Effect of Low Chloride and Sulfate Concentrations on Corrosion Behavior of Aluminum and Zinc Arc Thermal Sprayed Coatings

Abstract

The aim of this study was to determine the suitability of arc sprayed zinc and aluminum coatings as materials for protective coatings of different heating systems. The most aggressive chemical agents occurring in heating water are chloride and sulfate anions. Both ions are responsible for the corrosion of metals due to their high electronegativity and standard electrochemical potential. Water in heating systems should not contain more than 150 mg/L anions, including no more than 50 mg/L of chlorides and 100 mg/L of sulfates. To determine the corrosion resistance of three types of zinc and aluminum coatings, open circuit potential and linear polarization resistance (LPR) tests were conducted in eight alkaline solutions with different sulfate and chloride contents. The SEM/EDS structural properties of sprayed coatings at specific arc process parameters were investigated. Zinc coatings exhibit the most stable corrosion potentials in varying environments but have higher corrosion current density. Aluminum coatings exhibit much higher potential values in a chloride environment than in any other. A chloride environment also causes the lowest corrosion rates for aluminum-coated samples. A small addition of aluminum to the zinc coating (15 wt.%) does not appear to affect the stability of the corrosion potential but does result in a reduction in corrosion rates in chloride solutions.

1. Introduction

One of the most important parameters of materials used for the transmission of heating water is their resistance to corrosion. According to PN-93 C-04607 standard, water for central heating applications should be characterized by a low concentration of anions (<150 mg/L), particularly sulfates and chlorides [1]. Additionally, the heating medium should be alkaline to further inhibit corrosion processes. Alkaline solutions have different effects on the corrosion behavior of various coatings. Especially, aluminum, zinc, and their alloys are commonly used as thermal sprayed coating materials on steel substrate immersed in water. The sacrificial corrosion protection of these coatings materials, in combination with their relatively low corrosion rates, make them suitable for such harsh environments [2,3,4].

Zinc coatings show no significant change in corrosion resistance in acidic and alkaline solutions (pH = 4–10), whereas the formation of zinc hydroxide and crystalline zinc oxide in alkaline solutions has been reported [5]. A similar situation occurs on the aluminum surface, where the concentration of hydroxide anions is needed to precipitate Al³⁺ ions to form protective coatings. This effect was reported when aluminum alloys were tested at pH = 3–10 [6]. Chlorides and sulfides also have significant effects on the corrosion behavior of zinc and aluminum materials.

It was found that high sulfate concentrations (1–4%) in low alkaline solutions (pH about 8) lead to a decrease in corrosion rate with increasing concentration [7]. It has been also reported that sulfate anions contribute to the pitting corrosion of aluminum materials, however, the higher concentrations (2 mol/L) lead to the creation of protective layers [8]. In chlorides, zinc materials undergo pitting corrosion at ion concentration of 0.1 mol/L and pH values of 4–10. Outside these parameters, the pitting potential can even assume high positive values [9,10].

There are many methods of applying zinc-based coatings. These include sherardizing, plating, electroplating, hot-dip galvanizing (batch and strip), and thermal spraying (flame, cold gas, and arc) [11,12,13,14,15,16,17]. For large steel structures, the coating processes, such as batch galvanizing and thermal spraying are the only ones applicable. Batch galvanizing is still limited in terms of component size and elemental composition of the baths [18]. The arc spray process provides superior coating properties compared to other thermal spray methods for zinc and aluminum coatings [12,13,17]. The arc sprayed Zn and Al coatings are widely used due to their ability to withstand high temperatures, offer advantages in process efficiency, lower cost, and anti-corrosion protection.

Arc spraying is considered the most economical method for zinc-based coatings application due to its high (250 kg/h) deposition rate [19]. However, the most often suggested material, as having superior performance in seawater and freshwater environments is aluminum. Thermal sprayed aluminum coatings are applied to structural components and equipment made of steel and cast iron [20]. Aluminum-coated components are often exposed to potentially corrosive environments in water and the atmosphere, including combustion gases at high temperatures (900 °C) [16,17,20]. Due to the economics of application, aluminum and zinc coatings appear to have the potential for use in water transfer heating systems.

