Deposition of Al-Zn-Mg Alloy Coating on an A36 Steel Substrate: Evaluation of Microstructure, Porosity and Adherence

Abstract

Al and Zn-based metallic coatings are widely used to protect a significant diversity of steel components and structures from corrosion. The quality of coatings depends largely on their properties and spraying conditions. The purpose of this work was to find the optimal thermal spray parameters to manufacture Al-4.2%Zn-2%Mg alloy coatings on an A 36 steel substrate from diagnostic tests of porosity, adherence, and microstructure. Furthermore, adhesion of the coating onto the substrate after the precipitation heat treatment was investigated. Microstructural characterization was performed by scanning electron microscopy (SEM), the porosity was determined by Archimedes principle and the adhesion was evaluated by tensile testing. The optimum thermal spray distance, maintaining the thickness of the coating constant at 300 μm, was 0.20 m, in which the lowest porosity percent (9%) and the highest adherence (7.16 MPa) were attained. The coating increase of thickness and the post-heat treatment precipitation caused an adhesion decrease. However, heat-treated coatings with thicknesses between 150 and 300 µm could be used for cathodic protection of A36 steel because their adherence values (7.33 and 6.69 MPa, respectively) meet those required by the flame thermal spraying process.

Keywords:
Coatings; Aluminum alloy; Adhesion; Porosity; Heat treatment

1. Introduction

Currently, unalloyed carbon steels, such as 1018, A36 and 1040, are the most widely used structural materials due to their excellent mechanical properties, low cost, and wide availability. However, the resistance to atmospheric corrosion of these steels is extremely low in most environments, which leads to the loss of mechanical properties apart from the rather important surface integrity of designed components¹-³. It is usual to apply a metallic or non-metallic coating on the surfaces of the steels to create a continuous barrier that isolates them from the environment⁴. Coatings made of zinc and its alloys have been widely used to protect steels from atmospheric corrosion⁵. This element acts as a sacrificial anode on the surface of the steel, giving it effective protection against corrosion⁶, although the optimal is mostly dependent on physicochemical exposure conditions.

The study of coatings based on aluminum and its alloys for the cathodic protection of steels is in constant development⁷. Earlier studies have shown that some compositions of the Al-Zn-Mg alloy system have high efficiency in the cathodic protection of steels. The addition of Zn and Mg contributes to the depassivation of the thin layer of Al₂O₃ of aluminum sacrificial or otherwise electrodes, consequently shifting the operating potential of the protected metal toward a more electronegative direction⁸. In the design of this type of alloys, the formation and uniform distribution of intermetallic compounds (Mg-Zn binaries and Al-Mg-Zn ternaries) on the Al matrix is promoted, because they prevent the formation of the protective surface film of Al₂O₃ and increase the efficiency of the sacrificial anodes⁹.

Nasser Afify et al.¹⁰, conducted extensive research on the formation of diverse types of fine-scale precipitates in Al-Mg-Zn alloys with low concentrations of Mg and Zn. Their results show the thermal conditions for the growth of phases such as η' (MgZn₂), η (MgZn₂) and phase τ (Mg₃₂(Al, Zn)₄₉) obtained in alloys within the composition range: Al - 2%at. Mg- x %at Zn (x = 1.8, 2 and 4.2). Thus, in this same respect, Al-Zn-Mg alloys with these low Mg and Zn contents could be used to make coatings to protect steels from corrosion.

The protective efficacy of metal coatings for non-alloy steels depends on several factors, such as: 1) First and foremost, the ability to maintain adequate electrochemical protection, which for preliminary validation of the quality of the coating, this may best be additionally correlated through criteria of average service lives, much alike those reported in diverse applications for the said coatings, where they act as sacrificial anodes, 2) the compactness, 3) continuity and 4) adherence, which must be optimally correlated as a premium expression of microstructure¹¹. Porosity should be minimized to warrant the structural relevancy of this property in terms of the lasting operation of any coated component, lest it may turn into a curtailing duration determinant of the surface quality during exposure of coated surfaces to electrolytes as seemingly innocuous as common saline dew. Furthermore, adherence plays a critical role for the performance of the coating and depends on several factors such as chemical composition, thickness, microstructure, substrate roughness and thermal spray parameters.

