Mechanism of surface rust defects on tinplate strip steel during storage and transportation

Abstract

This study elucidates the rusting mechanism on tinplate during storage and transportation. It was found that the absence of a Sn layer on the sheared edges, combined with a thinner Sn layer in the centre compared to the edges, leads to increased ATC values and porosity at the centre. Consequently, this results in diminished rust resistance at both the sheared edges and central regions. Simulations of storage and transportation conditions, along with electroplating experiments, demonstrated that waviness in the tinplated substrate contributes to higher porosity and reduced thickness of the Sn–Fe alloy layers in both electroplating and reflowing processes. Moreover, the choice of electroplating processes significantly influences the rust resistance of tin-plated plates. The adherence of rust factors during the tinplating process, particularly when subjected to condensation during storage and transportation, exacerbates the rust of tinplate.

Keywords

rust tinplate rust factors temperature humidity

Introduction

Rust protection technology remains a pivotal area of study in industrial applications, leading to the development of various techniques such as electroplating, thermal spraying, hardening and novel protective coatings.^1–4 Tinplate, rendered rust-resistant through electroplating, is extensively utilised in the food and beverage industry.⁵ Recent years have seen rapid progress in the development of low-Sn layer tinplate strip steel and chromium-free passivation technologies, driven by the shift towards green and low-carbon initiatives.^6,7 The surface rust resistance of tinplate is particularly vital for the preservation of canned food. As demonstrated in Figure 1(c), the tinplate undergoes processes like electroplating, reflowing, passivation and oiling, resulting in the formation of tin, Sn–Fe alloy, oxide film, passivation film and oil film layers, substantially enhancing its rust resistance. Moreover, packaging plays a critical role in improving rust resistance during transportation, as illustrated in Figure 1(a) and (b). During canning, resin material application mitigates rust from acidic media. Despite these measures, rust can still occur in the production, post-canning, storage and transportation (as shown in Figure 2).

To enhance the rust resistance of tinplate, extensive analyses have been conducted on the factors affecting rust of the process parameters and the can material storage. Wu⁸ used scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS)to study the effects of current density on the uniformity, density and deposition rate of the tinplating layer. Li⁹ examined how variables such as assisting solvents, reflow temperature and time, and the characteristics of the quenching tank and alloy ratio impact rust resistance. Wang¹⁰ applied scanning electron microscopy, adhesion tests assessments to evaluate the effects of passivation process parameters—like resistance to flow, paint film adhesion and film thickness—on rust resistance. Arafat¹¹ examined the influence of additives on Sn coating grain size, hydrogen evolution, tin sludge and process defects like pinholes. Reis and others^12–15 used electrochemical impedance spectroscopy (EIS), x-ray diffraction and other techniques to analyse the rust rate and electrical parameters of tinplate in various food packaging environments, like corn and citric acid, determining the rust mechanisms under different real-world storage conditions. Martins¹⁶ confirmed through EDS and (fourier transform infrared spectroscopy testing that reduced thickness and poor uniformity of the tinplate coating can lead to rust in tin cans. Sachin¹⁷ identified the factors influencing rust in tin cans using SEM-EDS and simulated the impact of humidity during transportation on rust by a simplified experimental setup. These studies primarily focus on the production process parameters and the influence of canning solutions, with less research on the impact of environmental conditions during the storage and transportation.

To evaluate the rust resistance of tinplate during storage and transportation, studies have been conducted on accelerated rust testing methods and equipment. Zumelzu et al.^18–20 employed a range of techniques including microscopic morphology analysis, salt spray tests and electrochemical detection to assess rust resistance. Zhang et al.^21,22 advanced accelerated rust evaluation through external conditions such as salt spray tests, wet–dry cycle experiments, immersion tests and ultraviolet radiation exposure. Wang et al.^23,24 utilised EIS to monitor impedance changes during accelerated rusting, developing a quick evaluation method for the rust resistance of metallic materials. Xia^25,26 enhanced the detection of iron and tin dissolution from rusted tinplate strip steel using ICP-MS and innovative electrochemical sensors, facilitating a rapid assessment of rust severity in tinplated cans. Suzuki²⁷ introduced a six-step approach using a three-dimensional computational fluid dynamics program alongside an electrochemistry and oxide layer growth model, creating a new method for evaluating rust resistance. However, these methods are only suitable for small sample tests and fail to consider the impact of package, surface impurities on rust and atmosphere during storage and transportation.

