Analysis of Corrosion-Mechanical Properties of Electroplated and Hot-Dip Zinc Coatings on Mechanically Pre-Treated Steel Substrate

Abstract

This study evaluates the effects of three mechanical pre-treatment methods on S235JRG2 steel sheets: blasting with a synthetic corundum (F40), blasting with steel shot (S170), and grinding with synthetic corundum (P40). Untreated samples served as a reference. The analysis of mechanical pre-treatments focused on surface integrity, including measurements of surface roughness parameters Ra and Rz (ISO 21920-2) and subsurface microhardness (ISO 6507-1). Zinc coatings were assessed through mechanical testing (cupping test, ISO 1520) and corrosion testing in a neutral salt spray environment (ISO 9227), with results evaluated using digital image analysis. Experimental findings indicate that electroplated zinc deposition rates are influenced by surface roughness, while subsurface microhardness has no significant effect. In contrast, for hot-dip galvanizing, both parameters impact the process. The mechanical properties of electroplated zinc coatings are further affected by steel surface integrity, whereas hot-dip zinc coatings are primarily governed by intermetallic phase formation, making the influence of steel surface integrity statistically negligible. Corrosion testing revealed that blasting with a synthetic corundum is particularly unsuitable, as it leads to numerous inhomogeneities in both coating types, accelerating corrosion degradation.

1. Introduction

The effectiveness of an anti-corrosion coating largely depends on the proper pre-treatment of the substrate surface. This preparation can be performed through mechanical, chemical, or electrochemical methods, each ensuring the required cleanliness and surface roughness of the base material [1]. Mechanical pre-treatments, such as blasting or grinding, serve various industrial purposes, including rounding sharp edges after cutting, removing slag from weld joints, cleaning surfaces after flame cutting, and eliminating scale residues from hot-formed components—cases where chemical pre-treatment (e.g., pickling) may be insufficient. Additionally, surface defects such as laps or flakes, which commonly occur on forged semi-finished products, can be effectively addressed through mechanical methods [2]. Blasting treatments primarily modify the surface through mechanical action. This category includes both conventional and unconventional methods, such as machining with loose abrasive particles carried in a stream of water, air, or other gases [3]. In practice, abrasive particles are either angular or spherical and can be propelled using compressed air or rotating blades—both widely adopted in industrial applications. An alternative approach involves simulating blasting technology using specially shaped steel bristles [4,5]. Abrasive materials can be categorized into three groups:

• Natural abrasives (e.g., silica sand, flint, garnet, zircon).
• Agricultural by-products (e.g., walnut shells, corn cobs).
• Industrially manufactured abrasives (e.g., steel shots, aluminum oxide, silicon carbide, plastics, glass beads).

The choice of abrasive material is critical, as different abrasives create distinct surface profiles. The resulting surface roughness depends on multiple factors, including abrasive type, size, and hardness, as well as blasting parameters such as kinetic energy, impact angle, and standoff distance. Achieving a consistent surface profile requires proper operator training and process control [6,7,8,9,10]. In addition to blasting, grinding is another mechanical pre-treatment method, where material removal predominates over surface deformation. Unlike blasting, grinding is typically performed at lower cutting speeds with intensive cooling, ensuring minimal thermal influence on surface integrity. In cases where cooling is insufficient, the process may involve a combination of mechanical and thermal effects, leading to localized grain deformation and surface irregularities [11]. In this study, grinding was performed using new abrasive inserts and a custom-designed fixture to maintain a low cutting speed. Given the fundamental differences in their mechanisms, blasting and grinding treatments were expected to yield distinct effects on surface integrity.

Zinc coatings can be applied using either conventional or alternative methods, with the objective of optimizing production costs while maintaining coating quality. Conventional zinc coating processes involve multiple pre-treatment steps tailored to the specific material properties of the substrate. However, a major drawback of these methods is their extensive reliance on chemical substances, which pose environmental and health risks. Consequently, alternative mechanical pre-treatment techniques are being explored to minimize chemical usage in surface preparation [12,13,14,15].

The need for alternative methods is further emphasized by the significant environmental impact of traditional galvanic coating processes. Within the European Union, approximately 100,000 tonnes of sludge are generated annually from electroplating operations, while in China, this figure exceeds 10 million tonnes, with an additional 1 million tonnes produced globally. Furthermore, wastewater volumes from these processes are estimated to be 40 times greater than sludge production. Notably, wastewater treatment technologies vary significantly across regions, with Japan, the United States, and Western Europe implementing the most advanced solutions [16]. From a sustainability perspective, at least partial replacement of chemical pre-treatments with mechanical methods—such as abrasive blasting or grinding—could serve as a viable alternative.

This study presents a comprehensive comparison of three different mechanical pre-treatment methods applied to S235JRG2 steel substrates, assessing their effects on the corrosion-mechanical properties of zinc coatings. While previous studies have primarily examined pre-treatment and coating methods in isolation, this work offers a systematic evaluation of their combined effects. The findings provide valuable insights for industrial applications, particularly in optimizing mechanical surface treatments for improved corrosion protection.

The aim of this work is a comprehensive comparison of the effects of three different methods of the mechanical pre-treatment of steel substrates on the corrosion-mechanical properties of subsequently applied zinc coatings. In contrast to previous studies, which mostly focused on the individual pre-treatment and coating methods separately, this work presents a comprehensive systematic comparison of them. This comprehensive evaluation therefore has considerable added value in terms of the application of the knowledge gained to technical practice.

