Corrosion Characteristics and Tensile Property Evolution of Q235 Steel under Salt Spray Environment Based on DIC Technology

Corrosion Characteristics and Tensile Property Evolution of Q235 Steel under Salt Spray Environment Based on DIC Technology

Q235 low-carbon steel is one of the most widely used structural steel materials in building engineering, municipal facilities, offshore auxiliary structures and mechanical equipment, featuring low cost, excellent machinability and stable basic mechanical properties. However, Q235 steel lacks effective corrosion-resistant alloy elements, resulting in severe surface corrosion performance degradation in chloride-rich salt spray environments. Long-term salt spray erosion induces progressive pitting corrosion, loose rust layer accumulation and matrix thinning, which further deteriorate tensile mechanical properties, reduce structural ductility, and cause brittle fracture risks. Different from traditional macroscopic mechanical testing, Digital Image Correlation (DIC) technology can realize full-field non-contact strain monitoring, accurately capture heterogeneous strain distribution, stress concentration evolution and microscopic deformation differences of corroded steel. Based on neutral salt spray accelerated corrosion test and DIC full-field deformation detection principle, this document systematically elaborates the macroscopic and microscopic corrosion evolution characteristics of Q235 steel under salt spray conditions, dynamic variation law of corrosion rate, and tensile performance degradation mechanism based on full-field strain analysis, forming a standardized technical analysis system for corrosion damage evaluation and mechanical property prediction of carbon steel.

1. Fundamental Overview

1.1 Salt Spray Corrosion Mechanism of Q235 Steel

Neutral salt spray environment contains a large number of chloride ions, which can penetrate the initial passive film on the surface of Q235 steel, destroy the integrity of surface oxide layer, and form numerous microscopic corrosion cells on the steel matrix. Different from atmospheric natural corrosion, salt spray accelerated corrosion presents continuous wet-dry alternating characteristics, which accelerates the hydrolysis reaction of corrosion products and promotes the sustained development of matrix corrosion. The main corrosion products of Q235 steel include loose and porous γ-FeOOH, α-FeOOH and Fe₃O₄. The rust layer formed in the early stage has a certain isolation protection effect, while the late-stage cracked and loose rust layer accelerates the penetration of chloride ions, leading to intensified corrosion damage of the steel matrix.

1.2 Technical Advantages of DIC Testing

Traditional uniaxial tensile tests can only obtain macroscopic average mechanical indexes such as yield strength, ultimate tensile strength and elongation, which cannot reflect local microscopic deformation and uneven strain distribution of corroded specimens. DIC (Digital Image Correlation) technology realizes non-contact, full-field and high-precision strain measurement by collecting continuous surface speckle images of specimens during tensile deformation. It can effectively capture local strain concentration, strain field dispersion and premature necking deformation caused by corrosion pits and surface defects, revealing the essential degradation mechanism of mechanical properties of corroded Q235 steel that cannot be characterized by traditional testing methods.

2. Macro and Micro Corrosion Evolution Characteristics of Q235 Steel under Salt Spray

The salt spray corrosion process of Q235 steel presents obvious phased evolution characteristics, with regular changes in surface morphology, rust layer structure and microscopic pit distribution with the extension of corrosion time, which can be divided into three typical stages.

2.1 Early Corrosion Stage (1–3 Days)

In the initial salt spray corrosion stage, the surface of Q235 steel is uniformly covered with reddish-brown rust layers with a small amount of black Fe₃O₄ deposits. The microscopic morphology shows sparse and discrete pitting pits, with shallow pit depth and uniform distribution. The newly formed corrosion products preliminarily isolate the matrix from external chloride ions, which inhibits the continuous expansion of local corrosion to a certain extent. At this stage, the average corrosion rate decreases slightly, and the surface of the steel matrix remains relatively flat without obvious gully corrosion characteristics.

2.2 Medium Corrosion Stage (7–21 Days)

With the increase of salt spray erosion time, the surface rust layer gradually turns black-red, and strip-shaped yellow-black rust areas appear locally. Affected by salt spray gravity deposition and surface liquid flow, typical gully corrosion morphologies are formed on the steel surface, with deepened and widened corrosion gullies. Microscopically, discrete pitting pits develop into dense punctate corrosion clusters, and the original polishing scratches on the steel surface completely disappear. The corrosion pits expand and connect with each other, forming a transitional morphology from pitting to honeycomb corrosion. The protective performance of the rust layer decreases gradually, and the average corrosion rate begins to rise steadily.

2.3 Late Corrosion Stage (42 Days and Above)

In the late severe corrosion stage, the surface rust layer of Q235 steel is dark brown, thick and loose, with obvious cracks and bulges. The bonding force between the rust layer and the matrix is extremely weak, and the rust layer is easy to peel off under slight external force. Microscopically, the surface presents a typical honeycomb corrosion structure, with deep internal corrosion pits and new secondary pitting defects inside the original pits. The long-term penetration of chloride ions causes continuous thinning of the steel matrix, and the uneven surface morphology is further aggravated. The average corrosion rate increases significantly at this stage, forming a vicious cycle of rust layer cracking, ion penetration and matrix corrosion.

3. Dynamic Variation Law of Salt Spray Corrosion Rate

The corrosion depth of Q235 steel increases monotonically with the extension of salt spray time, while the average corrosion rate presents a unique dynamic law of first decreasing and then increasing.

