Assessment of corrosion behavior of novel stainless steels in coastal seawater environments

Thesis Abstract

Coastal infrastructure and marine industries face accelerated degradation of metallic components due to chloride-induced corrosion, impinging on service life, safety, and maintenance costs. The study addresses the critical gap in predictive understanding of corrosion performance for novel stainless steels in coastal seawater environments, where traditional alloys exhibit limited long-term durability under fluctuating salinity, temperature, and biofouling. The aim is to evaluate corrosion behavior, pit initiation/propagation, and passivation characteristics of selected novel stainless steels under simulated coastal seawater exposure, and to establish empirical relationships between alloy composition, microstructure, and corrosion response. Specific objectives include (i) determining corrosion rates and pitting susceptibility using electrochemical techniques, (ii) characterizing surface and subsurface corrosion features via microscopy and spectroscopy, (iii) assessing the influence of chloride concentration, temperature, and biofilm formation on corrosion performance, (iv) developing a predictive model linking alloying elements (Cr, Ni, Mo, N) and microstructural features to corrosion resistance, and (v) recommending alloy design and surface treatment strategies to enhance coastal durability. The methodology adopts an embedded empirical field-lacustrine design complemented by controlled laboratory simulations. The population comprises four novel stainless steel alloys developed for marine applications. A stratified sampling scheme yields n=120 specimens (30 per alloy) fabricated as 10 mm × 10 mm coupons, with varied heat-treatment conditions to induce representative microstructures. Data collection employs a combination of potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), and chronoamperometry in artificial seawater solutions with chloride concentrations ranging from 0.5 to 3.5 wt%. Complementary techniques include scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) for surface morphology and composition, transmission electron microscopy (TEM) for grain boundary and precipitate analysis, X-ray diffraction (XRD) for phase identification, and X-ray photoelectron spectroscopy (XPS) for passive film chemistry. Biofouling influence is approximated through controlled exposure to cultured marine biofilms on select coupons. Data reliability is ensured through triplicate measurements per condition, calibration traces for electrochemical instruments, and inter-laboratory validation of a subset of specimens. Analytical strategies combine descriptive statistics with inferential modeling. Descriptive analyses characterize baseline corrosion rates, pitting potentials, and impedance parameters. Inferential statistics include two-way ANOVA to examine the effects of alloy type and environmental variables on corrosion rate, followed by Tukey post hoc tests. Regression analysis is employed to model relationships between alloying composition, microstructural descriptors (grain size, precipitate volume fraction), and corrosion resistance metrics. EIS data are fit to equivalent electrical circuits to extract charge transfer resistance and double-layer capacitance, with goodness-of-fit assessed by chi-square criteria and residual analysis. Surface analytical data are integrated using multivariate principal component analysis (PCA) to identify key features distinguishing alloys under maritime exposure. A qualitative thematic synthesis guides interpretation of biofilm-related effects, framed by the ecological stoichiometry theory to explain interactions between surface chemistry and biofilm development. The study adheres to ethical standards for material testing and environmental risk assessment. Expected findings anticipate that alloys with optimized Cr-Ni-Mo-N compositions, refined microstructures with stable passive films, and reduced sensitization will exhibit lower corrosion rates (less than 0.2 mm/year) and higher pitting potentials in chloride-rich environments. It is projected that increased Mo and N content will enhance passivation stability, while certain heat treatments will mitigate intergranular attack. Biofilm presence is anticipated to modify local electrochemical conditions, potentially accelerating localized corrosion in select alloys but with specific surface treatments conferring resilience. The study contributes to knowledge by providing a robust dataset linking composition, microstructure, and environmental parameters to corrosion performance, enabling predictive service life assessments for novel stainless steels in coastal zones. It will offer practical guidance for alloy design, surface modification, and maintenance planning, including recommended chloride thresholds, protective coating strategies, and post-service inspection intervals. The main conclusion is that a targeted combination of alloy chemistry and thermomechanical processing can significantly improve coastal corrosion resistance, with implications for standardization of material specifications in marine infrastructure. Recommendations include adopting a design framework for marine-grade stainless steels that prioritizes Mo-N-C stabilizing elements, implementing passive-film-stabilizing surface treatments, and integrating ongoing in-situ monitoring using electrochemical sensors to anticipate degradation trajectories in coastal environments.