2. Materials and Methods

2.1. Base Substrate, Sample Dimensions, and Mechanical Pre-Treatments

The base material used in this study was ferritic–pearlitic steel (S235JRG2), with its fundamental properties summarized in

Table 1. Fundamental parameters of steel S235JRG2 [4].

Mechanical Property	Minimum Value, MPa
Tensile Strength (Rm)	360
Yield Strength (Re)	235
Chemical Composition	Maximum Value, wt.%
Carbon (C)	0.170
Manganese (Mn)	1.400
Phosphorus (P)	0.045
Sulphur (S)	0.045
Nitrogen (N)	0.014
Silicon (Si)	–
Iron (Fe)	Remainder

. The initial semi-finished product consisted of cold-rolled steel sheets measuring 2000 × 1000 × 1 mm, which were cut using guillotine shears to obtain final test specimens of 160 × 65 × 1 mm. To facilitate sample suspension, 5 mm diameter holes were punched using a manual press. Given the anisotropic properties of the steel (resulting from the rolling process), all samples were oriented uniformly, with elongated ferritic grains aligned along the longitudinal axis (Figure 1). Before undergoing further processing, the samples were degreased with technical ethanol (minimum 95 wt.% ethanol) to prevent contamination from preservation substances.

The mechanical pre-treatment by blasting was conducted manually within a blasting cabinet, utilizing both synthetic corundum and steel grit as abrasive media. The equipment operated on a pressure system, where the abrasive material was stored in a pressurized vessel and delivered through a compressed air hose to a nozzle. Identical blasting parameters were maintained for both abrasives:

• Pressure: 0.5 ± 0.05 MPa
• Nozzle-to-sample distance: 200–250 mm
• Abrasive impact angle: 45 ± 10°
• Blasting duration: 70–80 s per side

This procedure aligns with the methodology outlined in [17]. The specific properties of the blasting media are detailed in Table 2.

The mechanical pre-treatment by grinding was performed using P40 abrasive paper, featuring a circular section (75 mm diameter), open-coated grain structure, synthetic resin bonding, and paper backing, secured to the support plate via hook-and-loop fastening. The abrasive material (synthetic corundum) shared the same composition as specified in Table 2, except for the grain size, which ranged between 400 and 500 µm [18]. The difference in grain size arises from the wider distribution typically observed in F-grit series (loose abrasives) compared to P-grit series (coated abrasives).

During machining, the test specimen was clamped in a vice, and the initial developed length for grinding the first side was approximately 220 mm. After grinding, the sample was trimmed, bent at the opposite end, and ground on the reverse side (developed length: approximately 190 mm). Following the mechanical pre-treatment, final trimming to 160 mm and suspension hole creation were performed.

Grinding was conducted using a vertical knee-type milling machine (FA 3 AV) equipped with a custom-designed fixture (Figure 2). The cutting conditions were as follows:

• Tool rotational speed: n = 45 min⁻¹
• Workpiece feed rate (cross-feed of milling table): s = 450 mm·min⁻¹

The tool pressure was applied using a steel helical compression spring (EN 10270-1 SH material [19]), housed inside the fixture cavity. The spring characteristics were as follows:

• Wire diameter: 1.6 mm
• Outer diameter: 12 mm
• Free length: 45 mm
• Number of coils: 11

Table 2. Physical properties and chemical composition of blasting media.

Physical Property	Synthetic Corundum		Steel Grit
Hardness	9 Mohs		390–530 HV
Melting Point, °C	2040		1530
Bulk Density, kg·m−3	1800 ± 300		7000 ± 60
Grain Shape	Angular		Spherical
Grain Size Designation	F40 (355–500 µm)		S170 (350–710 µm)
Chemical Composition	Compound, wt.%		Chem. Element, wt.%
	Al2O3	99.52	C	0.80–1.20
	TiO2	0.07	Si	0.40–1.50
	SiO2	0.17	Mn	0.60–1.20
	Fe2O3	0.07	S	max. 0.05
	MgO	0.01	P	max. 0.05
	CaO	0.02	Fe	Remainder

Table 2. Physical properties and chemical composition of blasting media.

Physical Property	Synthetic Corundum		Steel Grit
Hardness	9 Mohs		390–530 HV
Melting Point, °C	2040		1530
Bulk Density, kg·m−3	1800 ± 300		7000 ± 60
Grain Shape	Angular		Spherical
Grain Size Designation	F40 (355–500 µm)		S170 (350–710 µm)
Chemical Composition	Compound, wt.%		Chem. Element, wt.%
	Al2O3	99.52	C	0.80–1.20
	TiO2	0.07	Si	0.40–1.50
	SiO2	0.17	Mn	0.60–1.20
	Fe2O3	0.07	S	max. 0.05
	MgO	0.01	P	max. 0.05
	CaO	0.02	Fe	Remainder

Figure 2. Principle of mechanical pre-treatment by grinding.

Figure 2. Principle of mechanical pre-treatment by grinding.

The spring characteristics were evaluated using a universal testing machine (ZDM 5/51). For the experimental setup, a normal force of F = 100 ± 5 N was applied, corresponding to a spring compression of 16.4 ± 0.3 mm. The tool itself exerted a force of approximately 2 N, which was accounted for within the ±5 N tolerance. The applied pressure, and consequently the spring compression, was adjusted using the quill feed of the milling machine. This methodology has been previously validated in experimental studies, with findings reported in [18,20].