The object of this research is to determine the usefulness of arc wire thermal sprayed zinc and aluminum coatings for heating water transfer applications.

2. Materials and Methods

The commercially available three types of feedstock solid cross-section wire materials with a diameter of 2 mm were the starting materials for arc-wire thermal spraying protective coatings. Commercially pure aluminum wire, produced by MigWeld (GmbH) Co. Ltd., Landau an der Isar, Germany, as well as commercially pure zinc wire from Metco^TM (Poznan, Poland) with 99.99% purity and Metco Zn15Al wt.% alloy wire, were used in this study. The coatings were deposited on a 5 mm thick S235JR plate using the EM-14M wire arc spray system (Kiev, Ukraine). The chemical composition of S235JR steel used as the substrate material is presented in

Table 1. Chemical composition of S235JR steel used as the substrate.

Alloying Element	C	P	S	N	Fe
wt.%	0.18	0.013	0.027	0.009	rem.

. For thermal arc spray, a roughness surface of around 110 microns is needed to provide greater adhesion bonding conditions and hinder rust spot formation. Therefore, the S235JR steel samples were cleaned in abrasive blasting directly before arc-wire spraying. Electric arc spraying using EM-14M metallizer was performed with the parameters shown in

Table 2. Arc-spraying parameters used to obtain three types coatings from of 2 mm feedstock wire materials.

Arc Spraying Parameters	Feedstock Wire Materials Used in Thermal Arc-Spraying Experiments
	Zn	Al	Zn-Al
Atomizing gas pressure, (MPa)	0.65	0.65	0.65
Arc current, (A)	50	50	100
Arc voltage, (V)	20	36	25
Power input, (kW)	1	1.8	2.5
Distance of spraying, (mm)	150–250
Rate of wire, (m/min)	3.5

.

The quality, morphological grain structure and physical–chemical composition of arc thermal sprayed coatings were analyzed with Quanta 3D FEG Dual Beam and Philips XL-30/LaB6 scanning microscopes (Warsaw, Poland) integrated with DX4i–EDAX X-ray microanalysis (Warsaw, Poland). In addition, the automatic X-ray spectrum phase analysis was performed using the Seifert X-ray phase analyzer XRD diffract meter 3003 with CoKα radiation (λ = 0.178897 nm). An angular step size of 0.02°/min and a step time of 5 s per point were used. The porosity of the coatings was assessed by photomicrograph quantitative analysis carried out with (SEM) Philips XL30/Lab6 programmed with SIS3.0 ^® software. The evaluation of the porosity level in the coatings volume was carried out by the quantitative image analysis with SIS analyzer (XL30/Lab6 equipment, Warsaw, Poland) which the level of inner porosity in the volume coatings is defined with the planimetric method as the ratio of the sum of pore surfaces to the total surface of the specimen, according to the Cavaleri-Hacquerta principle [21]. The coatings surface roughness was measured with PGM-1C profile meter with G250BS head. The measurements were carried out on the measuring length of l = 4 mm at the head movement velocity of v = 0.2 mm/s and the static load of the blade of 3 mN. Electrochemical properties of samples were characterized using Atlas 1131 Electrochemical unit and impedance analyzer. After testing samples were cleaned with isopropanol and dried under 60 °C, they were placed in a classical three-electrode electrochemical vessel. The sample was connected as a working electrode (WE), a silver chloride electrode (SCE) (Ag/AgCl) served as a reference electrode (RE) and the platinum electrode was a counter electrode (CE). For every sample, an open circuit measurement was performed in order to obtain values of steady-state potential (E_stat). After stabilization of the sample, linear polarization resistance (LPR) test was performed in order to obtain polarization curves and acquire values of corrosion potential (Ecorr), corrosion current density (I_corr) and approximated values of corrosion rate (r_corr). Corrosion rates are directly proportional to corrosion current density [22,23]. In this work corrosion rate values were calculated using Faraday’s law (1).