The purpose of this work is to obtain the optimal thermal spray parameters to manufacture a coating of based on the Al-4.2%at Zn-2%at-Mg alloy to coat a plain carbon A 36 steel on which diagnostic tests of porosity, adhesion, and microstructure were applied. In addition, evaluation of the adhesion of the coating was performed after an aging-like precipitation heat treatment. To conduct this work, the technique of flame spraying was used.

2. Experimental Procedure

2.1. Manufacture of the coating alloy

The Al-4.2%at Zn-2%at-Mg alloy coating was manufactured with commercial purity elements (Al-99.5%, Zn=98% and Mg=99.8%). The alloy was melted using an alumina crucible comprised in an induction furnace protecting the process with an inert Ar atmosphere. The alloy composition was assessed using atomic emission spectroscopy, to the following values: Al (88.34%wt.) - Zn (9.56%wt.) – Mg (1.69%wt.) – Fe (0.281%wt.) – Si- (0.018%wt.) – Cu (0.0016%wt.) – traces of other elements (0.109%wt.). This chemical composition complies with the alloy proposed in this work.

The thermal spraying technique used in the experiments was flame spraying that uses coating material in the form of rods. The rods with a diameter of 0.317 cm and a length of 0.8 m were cast in Pyrex glass tubes. A vacuum pump served to fill partially the tubes to collect a given volume of the liquid alloy that yielded the cylindrical samples.

2.2. Preparation of the substrate

ASTM A36 steel plates with dimensions: 0.05 m wide x 0.06 m long and 0.0127 m thick were used as a substrate material in thermal spraying experiments. The surface of the substrate was initially roughed with abrasive paper of the number 80 and later the mechanical sandblasting process was used to generate the final roughness. In the mechanical sandblasting process, silicon carbide powder with particle sizes between 180 μm (ASTM 80 sieve) and 212 μm (ASTM 70 sieve) was used. In the mechanical sandblasting procedure, the gun was placed at 0.05 m from the surface of the plates at an angle of 90° and using an exposure time of 15 minutes.

The surface roughness was measured using a portable surf test 402 surface analyzer, Mitutoyo brand. The average roughness measur ed in five zones on the surface of the plates was Ra = 7.5 μm. After mechanical sandblasting, the specimens were cleaned with acetone until the surfaces were completely free of abrasive dust and grease residues.

2.3. Thermal spraying procedure

The pressure of the gases used in the flame spraying equipment were oxygen 0.53 MPa (78 psi), acetylene 0.41 MPa (60 psi) and compressed air 0.48 MPa (70 psi). The feed rate of the rod inside the spray gun was 2.4 cm s^-1.

The study was first conducted by varying the spraying distances from 0.1 m to 0.6 m, keeping the coating thickness constant (300 μm, ±10 μm). Subsequently, coatings with different thicknesses were manufactured from 150 μm to 1200 μm, keeping the spray distance constant at 0.2 m. All coatings were applied without pre-heating the substrate.

2.4. Microstructural analysis

The cross-section of the specimens consisting of substrate - coating as well as the specimens of the filler metal (rod) were prepared by conventional metallographic methods. For roughing abrasive paper was used from grade 600 to 4000, polishing was done with alumina of 0.5 microns in a cloth MicroCloth. The development of the microstructure was conducted with the Keller reagent for 5 seconds.

Microstructural characterization and identification of microconstituents was conducted aided by scanning electron microscopy, SEM, using a Jeol JSM-7600F microscope equipped with energy dispersive X-ray spectrometer (EDS).

2.5. Porosity analysis

The porosity was analyzed in coatings that were detached from smooth substrates to which the mechanical sandblasting process was not applied. The porosity of the coatings was characterized by SEM photomicrographs and estimated aided by the following expression:

$\%P=\left(1-\frac{{\rho }_{coating}}{{\rho }_{alloy}}\right)x100$ (1)

Where:

P = Coating porosity

ρ_coating = Coating density

ρ_alloy = Alloy density

The ρ_coating and ρ_alloy (2.82 g/cm³) densities were obtained by the Archimedes method by using the density device installed in an analytical balance.

2.6. Precipitation heat treatment

The coated steel specimens were heat-treated, using the parameters reported in the literature¹⁰. The solution treatment started out at 520 °C for 1 h, after which water quenching was performed at 20 °C. Subsequently, the artificial aging treatments were conducted at 400 °C for 30 minutes to promote the formation of fine intermetallic compounds in the aluminum coating matrix. The said heat treatments were conducted in a horizontal tubular furnace with an Ar protecting atmosphere (2.5 SLPM).