The three mechanical pre-treatment methods employed in this study differ significantly in their technological impact on the steel substrate:

• Steel shot blasting induces significant plastic deformation, creating a surface morphology characterized by intersecting spherical depressions.
• Synthetic corundum blasting also results in notable plastic deformation; however, the impact on subsurface layers is less extensive due to the lower kinetic energy of angular abrasive particles. The interaction between the sharp-edged abrasive and the substrate produces distinct indentations and protrusions.
• Grinding predominantly involves material removal, with only minimal plastic deformation. Unlike blasting, plastic deformation in grinding is negligible, particularly when using new abrasive tools, as was the case in this experiment.

2.2. Evaluation of Surface Integrity of the Steel Substrate

Surface roughness was measured using a contact profilometer SURFTEST SJ-201 (Mitutoyo, Kawasaki, Japan). The analysis focused on two key parameters:

• Ra (arithmetical mean deviation of the assessed roughness profile)
• Rz (maximum roughness height, defined as the sum of the highest peak and the lowest valley depth)

Since Ra represents the average deviation from the mean profile line, it does not account for extreme protrusions or depressions. Therefore, Rz was also evaluated, as it provides a more comprehensive measure of surface irregularities. By definition, Rz is always greater than Ra, with the difference typically ranging from 4 to 8 times, depending on the surface type. Roughness profiles were filtered according to ISO 21920-2 [21], with the following measurement parameters:

• Sampling length (λc): 2.5 mm
• Number of sampling lengths: 5
• Total evaluation length (ln): 12.5 mm
• Filter type: Gaussian

For untreated (reference) and blasted surfaces, 25 measurements were taken along both the longitudinal and transverse axes of the samples. In contrast, for ground surfaces, 50 measurements were performed perpendicular to the tool marks. Each dataset was derived from five randomly selected samples to ensure statistical robustness.

The microhardness of the subsurface layers was assessed using a Hanemann microhardness tester, integrated into a Neophot 21 metallographic microscope (Carl Zeiss Jena, Jena, Germany). Measurements followed the Vickers hardness test (HV0.1, ISO 6507-1 [22]) using a diamond pyramid indenter with a 136° apex angle. For each dataset, microhardness measurements were conducted on five metallographic specimens, with five measurements per specimen taken at different depths from the surface. Due to limitations in the equipment, which prevent precise alignment of the indenter prior to testing, microhardness was measured first, followed by the determination of the indentation depth relative to the surface (with an accuracy of ±2 µm). Consequently, each dataset comprised 25 cumulative values per condition. Before measurement, each specimen was aligned parallel to the x-axis, as illustrated in Figure 3a. The measurement principle is depicted in Figure 3b. Given the influence of surface roughness, the reference (zero) position was established as the midpoint between the highest peak and the lowest valley. Accordingly, the indentation depth h (µm) was defined using the following relationship:

$$h={\displaystyle \frac{h_2-h_1}{2}}+h_1 (\mu m).$$

(1)

where

h₁ is the distance from the indentation apex to the lowest valley.

h₂ is the distance from the indentation apex to the highest peak.

Figure 3. Principle of microhardness measurement: (a) method of specimen alignment; (b) determination of the indentation apex position.

Figure 3. Principle of microhardness measurement: (a) method of specimen alignment; (b) determination of the indentation apex position.

The evaluated samples were sectioned using an Mikron 150 metallographic saw (MHT Hrazdil, Brno, Czech Republic), ensuring abundant cooling with a process fluid (emulsion oil mixed with water at a 1:20 ratio) to preserve the original microstructure. The specimens were then embedded in Dentacryl resin for further preparation. Metallographic grinding was performed using a Struers LaboPol-60 grinder (Struers, Ballerup, Denmark) in a sequential process with water as the process fluid. Abrasive papers of varying grit sizes were used in the following order:

• P200
• P400
• P1000
• P2500

The final polishing stage was conducted using diamond polishing paste (1–2 μm grit size) to achieve a smooth, reflective surface. To enhance microstructural visibility, the specimens were etched with Nital (3% nitric acid in ethanol). This procedure was applied consistently across all specimens, including those with zinc-coated steel substrates. The suitability of this methodology has been previously verified [23].

2.3. Electrolytic Zinc Coating Process

Electrolytic zinc plating was carried out under laboratory conditions in a commercial electroplating facility. Given the geometry of the test samples, a rack plating method was employed to ensure uniform deposition. The deposition process was conducted in a cyanide-free alkaline electrolyte bath, primarily consisting of an aqueous solution of Na₂[Zn(OH)₄], with a Zn²⁺ concentration of 15 ± 1 g·L⁻¹ at a temperature of 25 ± 1 °C. To maintain a stable pH range of 13.8–14.0, NaOH (160–170 g·L⁻¹) was added to the bath. Additionally, sulphites and gluconates were incorporated to enhance electrolyte conductivity and promote uniform coating deposition [24]. The current density was set at 1.0 A·dm⁻², and the zinc anode used in the process had a declared purity of 99.99%. To statistically evaluate the relationship between coating mass increase and plating duration, five deposition time intervals were selected:

• 900 s (15 min)
• 1800 s (30 min)
• 2700 s (45 min)
• 3600 s (60 min)
• 4500 s (75 min)

A detailed breakdown of the electrolytic plating procedure is provided in

Table 3. Electrolytic zinc plating process.

