Failure Mechanism And Life Prediction Of Solvent-Free Epoxy Primer And Matched Weather-Resistant Topcoat On Low-Alloy Steel Surface

Due to low-cost and effectiveness,organic coatings are widely used for metals corrosion protection.Because of the environmental problems caused by volatile organic solvents in solventborne organic coatings,the development of solvent-free epoxy coatings has become a hot direction for corrosion protection research.Clarifying the failure mechanism of coatings is the basis for coating development and application.Therefore,the research on the failure mechanism and life prediction of solvent-free epoxy coatings has important theoretical guidance and engineering application significance in the optimization of corrosion protection performance and maintenance of coatings.In this work,the failure process of environmentally friendly solvent-free general epoxy primer and matched weather-resistant topcoat（Aliphatic polyurethane matte topcoat）on 9440 steel surface with two substrate surface treatments and painting methods were investigated in 3.5%Na Cl solution,salt spray and cyclic temperature immersion test using electrochemical impedance spectroscopy（EIS）,open circuit potential（OCP）,adhesion test,gloss loss test and color difference test.The failure mechanisms of the coated samples in three accelerated environments were analyzed by the equivalent electrical circuit and combined with fourier transform infrared spectroscopy（FTIR）and scanning electron microscopy（SEM-EDS）.The failure criteria and life prediction methods for 9440 steel/coated samples were also investigated.The main conclusions are as follows:（1）The|Z|_0.01Hzvalue of the solvent-free epoxy primer samples during the deterioration process has a certain correlation with the change of the OCP value.When the|Z|_0.01Hzvalue of the coated sample drops below 1×10⁷Ωcm²,the OCP value of the sample drops suddenly.The OCP may reflect the protective properties of coatings to some extent,especially in the later stages.Therefore,the open circuit potential may be used to evaluate the degradation stage of the coating.（2）The corrosion of the substrate surface was observed under electron microscopy when the|Z|_0.01Hzof 9440 steel（Sa2.5）/solvent-free epoxy primer（sprayed）samples and 9440 steel（St3.0）/solvent-free epoxy primer（brushed）samples in the three environments decreased to 8×10⁶～2×10⁷Ωcm²,earlier than the appearance of bulging of the coating.Therefore,a reference criterion of failure for solvent-free epoxy primers on 9440 steel is around 1×10⁷Ωcm².（3）The deterioration speed and mechanisms of solvent-free epoxy primers were different in the three environments.The degradation speed of coated samples was faster in salt spray and cyclic temperature immersion test than in 3.5%Na Cl immersion.The degradation speed of the solvent-free epoxy primers in cyclic temperature immersion test was slower in the later stage compared to the salt spray,which is due to the post-curing of the solvent-free epoxy primers.The deterioration of the coating in 3.5%Na Cl immersion and salt spray was mainly a physical process,while the deterioration of the coating in cyclic temperature immersion test was also accompanied by changes in chemical structure.（4）The deterioration speed of solvent-free epoxy primer/polyurethane matte topcoat system and solvent-free epoxy primer in salt spray and cyclic temperature immersion test is consistent,while the anti-corrosion and weather-resistant coating system extends the protective effect on 9440 steel.In addition,although some blisters appeared on the coating surface after salt spray and cyclic temperature immersion test,the impedance of the whole coating system remained high,the infrared spectrum of the primer did not show significant changes compared with that before the test,the adhesion between the primer and the substrate was still high,and no rusting of the substrate under the coating was observed by electron microscopy.This shows that for anti-corrosion and weather-resistant coatings,the blisters occurred on the surface mainly reflects the deterioration of the topcoat,while the primer still has a protective effect on the substrate.（5）The key to predicting the coating life using the empirical linear relationship between low-frequency impedance modulus and time is the determination of the failure criterion.1×10⁷Ωcm²as the prediction of low-frequency impedance at failure is closest to the actual results.The empirical formula for coating life prediction was extended and verified based on the variation of the 15 k Hz phase angle（φ）with time.The predicted results were in good agreement with the experimental results,and the effect of the environment on the deterioration speed of the coating was consistent with the order of coating deterioration in the actual experiment.In addition,the weight coefficients of two indicators,color difference and adhesion,in predicting the coating service life were determined,and a life prediction model of solvent-free epoxy primer was established and validated in different environments.