Critical Challenges in the Anodizing Process of Aluminium–Silicon Cast Alloys—A Review

Abstract

The microstructure of the substrate plays a crucial role in the anodizing process. Anodizing cast aluminum alloys is quite challenging due to the higher levels of alloying elements present compared to pure aluminum. Elements such as silicon, iron, and copper significantly impact the growth and quality of the anodic layer. Additionally, anodizing parameters such as electrolyte composition, current density, and temperature are critical in determining the morphology and thickness of the anodic film. The casting process, surface condition, and post-treatment also affect the properties of the anodic layer. Optimizing these parameters is essential to achieve a durable and high-quality anodic layer. This work aims to provide a comprehensive understanding of the various factors affecting the anodizing of cast aluminum alloys and the properties of the anodic layer, including its thickness, corrosion resistance, and wear resistance.

1. Introduction

Aluminium and its alloys, known for their lightweight properties, exhibit natural resistance to atmospheric corrosion. This resistance occurs because a protective film forms immediately upon exposure to air. The film is either an oxide (Al₂O₃) or a hydroxide. This natural oxide layer is usually 2.5 to 10 nm thick [1] and provides only limited protection in harsh environments, such as marine coastal areas. To enhance these protective properties, the surface can be further oxidized through thermal, chemical, or electrochemical means. Anodic oxidation, i.e., anodizing, is a widely used method to create a much thicker porous anodic film that acts as a primer layer, significantly improving the corrosion resistance of aluminium alloys and enhancing the adhesion strength of additional surface coatings [2]. Despite the challenges associated with anodizing Al-Si alloys, this process is still utilized to enhance corrosion and wear resistance, especially for severe environments, and to improve surface hardness.

Anodizing is an electrochemical process that uses an electrical current to induce the growth of an aluminum oxide layer on an aluminum surface. This layer is created through the reaction of the aluminum anode with the electrolyte, causing a continuous consumption of the aluminum anode. The microstructure and composition of the alloy strongly influence the continued growth of the anodic layer [3,4]. During anodizing, the main element that dissolves in this electrochemical surface treatment is the base metal, which is aluminum. Practically, all aluminum alloys can be surface oxidized through this process. However, practical experience indicates that the anodization process is influenced by the presence of certain alloying elements, making it more challenging when the alloy has high amounts of these elements. Therefore, it is interesting to note that cast alloys are significantly more challenging to anodize compared to high-quality wrought alloys [4].

The anodization of pure aluminum in aqueous solutions involves the movement of Al³⁺ cations and either O²⁻ or OH⁻ anions [4,5,6]. At the interface between the aluminum and oxide, aluminum oxidation occurs, leading to the generation of Al³⁺ cations. Simultaneously, at the interface between the oxide and solution, either O²⁻ or OH⁻ anions form as H⁺ ions are removed from H₂O molecules [4].

The progression of the anodizing process, resulting in the formation of a porous oxide layer, can be succinctly outlined as follows:

• Initially, aluminum cations (Al³⁺) are generated from the aluminum substrate acting as the anode.
• In the presence of a strong electric field, aluminum cations undergo migration toward the cathode, while anions present in the aqueous solution (such as O²⁻, OH⁻, and electrolyte anions) move in the opposite direction. At the interfaces of the metal/oxide and oxide/electrolyte, the Al³⁺ cations react with the anions, giving rise to the formation of aluminum oxide (Al₂O₃).
  2Al³⁺ + 3O²⁻ = Al₂O₃ (at the metal/oxide interface)
  2Al³⁺ + 3H₂O = Al₂O₃ + 6H⁺ (at the oxide/electrolyte interface)
• Simultaneously, at the oxide/electrolyte interface, the aluminum oxide has the potential to dissolve within the electrolyte, resulting in the creation of a porous layer. The following equation governs this chemical dissolution process:
  Al₂O₃ + 6H⁺ = 2Al³⁺ + 3H₂O

Consequently, a meticulously organized hexagonal cellular structure, characterized by self-assembly, is formed. Each cell in this structure is enclosed at its base and has a central pore extending from the base to the apex. The oxide structure can be divided into two distinct regions: the barrier layer at the base and the porous layer (see Figure 1). The thickness of the cell walls, which is equal to that of the base, is determined by the electrolysis parameters, specifically the applied voltage and current density. In contrast, the expansion of the porous layer, including the size of the hexagonal cells and the internal pores, depends on various anodizing parameters such as the electrolyte type, current density, and anodizing duration [7]. The careful selection of anodizing parameters dramatically influences the performance of an anodized component. The initial microstructure of the alloy also plays a significant role in this performance. Additionally, any pre- and post-anodizing treatments can have a notable impact [4].