Scheme	Brief Description of the Operation
1	Degreasing in an 8% NaOH solution (immersion time: 480 s, temperature: 60 ± 2 °C)
2	Rinsing (2×) in demineralized water (immersion time: 30 s per rinse, temperature: 21 ± 1 °C)
3	Electrolytic degreasing in a 5% NaOH solution (immersion time: 240 s, temperature: 60 ± 2 °C)
4	Rinsing (2×) in demineralized water (immersion time: 30 s per rinse, temperature: 21 ± 1 °C)
5	Pickling (acid cleaning) in a 10% HCl solution (immersion time: 120 s, temperature: 25 ± 1 °C)
6	Rinsing (2×) in demineralized water (immersion time: 30 s per rinse, temperature: 21 ± 1 °C)
7	Electrolytic zinc deposition (as described above)
8	Rinsing (3×) in demineralized water (immersion time: 30 s per rinse, temperature: 21 ± 1 °C)
9	Drying of samples in a continuous dryer (drying time: 720 s, temperature: 60 ± 5 °C)
10	Final visual inspection

.

Given the planar dimensions of the test samples (160 × 65 mm), the total coated surface area was calculated as 0.0208 m² (0.160 × 0.065 × 2), excluding the suspension hole and sheared edges. Accordingly, 1 m² of zinc coating was sufficient to cover approximately 48.1 steel samples on both sides (0.0208⁻¹). This factor was incorporated into the calculation of the surface mass of deposited zinc (m_Zn, g·m⁻²), determined as the difference between the mass of a single steel substrate sample before (m_S) and after coating (m_S+Zn). The constant 48.1 m⁻² was applied to scale the deposited zinc mass to 1 m² of surface area:

$$m_{Zn}=\left(m_{S+Zn}-m_S\right)·48.1 (g·m^{-2})$$

(2)

2.4. Hot-Dip Galvanizing Process

Hot-dip galvanizing was performed under laboratory conditions at the Department of Technology and Automobile Transport, Mendel University in Brno. The zinc coating process was carried out in a molten zinc bath with the following composition:

• 99.50 wt.% Zn
• 0.30 wt.% Al
• 0.15 wt.% Sn
• 0.05 wt.% Bi

The metals were melted in a graphite crucible, housed within a muffle resistance furnace (MP 05-1.1 type), and maintained at a bath temperature of 450 ± 2 °C. The declared purity of the input metals was 99.99%. To statistically evaluate the average mass gain of the zinc coating as a function of immersion time, five time intervals were selected:

• 70 s
• 105 s
• 140 s
• 175 s
• 210 s

The conversion of deposited zinc mass to a surface area of 1 m² was performed using Equation (2). A detailed specification of the hot-dip galvanizing procedure is provided in

Table 4. Hot-dip galvanizing process.

Step No.	Brief Description of the Operation
1	Degreasing in a solution of NaOH (20 g·L−1) and K3PO4 (30 g·L−1) (immersion time: 300 s, temperature: 60 ± 2 °C)
2	Rinsing (2×) in demineralized water (immersion time: 30 s per rinse, temperature: 22 ± 1 °C)
3	Pickling in a 14% HCl solution (immersion time: 180 s, temperature: 50 ± 2 °C)
4	Rinsing (2×) in demineralized water (immersion time: 30 s per rinse, temperature: 22 ± 1 °C)
5	Immersion in a flux solution composed of ZnCl2 and NH4Cl in a 3:2 ratio (immersion time: 120 s, temperature: 45 ± 2 °C)
6	Drying of samples in an electric annealing furnace (drying time: 600 s, temperature: 65 ± 2 °C)
7	Hot-dip galvanizing process (as described above)
8	Final visual inspection

.

2.5. Methodology of Analysis of Deposited Zinc Mass and Zinc Coating Quality

In this phase, the surface mass of deposited zinc (m_Zn) for both electrolytic and hot-dip galvanizing will be calculated using Equation (2). Additionally, surface roughness parameters (Ra and Rz) will be measured following the methodology outlined in Section 2.2 to evaluate the degree of surface levelling.

The quality of the zinc coatings will be further assessed through metallographic analysis, with particular attention to structural inhomogeneities. These findings will also be considered in the corrosion test evaluation. The methodology for metallographic analysis has been previously described in Section 2.2.

2.6. Methodology of Mechanical Testing of Zinc Coatings

The mechanical properties of the coatings were evaluated using the cupping test in accordance with ISO 1520 [25], utilizing the Elcometer 1620 testing device (Elcometer, Manchester, United Kingdom). As the Elcometer 1620 is equipped with an optical system featuring fixed 15× magnification, the onset of coating cracks was reliably detected in real-time during testing. For microscopic imaging, additional analysis was performed using a VHX-5000 digital microscope (Keyence, Mechelen, Belgium), as shown in Figure 4. Each sample set was tested five times to ensure statistical reliability.

2.7. Methodology of Corrosion Testing of Zinc Coatings

Corrosion resistance tests were conducted in a neutral salt spray environment following ISO 9227 [26]. The experiments were performed in a Liebisch S400M-TR chamber, with each test cycle lasting 24 h. Each cycle consisted of the following:

• A total of 16 h of continuous salt spray exposure.
• A period of 8 h for the natural evaporation of the deposited solution from the sample surface (ambient temperature: 20–24 °C, relative humidity: 40–60%).
• Regular photographic documentation and additional analyses.