2.7. Adhesion test

The adhesion testing was performed under the ASTM (C633 -79) standard¹² that consists of finding the maximum load that the coating resists when detached from the substrate by means of a tensile test. The cylindrical test samples were glued together with epoxy resin, as shown in Figure 1. After applying the resin, the specimens were left to dry for 24 h at room temperature 25 °C. Adhesion tests were carried out using a universal tension machine (Instron, model 1125) at a head speed of 0.013 mm s^-1. Adhesion values were obtained by dividing the force applied through uniaxial displacement of the specimen grips attached to the controllable moving crossheads by the surface area of the normalized specimen (0.0254 m diameter); values are given in MPa.

Figure 1
Adhesion test specimens designed for a universal mechanical testing machine.

3. Results

3. 1. Coating porosity

Figure 2 shows the cross section of the specimens where the ASTM A36 steel substrate comprises the bottom grey part of each micrograph while the coatings obtained consist of the juxtaposition of rounded, flattened droplets, which were deformed to an average thickness of 300 μm (±10 μm). Significant changes, however, feature in the size and morphology of the droplets deposited on the substrate as showed in this figure, after the spraying distance was increased from 0.1 m to 0.4 m. At distances between 0.2 m and 0.3 m (Figure 2: b, c) more homogeneous coatings would appear to have resulted, formed by layers of droplets deformed plastically as result of the throwing impact, initially against the steel substrate and subsequently against the newly deposited adjoining layers. At spray distances of 0.1 m and 0.4 m (Figure 2: a, d) less compact coatings resulted. At the shortest distance (0.1 m) between the substrate and the spray gun, the great turbulence produced by the gases flow led to non-uniform deposit of the droplets. Furthermore, the coatings manufactured at distances greater than or equal to 0.4 m displayed a lower plastic deformation of the droplets, generating thus a greater number of voids and insterstices between the layers. This can be attributed to incipient though effective, solidification of droplets during long flight distance. It is expected that the non-uniform deposit of the droplets observed through the thickness of the coatings may also occur at the substrate - deposit interface, which could affect the properties of the coating.

Figure 2
SEM-Micrographs of the cross section of the coatings deposited on the surface of the substrate (ASTM A36 steel) at the spray distances of a) 0.1 m, b) 0.2 m, c) 0.3 m and d) 0.4 m.

The porosity of metal sprayed coatings has been classified according to their sizes and formation mechanisms as type A and type B¹³. The size range for type A is 0.1 to 10 μm and the size range for type B is 10 to 25 μm. The largest number of holes that are seen in the resulting coatings are the type B pores (inter – lamellar pores), which are formed through incomplete filling of the interstices between the layers of solidified droplets, producing thereby this sort of pores with an irregular morphology. Also, small spherodial pores type A are seen, formed by the interaction between the gaseous medium and the liquid droplets. The highest amount of type A porosity was obtained in the coating sprayed at the distance of 0.1 m. The least amount of type B pores was obtained in the coating sprayed at a distance of 0.2 m with average thicknesses of 5.77 μm (±1 μm). The size and amount of this type of porosity increased with spraying distance.

Inasmuch as the presence of the two types of porosity (A and B) affect the degree of compaction and the quality of the coatings, it is necessary though to minimize it fully. Table 1 presents the size and porosity percentage of the coatings obtained at different flame spraying distances. The porosity percentage was calculated with the Equation (1), for which both the density of the alloy and the coatings were experimentally measured.

Table 1
Size and percentage of porosity of coatings manufactured at different spray distances.

Spray distance (m)	Pore size				Alloy density (g cm-3)	Coating density (g cm-3)	Porosity (%)
	Type A		Type B
	Diameter (μm)	Area (μm2)	Thickness (μm)	Area (μm2)
0.1	1.92	1.678	9.33	193.87	2.82	2.36	16
0.2	1.64	1.457	5.77	150.9	2.82	2.56	9
0.3	1.22	1.021	10.83	239.31	2.82	2.48	12
0.4	0.951	0.894	11.86	262.61	2.82	2.42	14
0.5	0.91	0.850	11.97	284.33	2.82	2.38	15
0.6	0.88	0.80	12.22	303.2	2.82	2.31	18

The lowest percentage of porosity (9%) was obtained in the coating sprayed at a distance of 0.2 m. At spray distances smaller and greater than 0.2 m not only there occurred an increase in the size of the pores but also of porosity percentage. The smallest amount of porosity reported in the literature for the flame spraying process is between 8 - 15%, when process conditions such as spray distance, droplet size and deposit rate are optimized¹⁴. Therefore, spray distances between 0.2 and 0.3 m would seem appropriate proper for manufacturing coatings of the Al-Zn-Mg alloy with an acceptable porosity level.

