layer changes and corrosion mechanisms.

Contribution

The study provides new insights into the corrosion behavior and protective properties of rust layers on Q235 steel in subtropical marine environments.

Findings

The corrosion rate of Q235 steel initially increased and then decreased over two years of exposure.

Rust layer composition shifted from γ-FeOOH to α-FeOOH and Fe3O4, improving protective properties over time.

Chloride ions at rust layer defects caused local acidification, promoting pitting corrosion despite enhanced electrochemical stability.

Abstract

The corrosion behavior and mechanism of Q235 steel during a two-year exposure to the subtropical marine atmospheric environment on an offshore platform in the East China Sea were investigated in this study. Methods including corrosion weight loss measurement, macro/micro-morphological observation (using a digital camera, SEM, and 3D-CLSM), composition analysis (XRD and XPS), and electrochemical tests (EIS and Tafel polarization curves) were employed to systematically examine corrosion kinetics, rust layer evolution, and electrochemical performance. The results indicated that the corrosion rate of Q235 steel initially increased and subsequently decreased with prolonged exposure, with the atmospheric corrosivity reaching CX level as defined (according to the ISO 9223 standard). The corrosion products transitioned from an early-stage rust layer predominantly consisting of γ-FeOOH to a later-stage layer primarily composed of α-FeOOH and Fe3O4. XPS analyses revealed that both the α*/γ* ratio and the Fe(II)/Fe(III) ratio increased over time, demonstrating a progressive improvement in the protective properties of the rust layer. The polarization resistance of the rust layer gradually rose, while the corrosion current density declined significantly, further confirming the enhanced stability and protective performance of the rust layer following long-term exposure. Chloride ions accumulated at defects within the rust layer, inducing local acidification, which played a key role in promoting the initiation and propagation of pitting corrosion. This study elucidated the corrosion behavior and mechanism of Q235 steel in the marine atmospheric environment of the East China Sea. Despite the increase in exposure time from 6 to 24 months, during which the electrochemical stability of the rust layer enhanced over time, it failed to prevent the initiation and propagation of severe localized corrosion—an issue of critical importance for load-bearing structures. The findings provide important theoretical and data support for service-life assessment and corrosion protection design of offshore photovoltaic steel structures.

Figures