Test Parameters:

• Salt fog environment temperature: 35 ± 2 °C
• Sodium chloride concentration in distilled water: 50 ± 5 g·L⁻¹
• pH of the sodium chloride solution (at 20 ± 2 °C): 6.5–7.2
• Consumption rate of sodium chloride solution: 0.5–0.6 L·h⁻¹
• Air pressure at spray nozzles: 120 kPa
• Inclination angle of test samples: 20 ± 2° (relative to the vertical plane)
• Number of test samples per set: 5

The degree of sample degradation was evaluated based on the formation of red rust, indicating perforation of the anti-corrosion coating down to the steel substrate. This criterion was selected due to the distinct colour contrast between zinc corrosion products (white) and steel corrosion products (red/brown), allowing for accurate visual identification. To quantify corrosion damage, digital image analysis was performed using the ImageJ software (version 1.54) to determine the percentage of the surface area affected by red rust (Figure 5).

Since ISO 10289 [27] requires that the edges of the samples be excluded from the evaluation within a 5 mm margin, the evaluated surface dimensions were 143 × 55 mm (total area: 7865 mm²).

3. Results and Discussion

3.1. Evaluation of the Surface Integrity of the Steel Substrate

The measured roughness values (

Table 5. Surface roughness parameters of the steel substrate.

Mechanical Pre-Treatment	Mean Roughness Parameter ± Standard Deviation
	Ra, µm	Rz, µm
Synthetic Corundum Blasting	5.41 ± 1.51	44.97 ± 11.83
Steel Granulate Blasting	3.49 ± 0.82	25.17 ± 8.55
Grinding	3.97 ± 0.69	26.78 ± 6.14
Reference Sample (Without Pre-treatment)	2.48 ± 0.45	16.46 ± 3.78

) indicate that the rolled surface (reference sample without mechanical pre-treatment) exhibited the lowest average values for both Ra and Rz parameters. The selected process parameters for blasting and grinding technologies were considered optimal, as they produced measurable plastic deformation, which is also statistically comparable to findings in previous studies [5,18]. Based on standard deviation values for Ra and Rz, grinding was identified as the most suitable mechanical pre-treatment, as it yielded the most homogeneous steel surface texture. This can be attributed to the consistent cutting conditions during machining. In contrast, both blasting methods exhibited higher standard deviations, primarily due to manual operation of the blasting equipment. However, these variations were not statistically significant and align with technical practice. Among the tested methods, synthetic corundum blasting produced the least homogeneous surface, which can be attributed to the aggressiveness of angular abrasives [6,7]. Conversely, the spherical shape of steel grit particles contributed to lower roughness values compared to the other mechanical pre-treatments. A comparative analysis of Ra and Rz parameters further confirmed that the average Rz values were approximately 6.6× (reference sample) to 8.3× (synthetic corundum blasting) higher than Ra values. The accuracy of Rz measurements was objectively verified using metallographic specimens (Figure 6), while the appearance of individual sample types is illustrated in Figure 7.

Microhardness measurements (HV0.1, Figure 8) confirmed the highest degree of plastic deformation in samples blasted with steel grit, with hardness increases observed up to a depth of approximately 375 µm from the theoretical surface position. This finding aligns with the results of [28], which also examined the effects of blasting on the properties of carbon steel AISI 1045. In comparison, samples blasted with synthetic corundum exhibited a less pronounced effect on subsurface hardness, with hardness increases detected only up to a depth of approximately 175 µm. This difference is primarily due to the higher specific density of steel grit, which enables greater kinetic energy transfer during blasting [10]. This characteristic can be particularly beneficial for blasting high-strength martensitic steels, which exhibit high toughness, as demonstrated in [29]. In that study, steel grit blasting affected material hardness only up to a depth of approximately 120 µm. The final hardness values of ground samples and the reference sample were found to be equivalent, which is a direct result of the grinding process itself. Unlike blasting, grinding involves a different abrasive-substrate interaction, primarily governed by material removal rather than plastic deformation. However, both ground and reference samples exhibited a slight decrease in hardness toward the core, likely due to reduced deformation hardening in the rolled material. An example of the sample appearance after hardness testing is shown in Figure 3, as part of the measurement methodology.

3.2. Analysis of Deposited Zinc Mass and Zinc Coating Quality

Surface roughness and hardness measurements confirmed that mechanical pre-treatments altered the integrity of the steel surfaces. Experimental results further demonstrated that these changes had a direct impact on the deposition rate of both electrolytic zinc (

Table 6. Surface mass of deposited zinc (arithmetic mean ± standard deviation, g·m⁻²).

Type of MP	Electrolytic Zinc Deposition Time (s)
	900	1800	2700	2600	4500
SCB	33.62 ± 0.71	59.48 ± 0.97	81.93 ± 1.22	102.59 ± 1.57	116.45 ± 1.96
SGB	28.72 ± 0.49	52.13 ± 0.70	69.88 ± 0.92	87.47 ± 0.99	96.27 ± 1.16
G	30.97 ± 0.52	55.46 ± 0.74	72.44 ± 0.89	90.23 ± 1.02	102.45 ± 1.19
E	24.21 ± 0.39	43.18 ± 0.57	60.53 ± 0.71	75.14 ± 0.83	78.80 ± 0.87
	Hot-Dip Zinc Deposition Time (s)
	70	105	140	175	210
SCB	164.17 ± 3.77	278.24 ± 5.29	356.32 ± 6.73	439.84 ± 8.02	507.91 ± 9.81
SGB	145.85 ± 2.93	242.80 ± 4.36	326.37 ± 5.87	409.58 ± 7.39	458.44 ± 8.23
G	209.83 ± 3.94	297.51 ± 5.07	368.74 ± 5.98	424.69 ± 6.78	477.13 ± 7.87
E	184.39 ± 3.31	266.89 ± 4.17	334.28 ± 4.80	393.95 ± 5.53	442.21 ± 6.09

MP—mechanical pre-treatment; SCB—synthetic corundum blasting; SGB—steel granulate blasting; G—grinding; E—etalon (reference sample).

