The Effect of Yellowing on the Corrosion Resistance of Chromium-Free Fingerprint-Resistant Hot-Dip Al-Zn-Coated Steel

Abstract

Inorganic/organic composite passivation film can significantly improve the corrosion resistance performance of hot-dip Al-Zn-coated steel. However, yellowing of the passivation film always leads to obvious performance degradation in corrosion resistance. Investigating the yellowing mechanism of the passivation film and its impact on corrosion resistance would provide a foundation for enhancing its yellowing resistance property. This study primarily focuses on the yellowing mechanism of the passivation film based on the copolymer of N-vinylpyrrolidone and N-vinylcaprolactam. It is found that the oxidation and semi-carbonization of butyramide and valeroamide generated by C–N bond cleavage in the copolymer at high temperatures are responsible for the yellowing of the passivation film. The cracking of the passivation film caused by yellowing degree exposes more of the bare Al-Zn coating, further accelerating the degradation in the corrosion resistance. Additionally, it is observed that the impact of yellowing on the corrosion resistance is negligible when the color difference (ΔE*) caused by yellowing is less than 3.0, whereas ΔE* values above 3.0 result in rapid degradation in the corrosion resistance of the passivation film. The formula y = 0.77 − 0.07x + 0.023x² + 0.0039x³ effectively expresses the relationship between corrosion area (y) and ΔE* (x) (R² = 0.995).

1. Introduction

Hot-dip Al-Zn-coated steel (HDAZCS) is a type of cold-rolled steel plate coated with Al-Zn alloy through a hot-dip coating process. Due to the excellent corrosion resistance provided by the oxide film formed from the aluminum in the hot-dip Al-Zn coating, HDAZCS exhibits superior corrosion resistance compared to traditional hot-dip galvanized steel [1]. Typically, the corrosion resistance of HDAZCS is 6–8 times greater than that of hot-dip galvanized steel, and it also has excellent resistance to acids, alkalis, chlorides, high temperature, and erosion [2]. Owing to these outstanding properties, HDAZCS is widely used in industries, such as home appliances, automotives, and power equipment. According to market research by Global Info Research, the global market for HDAZCS reached approximately USD 62 billion in 2023 and is expected to grow to around USD 74.6 billion by 2029, with a compound annual growth rate (CAGR) of nearly 2.7% over the next five years.

Passivation is an essential process in the production of HDAZCS and has a significant impact on the performance of HDAZCS [3]. Passivation film formed on the surface of HDAZCS can effectively avoid direct contact of corrosive gases and liquids with the Al-Zn coating, thus enhancing the corrosion resistance of HDAZCS. Additionally, passivation film improves the HDAZCS’s coating performance and resistance to acids, alkalis, and chlorides [4]. Chromate passivation is a traditional passivation process for HDAZCS. A dense oxide film is formed on the surface of hot-dipped Al-Zn coatings through chromate passivation, which inhibits the anodic dissolution of Zn during the electrochemical process, thereby enhancing the corrosion resistance of HDAZCS [5]. However, chromate passivation is the cause of severe environmental issues due to the use of hexavalent chromium, which is highly carcinogenic and teratogenic to humans, causing the gradual phase-out of chromate passivation in the production of HDAZCS. Thus, developing environmentally friendly Cr-free passivation processes has become the focus of current research and application. Based on the component, Cr-free passivation processes can be categorized into three types: inorganic passivation, organic passivation, and organic/inorganic composite passivation [6]. Inorganic passivation salts, including molybdates, tungstates, silicates, titanates, and rare earth metal salts, form an insoluble passivation film on the surface of hot-dipped Al-Zn coatings by reacting with corrosive media or metal corrosion products. This process hinders the dissolution of the anodic coating and reduces the electrochemical corrosion rate of the hot-dipped Al-Zn coating [7]. However, the corrosion resistance of inorganic passivation films is currently still inferior to that of traditional chromate passivation films. Additionally, inorganic passivation films often face difficulty in meeting customers’ high requirements for fingerprint resistance and coating properties. Organic passivation primarily relies on a crosslinked organic film on the surface of a hot-dipped Al-Zn coating constructed by the chelation reaction between organic compounds and the metal substrate, forming a barrier between the corrosive media and hot-dipped Al-Zn coating, thereby enhancing the corrosion resistance of HDAZCS [8]. Additionally, the strong chemical action between the organic groups in organic passivation films and paints or adhesive tapes enhances the paintability and adhesion properties of the HDAZCS surface. Common organic components used for the organic passivation of HDAZCS include various silane coupling agents, tannic acid, nitrogen-containing heterocyclic derivatives, phytic acid, and water-soluble resins [9]. As the application fields of HDAZCS continue to expand, customers are demanding more sophisticated requirements not only for conventional properties such as corrosion resistance, environmental friendliness, and adhesion, but also for special functional features, including paintability, fingerprint resistance, and conductivity. A passivation film with multiple functions is required. The existing inorganic and organic passivation processes struggle to meet customers’ multiple demands. The composite passivation process, which combines the advantages of both inorganic and organic passivation, shows greater potential for meeting the diverse requirements of customers compared to solely inorganic or organic processes due to the synergistic and complementary effects of the inorganic and organic components, establishing composite passivation as the mainstream passivation process for HDAZCS today.