When anodizing is carried out in electrolytes with a neutral pH and low reactivity toward the anodic film, a specific type of oxide film known as a barrier-type anodic film is formed. This occurs, for instance, in borate or tartrate solutions where the resulting oxide is insoluble [8]. Under these conditions, there is no loss of Al³⁺ cations into the electrolyte, enabling the growth of barrier oxides with high current efficiencies, reaching close to 100% [9]. Current efficiency is determined by calculating the ratio of the current utilized for oxide formation to the total current applied throughout the process [10]. During the process of barrier anodizing, the growth of the oxide layer transpires at both the aluminum/oxide interface and the oxide/electrolyte interface. Through investigation, it has been determined that approximately 60% of the total oxide growth occurs at the aluminum/oxide interface, while the remaining 40% of the film thickness is formed at the oxide/electrolyte interface [11], as visually represented in Figure 2. The film continues to grow until its resistance impedes the flow of current to the anode [12]. At this juncture, the barrier-type anodic film experiences a breakdown in dielectric properties [6].

The thickness of the barrier layer is directly proportional to the applied voltage, and once established, it remains constant throughout the anodizing process [8]. Despite its thinness, the barrier layer allows for continuous ionic flow [6]. This continuous ionic flow, in conjunction with the electrolyte’s accessibility and subsequent current flow to the oxide/metal interface via the porous structure, facilitates ongoing film growth [12]. It is important to note that continuous film growth does not imply a constant increase in film thickness. The rate of film growth gradually diminishes as electrical resistance escalates with the accumulation of film thickness [12,13]. When film growth occurs at the same rate as film dissolution, the film maintains a constant thickness. This phenomenon enables the creation of a much thicker porous anodic oxide layer compared to the barrier oxide layer. This porous layer can reach typical thicknesses of several hundred microns.

During the formation of a porous anodic film, Al³⁺ cations do not undergo a reaction with O²⁻ anions at the oxide/electrolyte interface. Instead, they are expelled into the electrolyte through field-assisted dissolution or field-assisted direct anion ejection mechanisms [6,14]. As a result, the growth of the oxide exclusively occurs at the oxide/metal interface. The overall process efficiency is approximately 60% since approximately 40% of the Al³⁺ cations are lost into the electrolyte [14,15]. As the film growth progresses, the formed oxide is pushed away from the oxide/metal interface. Consequently, the outer portion of the film, which corresponds to the oxide generated during the initial stages of the process, remains in contact with the electrolyte throughout the anodizing duration. This scenario can result in significant chemical attack on the outer part of the film [12,13]. This attack leads to the thinning of pore walls and the widening of pore openings.

In the development of Al-Si alloys, the surface’s suitability for anodization is often overlooked. The main focus is on creating cast alloys that meet mechanical requirements like strength, hardness, and resistance to wear and fatigue while considering factors such as castability and structural integrity. Alloys are engineered to improve fluidity, minimize gas entrapment during melting, and make the casting process smoother upon solidification [3]. The mechanical properties heavily rely on the quality of the casting, so alloy formulation and processing methods are adjusted to reduce porosity and enhance surface quality. The mechanical attributes of aluminum castings, such as strength, hardness, and resistance to wear and fatigue, are primarily achieved through two metallurgical mechanisms: solid solution hardening and precipitation hardening. The evolution of microstructure, influenced by alloy composition, cooling rates, and tempering procedures, plays a crucial role in achieving these properties [4]. The microstructure of castings not only determines the component quality but also significantly impacts the quality of the resulting anodic oxide layer. Due to their higher alloy content, castings often have more complex surfaces with reduced free aluminum compared to wrought alloys. This difference leads to a broader range of surface chemical potentials, which poses challenges for the anodization process. The response to anodization is influenced by the composition, casting technique, and casting quality. Therefore, achieving a casting with a uniform microstructure and fine particle size becomes crucial when aiming for a specific anodized finish for a particular application [16,17,18].