, Figure 9) and hot-dip zinc (Table 6, Figure 12).

3.2.1. Electrolytic Zinc Coating

In the context of electrolytic deposition, ref. [30] states that the deposition rate of the coating metal generally depends not only on the microstructure of the substrate but also on the temperature of the plating bath (electrolyte). Furthermore, ref. [31] adds that increasing the bath temperature enhances electrical conductivity, thereby accelerating metal transfer to the substrate. However, since all electroplating experiments in this study were conducted at a constant electrolyte temperature of 25 ± 1 °C, the influence of temperature on the zinc deposition rate can be considered negligible in this case. Regardless of the corrosion-mechanical properties of the deposited coatings, the graphical representation in Figure 9 suggests that for electroplating applications involving localized mechanical pre-treatment, the most suitable method is steel grit blasting. This is because the resulting coating thickness after steel grit blasting is statistically closest to that of the reference sample. Consequently, the outer layer of the coating is more uniform, with a smoother transition in thickness between different substrate areas compared to other mechanical pre-treatments. In technical practice, partial mechanical pre-treatment is commonly applied for the following:

• Slag removal from weld joints.
• Addressing localized corrosion damage.
• Cleaning areas contaminated with residual old coatings.

Additionally, results indicate that grinding produces a zinc mass increase nearly comparable to steel grit blasting. This suggests that combining these two techniques could be beneficial when applying multiple mechanical pre-treatments. From a practical perspective, steel grit blasting can be used for cleaning hard-to-reach areas (e.g., fillet welds), while grinding is better suited for easily accessible surfaces (e.g., flat rolled semi-finished products). However, it is important to note that these recommendations are based solely on coating thickness analysis. Whether similar conclusions apply to corrosion-mechanical properties will be examined in the subsequent section of this study.

Experimental measurements reported in [13] demonstrated that coating adhesion was highest on substrates with the greatest average Ra roughness value, making synthetic corundum blasting the most effective mechanical pre-treatment in this regard. However, the same authors also noted that higher adhesion can be achieved on surfaces with a homogeneous texture. Based on this assertion, grinding appears to be the most effective pre-treatment method, a conclusion further supported by previous studies [32,33]. Findings from these studies suggest the existence of an optimal roughness profile, as excessive surface roughness can lead to localized variations in coating adhesion. Ultimately, coating adhesion is a critical factor influencing both corrosion resistance and the overall effectiveness of anti-corrosion protection for the underlying substrate [34].

Figure 9. Mass increase of electrolytic zinc coating on different substrate types as a function of immersion time in the zinc bath.

Figure 9. Mass increase of electrolytic zinc coating on different substrate types as a function of immersion time in the zinc bath.

Further roughness analysis revealed that electrolytic zinc coating had a statistically insignificant levelling effect (Figure 10). Although a slight decrease in average Ra and Rz values was observed across all cases—likely due to the optimized zinc bath composition [24,35]—this reduction was statistically negligible given the standard deviation values. This phenomenon can be attributed to the low electrolyte temperature used during the electrolytic plating process, which results in minimal Fe/Zn atomic diffusion [36]. Consequently, the smoothing of steel surface protrusions through diffusion does not occur. Another contributing factor is the relatively thin electrolytic zinc coating layer, typically measuring 6–20 µm, which primarily replicates the underlying steel surface geometry [37].

The findings were further validated through metallographic specimen analysis (Figure 11). This final phase of the study confirmed the presence of a continuous and homogeneous zinc coating not only on the reference (untreated) substrate but also on substrates blasted with steel grit and ground surfaces. In contrast, the zinc coating on the synthetic corundum-blasted substrate exhibited a higher occurrence of microcracks, predominantly located within surface depressions formed by the impact of angular abrasive particles [6,10]. This irregular surface microgeometry poses challenges for uniform coating deposition and may also reduce the effectiveness of chemical pre-treatment [9]. Additionally, increased surface roughness has been associated with a higher tendency for corrosive agents to adhere, as reported in [38,39]. The observed differences in coating quality between synthetic corundum-blasted and ground substrates, despite the use of a comparable abrasive material, can be attributed to the fundamental differences in the mechanisms by which these mechanical pre-treatment methods interact with the steel surface [20].

3.2.2. Hot-Dip Zinc Coating

As illustrated in Figure 12, the hot-dip zinc coating process differs significantly from electrolytic zinc plating. Based on the approximation function curves, the hot-dip galvanizing process can be divided into two distinct phases. The first phase occurs immediately after sample immersion in molten zinc, where the highest zinc mass increase was recorded for the ground substrate. This phenomenon can be attributed to the following factors:

• Higher surface roughness.
• Lower subsurface hardness.
• A favourable residual stress state, which strongly facilitates Fe/Zn atomic diffusion [40,41].