r_corr = Icorr·t·M·n⁻¹·F⁻¹,

(1)

where:

• r_corr—corrosion rate (g·m⁻²·year⁻¹),
• I_corr—corrosion current density (A·m⁻²),
• M—molar mass of metal (g·mol⁻¹),
• n—number of electrons exchanged in dissolution reaction,
• F—Faradays constant (96,485 C·mol⁻¹),
• t—seconds per year (31,557,600 s·year⁻¹).

For zinc coatings, a molar mass of 65.38 g·mol⁻¹ and two electrons exchanged were used for corrosion rate calculation. Calculating the same values for aluminum coatings required a molar mass of 26.98 g·mol⁻¹ and three electrons exchanged during the electrochemical process. In the case of mixed coatings average molar mass and number of electrons exchanged were assessed by calculating average atomic ratios in wires used for arc spraying. The values of molar mass 53.6 g·mol⁻¹ and two electrons were exchanged. A linear polarization resistance (LPR) test was performed by scanning current responses of the sample in the potential range of < E_stat − 0.1 V; E_stat + 0.1 V >. The test was selected in order to provide minimal change to the structure of samples, due to the possibility of obtaining electrochemical parameters for low values of forced potential changes. Scanning was performed at 1 mV/s rate, and the material exposed area was 0.8 cm². The electrochemical tests were performed in 8 types of solutions with chemical composition presented in

Table 3. Composition of solutions used in electrochemical testing.

Solution Number	Concentration of Chloride Anions Cl− (mg/L)	Concentration of Sulfate Anions SO42− (mg/L)	pH
1	50	100	8
2	0	100	8
3	50	0	8
4	0	0	8
5	50	100	8.5
6	0	100	8.5
7	0	0	8.5
8	50	0	8.5

. Solutions were prepared by dissolution of sodium chloride (99.8% purity), and sodium sulfate (98.5% purity) from Chempure company (Piekary Śląskie, Poland). Solutions were prepared in distilled water produced with DE 5 Polna laboratory distiller (Przemyśl, Poland). The pH value was reached by titration before every experiment with 0.1 M sodium hydroxide solution performed with TitroLine 5000 titration device (Mainz, Germany) until the selected pH value was reached. Every material was tested three times in each solution. The layout of Table 3 is a basic matrix for 2³ experimental plans on basis of which regression coefficients were calculated for electrochemical parameters of every material as a function of environmental factors, such as chlorides concentration, sulfate concentration, and pH value. Statistical significance of environmental factors was assessed using a double-sided t-student test for α = 0.05.

3. Results and Discussion

3.1. Structure Analysis

Detailed research and analyses of the SEM/EDS results of the certain Zn, Al, and Al-Zn coatings arc-wire deposited onto S235JR steel substrate were presented in Figure 1a–f and in Table 3 and

Table 4. Properties of obtained arc sprayed coatings.

Arc-Wire Sprayed Coatings	Thickness (µm)	Porosity (%)	Phase Composition (XRD Patterns)
Zn	398 ± 32	0.2 ± 0.05	zinc, zinc oxides (EDS)
Al	648 ± 49	2.5 ± 0.3	α-Al0.974Si0.026
Zn-Al	521 ± 63	0.25 ± 0.1	zinc, α-Al0.99Si0.01

. Arc-wire spraying exhibits a close correlation between the spraying conditions and the physical–chemical properties of the coating material where the microstructure of the Zn, Al, and Zn-Al coatings is composed of splats with various porosity degrees without visible cracks (Figure 1), and which melted droplets produce a surface roughness in the as-arc sprayed condition.