In some applications of HDAZCS, such as solar power generation equipment and the outer shells of electrical devices, inorganic/organic composite passivation film often experiences yellowing after prolonged exposure to high-temperature environments. This yellowing can compromise the structural completeness of the passivation film, leading to a degradation of its corrosion resistance and other properties. Studying the yellowing mechanism of inorganic/organic composite passivation films and its impact on corrosion resistance will provide guidance for passivation liquid manufacturers to optimize their formulations, while also giving users a basis to assess the reliability of yellowed Al-Zn-coated steel plates in practical applications. Unfortunately, it is rare that research focuses on the yellowing mechanism of passivation film and its impact on the corrosion resistance of HDAZCS at present. Henkel’s Granocoat 621 inorganic/organic composite passivation solution, which consists of a copolymer of N-vinylpyrrolidone (VP) and N-vinylcaprolactam (VCL), is widely used by major producers of HDAZCS due to its excellent overall performance. This study reveals, for the first time, the yellowing mechanism of a passivation film based on the copolymer of N-vinylpyrrolidone and N-vinylcaprolactam. The oxidation and semi-carbonization of butyramide and valeroamide generated by C–N bond cleavage in the copolymer at high temperatures are found to be responsible for the yellowing of the passivation film. Additionally, the relationship between the corrosion area and the color difference (ΔE*) caused by yellowing is established for the first time. The passivation liquid manufacturers and their users are expected to benefit from this study.

2. Materials and Methods

2.1. Materials

The hot-dipped Al-Zn-coated carbon steel with a thickness of 1.0 mm produced by JISCO (Jiuquan Iron and Steel Group Co., Ltd., Jiayuguan, China) was used as the experimental substrate. The hot-dipped Al-Zn coating of the substrate consisted of 55% Al and 43.6% Zn. As per the requirements, the substrate was laser-cut into specimens with size of 150 × 100 × 1 mm, which were deburred using a trimming machine to remove edge burrs, soaked in acetone for 2 h to remove surface greasy dirt, and dried with lint-free cotton before use. The Cr-free fingerprint-resistant passivation solution was provided by Henkel (Granocoat 621, HG). The primary effective component of this passivation film ws a copolymer of N-vinylpyrrolidone and N-vinylcaprolactam. The element composition of the passivation film is presented in

Table 1. The elements composition of the passivation film.

Elements (mol %)	Fe	C	O	N	Si	P	S	Al	Zn	Na
Granocoat 621	0.75	46.90	16.87	29.40	0.66	0.22	0.46	0.29	4.36	0.09

.

2.2. Experimental Methods

The passivation solution was dried in an oven at 100 °C for 60 min to remove the solvent and then heated at 240 °C for 0–120 min for yellowing treatment. The resulting yellowed sample was labeled as HG-Yx, where x represents the duration of the yellowing treatment. The yellowing mechanism of the passivation film was investigated using thermogravimetric analysis under air atmosphere (TGA, STA200, Hitachi, Ltd., Chiyoda, Japan), X-ray photoelectron spectroscopy (XPS, AXIS SUPRA+, Shimadzu Corporation, Kyoto, Japan), Fourier-transform infrared spectroscopy (FTIR, iS50, Thermo Nicolet Corporation, Madison, WI, USA), and X-ray powder diffraction (XRD, ARL EQUINOX 3000, Thermo Fisher Scientific, Waltham, MA, USA).