In contrast, the steel grit-blasted surface exhibited the least favourable conditions for diffusion, despite its statistically comparable roughness to the ground surface. This suggests that high subsurface hardness plays a dominant role, as it induces significant compressive stresses, which act as a barrier to diffusion [42]. However, with material recrystallization, these compressive stresses gradually decrease, accelerating diffusion processes [43]. This marks the transition to the second phase, where diffusion is primarily supported by high dislocation densities and fine-grained structures [44,45]. As a result, the synthetic corundum-blasted substrate exhibited the highest zinc deposition rates after 175 s and 210 s of immersion in the molten zinc bath. Similar to electrolytic zinc coating, the influence of localized mechanical pre-treatment was briefly examined. In this case, steel grit blasting was again identified as the most suitable variant, regardless of the corrosion-mechanical properties of the hot-dip zinc coatings. This conclusion is based on the statistically comparable zinc deposition rates recorded for the reference sample after approximately 140–210 s of immersion, which aligns well with technical practice. By contrast, the other two mechanical pre-treatment methods appear less suitable for localized application. However, this does not apply when combining two mechanical pre-treatment techniques. The most effective combination was grinding followed by steel grit blasting, as evidenced by the higher zinc mass deposition observed after 175 s and 210 s. These time intervals correspond well with standard industrial practices [2].

Figure 12. Mass increase of hot-dip zinc coating on different substrate types as a function of immersion time in the zinc bath.

Figure 12. Mass increase of hot-dip zinc coating on different substrate types as a function of immersion time in the zinc bath.

The roughness analysis revealed a statistically significant leveling effect of hot-dip zinc coating (Figure 13). The most pronounced decrease in average Ra and Rz values was observed for coatings applied to synthetic corundum-blasted substrates. This phenomenon can be attributed to the faster heating of sharp surface protrusions compared to the bulk material, which leads to earlier initiation of Fe/Zn diffusion processes in these regions [46]. This effect represents a fundamental difference from electrolytic zinc deposition [47]. After 210 s of immersion, the Ra and Rz values became statistically comparable, as indicated by their standard deviations. This can be further explained by the higher zinc deposition rate observed on substrates with greater initial roughness. In contrast, the reference sample exhibited the least reduction in roughness values, likely due to the manufacturing process of the semi-finished product. Since cold rolling inherently produces surfaces with an optimized microgeometry, minimal surface leveling occurs during hot-dip galvanizing [4,20].

Metallographic analysis (Figure 14) further revealed the negative effects of blasting technologies on coating homogeneity. On the steel grit-blasted surface, pronounced zeta-phase outgrowths were observed, surrounded by an eta-phase matrix. However, these outgrowths remained mostly continuous with the lower part of the zeta phase. In contrast, the synthetic corundum-blasted substrate exhibited more pronounced inhomogeneities, with zeta-phase outgrowths largely detached from their lower portion and completely surrounded by the upper eta phase. Additionally, defects within the delta phase were more prominent in these samples compared to other categories. Similar changes in intermetallic phases have been reported in previous studies [43,44] and can be explained by the sequential nucleation of the zeta phase, followed by the delta phase [46]. The high frequency of microcracks within the coating may accelerate corrosion degradation [48]. In this respect, the leveling effect of hot-dip zinc can be considered detrimental. However, from an aesthetic perspective and in terms of reducing surface contamination, a smoother surface may contribute to enhanced corrosion resistance [49].

3.3. Analysis of Mechanical Properties

The mechanical properties of both electrolytic and hot-dip zinc coatings were evaluated using the Erichsen cupping test, which was conducted prior to corrosion testing (Figure 15). This sequence was chosen to prevent voluminous corrosion products from hindering the objective assessment of the zinc-coated samples after salt spray exposure [18]. The cupping test measures the ability of both the coating and substrate to undergo plastic deformation. The later the sample fails, the greater the plasticity of the coating or substrate. During testing, the sample is clamped between a holder and a punch, where a spherical-ended punch (20 mm diameter) indents the surface until a visible crack appears.

For electrolytically deposited zinc, the lowest elasticity was observed in samples with steel grit-blasted substrates. This effect is attributed to the high hardness of the subsurface layers, which increases brittleness in the affected steel region. Although pure zinc is a ductile metal, the 350 µm-thick hardened steel layer significantly exceeds the thickness of the zinc coating [37]. As a result, cracks in the zinc coating formed network-like patterns, similar to those observed in hot-dip zinc coatings. Quantitatively, this similarity was evident as cracks appeared at an indentation depth of approximately 2 mm in both cases, indicating very low ductility [50]. Among electrolytically deposited zinc coatings, the highest ductility was observed in the reference sample, which correlated with the lowest subsurface hardness. The ground surface exhibited similar substrate hardness, though its surface microgeometry contributed to a minor statistical reduction in elasticity. For hot-dip zinc coatings, the low ductility is primarily due to the inherent brittleness of intermetallic compounds and the relatively high coating thickness. However, hot-dip zinc coatings without intermetallic phases can be achieved through continuous galvanizing using the Sendzimir method, which is known to improve coating elasticity. The main drawback of this method is the reduced zinc coating thickness, which is comparable to electrolytic zinc deposition [2,20,37,44]. As shown in previous studies [51,52,53], the cupping test is widely used for objective assessment of coating ductility. Due to the greater elasticity of electrolytic zinc coatings, imaging required a microscope with a greater depth of field. Consequently, images were captured using a digital microscope (Figure 16). In contrast, for hot-dip galvanized samples, a metallographic microscope was sufficient for imaging (Figure 17).