Generally, the Zn, Al, and Zn-Al coatings obtained are characterized by consistently maintained thickness (different for specific types of coatings—Table 4) and they are cohesive in their volume (porosity less than 0.5% for Zn coating and 2.5% for Al coating). It is confirmed that among the three specimens, zinc coating has the lowest thickness of 398 ± 32 µm compared to aluminum coating with the highest thickness of 648 ± 49 µm (Table 3). However, the aluminum coating shows a morphology with more porosity of 2.5 ± 0.3 % as compared to zinc coating with significantly lower porous of 0.2 ± 0.05 %, as shown in Figure 1a–d and Table 4.

At the same feed rate of Zn and Al wires during arc-spraying coatings at about 5000 °C, the higher thickness of the aluminum coating was most probably due to the lower vapor pressure of aluminum than that of zinc which is evaporated during arc deposition due to its high vapor pressure [3]. The same phenomenon may occur also for Zn-Al coating with a relatively lower thickness of 521 ± 63 µm and much lower porosity of 0.25 ± 0.1% than aluminum coating. Although the chemisorption process is particularly intense in areas of increased surface energy (inclusions, voids, areas of plasticization, and stress concentrations), i.e., the surface layer of the S235JR steel, the analyzed coating/steel substrate interface did not show a tendency to accumulate impurities, porosity, microcracks, and other discontinuities, and the wavy line indicates a typical adhesive bond (Figure 1).

According to the SEM/EDS results it, was found that the arc-sprayed coatings showed practically the chemical-phase composition of the wire material, regardless of whether they were sprayed from the Zn (Figure 1a,b), Al (Figure 1c,d), or Zn-Al (Figure 1e,f) wire. This is a microstructure typical for the arc-spraying method comprising of the layered and slightly oxidized grains produced from the wires which in the arc spray process become melted and change their geometry as they are converted into the splats of the coating. In the arc spraying process, the zinc wire is chemically active, which causes oxidation of the molten particles, and the resulting coatings contain dispersed spheroidal oxides and some strip oxides in the coating volume (Figure 1a,b). Based on the point EDS analysis (Figure 1b and

Table 5. Semiquantitative EDS analysis (at.%) of as-arc wire sprayed with three types of coatings (Zn, Al, and Zn-Al).

Designation of Grain Area according to Figure 1	Content, at.%
	Zn	Al	Si	O
Zn coating—Figure 1b
1—medium gray	82.5	–	–	~17.5
2—gray	68.6	–	–	~31.4
3—bright gray	97.5	–	–	~2.5
mapping	85.3	–	–	~14.7
Al coating—Figure 1d mapping	–	97.2	0.2	~2.6
Zn-Al coating
1—light	96.8	0.7	–	~2.5
2—bright gray	24.7	74	–	~1.3
3—dark gray	2.3	96.7	–	~1.0
4—medium gray	4.8	93.7	–	~1.5
mapping	17.1	80.7	–	~2.2

) besides Zn oxides (point 2), different zinc oxidation degrees (medium gray strip—point 1, and bright grey area—point 3) were also identified. The aluminum with high affinity to oxygen is also oxidized at arc sprayed conditions, however less prominent in the homogeneous structure of the aluminum coating where the melted Al particles create dark-gray homogeneity grains (Figure 1c,d). EDS analysis on the cross-section of the Al coating also revealed the presence of 0.2 at.% Si (Table 5).

The chemical composition identification based on the EDS microanalysis confirmed that in the case of Zn–Al arc sprayed coating in the lamellar structure of flattened grains the predominant is Al phase (grey area in the BSE pictures, Figure 1e,f). EDS elemental mapping analysis indicates that the chemical composition of the arc-sprayed Zn-Al coating consists of 17.1 at.% Zn, 80.7 at.% Al, and an average of 2.2 at.% O distributed in grains with non-uniform, chemical diverse lamellar structure (Table 5).

