Seismic Performance Degradation of Square CFST Columns Under Simulated Acid Rain Corrosion

Literature Overview

This paper by Chen Mengcheng, Zhang Fanmeng, Huang Hong, and Wang Chao, published in the Journal of the Railway Society of China in 2018 (Vol. 40, No. 6, pp. 106-114), investigates the influence of acid rain corrosion on the seismic performance of square steel tube concrete-filled columns. The study is particularly relevant to engineers working in regions with significant acid precipitation, where structural longevity and seismic resilience are critical design considerations. Twelve square CFST specimens were fabricated, of which nine were subjected to accelerated corrosion by immersion in simulated acid rain solution combined with electrochemical acceleration. All specimens were then tested under low-cycle reversed loading to evaluate their seismic behavior. The experimental program is well-designed and provides valuable quantitative data on corrosion-induced degradation mechanisms.

Core Experimental Design and Test Parameters

The experimental matrix is built around two primary variables: corrosion level (expressed as corrosion rate) and axial compression ratio. The test specimens were divided into groups based on corrosion duration and applied current intensity to achieve controlled corrosion rates. The electrochemical acceleration method, which combines chemical immersion with impressed current, is a well-established approach for accelerating corrosion in laboratory settings, though it must be acknowledged that accelerated corrosion may not perfectly replicate long-term natural corrosion patterns.

The selection of square tubes rather than circular tubes introduces an important consideration from a welding and fabrication standpoint. Square CFST members require longitudinal and transverse butt welds, as well as potential corner welds, which represent stress concentration zones. The corrosion environment preferentially attacks these weld regions, making them critical locations for damage initiation.

Failure Modes and Welding Implications

The most striking finding from this study is that all specimens failed rapidly after weld cracking. This observation carries profound implications for welding quality control in CFST structural applications. The weld zones, which are inherently the weakest links due to heat-affected zone (HAZ) microstructural changes, residual stress concentrations, and potential micro-defects such as porosity or lack of fusion, become the primary failure initiators under combined corrosion and cyclic loading.

From a welding metallurgy perspective, the HAZ of low-carbon and low-alloy structural steels (such as Q235 or Q345 commonly used in CFST construction) undergoes grain coarsening and potential martensitic transformation in the coarse-grained heat-affected zone (CGHAZ). Under corrosive attack, these regions are particularly susceptible to pitting and intergranular corrosion. The combination of cyclic stress and corrosion damage leads to corrosion fatigue, where crack initiation life is significantly reduced compared to either corrosion or fatigue acting alone.

The progression of failure can be understood through the following sequence: corrosion initiates at weld toes and HAZ boundaries, leading to localized material thinning; under cyclic loading, these thinned regions experience elevated stress concentrations; cracks initiate at corrosion pits or pre-existing micro-defects; once a crack forms, rapid propagation follows due to the reduced cross-sectional area and the stress intensity factor amplification at the crack tip.

Quantitative Performance Degradation

The study presents several quantitative findings that are essential for engineers to understand the magnitude of corrosion effects:

The transition of the hysteresis curve from a full, well-filled shape to a spindle-shaped form is a classic indicator of stiffness degradation and strength loss. In a full hysteresis loop, the structure can undergo large inelastic deformations while maintaining significant residual strength. The spindle shape indicates that the structure loses stiffness rapidly after initial yielding, with minimal energy dissipation capacity remaining.

Standards Comparison and Engineering Practice

The comparison between test results and predictions from GB 51248-2017 and the Jiangxi Province local code reveals that both standards provide reasonably good predictions, with GB 51248-2017 being conservative. This is an important finding for design practice, as it suggests that the current national code may adequately capture the seismic behavior of corroded CFST members, at least within the parameter range studied. However, the conservatism of the national code may mask the true severity of corrosion-induced degradation, particularly in cases where corrosion rates exceed those studied here.

From an engineering practice standpoint, several recommendations emerge:

1. Weld quality must be prioritized in CFST construction, particularly in corrosive environments. Full-penetration butt welds with proper pre-heat and post-weld heat treatment (PWHT) are essential to minimize HAZ susceptibility to corrosion.
2. Non-destructive testing (NDT) protocols should include ultrasonic testing (UT) and magnetic particle testing (MT) of all welds before and after corrosion exposure, with particular attention to weld toes and HAZ boundaries.
3. Corrosion protection systems, including hot-dip galvanizing, epoxy coating, or cathodic protection, should be specified for CFST members in acid rain-prone regions, with specific attention to weld areas where coating adhesion may be compromised.
4. Periodic inspection and monitoring of CFST structures in corrosive environments should include measurement of wall thickness reduction at weld locations using ultrasonic thickness gauging.

Key Questions and Reflections

Several important questions arise from this study that warrant further investigation. First, the study focuses on square tubes, which have inherent stress concentration at corners. How would the corrosion effects differ for circular tubes, where the weld geometry is simpler and stress distribution more uniform? Second, the accelerated corrosion method used here may not fully capture the localized corrosion patterns (such as crevice corrosion at weld roots or under coatings) that occur in real structures. Third, the study does not address the interaction between corrosion and fire exposure, which is a critical concern for CFST structures in multi-hazard scenarios.

The finding that weld cracking initiates failure in all specimens underscores the critical importance of welding quality in CFST construction. As a welding engineer, I find this result particularly compelling because it validates the need for rigorous welding procedure qualification (WPQ) and welder performance qualification (WPQ) programs, especially for CFST applications. The welding process should be selected to minimize HAZ width and residual stress, such as using narrow-gap submerged arc welding (SAW) or gas metal arc welding (GMAW) with low heat input settings.

Study Insights and Implications

This study provides valuable quantitative data on the degradation of seismic performance of square CFST columns under simulated acid rain corrosion. The finding that weld cracking is the universal failure initiation mechanism highlights the critical role of welding quality in the long-term seismic resilience of CFST structures. Engineers should integrate corrosion considerations into the design phase, specifying appropriate weld details, corrosion protection systems, and inspection protocols. The conservative nature of GB 51248-2017 predictions offers some confidence in current design codes, but the margin of safety may not be sufficient for severely corroded members. Future research should extend to circular tubes, include more realistic corrosion simulation methods, and address multi-hazard scenarios involving combined corrosion, fatigue, and fire exposure. The study also emphasizes the need for periodic structural health monitoring of CFST structures in corrosive environments, with particular focus on weld integrity assessment.