This article establishes a comprehensive environmental testing apparatus featuring temperature, humidity and rust factor functionalities by collecting data on corrosion factor concentrations, temperature and humidity from tinplating units and during the storage and transportation. By analysing the quality and rust constituents at the sheared edges and central parts of the tinplate, it identifies the rust mechanisms, thereby laying the groundwork for improvements in the tinplating.

Materials, methods and equipment

Detection of tinplate at sheared edge and central part

SEM and EDS were utilised to examine the composition and microstructure at and beyond the sheared edges. The central part was assessed through measurements of Sn thickness, ATC values of the Sn–Fe alloy, blue dot experiments. The sampling method for the central part is depicted in Figure 3 and sample parameters displayed in Table 1.

Table 1.

The parameters of tinplate strip steel.

Detect position	Length (mm)	Width (mm)	Thickness (mm)	Amount of Sn (g/m²)
Before and after sheared edge	10	5	0.18	1.1
Central part	3	3	0.18	1.1

Simulation method for storage and transportation conditions

The rust resistance of tinplate strip steel is significantly influenced by the electroplating process. Additionally, environmental conditions during production, storage and transportation also impact their rust resistance. An LFS-113 air sampling pump and a gas absorption bottle were utilised for gas collection in tinplating unit and during storage and transportation. Subsequently, the AquaMate8100 UV-visible spectrophotometer was employed to analyse the presence of Cl⁻, NO2⁻, NO3⁻ and SO₄²⁻. Temperature and humidity were monitored using the RC-4HC hygrothermograph during storage and transportation, as illustrated in Figure 4.

The acidic in solution components may evaporate and adhere to the tinplate surface, elevating rust risk. Figure 5 illustrates eight critical production processes in the tinplating unit that influence the rust resistance.

In environments of high temperature, high humidity and high salinity, tinplate strip steel is susceptible to rusting.^28,29 Thus, the internal and external temperature, humidity and rust-inducing factors were monitored during storage and transportation to Southeast Asia, as depicted in Figure 6.

A comprehensive environmental apparatus was designed to simulate the atmosphere during storage and transportation, with the parameters detailed in Table 2. The characteristics of the experimental tinplate, prone to rust defects, are specified in Table 3. The experimental process is depicted in Figure 7.

Table 2.

The parameters of comprehensive environmental testing equipment.

Function	Parameter types	Parameters
Temperature (°C)	Range	−15–70
Temperature (°C)	Control accuracy	≤±2
Humidity (%RH)	Range	20–95
Humidity (%RH)	Control accuracy	≤±3
Temperature variation (°C/min)	Heating rate	1.0
Temperature variation (°C/min)	Cooling rate	0.6
Rainfall flow (L/min)	Range	20–30
Rust factors (mg/m³)	Range	0–0.5
Bearing capacity (t)	Range	≥18

Table 3.

The parameters of tinplate coil.

Material	Width (mm)	Thickness (mm)	Amount of Sn (g/m²)	Package
Tinplate coil	870	0.2	1.1	Rust-proof paper and polyethylene

The gathered data on temperature, humidity and rust-inducing factors serve as control parameters for this apparatus. Its ability to independently manage these variables enables an accurate simulation of the atmosphere during transportation and storage.

Tinplating experiments

The tinplating test and polarisation curves were detected through electroplating experiments under various conditions, including different Sn thicknesses, the temperatures and flow rates of solution, current densities and tin sludge concentrations. The components and concentrations of solution are listed in Table 4, and the experimental equipment and process are depicted in Figure 8.

Table 4.

Concentration and temperature of solution.