Based on the XRD results for Zn, Al, and Zn-Al coatings (Figure 2), it was found that in the Zn coating structure, the matrix is a solid solution based on Zn phase (Figure 2a), although quantitative EDS analysis revealed dispersed Zn oxides grains with varied oxygen, as shown in Table 5 connected to Figure 1b. Based on the SEM/EDS results, it can be concluded that the preferential site for the formation of dispersion Zn oxides in the coating volume is the matrix grains, where perhaps the Zn oxide content is very low and beyond the detection limit of XRD analysis, which has been also reported in the literature for this type of arc-sprayed Zn-Al coatings [2,3].

For the arc-sprayed Al coating, similar to the Zn coating, no aluminum oxides were found in the dark gray structure with homogeneous grain morphology based on the Al0.974Si0.026 phase as confirmed by the XRD pattern (Figure 2b). Although the aluminum wire material is chemically active during arc sputtering, the molten wire particles do not lead to the formation of oxide layers at the grain boundaries of the Al coating as in the gas detonation spraying process (GDS), in which highly softened Al-rich intermetallic powder particles are coated with thin α-Al₂O₃ oxide layers to form a lamellar structure of Fe-Al intermetallic coatings similar to a composite [24,25].

By analyzing the XRD patterns for the Zn-Al coating, it can be concluded that the molten zinc and aluminum particles did not react with each other during the arc spraying process, forming Zn-based solid solutions and an aluminum-silicon phase (Al0.99 Si0.01)—Figure 2c, which showed a diverse chemical composition with the range shown in Table 5.

Although no intermetallic phase or other oxide phases were detected, it is possible that an Al0.403Zn0.597 compound was formed because its peaks are consistent and overlap with those of Al [3,26].

3.2. Electrochemical Studies of Zn, Al, and Zn-Al Coatings

Electrochemical parameters obtained from the open circuit and LPR experiments are presented in

Table 6. Electrochemical properties of arc sprayed Zn, Al, and Zn-Al coatings in solutions.

Zn
Solution No.	Estat (V)	Ecorr (V)	Icorr (nA·cm−2)	r (g·m−2·year−1)
1	−1.04 ± 0.002	−1.07 ± 0.002	348 ± 23	37 ± 2.5
2	−0.97 ± 0.01	−0.95 ± 0.02	618.5 ± 15	65.8 ± 1.6
3	−0.99 ± 0.003	−1.01 ± 0.003	425 ± 19	45.25 ± 2
4	−0.98 ± 0.01	−1.01 ± 0.02	587.5 ± 62	73 ± 3
5	−1.04 ± 0.002	−1.07 ± 0.003	514 ± 28	54.6 ± 3
6	−0.98 ± 0.01	−0.99 ± 0.009	941.5 ± 137	99.8 ± 14
7	−1.05 ± 0.004	−1.05 ± 0.009	1505.5 ± 27	155.3 ± 1.7
8	−1.03 ± 0.001	−1.08 ± 0.002	339.3 ± 4	36.3 ± 0.5
Al
Solution No.	Estat (V)	Ecorr (V)	Icorr (nA·cm−2)	r (g·m−2·year−1)
1	−0.65 ± 0.05	−0.66 ± 0.05	27.5 ± 3	0.8 ± 0.08
2	−0.66 ± 0.02	−0.69 ± 0.02	13.5 ± 0.2	0.4 ± 0.005
3	−0.71 ± 0.03	−0.72 ± 0.03	31 ± 6	0.9 ± 0.17
4	−0.67 ± 0.04	−0.7 ± 0.04	19.85 ± 0.4	0.6 ± 0.01
5	−0.85 ± 0.08	−0.88 ± 0.08	45.5 ± 11	1.3 ± 0.32
6	−0.74 ± 0.05	−0.78 ± 0.04	26.15 ± 3	0.8 ± 0.09
7	−0.74 ± 0.007	−0.83 ± 0.02	14.71 ± 3	0.4 ± 0.09
8	−0.49 ± 0.02	−0.51 ± 0.02	5.18 ± 0.5	0.15 ± 0.01
Zn-Al
Solution No.	Estat (V)	Ecorr (V)	Icorr (nA·cm−2)	r (g·m−2·year−1)
1	−0.96 ± 0.002	−0.95 ± 0.005	1061 ± 3	92.9 ± 0.3
2	−0.92 ± 0.002	−0.91 ± 0.005	1225 ± 32	107.15 ± 2.8
3	−0.98 ± 0.001	−0.94 ± 0.005	367 ± 11	32.15 ± 1
4	−0.91 ± 0.002	−0.9 ± 0.002	604 ± 32	52.9 ± 2.9
5	−0.95 ± 0.005	−0.94 ± 0.009	770 ± 56	67.5 ± 4.9
6	−1 ± 0.02	−1.02 ± 0.03	727 ± 176	78.45 ± 10
7	−0.96 ± 0.05	−0.98 ± 0.06	696.3 ± 212	60.7 ± 11.5
8	−1 ± 0.05	−1.05 ± 0.04	60.5 ± 5	9.7 ± 1.1

