This study focuses on the corrosion behavior of Integrated spiral finned tubes in low-temperature sulfuric acid flue gas environments, with an emphasis on the effects of different pitch distances on the corrosion resistance of A106 Gr.B and ND steel finned tubes, as well as the evolution of their microstructures. The samples were taken from an industrial hot-rolling production line, with A106 Gr.B steel having pitch distances set at 8 mm, 11 mm, and 13 mm, and ND steel at 8 mm and 11 mm. All samples were ground to 1200 grit and then subjected to immersion corrosion tests in a constant temperature and humidity chamber. The experimental parameters covered temperatures ranging from 30 to 140 ℃, sulfuric acid mass fractions from 30 to 80 %, and corrosion times from 2 to 4 h. The corrosion rate was calculated using the weight loss method, with units of mg·cm^⁻²·h^⁻¹. The number of parallel samples was ≥3, and the relative standard deviation was controlled within 5 %.

Metallographic sample preparation followed the standard procedures of embedding, polishing, and etching with 4 % nitric acid alcohol. Observations were made using an Axio Scope A1 optical microscope and a GeminiSEM 500 field emission scanning electron microscope. Grain size was evaluated according to the ASTM E112 intercept method, and the pearlite area fraction was obtained by averaging five points in a 500× field of view using ImageJ software.

1. The sample with an 11 mm pitch made of A106 Gr.B material had the lowest pearlite area fraction (9.1 %), a grain size level of 7.5, and the highest microstructural uniformity. The corresponding corrosion rate under conditions of 60 ℃ and 35 % H_₂SO_₄ was 35.7 mg·cm^⁻²·h^⁻¹, significantly lower than that of the 8 mm pitch sample (63.5 mg·cm^⁻²·h^⁻¹).
2. The ND steel sample with an 8 mm pitch had a grain size level of 8.0 and a pearlite area fraction of 10.4 %. Its corrosion rate under the same conditions was 4.6 mg·cm^⁻²·h^⁻¹, only 13 % of the minimum value of 20G steel. Energy dispersive spectroscopy (EDS) analysis indicated that a continuous passivation film containing Cr, Cu, and Sb was formed on the surface of ND steel, with a thickness of approximately 80–120 nm, effectively inhibiting anodic dissolution.
3. When the temperature was ≥80 ℃ and the sulfuric acid concentration was ≥50 %, all three materials entered the passivation zone, with corrosion rates dropping below 5 mg·cm^⁻²·h^⁻¹. The effect of pitch distance on weight loss results was reduced to within the error range.

The relationship between microstructure and properties indicates that pearlite, as the cathodic phase, has a potential difference of about 60 mV with ferrite, making it the site for pitting initiation. For every 1 % decrease in pearlite volume fraction, the corrosion rate is reduced by an average of 2.3 mg·cm^⁻²·h^⁻¹. Grain refinement accelerates corrosion in the active dissolution zone but reduces corrosion in the passivation zone by increasing the density of the film layer. For every one level increase in grain size, the corrosion rate in the active zone increases by 1.8 mg·cm^⁻²·h^⁻¹, while in the passivation zone it decreases by 0.7 mg·cm^⁻²·h^⁻¹.

Based on the comprehensive experimental data, it is recommended to use Integrated spiral finned tubes with an 11 mm pitch of A106 Gr.B or an 8 mm pitch of ND steel in boiler tail flue gas conditions where the flue gas temperature is ≤70 ℃ and the H_₂SO_₄ mass fraction corresponding to the sulfuric acid dew point temperature is ≤45 %, to balance cost and corrosion resistance. The rolling process should control the final rolling temperature at 880–920 ℃ and the cumulative deformation at ≥60 % to reduce the pearlite volume fraction and improve microstructural uniformity.