Experimental section	Solution	Concentration (g/L)	Temperature (°C)
Alkaline washing	NaOH	15–20	45
Pickling	H₂SO₄	65–75	25
Pickling	PH	≥3	25
Tinplating	Sn²⁺	25–30	60–80
	ENSA	2–4
	EN	4–6
	SO₄²⁻	≤8
Tafel testing	NaCl	3.5	25

Results

Results for tinplate at sheared edge and central part

The composition and microstructure at and beyond the sheared edges are presented in Figure 9 and Table 5. The findings reveal a low presence of Sn elements at the sheared edge, indicative of inferior rust resistance.

Table 5.

Specific gravity of detected elements before and after sheared edge (%).

Elements	C	Al	Si	Cr	Fe	Sn	Total
Before shearing	7.13	0	0	0	0	92.85	100
After shearing	5.64	0	0	0	92.14	2.22	100

As depicted in Table 6 and Figure 10, the Sn layer is thinner in the centre compared to the edge, with a higher ATC value and more blue dots than at the edge. This indicates that the quality of both the Sn layer and the Sn–Fe alloy layer on the tinplate's central surface is lower than that at the edge, making the central surface more susceptible to rust defects.

Table 6.

The results of pinhole, Sn thickness and ATC values on tinplate.

Detection position	Sn thickness (μm)	ATC values (μA/cm²)	Blue dots (number/cm²)	Baseplate thickness (mm)
D	0.133	0.126	0.8	0.182
C	0.111	0.167	2.5	0.178
W	0.139	0.127	0.6	0.181

Simulation results during storage and transportation

Table 7 presents the concentrations of rust-inducing factors, with the highest levels observed in the pickling section due to the predominant use of sulphuric acid. Conversely, concentrations are lower in the packaging area, benefiting from superior gas circulation.

Table 7.

The concentration of rust factors in tinplating unit.

Detection position	Cl⁻ (mg/m³)	NO₂⁻ (mg/m³)	NO₃⁻ (mg/m³)	SO₄²⁻(mg/m³)
1	0.014	0.000	0.000	0.000
2	0.028	0.006	0.220	0.340
3	0.035	0.009	0.008	0.015
4	0.022	0.009	0.007	0.009
5	0.029	0.000	0.008	0.011
6	0.025	0.006	0.008	0.013
7	0.058	0.011	0.013	0.017
8	0.000	0.000	0.000	0.000

Figure 11 shows a uniform decrease in both internal and external temperatures from 37.2°C to 23.5°C, while internal humidity rises from 52.1% RH to 76.7% RH. The initial conditions suggest a dew point of 25.7°C, highlighting the risk of internal condensation and potential rust on the tinplate. Moreover, the chloride ion content significantly surpasses that of other salts, attributed to marine environmental exposure. These factors not only reduce the surface dew point of the tinplate but also accelerate rusting.

At the end of the simulation, macroscopic examination of the rusted areas revealed rust defects at both the sheared edge and the central part, as shown in Figure 12. SEM and EDS were used for the microscopic examination and composition analysis of rusted areas at the sheared edge and central part of the tinplate strip steel. The types and components of rust identified in these areas are illustrated in Figure 13 and Table 8.

Table 8.

Specific gravity of detected elements in rust section (%).

Rust part	C	O	Fe	Sn	Si	S	K	Cl	Total
Sheared edge	5.04	25.99	65.78	1.38	0	0	0	0	100
Central part	0	0.32	1.26	32.21	23.65	25.35	1.25	15.96	100

The results reveal that the primary rust components at the sheared edge of tinplate are Fe, Sn, O and C. Sn adheres to the sheared edge during cutting, while C and Fe are inherent to the tinplated substrate. O is likely associated with rust formation. The rusted sections appear as yellowish blocks, indicating iron oxide as the principal rust component.

In the central part, rust constituents include Fe, Si, K, Cl, S, Sn and O. Fe, Si, K and S, being significant components of the substrate, have heightened levels of Si, K and S, suggesting foreign substance accumulation. Cl and S, aligned with collected rust factors, are identified as primary contributors to rust. Central rust primarily manifests as black spot rust, excluding O as part of iron oxide. The diminished Sn content in rusted areas indicates the central Sn layer's inferior quality.