The passivation solution was uniformly coated onto the clean HDAZCS surface using a coating bar (OSP-04, OSP Corporation, Osaka, Japan) and then dried in an oven at 130 °C for 10 min. The thickness of the dry passivation film was controlled at approximately 1.2 g/m², corresponding to a physical film thickness of about 1.0 μm. The yellowing test was conducted based on the national standard of China (GB/T 1740-2007, Determination of resistance to heat and humidity of paint films [10]). After being heated in an oven at 240 °C for 0–120 min, the color difference (ΔE*) of the HDAZCS with the passivation film after yellowing treatment was recorded using a colorimeter (CR10 Plus, Konica Corporation, Tokyo, Japan) and calculated using the formula ΔE* = [(ΔL)² + (Δa)² + (Δb)²]^¹/₂, where L, a, and b are the tristimulus color coordinates. All the test data are the average of three parallel tests. The conductivity and resistivity were measured using a four-point probe resistivity tester (Four Point Probe, Filmetrics R50, KLA Instruments, Milpitas, CA, USA). The neutral salt spray test to assess corrosion resistance was performed according to the national standard of China (GB/T 10125-2012, Artificial Atmosphere Corrosion Test—Salt Spray Test [11]) in a salt spray tank. The surface morphology of the HDAZCS with the yellowed passivation film after the corrosion resistance test was observed using scanning electron microscopy (SEM, Hitachi, SU8600), metallographic microscopy (DM2700 M, Leica Camera AG., Wetzlar, Germany), laser scanning confocal microscopy (LSCM, Olympus, OLS5000, Japan), and optical camera (EOS R7, Canon Corporation, Tokyo, Japan). The Tafel polarization curves and electrochemical impedance spectroscopy (EIS) of the HDAZCS with the yellowed passivation film were analyzed using an electrochemical workstation (660I, CH Instruments Inc., Shanghai, China).

3. Results and Discussion

3.1. Investigation of the Yellowing Mechanism

The weight loss of the dried passivation solution powders without yellowing treatment (HG-Y0) and after yellowing treatment at 240 °C for 120 min (HG-Y120) was measured at a constant temperature of 240 °C. As shown in Figure 1a, HG-Y0 exhibited continuous weight loss over the test period. Rapid weight loss of 13.1% occurred within the first 0–100 s primarily due to the evaporation of residual solvent water, while weight loss of 4.5% from 100 to 154 s was caused by the volatilization of some small molecules such as pyrrolidone, amides, fatty acids, and carbon oxides generated from the oxidation and depolymerization of certain low-molecular-weight oligomers of VP and CAL [12]. Slow weight loss of 4.8% within the 154–500 s range was due to the localized carbonization of the passivation solution powder. The oxidation and pyrolysis of the copolymer of VP and CAL led to fractures in the passivation film and numerous structural defects, reducing the protective effect of the passivation film on HDAZCS. The subsequent partial carbonization caused yellowing and fragmentation of the passivation film, further degrading the corrosion resistance of HDAZCS. After yellowing treatment for 120 min at 240 °C, the powder HG-Y120 showed only small weight loss of 1.9% during the 500 s constant-temperature process, indicating that the oxidation, depolymerization, and partial carbonization of the passivation solution powder were already completed during the yellowing treatment process. Figure 1b shows the weight loss of HG-Y0 and HG-Y120 under the temperature range from 25 to 400 °C. As shown in Figure 1b, weight loss of 18.6% was observed as the temperature increased from 100 to 210 °C. This weight loss was mainly due to the oxidation and depolymerization of low-degree oligomers of VP and VCL copolymers, causing localized breakage of the polymer chain and generating low-molecular-weight compounds such as pyrrolidone, caprolactam, amides, and acids [13]. The volatilization of these low-molecular-weight compounds was responsible for this weight loss. Additional weight loss of 7.5% between 210 and 292 °C was caused by the ring-opening reactions of pyrrolidone and caprolactam; the radicals generated during these reactions further induced the acidic depolymerization reactions and formation of oxycarbides with low molecular weight and high volatileness, such as CO₂ and CO. When the temperature was higher than 292 °C, significant carbonization of the power induced rapid and sustained weight loss. The HG-Y120 powder, which had already undergone yellowing treatment before the thermogravimetric test, exhibited no noticeable weight loss as the temperature was below 292 °C, suggesting the accomplishment of the depolymerization of the VP and VCL copolymer during the yellowing treatment; the depolymerized products with low molecular weight were removed from the passivation solution powder before the thermogravimetric test. Rapid weight loss above 292 °C was attributed to the carbonization of the passivation film. The above results suggest that yellowing of the passivation film primarily occurs above 210 °C and intensifies above 292 °C, leading to rapid degradation in the protection of the passivation film on HDAZCS.