3.2. Characterization of the microstructures

The microstructure of the coating material (Al - 4.2%at Zn - 2%at Mg alloy) solidified in Pyrex tubes is shown in Figure 3 (a - c), where equiaxed grains can be seen, consisting mainly of an α-Al matrix and Al - 8.07Zn- 6.36 Mg type compound (in %at) distributed in the interdendritic regions.

Figure 3
a) SEM-micrograph of the coating material, b) enlargement of the micrograph (a), and c) punctual SEM-EDS analysis on the matrix and in the interdendritic regions.

Figure 4a shows a micrograph of the coating surface produced at a spray distance of 0.2 m. It becomes apparent the overlapping amidst fluid drops that resulted upon arrival to the cooling zone where they deformed them as result of impact. Figure 4b shows the solidification shrinkage of the droplets deposited on the substrate and the B-type holes or pores between solidified droplets.

Figure 4
a) SEM micrograph taken from the surface of the coating of the Al - 4.2%at Zn - 2%at Mg alloy manufactured at 0.2 m spray distance, b) close-up of deposited drops.

Figure 5 (a - b) shows SEM -micrographs of the surface of the sprayed coating at 0.2 m, which was thermally treated (solubilized and artificially aged). It becomes noticieable the fine white particles that can be seen distributed throughout the coating formed by the precipitation heat treatment. Figure 5c presents the punctual microanalysis of the formed compounds; the finest compound (with an average size of 0.43 microns) has a composition of Al-18.24Zn-16.82Mg (in %at), while the coarsest compound has a chemical composition of Al-40.55Zn -23.32Mg (in %at). Based on the Al-Zn-Mg phase diagram, the latter compound corresponds to the phase τ (Mg32 (Al, Zn) 49)¹⁵. The τ compound has an average size of 1.07 microns. The presence of these fine intermetallic compounds is vital for the coating to function as a sacrificial anode for the cathodic protection of steel.

Figure 5
SEM – micrographs of the thermally treated coating taken at different magnifications a) (500X), b) (1000X), and c) compositional analysis of the precipitates.

3.3. Coating adhesion

The tensile test was used to determine the adhesion of the deposit to the substrate. For all the tensile tests conducted, the coating detached completely from the surface of the test specimen. It is worth mentioning that there was no failure through the thickness of the coating or the glue (resin), therefore, the resistance of the substrate - deposit union obtained is of an adhesive nature. The adhesion was evaluated under two conditions: 1) varying the spray distance (at constant thickness (300 μm, ±10 μm)) and 2) varying the thickness of the coating (at a constant spray distance, 0.2 m). In this last condition, both the adhesion of the coating in simple as-spraying condition and that of the heat-treated coating were evaluated.

Table 2 shows the adhesion results as a function of spray distance, where the maximum value of 7.16 MPa was obtained for the spray distance of 0.2 m. In this table, a decreasing behavior of adhesion can be observed at distances smaller and larger than 0.2 m. The slight decrease in adhesion at longer spray distances may be due to the high degree of solidification of the droplets in flight and the decrease in the speed of impact of the drops on the substrate¹⁶. On the other hand, the inhomogeneity of the coating, the presence of high porosity in the substrate-deposit interface obtained at short spraying distances < 0.2 m had a negative effect on the adhesion of the coating.

Table 2
Adhesion as a function of the spray distance of the Al - 4.2%at. Zn - 2% at .Mg coating on A36 steel substrate.

Spray distance (m)	Adhesion (MPa)
0.1	5.84
0.2	7.16
0.3	6.41
0.4	6.12
0.5	5.24
0.6	4.92

Based on the literature, the most likely mechanism of adhesion in this type of coating is mechanical anchoring, where plastically deformed droplets adhere to substrate irregularities by the force resulting from the contraction of the liquid¹⁷. A metallurgical bond is unlikely to form between the aluminum coating and the steel substrate because the high cooling speed of the droplets (about 10⁶ °C s^-1) limits the diffusion between them.