Tinplating results

Analysis from Figure 14 and Table 9 shows that as the Sn thickness, current density and solution flow rate increase, both the rust potential and rust current on the tinplate surface decrease. However, as solution temperature rises, rust potential and rust current initially decrease and then increase. Moreover, an increase in tin sludge concentration leads to a gradual increase in both rust potential and rust current.

Table 9.

The results of pinhole, Sn thickness and ATC values on tinplate.

Experimental condition	Parameters	Rust current (μA/cm²)	Rust voltage (V)
Sn thicknesses	0.5	13.96	−0.636
	1.1	11.65	−0.584
	2.8	4.7	−0.546
	5.6	3.4	−0.486
Current densities	10	13.78	−0.679
	15	11.65	−0.584
	25	8.96	−0.531
	35	6.29	−0.478
Flow rates of solution	10	13.26	−0.618
	20	11.65	−0.584
	30	9.34	−0.571
	40	8.32	−0.543
Solution temperatures	50	16.78	−0.679
	60	7.29	−0.524
	70	11.65	−0.584
	80	11.24	−0.668
Tin sludge concentrations	0	11.65	−0.584
	1	12.78	−0.597
	2	14.65	−0.654
	3	18.03	−0.684

A thicker Sn layer enhances coverage on the tinplate, reducing porosity and improving rust resistance. Higher current density refines the grains of the tin layer, increasing its density and rust resistance. Improved solution flow rate and temperature boost the tin ion deposition rate on the substrate surface, further refining tin layer grains. However, tin sludge adheres to the surface, lowering the Sn layer's density; thus, its increased concentration negatively impacts rust resistance.

Discussion

From the above detection, testing and simulation results of tinplate during storage and transportation, the following conclusions can be drawn:

Rust defect mechanism at sheared edge

The absence of Sn layer at the tinplate's sheared edge is mitigated by slow-releasing factors from rust-preventive paper, which adhere to the edge, forming a protective layer to enhance rust resistance.

HCOOH + {NH}_{3} \to {HCONH}_{2}

(1)

However, water vapour within the packaging condenses at low temperatures. This condensation dissolves the protective layer, exposing the sheared edge to oxygen and moisture. Analysis of the rust components and morphology at the sheared edge indicates that rust occurs in a neutral environment. When oxygen is abundant, the anode undergoes iron oxidation, while the cathode primarily experiences oxygen reduction. This rust mechanism at the sheared edge is depicted in Figure 15.

Anodic reaction : Fe - 2 e \to {Fe}^{2 +}

(2)

Cathodic reaction : O_{2} + 4 e \to 2 O^{2 -}

(3)

When oxygen is scarce, the cathodic reaction shifts to the reduction of water.

Cathodic reaction : O_{2} + 2 H_{2} O + 4 e \to 4 {OH}^{-}

(4)

Based on the mechanism of defect formation at the sheared edges of tinplate, rust can be mitigated through two approaches: controlling humidity in the packaging area to reduce the absolute moisture content within the package and enhancing the release rate of rust inhibitors from rust-preventive paper to promote the continuous formation of a protective film.

Rust defects mechanism at the central areas

Based on the analyses of Sn and substrate thickness, as well as ATC values in the central part of the tinplate, it is evident that the thickness of the Sn layer, substrate and alloy layer is less than at the edges. Electroplating principles suggest that the distance from the anode to the tinplate substrate can cause variations in layer thickness, as illustrated in Figure 16(b).

During resistance reflowing, the process heats and melts the Sn layer using the current from the conductive roller through the strip's electrical resistance. However, centre waves in the substrate prevent direct contact with the conductive roll, resulting in a heating temperature below Sn's melting point, as shown in Figure 16(c). This leads to a thinner Sn layer in the centre, with inadequate reflowing causing less grain refinement, reduced coating density and a thinner alloy layer.

