Analysis of Corrosion Rate and Mechanical Property Degradation on Hot Dipped Galvanised Pipes Exposed to NaCl Environment

ABSTRACT The durability of hot-dipped galvanized (HDG) steel pipes is largely dependent on their resistance to corrosive environments, particularly those containing chlorides. This study investigates corrosion rate and degradation of mechanical properties of HDG pipes when exposed to sodium chloride (NaCl) environment. Hot dipped galvanised pipe samples were immersed in controlled NaCl solution of varying concentrations and exposure times of 7, 14, 28 and 56 days. The corrosion rates were evaluated through surface morphology and microstructural analysis, mechanical property degradation of the samples was also examined after each exposure time. SEM was employed to examine both the corrosion products and pitting, while EDX was also conducted to analyse the elemental composition of the samples. Surface morphology of samples revealed early stages of localized initiation, while later stages exhibit build-up of protective yet flawed layers, with iron oxyhydroxides (FeOOH variants) being the typical phase. Ferritic-pearlitic grain structure, with equiaxed or elongated ferrite grains (10-50 μm), as well as predominantly ferritic (>90%) with sparse pearlite and defects like MnS inclusions were noticed, which disrupted the phase uniformity, acting as cathodic sites in corrosive media. Further results revealed that corrosion rates increased with both chloride concentration and exposure duration, leading to progressive thinning of the zinc coating and exposure of the underlying steel substrate. Mechanical testing demonstrated a progressive reduction in ultimate tensile strength, yield strength, hardness as well as elongation as the sample(s) exposure time to NaCl increased, highlighting direct correlation between corrosion damage and deterioration of structural integrity. The findings revealed the limitations of HDG coatings in aggressive saline environments, while providing insights for service life prediction, maintenance scheduling, and material selection in engineering applications where chloride exposure is prevalent. Keywords: Corrosion rate, mechanical properties, galvanised pipes, NaCl environment.