Figure 1c shows the XRD patterns of the dried passivation solution powders that had not undergone yellowing treatment and those treated at 240 °C for 40, 80, and 120 min. As indicated in Figure 1c, the XRD pattern of sample HG-Y0 reveals the amorphous nature of the copolymer of VP and VCL, as evidenced by the weak and broad diffraction peak [14]. The samples treated for 40 to 120 min exhibited broad diffraction peaks near the 2θ angles of 19.4° and 21°. The peak at 19.4° corresponds to the characteristic diffraction peak of pure polyvinylpyrrolidone (PVP), while the peak near 21° is associated with saturated structures linked to the edges of amorphous carbon, such as aliphatic side chains [15]. The presence of diffraction peaks from pure PVP and amorphous carbon indicates that the copolymer of VP and VCL underwent depolymerization and localized carbonization, leading to yellowing and degradation in the protective capability of the passivation film on HDAZCS. Figure 1d shows the FTIR spectra of samples HG-Y0 and HG-Y120. The infrared absorption peak at 3436 cm⁻¹ corresponds to the stretching vibration of O-H; the absorption peaks at 2929 cm⁻¹ and 2857 cm⁻¹ are attributed to the symmetric stretching vibrations of -CH₂ groups within the polymer chains, while the peak at 2891 cm⁻¹ is due to the asymmetric stretching vibration of -CH₂; the absorption peaks at 1735 cm⁻¹ and 1635 cm⁻¹ are related to the C=O functional groups in the copolymer; the peak at 1463 cm⁻¹ results from the C-N stretching vibration in the caprolactam ring; the peak at 1390 cm⁻¹ is associated with in-plane vibrations of aliphatic C-H, C-C, and C-O bonds; the peaks at 1087 cm⁻¹ and 942 cm⁻¹ are caused by the bending vibrations of C-N bonds and CH₂ groups connected to the pyridine ring; the peak at 1052 cm⁻¹ corresponds to the stretching vibration of C-O. Compared to the FTIR spectrum of HG-Y0, the FTIR spectrum of sample HG-Y120 shows a stretching vibration peak of the primary amine -NH₂ at 2973 cm⁻¹, while the asymmetric stretching vibration of -CH₂ at 2891 cm⁻¹ disappears, and the absorption peak for the C-O stretching vibration at 1052 cm⁻¹ is enhanced [16,17].

Figure 2 shows the XPS spectra of the dried passivation solution powders before and after the yellowing treatment. As shown in Figure 2a,b, the high-resolution C 1s XPS spectra of samples HG-Y0 and HG-Y120 can be deconvoluted into four peaks at 284.7, 285.1, 286.1, 286.7, and 288.9 eV, corresponding to C-C, N-sp² C, N-sp³ C, C=O, and O-C=O bonds, respectively [18]. The relative content of each type of chemical bond is shown in

Table 2. Deconvolution fitting results of the C 1s XPS orbital and the relative percentages of various chemical bonds.

Samples	284.7 eV (C-C)	285.1 eV (N-sp2 C)	286.1 eV (N-sp3 C)	286.65 eV (C=O)	288.9 eV (O-C=O)
HG-Y0	32.71%	21.26%	10.96%	25.38%	9.69%
HG-Y120	24.00%	30.34%	14.70%	22.00%	8.96%