. In every solution potentials of zinc materials are significantly lower than the potential of aluminum coatings. Values of steady-state potential range from −1.05 V to −0.97 V for zinc coatings, corrosion potential values range from −1.08 V to −0.95 V. For aluminum coatings steady-state potentials values range from −0.85 V to −0.49 V and corrosion potentials are measured between −0.88 V and −0.51 V. Aluminum materials also have lower values of current density which range from 5.18 nA·cm⁻² to 45.5 nA·cm⁻² this value influences calculated rate of corrosion, which for aluminum ranges from 0.15 g·m⁻²·year⁻¹ to 1.32 g·m⁻²·year⁻¹. Current density and corrosion rate values are significantly higher for Zinc coatings. Current density values for zinc range from 339.3 nA·cm⁻² to 1505.5 nA·cm⁻², with corrosion rate values between 36.3 g·m⁻²·year⁻¹ and 155.3 g·m⁻²·year⁻¹. Values of electrochemical parameters for mixed zinc-aluminum coatings are closer to values obtained for zinc materials due to the higher content of the heavier metal. Steady-state potential values range from −1.00 V to −0.91 V and corrosion potential values are between −1.05 V and −0.91 V. Current density values for mixed coatings range from 60.5nA·cm⁻² to 1225 nA·cm⁻² corrosion rate is calculated between 9.7 g·m⁻²·year⁻¹ and 107.15 g·m⁻²·year⁻¹.

The polarization curves obtained from the linear polarization resistance (LPR) test are shown in Figure 3. In water solution, increasing the pH value from 8 to 8.5 does not change the corrosion current density for all materials. An increase in pH causes a decrease in the potential for single metal coatings and an increase in the potential for mixed coatings. In the presence of chlorides, an increase in pH causes an increase in the potential for aluminum coatings and a decrease in zinc and mixed coatings. The increase in chloride pH causes a decrease in current density and corrosion rate for aluminum and mixed coatings and does not change significantly for zinc coatings. In sulfate solutions, changing the pH value does not change the corrosion current density and corrosion rate in all coatings. Increasing the pH value leads to a decrease in the corrosion potential of all coatings, but the decrease in corrosion potential is proportional to aluminum content in the coating. In a solution containing both sulfate and chloride anions, both zinc and mixed coatings show almost no change in their polarity curves with a change in pH. Aluminum coatings show a decrease in corrosion potential and an increase in corrosion current and corrosion rate in higher pH solutions.

Statistical evaluation of the results was performed, and the significance of the coefficients was calculated using t-student test. The t-student coefficients for each coating are shown in

Table 7. t-Student coefficients for the influence of parameters of coatings.

