Effect of Calcium Nitrate on Microstructure and Anti-Corrosion Properties of Zinc Phosphate Coatings on Stainless Steel

Abstract

Hopeite (Zn₃(PO₄)₂·4H₂O) coatings, fabricated via zinc phosphate chemical conversion (ZPCC), have attracted considerable interest in biomedical applications owing to their excellent corrosion resistance and biocompatibility. However, the influence of calcium nitrate (CN) on coating properties remains poorly understood. This study systematically investigates the effect of CN concentration on the microstructure and corrosion behavior of ZPCC coatings deposited on stainless steel (SS). The phase composition, surface morphology, and elemental distribution were characterized using X-ray diffraction (XRD) and scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS). Electrochemical corrosion performance was assessed via potentiodynamic polarization in a 0.9 wt.% NaCl solution. The results indicate that CN concentration critically influences coating morphology, with higher concentrations leading to reduced crystal size and increased coating mass. Notably, the coating prepared with 6 g/L CN exhibited a dense, uniform, and fine-grained microstructure, resulting in superior corrosion resistance. Additionally, the optimized coating demonstrated strong interfacial adhesion, with a shear strength of 10.05 ± 1.2 MPa.

1. Introduction

Zinc phosphate chemical conversion (ZPCC) has emerged as a widely adopted technique in metal surface engineering due to its rapid coating deposition, economic viability, and compatibility with geometrically complex substrates [1,2,3]. The process facilitates the formation of a hopeite-based coating, which markedly improves the substrate’s corrosion resistance while offering robust adhesion and favorable biocompatibility [4,5], making them particularly valuable for steel protection. The corrosion resistance of ZPCC coatings is governed by multiple critical parameters, including coating mass, thickness, porosity, and structural homogeneity [2,6]. Among these, a high coating mass and low porosity are particularly essential for enhancing corrosion protection, as they promote a more uniform and defect-free microstructure. Consequently, optimizing these factors through surface modification is crucial to achieving superior corrosion resistance. Optimizing coating performance necessitates precise control over process parameters, wherein additives and accelerators play a pivotal role. Transition metal cations (e.g., Mn²⁺, Ca²⁺, Ni²⁺, and Co²⁺) are frequently employed to enhance coating uniformity and corrosion resistance, particularly on mild steel, magnesium alloys, and aluminum substrates [7,8,9,10]. Additionally, oxidizing agents such as NO₃⁻, NO₂⁻, and ClO₃⁻ are incorporated into phosphate solutions to promote nucleation kinetics and accelerate coating growth [2]. Zuo et al. developed an in situ Ca-doped Sr-phosphate chemical conversion coating on titanium, demonstrating that Ca incorporation markedly reduces SrP crystal size [11]. Hosseinirad et al. demonstrated that incorporating Co into zinc phosphate conversion coatings on steel substrates significantly improves microstructural homogeneity and corrosion resistance [12]. Empirical evidence suggests that the strategic selection of these additives significantly governs critical coating properties, including coating mass, microstructural morphology, and long-term anti-corrosion performance.

Stainless steels (SSs) are widely employed in industrial applications due to their favorable combination of cost-effectiveness and functional properties, including high mechanical strength and exceptional corrosion resistance [13,14]. However, despite these advantages, SS remains vulnerable to corrosive attacks in highly aggressive environments, limiting its long-term durability [15]. To address this limitation, surface engineering strategies, such as electrodeposition, sol–gel processing, and electrochemical modification, have been extensively explored to enhance surface protection [15,16,17]. Among these techniques, our previous work demonstrated the successful deposition of zinc phosphate conversion coatings (ZPCCs) on SS substrates via an innovative galvanic coupling approach [4,18].

Previous studies [18,19] have primarily investigated the effects of key parameters such as temperature, processing time, and pH on the formation of hopeite coatings. While these works established fundamental relationships between the additives in the solution and coating characteristics, the role of additive concentration (such as calcium nitrate) remains insufficiently explored. This study systematically examines the influence of calcium nitrate concentration on critical coating properties, including coating mass, phase composition, morphology, and electrochemical behavior (as evaluated through polarization curves). The present work aims to identify the optimal calcium nitrate concentration for hopeite coating formation, thereby expanding the parameter space for controlling coating performance. By correlating precursor concentration with structural and electrochemical properties, this research provides actionable insights for tailoring hopeite coatings to specific functional requirements.