3.4. Analysis of Corrosion Resistance

Due to the significant difference in the amount of deposited zinc between the two coating types (consistent with technical practice), the electrolytic zinc coating underwent 7 degradation cycles, while the hot-dip zinc coating endured 33 cycles. As indicated by the corrosion test results (Figure 18), when comparing the corrosion resistance of mechanically pre-treated and untreated samples, two main factors must be considered:

• Coating structure quality.
• The mass of deposited zinc, which represents the quantitative aspect.

For the electrolytically galvanized reference sample, no structural defects were observed in the coating. However, the highest degree of degradation was recorded during corrosion testing. This can be attributed to the lower zinc surface mass, as the reference sample contained approximately 1.5× less zinc than the synthetic corundum-blasted substrate. When evaluating zinc coatings in a salt spray environment, the relatively rapid reduction in zinc layer thickness must also be considered [54]. This was further confirmed by the results, where the first signs of red rust in the electrolytic zinc coating appeared uniformly after six cycles, with corrosion spots typically up to 3 mm in diameter. The only exception was the reference sample, where corrosion spots reached diameters of up to 5 mm. Corrosion testing was terminated after the following cycle (Figure 19) due to rapid degradation of the coating, specifically its thin layer [37]. For all three types of mechanical pre-treatment, the corrosion degradation rate was statistically comparable. However, based on these findings, synthetic corundum blasting can be considered the least suitable method, while grinding appears to be the most effective. The key factor influencing these differences is the varying zinc deposition on different substrates. The fastest coating degradation occurred on the synthetic corundum-blasted substrate, primarily due to coating discontinuities, which were confirmed through metallographic specimen analysis. A similar trend was observed in hot-dip zinc coatings, where inhomogeneities within the coating contributed to faster corrosion degradation. Once again, synthetic corundum blasting proved to be the least suitable pre-treatment method. In this case, the zeta phase exhibited high heterogeneity, accompanied by numerous microcracks in the delta phase. This intermetallic phase structure not only promotes salt solution penetration to the steel substrate but also accelerates red rust formation [2,44]. The first signs of red rust appeared in synthetic corundum- and steel grit-blasted samples after 30 cycles, with corrosion spots generally up to 4 mm in diameter. In the remaining two sample types, similar corrosion patterns emerged after 31 cycles. Exposure in the salt spray environment was terminated after 33 cycles (Figure 20). This number of degradation cycles is comparable to those used in testing organic coatings [18,50,55]. As indicated by the extent of corrosion damage in the reference sample and ground substrate, coating quality in hot-dip zinc coatings is more critical than zinc mass. For this reason, grinding can once again be identified as the most suitable mechanical pre-treatment method. Another factor supporting this conclusion is the relatively favourable microgeometry of the steel substrate, which reduces the adhesion of corrosive agents to the zinc coating surface [38,39]. If blasting with angular abrasives is necessary, additional measures should be implemented to enhance the corrosion resistance of zinc coatings. In technical practice, such measures commonly include the following:

• Application of conversion layers [56,57].
• Use of organic coatings [58].
• A combination of both methods [59].

The corrosion tests, consistent with the mechanical tests, further confirmed that the choice of mechanical pre-treatment technology is crucial for the overall corrosion protection performance of the component. Additionally, the measurable differences in corrosion-mechanical properties among the four tested substrates validate the effectiveness of the technologically distinct mechanical pre-treatment methods.

4. Conclusions

Based on the comprehensive corrosion-mechanical assessment of the test samples, the highest coating quality for both electrolytic and hot-dip zinc coatings was achieved using conventional zinc coating methods that exclude mechanical pre-treatments from the process. This can be attributed to decades of development and refinement, which have optimized these conventional methods to their current level of effectiveness. However, a major drawback of conventional technologies is their reliance on chemical pre-treatment processes, which involve the extensive use of chemical substances. From an environmental and sustainability perspective, it is therefore essential to explore innovative alternatives. One promising approach is the partial replacement of chemical pre-treatments with mechanical surface preparation methods. The experimental results indicate that, with proper optimization, mechanical pre-treatments can produce high-quality zinc coatings. For both zinc coating methods, grinding emerged as the most suitable pre-treatment, as coatings applied to ground substrates exhibited corrosion-mechanical properties statistically comparable to reference samples. Conversely, the least suitable pre-treatment was blasting with angular synthetic corundum, as it induces significant structural inhomogeneities in both coating types. These heterogeneous regions ultimately accelerate the corrosion degradation of the zinc layers. Compared to synthetic corundum blasting, steel grit blasting appears to be a more suitable alternative. However, a notable drawback of this method is the high hardness of the subsurface layers, which increases substrate brittleness and negatively impacts the ductility of electrolytic zinc coatings. As demonstrated by surface roughness analyses and metallographic evaluations, the spherical shape of steel grit particles provides a favorable surface microgeometry. To further optimize the blasting process, future research should focus on the following:

• Selecting spherical particles with lower specific density.
• Reducing subsurface hardness through appropriate heat treatment.

Such an approach could mitigate the negative effects of mechanical pre-treatments while preserving the benefits of alternative surface preparation methods.