. Compared to HG-Y0, the relative content of C-C bonds in sample HG-Y120 decreased from 32.71% to 24.00%, while the relative content of C=O bonds dropped from 25.35% to 22.00%. This indicates that the carbon chain of the copolymer of VP and VCL underwent localized oxidation and cleavage, generating small molecules, such as CO, CO₂, pyrrolidone, and caprolactam. Due to the significant decrease in the relative content of C-C bonds in HG-Y120, an increase in the relative contents of N-sp² C and N-sp³ C could be observed compared to HG-Y0. However, the increase in N-sp² C content was much greater than that of N-sp³ C, suggesting that the N-C bond in the N-C=O group located in the pyrrole and caprolactam rings was more susceptible to cleavage than the N-C bonds in the C-N-C group, leading to the ring opening of pyrrole and caprolactam and the formation of small molecules, such as carbon oxides and fatty acids. The high-resolution N 1s XPS spectra of HG-Y0 and HG-Y120 can be fitted with two peaks at 397.2 and 399.0 eV (Figure 2c,d), corresponding to sp² C-N and sp³ C-N bonds in the caprolactam and pyrrole rings, respectively [19]. Compared to HG-Y0, the relative content of sp³ C-N bonds in HG-Y120 increased from 17.6% to 61.7%, further indicating that the N-C bond in the N-C=O group was more susceptible to cleavage than that in the C-N-C group. For the O 1s orbital, the high-resolution XPS spectrum can be fitted with two peaks at 530.3 eV and 529.8 eV, corresponding to the O=C-N bonds in the caprolactam and pyrrolidone rings, respectively (Figure 2e,f) [20]. After yellowing treatment at 240 °C for 120 min, the relative content of the O=C-N bond in the pyrrolidone (529.8 eV) decreased from 28.88% for HG-Y0 to 21.65%, indicating that the O=C-N bond in the pyrrolidone ring decomposed before that in the caprolactam ring.

Based on TGA, FTIR, XRD, and XPS analyses, the possible decomposition and yellowing mechanisms of the copolymer of N-vinylcaprolactam and N-vinylpyrrolidone are proposed, as shown in Figure 3. When the temperature exceeded 100 °C, C-N bonds connected to the main chain in some lower molecular weight copolymers initially broke, generating caprolactam, α-pyrrolidone, and polyvinyl compounds. As the temperature increased, the sp³ C-N bonds in caprolactam and α-pyrrolidone underwent cleavage, leading to the ring opening of caprolactam and α-pyrrolidone, resulting in the formation of butyramide and pentanamide. When the temperature exceeded 210 °C, butyramide and pentanamide further pyrolyzed in the presence of O₂, producing semi-carbonized products, carbon oxides, and water. The carbon oxides and water quickly escaped from the passivation film, forming gas pores within the network of the film, while the semi-carbonized products caused the passivation film to harden, shrink, and undergo yellowing.

3.2. The Effect of Yellowing on the Surface Morphology

Figure 4a–d show optical photographs of HDAZCS coated with Henkel passivation film after yellowing tests of 0, 40, 60, and 80 min. After 40, 60, and 80 min of yellowing tests, the color differences (∆E) of the passivation film compared to the untested film were 3.82, 5.17, and 5.69, respectively. Yellow spots gradually appeared on the surface of the passivation film with increasing yellowing test duration along with the increase in the yellowing area and yellowing intensity. The metallographic images of the passivation film before yellowing tests show a clear and transparent network (Figure 4c). As the duration of the yellowing test increased, the passivation film network progressively darkened, with the appearance of pores and cracks (Figure 4f–h). SEM images reveal that the passivation film exhibited a pine needle-like morphology and uniformly covered the Al-Zn coating surface before the yellowing test (Figure 4i,j). However, after 80 min of yellowing testing, the passivation film network was locally damaged. The rupture of the passivation film caused lots of large fragments of passivation film on the Al-Zn coating surface (Figure 4k,l). EDS surface scanning analysis indicated the uniform distribution of C and O elements in the passivation film. However, the C content in the passivation film increased from 18.5% to 26.4% after 80 min of yellowing testing, while the O content rose from 3.9% to 8.7%, indicating the localized carbonization and oxidation of the passivation film.