2. Materials and Methods

Commercially available 316 SS substrates (Φ10 × 1 mm) were employed in this study. The ZPCC was synthesized according to a previously established protocol [18], with comprehensive process parameters and a schematic illustration presented in Figure 1. The coating process involved immersing the samples in an aqueous solution composed of 20 g/L ZnO, 8 mL/L H₃PO₄, and 30 mL/L HNO₃. To systematically evaluate the influence of Ca(NO₃)₂ concentration on coating formation, ZPCC treatments were performed across a concentration range of 1.5–6.0 g/L.

The coating mass was determined gravimetrically using a high-precision digital balance (±0.1 mg accuracy), with the final value representing the average of five replicate measurements.

The coating mass (M, g/m²) was determined using Equation (1):

$$M={\displaystyle \frac{m_1-m_2}{A}}$$

(1)

where m₁ and m₂ (g) represent the weights before and after the coating removal, respectively, and A (m²) denotes the substrate area. The coatings were stripped by immersing the samples in a 70 °C alkaline solution (100 g/L NaOH, 4 g/L C₆H1₅NO₃, and 90 g/L C₆H₁₀N₂Na₄O₄) for 15 min [2].

The surface morphology and microstructure of the coatings were characterized by field-emission scanning electron microscopy (FE-SEM, SU-70, Hitachi High-Tech, Singapore). Phase composition analysis was performed using an X-ray diffractometer (XRD, Rigaku D/max-γB, Rigaku, Wilmington, MA, USA) with CuKα radiation (λ = 1.5406 Å) operated at 40 kV and 100 mA. XRD scans were conducted over a 2θ range of 5–70° at a scan rate of 10°/min.

The corrosion resistance of the specimens was assessed through potentiodynamic polarization measurements employing a conventional three-electrode configuration. A platinum sheet served as the counter electrode, while a saturated calomel electrode (SCE) was utilized as the reference electrode. The working electrode consisted of either uncoated or coated samples with an exposed surface area of 1 cm². All electrochemical tests were conducted in a 0.9 wt.% NaCl aqueous solution as the corrosive electrolyte. The polarization scans were performed using a PARSTAT 2273 potentiostat at a constant scan rate of 1 mV/s.

The adhesive bond strength between the ZPCC coating and the substrate was evaluated through shear strength testing [18]. To ensure reliability, the reported values represent the average of three consistent measurements obtained from five independent samples.

3. Results and Discussion

3.1. Coating Mass

Figure 2 presents the coating mass measurements of ZPCC prepared from baths containing varying concentrations of CN. A clear positive correlation is observed between CN content and coating mass deposition. When the CN concentration was increased from 1.5 g/L to 4.5 g/L, the coating mass exhibited a significant enhancement from 6.39 ± 0.42 g/m² to 14.65 ± 0.72 g/m². Further increasing the CN content to 6 g/L resulted in a coating mass of 16.55 ± 0.94 g/m², demonstrating continued crystallization and a more gradual growth trend. From an industrial perspective, coating mass serves as a critical quality control parameter, as it directly influences both the coating homogeneity and corrosion protection performance. These findings suggest that CN incorporation effectively modulates the deposition characteristics of ZPCC coatings.

3.2. Phase Composition

Figure 3 presents the XRD patterns (a) and the plane variation (b) of ZPCC coatings treated with 4.5 g/L and 6 g/L CN. The incorporation of CN in the bath did not alter the phase compositions or primary crystallographic orientation but rather influenced the relative peak intensities. Both coatings predominantly consisted of hopeite (Zn₃(PO₄)₂·4H₂O) with minor phosphophyllite (FeZn₂(PO₄)₂·4H₂O) phases. Notably, increasing the CN concentration from 4.5 to 6 g/L enhanced the diffraction peak intensities while maintaining similar phase spectra. As shown in Figure 3b, the diffraction intensities of the (020), (040), and (241) planes (located at 9.7°, 19.4°, and 31.31°, respectively) increase with higher CN additions. When the CN concentration rises from 4.5 to 6.0 g/L, the enhanced intensities of these planes suggest a potential influence on the coating’s properties. Notably, the (040) plane exhibits the most pronounced increase in diffraction intensity at 6.0 g/L CN (Figure 3a), indicating a preferential epitaxial growth of hopeite along this crystallographic orientation.