The adhesion values obtained for the different spray distances evaluated are in a range of 4.92 – 7.16 MPa. These values agree with the results presented by RSC Paredes et al.¹⁸. They have reported adhesion values between 9.2 - 11.7 MPa for aluminum coatings (thickness of 386 μm) on a 1020 steel substrate with higher substrate roughness (Ra of 50 - 60 microns) manufactured by the flame spraying process. To increase the adhesion, they suggest preheating the substrate as well as optimizing roughness.

Table 3 shows the results of adhesion as a function of coating thickness in the simple as-spraying and heat treatment conditions. In the coatings obtained after simple as-spraying condition a clear decrease in adhesion can be seen by increasing the thickness from 7.61 MPa to 5.53 MPa for thicknesses of 150 and 1200 μm, respectively.

Table 3
Adhesion curves of coatings in single as-spraying and heat-treated conditions as a function of coating thickness.

Coating thickness (mm)	Adhesion (MPa)
	As -Spraying	Heat treated coating
150	7.61	7.33
300	6.91	6.72
600	6.41	6.15
900	5.82	5.61
1200	5.53	4.96

In previous works, it has been reported that the decrease in adhesion occurs when the thickness of the coatings increases, mainly due to the increase in residual stresses¹⁹,²⁰. Clyne et al.²¹ concluded that the stresses in the coatings can be generated by three factors: 1) tempering of the deposited droplets, 2) joint cooling between the substrate and the deposit, and 3) phase transformations.

The maximum adhesion achieved for heat-treated coatings was 7.33 MPa for the thickness of 150 μm, this value is 3.7% lower than the value obtained for the coating in simple as-sprayed condition. In general, a decrease in the adhesion values of heat-treated coatings can be seen in Table 3 compared to those of simple as-sprayed condition. The difference in adhesion increases up to 11% for coatings with the highest thicknesses (1200 μm).

Several studies have reported that the annealing heat treatment increases the adhesion of the coating due to a decrease in residual stresses¹⁹,²²,²³. In the present work, the decrease in adhesion in the thermally treated coating may be due to the formation of intermetallic compounds (Al-Zn-Mg base). The effect of precipitates on the adhesion of metal coatings has been extensively analyzed by Jong Min Byun et al.²⁴, they found that the decreased adhesion of Zn-Mg coatings on a steel is due to the presence of intermetallic compounds, which are brittle in nature and harden the matrix of the coating metal.

Despite the decrease in adhesion due to the heat treatment conducted to promote the formation of fine compounds, which are essential for the coating to serve as a sacrificial anode, the adhesion values obtained (7.33 - 6.72 MPa) in the thin thicknesses (150 -300 μm) are sufficient to keep deposit bound to the substrate. These values meet the minimums required by the thermal flame projection technique²⁵. Deposit adhesion could be increased by preheating and optimizing the roughness of the substrate.

The results obtained in this study open the possibility of carrying out several future studies, such as evaluating the effect of substrate roughness and preheating temperature to increase adhesion, evaluating the corrosion resistance of steel coated with the Al-4.2%Zn-2%Mg alloy with its respective microstructural characterization using XRD, SEM and TEM techniques.

4. Conclusions

The optimum thermal spray distance for the manufacture of Al-4.2%at Zn-2%at Mg coating on an A36 steel substrate was 0.2 m. At this spray distance, the lowest porosity percentage (9%) and the highest adherence (7.16 MPa) were obtained for one as- sprayed coating with an average thickness of 300 microns.

The increase in thickness from 150 to 1200 microns and the post-heat treatment (precipitation) of the coating caused a decrease in adhesion. Heat-treated coatings with thicknesses of 150 and 300 microns presented adhesion values 7.33 and 6.72 MPa, respectively, which are within the technological acceptable values by the thermal spraying technique. These adhesion values are sufficient to hold the coating together on the surface of A36 steel.

5. Acknowledgments

The authors would like to acknowledge the support of the scholarships provided by CONAHCYT and SIP-IPN. Edgar Luna and Raúl Cruz are also recognized for their technical support.

• Data Availability
  All data supporting the findings of this study have been published within the article itself.

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Edited by

• Associate Editor:
  Ana Sofia de Oliveira.

• Editor-in-Chief:
  Luiz Antonio Pessan.

Data availability

All data supporting the findings of this study have been published within the article itself.

Publication Dates

• Publication in this collection
  13 Mar 2026
• Date of issue
  2026

History

• Received
  05 Sept 2025
• Reviewed
  18 Nov 2025
• Accepted
  17 Dec 2025