During 1 to 3 days of early corrosion, the average corrosion rate drops from 4.60 μm/d to 4.24 μm/d. The newly generated fine corrosion products cover the steel surface, which blocks the diffusion channel of chloride ions and plays a certain protective role on the matrix, resulting in reduced corrosion kinetics speed. From 7 days to 42 days of medium and late corrosion, the loose structure and surface cracking of the rust layer weaken the protective effect significantly. The average corrosion rate rises continuously from 4.36 μm/d to 5.82 μm/d. The long-term salt spray erosion destroys the structural integrity of the rust layer, and the porous corrosion products accelerate the infiltration of corrosive media, leading to accelerated corrosion damage of the Q235 steel matrix.

4. Tensile Mechanical Property Degradation Based on DIC Full-Field Strain Analysis

4.1 Macroscopic Mechanical Index Degradation Law

After salt spray corrosion, the core tensile mechanical indexes of Q235 steel decrease linearly with the increase of corrosion weight loss rate, including elastic modulus, yield strength, ultimate tensile strength and breaking elongation. With the extension of corrosion time, the yield platform of the stress-strain curve gradually shortens and even disappears in the late corrosion stage. The linear fitting results show that the yield strength and tensile strength have a significant negative linear correlation with the weight loss rate, with stable fitting accuracy, which can quantitatively predict the mechanical degradation degree of corroded Q235 steel.

Salt spray corrosion significantly reduces the ductility of Q235 steel. The strain difference between the yield stage and fracture stage decreases continuously, and the material presents obvious brittle transition characteristics. The macroscopic fracture morphology changes from oblique shear fracture of intact steel to stepped brittle fracture of corroded steel.

4.2 DIC Full-Field Strain Evolution Characteristics

DIC full-field strain cloud images intuitively reveal the essential deformation difference between corroded and intact Q235 steel specimens. For uncorroded specimens, the surface strain field is uniform in the elastic and yielding stages, and the strain concentration only occurs in the local necking area in the late tensile stage.

With the aggravation of salt spray corrosion, the surface strain field uniformity of Q235 steel deteriorates significantly. In the elastic and hardening stages, high-strain areas are dispersed and randomly distributed, and local strain concentration appears at corrosion pits and gully defects in advance. The uneven strain distribution becomes more prominent with the extension of corrosion time. The irregular surface corrosion defects break the uniform stress transmission of the matrix, induce premature local plastic deformation, and eventually lead to earlier fracture failure of specimens under lower load and deformation.

4.3 Corrosion Defect Induced Failure Mechanism

Honeycomb pitting corrosion and gully morphology formed by long-term salt spray corrosion are the fundamental causes of mechanical performance degradation and strain field heterogeneity. Corrosion pits form local stress concentration points during tension, which preferentially induce micro-plastic deformation and expand continuously. The superposition of multiple dispersed strain concentration areas accelerates the plastic instability of the specimen, shortens the yield platform, reduces the ultimate deformation capacity, and finally realizes the transition from ductile failure to brittle failure of Q235 steel.

5. Standard Test and Analysis Methodology

5.1 Salt Spray Accelerated Corrosion Test

Adopt neutral salt spray test in accordance with GB/T 10125 standard, with 5% NaCl solution, pH value controlled at 6.5–7.2, and continuous spray mode. Set multiple corrosion time gradients of 1d, 3d, 7d, 21d and 42d. After corrosion, clean and dry the specimens, test the weight loss rate and corrosion depth, and record macroscopic and microscopic surface morphology changes.

5.2 DIC Tensile Test Procedure

Arrange artificial speckle patterns on the surface of corroded Q235 standard tensile specimens. Conduct uniaxial tensile test at a constant loading rate, synchronously collect surface speckle images through high-resolution cameras, and calculate full-field strain distribution, local strain evolution and overall deformation characteristics by DIC algorithm. Compare and analyze the differences in strain field uniformity, strain concentration position and deformation failure process of specimens with different corrosion degrees.

6. Key Technical Conclusions

First, the salt spray corrosion morphology of Q235 steel evolves regularly with time: the macroscopic rust layer changes from reddish-brown to black-red and dark brown, and the microscopic morphology develops from sparse pitting to dense punctate corrosion, and finally to honeycomb gully corrosion. The corrosion gully gradually evolves from shallow and wide to deep and narrow.

Second, the corrosion rate of Q235 steel has obvious phased characteristics: the early rust layer has a protective effect leading to decreased corrosion rate, while the late loose and cracked rust layer loses protection, resulting in continuous acceleration of corrosion and increased matrix damage degree.

Third, salt spray corrosion induces linear degradation of tensile mechanical properties of Q235 steel. The yield platform shortens or disappears, and the material undergoes ductile-brittle transition. DIC full-field detection verifies that corrosion defects cause dispersed high-strain areas and uneven strain field distribution, which is the core microscopic mechanism leading to premature failure and reduced bearing capacity of corroded steel.

Fourth, DIC technology can effectively characterize the heterogeneous deformation characteristics of corroded steel, make up for the limitations of traditional macroscopic mechanical tests, and provide accurate technical support for corrosion damage evaluation, residual life prediction and safety assessment of Q235 steel structural components in coastal and salt fog environments.