Hopeite has been widely investigated for biomedical applications due to its excellent substrate adhesion, biocompatibility, and osteogenic properties [4,11]. In vitro studies further demonstrate that hopeite coatings on titanium substrates support favorable attachment and spreading of both osteogenic and human fibroblast cells [20]. The presence of phosphophyllite is particularly advantageous for corrosion protection, as this phase exhibits superior chemical stability in chloride-containing environments compared to hopeite [21]. This characteristic contributes to the enhanced corrosion resistance of ZPCC coatings.

3.3. Microstructure Characterization

Figure 4 presents the FE-SEM micrographs and EDS spectra of ZPCC coatings treated under varying conditions. The incorporation of CN in the deposition bath significantly influenced the crystalline morphology and size. Compared to CN-free coatings, those prepared with CN exhibited markedly denser and more compact microstructures. Increasing the CN concentration from 0 to 6 g/L led to a progressive reduction in crystal size, accompanied by distinct morphological changes, as evidenced by the inset images in Figure 4.

In the absence of CN, the coating comprised large plate-like crystals with incomplete substrate coverage (Figure 4a,b). As the CN concentration increased, the coatings displayed enhanced surface coverage, improved crystallinity, and greater densification, critical factors governing corrosion resistance. At 1.5 g/L CN, the average crystal size measured about 100 μm (Figure 4d), while further increasing CN to 3 g/L reduced the size to 30–60 μm (Figure 4g). The most homogeneous and fine-grained microstructure was achieved at 6 g/L CN (Figure 4m), featuring minimal crystal dimensions and optimal compactness. Such refined grain structure and high density are known to enhance the corrosion resistance of ZPCC coatings.

The presence of CN also promoted secondary crystallization, as evidenced by granular crystal formations (Figure 4m), contributing to increased coating mass. Crystal morphology and dimensions are governed by solution composition, processing parameters, and substrate conditions. In this study, elevated CN concentrations facilitated nucleation and grain refinement, likely due to synergistic effects between Ca²⁺ and NO₃⁻ ions. Calcium, widely utilized in conversion coatings for its cost-effectiveness and environmental compatibility [2], has been reported to yield dense, low-porosity ZPCC layers [22].

As shown the EDS spectra of coatings deposited with varying CN contents, all coatings primarily consisted of Zn, P, and O, with trace Fe and Cr originating from the substrate. Notably, Ca was undetectable, aligning with XRD findings (Figure 3). The detection of Cr (Figure 4c) suggests limited coating thickness.

Cross-sectional morphological characteristics of the sample treated with 6 g/L CN concentration is displayed in Figure 5. The cross-sectional morphology of the specimen exhibits a strong correlation with its surface (Figure 4m). The thickness of the cross-section exhibits minimal variation, with measurements ranging from 28 µm to 35.5 µm.

The formation of ZPCC typically progresses through four distinct stages: electrochemical dissolution of the substrate, amorphous precipitation, crystallization and growth, and crystal reorganization [2]. Initially, substrate dissolution at the solution interface releases iron ions, facilitating the preferential precipitation of phosphophyllite. Subsequently, hopeite formation occurs through nucleation and crystal growth. The final stage involves recrystallization and morphological evolution of the crystalline structure. In this study, the curing process effectively replenished the Fe²⁺ ions concentration to an optimal level. With increasing CN, the substrate/solution interface accumulates higher concentrations of Ca²⁺ and Fe²⁺ ions, promoting enhanced crystal nucleation and growth [11,23]. This phenomenon results in both greater coating mass (Figure 2) and improved coating uniformity (Figure 4). It has been reported that ZPCC solution significantly influences the phase composition of coatings, with Ca-modified coatings being formed within the pH range of 3.5–4.25 [3,11]. In the present study, however, the solution pH was maintained at 2.65. Under this acidic condition, Ca²⁺ ions primarily functioned as an accelerator rather than a major constituent of the coating phase. As a result, no calcium was detected in the coatings by either XRD (Figure 3a) or EDS (Figure 4).

3.4. Electrochemical Investigation

Figure 6 presents the polarization curves of the bare substrate and ZPCC coatings deposited from baths containing varying CN concentrations in a 0.9 wt.% NaCl solution. The corresponding electrochemical parameters, including corrosion current density (I_corr), corrosion potential (E_corr), polarization resistance (R_p), and porosity percentage (P), derived from Tafel extrapolation, are summarized in

Table 1. The results of potentiodynamic corrosion tests of the ZPCC coatings with containing different contents of CN in a 0.9 wt. % sodium chloride solution ^a.