Figure 17 shows that during coiling, centre waves can cause cavitation, where tension prevents gas exchange with the external environment, locking in the temperature and humidity conditions of the coiling process. High temperature and humidity during coiling, followed by exposure to cooler environments during storage and transportation, can cause condensation within cavitation areas. Pinhole defects in the Sn layer at centre waves, along with adhered rust factors, significantly heighten the risk of rust in these areas.

Within the sealed space of tinplate, water condensation chemically reacts with rust factors to form acidic solutions including H₂SO₃, H₂SO₄, HCl, HNO₂ and HNO₃, predominantly HCl. Rust initiates with the Sn layer, where the anodic reaction is the oxidation of Sn and the cathodic reaction is the reduction of hydrogen ions.

Anodic reaction : Sn - 2 e \to {Sn}^{2 -}

(5)

Cathodic reaction : 2 H^{+} + 2 e \to H^{2}

(6)

As the tin layer rusts, the baseplate begins to rust at positions with less Sn. When pinhole defects are present on the tinplate, they directly corrode the baseplate. The anodic reaction involves the oxidation of Fe, while the cathodic reaction remains the reduction of H⁺.

Anodic reaction : {\begin{matrix} Fe - 2 e \to {Fe}^{2 +} \\ Fe - 3 e \to {Fe}^{3 +} \end{matrix}

(7)

Cathodic reaction : 2 H^{+} + 2 e \to H^{2}

(8)

As O₂ are continuously consumed, the electrochemical reactions shift to the reduction of O₂ and H₂O.

Anodic reaction : Fe - 2 e \to {Fe}^{2 +}

(9)

Cathodic reaction : {\begin{matrix} O_{2} + 4 e \to 2 O^{2 -} \\ O_{2} + 2 H_{2} O + 4 e \to 4 {OH}^{-} \end{matrix}

(10)

Based on the analysis of the rust defect elements in the central part of the tinplate strip steel and the above discussion,³⁰ the rust mechanism is illustrated in Figure 18. Based on the rust mechanism and the results of tinplating experiments, reducing the center wave of the baseplate can enhance the uniformity of Sn and the Sn–Fe alloy layer. Factors such as solution temperature, flow rate and tin sludge concentration further decrease the compactness of the Sn layer and the alloy layer thickness in the centre of low-Sn-layer tinplate, diminishing rust resistance.

Conclusions

This study aimed to develop a comprehensive environmental experimental device to assess the rust resistance of tinplate strip steel and to understand the rust formation mechanism during storage and transportation, integrating tinplate detection, tinplating tests and the production process. The key findings of this study include:

Cl⁻, NO²⁻, NO³⁻ and SO₄²⁻ are present during critical tinplating processes, adhering to the tinplate strip steel surface. Condensation on this surface significantly increases rust risk.

Centre waves in the tinplated substrate not only lead to pinhole defects during electroplating and reflowing but also trap high-temperature and high-humidity gases. These can lead to rust defects after condensing in cooler conditions during storage and transportation.

Factors such as Sn layer thickness, current density, solution flow rate and temperature and tin sludge concentration critically affect tinplate's rust resistance. It is crucial to tailor tinplating parameters to the specific needs of different tinplate products during production.

Research on the mechanisms of rust defects on tinplate provides a direction for addressing these defects. However, simulating test requires significant material and lengthy experimental cycles. Several challenges need addressing:

Increase the dimensions of simulating test, such as tension, impact and rust inhibitor concentration to enhance the simulation testing accuracy of small sample. This could help reduce material consumption and experimental durations.

Limited research has been conducted on the impact of electroplating process parameters on rust. Reflowing, passivation and oiling also affect rust, and upstream processes impact downstream ones. A deep and systematic research on process parameters at each stage is needed, which could help improve the rust resistance of tinplate.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Funding for Science and Technology Research Project of Higher Education Institutions in Hebei Province, the Major Scientific and Technological Achievements Transformation Project of Hebei Province, the Central Guidance for Local Scientific and Technological Development Funding projects, and the Science and Technology Research and Development Plan - Science and Technology Support Plan Project in Hebei Province (grant number CXY2023012, 22281001Z, 236Z1024G, and 23280101Z).

ORCID iD

Wenjun Zhang

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