3.3. The Effect of Yellowing on the Corrosion Resistance

The corrosion potential and corrosion current density of samples subjected to different yellowing test durations at 240 °C were evaluated using the Tafel polarization curve. As shown in Figure 5a, the corrosion potential and corrosion current density of the untreated passivation film HG-Y0 were −0.663 V and 9.71 × 10⁻⁷ A cm², respectively. In contrast, the sample HG-Y60, which underwent yellowing treatment for 60 min exhibited a corrosion potential of −0.667 V and a corrosion current density of 9.98 × 10⁻⁷ A cm². For the sample HG-Y100 subjected to 100 min of yellowing treatment, the corrosion potential further decreased to −0.671 V, while the corrosion current density rose to 1.032 × 10⁻⁶ A cm². The increase in corrosion current density and decrease in corrosion potential with greater yellowing indicates a gradual deterioration in the corrosion resistance of the HDAZCS coated with passivation film. Yellowing of the passivation film led to structural defects and fragmentation of the organic polymer networks on the surface of HDAZCS, facilitating the diffusion of corrosive gases and solutions through the passivation film. This interaction accelerated the corrosion process by allowing corrosive agents to reach the underlying aluminum–zinc layer, thereby degrading the corrosion resistance of the passivation film-coated substrate. The EIS spectra further confirm the degradation in the corrosion resistance of the passivation film (Figure 5b). The inset in Figure 5b presents the equivalent circuit that is suitable for modeling the experimental results, where R_s is the solution resistance, Q_dl is the constant phase element (CPE) of the electric double layer between the passivation film and solution, and R_ct is the charge transfer resistance. The fitted results of EIS reveal Rct values of 21,194 Ω, 6325 Ω, and 4009 Ω for specimens HG-0, HG-60, and HG-100, respectively. Generally, the R_ct value is directly proportional to corrosion resistance [21]. As shown in Figure 5b, the rapid decrease in the R_ct value with the progression of the yellowing test demonstrates a significant degradation in the corrosion resistance of the passivation film. Figure 5c illustrates the relationship between the resistivity and conductivity of the passivation film and the duration of the yellowing test. It is clear that as the duration of the yellowing test increased, the resistivity of the passivation film decreased while the conductivity correspondingly increased. This change is attributed to the gradual cracking of the passivation film over time, which exposed more of the aluminum–zinc layer and thus raised conductivity. Furthermore, with extended yellowing treatment, the passivation film experienced dehydration, leading to the formation of semi-carbonized products that had higher conductivity, thereby further contributing to the increase in the conductivity of the passivation film.

The corrosion resistance and color difference (ΔE*) of HDAZCS coated with a passivation film after yellowing tests were evaluated according to Chinese national standards GB/T 10125-2021 (Artificial Atmosphere Corrosion Testing—Salt Spray Test) [22], and GB/T 11186.2-1989 (Methods for Measuring the Color of Coatings—Part 2: Color Measurement) [23], respectively. After yellowing treatments at 240 °C for 0, 20, 40, 60, 80, and 100 min, the ΔE* values of HDAZCS coated with a yellowed passivation film were 0, 1.30, 3.0, 4.89, 6.18, and 6.80, respectively. Following a subsequent 72 h standard salt spray test, the surface corrosion areas of the corresponding samples reached 0.756%, 0.765%, 0.847%, 1.424%, 2.206%, and 2.637% (Figure 6a–f), demonstrating an increase in corrosion area with the increasing ΔE* values. Notably, when ΔE* was below 3.0, yellowing had minimal impact on corrosion resistance, but at ΔE* values above 3.0, a marked rise in surface corrosion areas was observed. The relationship between the corrosion area (y) and ΔE* (x) is expressed by the equation y = 0.77 − 0.07x + 0.023x² + 0.0039x³ (R² = 0.995) (Figure 6g). Figure 6h,i show the metallographic images of samples HG-Y0 (untreated) and HG-Y100 (yellowing treated for 100 min) after 72 h of standard salt spray corrosion testing. In the metallographic image of HG-Y0, the passivation film remains largely intact, with only minor pitting observed on the surface. By contrast, HG-Y100 shows significant damage to the passivation film and extensive corrosion. Laser confocal microscopy images further reveal that HG-Y0 exhibits minimal pitting and maintains a smooth surface with a maximum height difference of only 6.73 μm after corrosion testing. In contrast, HG-Y100 has a rougher surface, with a surface height difference of 17.46 μm, and displays significant pitting on the surface of the yellowed passivation film after 72 h of salt spray corrosion testing.

4. Conclusions

The yellowing mechanism of the N-vinylpyrrolidone and N-vinylcaprolactam copolymer-based passivation film was successfully revealed. The oxidation and semi-carbonization of butyramide and valeroamide generated by C–N bond cleavage in the copolymer at high temperatures are found to be responsible for the yellowing of the passivation film. The cracking of the passivation film caused by the yellowing contributes to the degradation of corrosion resistance. The relationship between corrosion area (y) and ΔE* (x) is expressed by the following equation: y = 0.77 − 0.07x + 0.023x² + 0.0039x³) (R² = 0.995). This study will provide guidance for passivation liquid manufacturers to optimize their formulations, while also giving users a basis to assess the reliability of yellowed Al-Zn-coated steel sheets in practical applications.