Introduction	Ecorr (VSCE)	Icorr (μA/cm2)	Rp, × 103 (Ω·cm2)	P (%) b	Pe (%) c
Bare	−0.922 ± 0.18	1.68 ± 0.46	2.20 ± 1.64	--	--
1.5 g/L CN	−0.851 ± 0.009	0.629 ± 0.25	27.3 ± 6.46	2.96	62.56
3 g/L CN	−0.686 ± 0.04	0.340 ± 0.34	40.65 ± 3.35	0.193	79.76
4.5 g/L CN	−0.465 ± 0.008	0.356 ± 0.046	49.5 ± 2.97	0.007	79.81
6 g/L CN	−0.414 ± 0.011	0.236 ± 0.026	70.36 ± 2.23	0.0024	85.95

^a Entries are average values, n = 3; ^b calculated with the average Rp value; ^c calculated with the average I_corr value.

[18].

The polarization resistance (R_p), porosity percentage (P), and corrosion protection efficiency (P_e) were determined through electrochemical analysis using Equations (2)–(4) [18]:

$$R_p={\displaystyle \frac{{\beta }_a·{\beta }_c}{2.303 I_{corr}({\beta }_a+{\beta }_c)}}$$

(2)

$$P=(R_{p,s}/R_{p,c})\times {10}^{-{\displaystyle \frac{{∆E}_{corr}}{{\beta }_{a,s}}}}\times 100\%$$

(3)

$$P_e\%=(1-{\displaystyle \frac{I_{corr}^c}{I_{corr}^s}})\times 100$$

(4)

where

I_corr represents the corrosion current density (μA/cm²);

βa and βc denote the anodic and cathodic Tafel slopes (mV/decade), respectively;

R_p,s and R_p,c correspond to the polarization resistance (Ω·cm²) of the bare substrate and coated substrate;

ΔE_corr indicates the potential difference (mV) between the coated and bare substrates;

β_a,s is the anodic Tafel slope (mV/decade) of the bare substrate;

$I_{corr}^c$ and $I_{corr}^s$ are the corrosion current densities (μA/cm²) of the coated sample and the bare substrate, respectively.

All ZPCC coatings exhibit similar polarization behavior, demonstrating enhanced corrosion resistance compared to the bare SS substrate (Figure 6 and Table 1). This improvement can be attributed to the incorporation of calcium and nitrate, known corrosion inhibitors in phosphating processes [2,22]. As the CN concentration increases from 1.5 to 6.0 g/L, a systematic shift in E_corr toward more noble values and a continuous reduction in I_corr are observed (Figure 6). This trend correlates with the formation of finer crystallites and higher coating mass (Figure 2 and Figure 4), which enhance barrier properties. Notably, the coating obtained at 6.0 g/L CN exhibits optimal performance, characterized by the lowest I_corr and porosity, alongside the highest E_corr, Rp, and protection efficiency. These findings are consistent with its denser microstructure, increased deposition mass, and minimal porosity (Figure 2 and Figure 4 and Table 1).

3.5. Adhesion Strength Test

The adhesive strength test revealed that the ZPCC coating deposited from a bath containing 6 g/L CN at 65 °C for 30 min exhibits a shear strength of 10.15 ± 1.2 MPa at the coating–substrate interface.

4. Conclusions

Zinc phosphate chemical conversion (ZPCC) coatings were successfully fabricated on stainless steel substrates using a conventional chemical conversion process with calcium nitrate (CN) as an additive. X-ray diffraction analysis confirmed that all coatings, regardless of CN concentration, consisted primarily of hopeite (Zn₃(PO₄)₂·4H₂O) with trace amounts of phosphophyllite (Zn₂Fe(PO₄)₂·4H₂O), indicating minimal influence of CN on phase formation. The incorporation of CN at an optimal concentration (6 g/L) significantly refined crystallite size, producing a dense, uniform, and fine-grained coating structure. The optimized coating exhibited superior corrosion protection, attributed to its compact microstructure and reduced defect density. Furthermore, adhesion testing confirmed strong interfacial bonding between the ZPCC coating and the substrate. These results highlight the critical role of CN in tailoring coating morphology and performance, offering a practical strategy for improving the corrosion resistance of stainless steel via ZPCC coatings.