Zn
Parameter	Estab (V)	Ecorr (V)	Icorr (nA·cm−2)	r (g·m−2·year−1)	Σ
Cl−	0.71	9.32	1.58	1.76	13.37
SO42−	0.19	8.62	0.34	0.42	9.57
pH	0.73	7.60	1.03	1.00	10.36
Al
Parameter	Estab (V)	Ecorr (V)	Icorr (nA·cm−2)	r (g·m−2·year−1)	Σ
Cl−	0.12	0.24	0.33	0.33	1.02
SO42−	0.32	0.25	0.39	0.39	1.35
pH	0.13	0.24	0.003	0.005	0.38
Zn/Al
Parameter	Estab (V)	Ecorr (V)	Icorr (nA·cm−2)	r (g·m−2·year−1)	Σ
Cl−	0.16	0.12	0.44	0.55	1.26
SO42−	0.03	0.05	0.90	1.08	2.05
pH	0.23	0.42	0.44	0.39	1.48

. The chloride sulfide concentration and pH value have a significant effect only on the corrosion potential of the zinc coatings. For all other electrochemical parameters, the Student’s t-ratios are too low (critical value 4.303). In order to determine which environmental factor (chloride concentration, sulfate concentration or pH) is the most significant, the sum of t-Student coefficients was calculated for each material and each environmental factor. For zinc coatings, chloride concentration is the most significant factor for corrosion processes and sulfate concentration is the least significant. For the other coatings, sulfate concentration is the most significant environmental factor. The least important for the corrosion process of aluminum coatings is the pH value, and for mixed coatings, the least important parameter is the chloride concentration.

Investigations performed by Zaid et al. on AA6061 [27] aluminum alloy showed that in chloride solutions higher concentrations of hydroxide anions (higher pH values) cause higher rates of corrosion due to the dissolution of aluminum. Our experiments have shown a similar effect on aluminum coatings where corrosion current density increases with pH increase. According to Li and Church [18] in slightly alkaline solutions, protective layers of aluminum oxide and aluminum peroxide are formed. Additionally, the presence of sulfate anions leads to the creation of hydroxyl aluminum sulfate which is an ion-selective protection layer. Therefore, the decrease in aluminum ions can lead to a decrease in potentials and corrosion current densities in higher pH values and sulfate content. This effect seems to be present in aluminum coatings and partially (decreases corrosion potential) in mixed coatings. According to Thomas et al. [5], corrosion of zinc does not change significantly in alkaline solutions which are supported by performed research. For all investigated solutions polarization curves of zinc in two pH values are close to each other. According to Mouanga et al. [28] in alkaline pH, zinc is covered with zinc oxide, zinc hydroxide, and zinc carbonate. In presence of chlorides, zinc hydroxy chloride is also present resulting in a layer with high electrochemical stability. In the presence of sulfate anions, zinc sulfate is formed, which is a reported corrosion inhibitor [29]. The formation of this compound probably leads to a slight increase in the potentials of zinc coatings where sulfates are present in comparison to similar solutions without sulfates.

The increased corrosion resistance of aluminum coatings in sulfate may also be due to the higher porosity of these coatings. A study by Jeong et al. showed that microporosity can form an oxygen barrier that is protective in corrosive environments [30]. This effect may be important in the presence of sulfate, where one of the corrosion mechanisms is oxidation due to the reduction in sulfate ions. According to Liu et al., pore size affects the corrosion resistance of zinc, where corrosion protection increases with decreasing pore size due to the possible reduction in precipitation of corrosion products into solutions [31].

The results presented in this paper also indicate that thicker coatings of aluminum and mixed materials provide greater corrosion protection, which is consistent with the findings of Penney et al. [32].

4. Conclusions

The presented results can lead to a conclusion that arc sprayed aluminum coatings are suitable materials for heating water pipes. Zinc materials are less favorable due to lower corrosion potentials and higher corrosion rates but the mixing of aluminum seems to improve their properties. Further investigation should be carried out for coatings with higher aluminum content. The research also shows that within critical parameters of heating water there are no significant differences in corrosion behavior of investigated materials. Further research should also be conducted in higher concentrations of chlorides and sulfates in order to assess the usefulness of protective zinc and aluminum coatings for more corrosive environments, such as seawater pipelines.