Recently, certain aluminum–silicon alloys that are eutectic or near-eutectic, for example AlSi12 and AlSi10Mg, are becoming more favorable for manufacturing aluminum alloy products [19,20]. However, these Al-Si alloys have limitations due to their relatively low hardness and inadequate corrosion and abrasion resistance properties [21,22]. Consequently, various surface technologies, including anodizing [23], laser remelting [24], plasma electrolytic oxidation (PEO) [25], and cathode plasma electrolytic deposition [26], have been employed to address these challenges. Scampone et al. [27] provided a thorough examination of the chemical and microstructural factors affecting the anodizing process, while this review focuses more on the challenges and potential solutions through the optimization of anodizing parameters and post-treatment processes. Also, this review discusses the impact of secondary phase particles, casting processes, and anodizing parameters on anodic film formation. It emphasizes the role of microstructural consistency, alloy composition, and controlled anodizing conditions in improving the anodic layer quality. Additionally, post-treatment processes like hydrothermal sealing and plasma electrolytic oxidation (PEO) were analyzed.

2. Effect of the Alloy Second-Phase Particles

The anodization process of as-cast products can be affected by the structure and phases present in the substrate. Eutectic Si and intermetallic compounds, such as Mg₂Si, $\beta$-Al₅FeSi, $\alpha$–Al(Fe, Mn, Cr)Si, and Al₂Cu phases, prove detrimental to the anodization process [28,29,30,31]. Local changes in the composition and structure of the boundary between the bulk material and the oxide occur due to these secondary phases, affecting the thickness of the anodic layer [9]. Also, some phases might decrease the hardness and thickness of the anodic layer [28,30]. The primary components of the as-cast microstructure in Al-Si alloys consist of large grains of primary $\alpha$-Al, eutectic Si, coarse primary Si, and other harmful intermetallic phases such as needle-like $\beta$-Al₅FeSi. Furthermore, these alloys display uncontrolled and unevenly distributed porosities [32,33,34]. The phase precipitation sequence in hypoeutectic Al-Si alloys is characterized by the following specifics at various temperatures:

• At 650 °C, Primary Al₁₅(Mn, Fe)₃Si₂ (commonly referred to as sludge) precipitates.
• At 600 °C, Al₁₅(Mn, Fe)₃Si₂ and/or Al₅FeSi precipitate.
• At 550 °C, eutectic Al+Si, Al₅FeSi, and Mg₅Si precipitate.
• At 500°C, Al2_22Cu and more complex phases precipitate [35].

The interdendritic regions and grain boundaries may contain intermetallic compounds, depending on the alloy’s chemical composition and the speed at which it solidifies [36]. The introduction of alloying elements into solid solution generally does not noticeably impact the anodizing behavior of Al alloys. Nonetheless, the creation of precipitates or intermetallic particles within the $\alpha$-Al matrix or at grain boundaries may undermine the integrity of the oxide layer [37]. Intermetallic phases possessing redox standard potentials more positive than the $\alpha$-Al matrix demonstrate slower oxidation rates and remain as unanodized particles following the anodization process. Conversely, intermetallic compounds with higher oxidation energies dissolve entirely during anodization, forming extra porosities within the oxide layer [4].

2.1. Silicon Particles

During the anodization process, eutectic silicon particles may lead to the forming of several defects within the anodic layer such as the following:

• Formation of oxygen gas-filled voids: When the oxide front comes into contact with the silicon phase, it causes the creation of both SiO₂ and gaseous oxygen because of the semiconductor characteristics of the Si-O bond. As a result, voids filled with oxygen gas appear near the Si particles [38,39,40], as depicted in Figure 3.
• Formation of un-anodized regions: Un-anodized regions form when the oxide layer does not completely surround the Si phase, potentially due to its morphology or reduced particle spacing. As a result, the eutectic silicon phase functions as a barrier, protecting the nearby Al matrix from the oxide layer and thereby preserving it in an un-anodized state. Residual metallic Al phases are primarily observed under or among large, interconnected Si eutectic particles [39,40].
• Influence on film cracking and intrinsic stress: The volumetric expansion of the oxidizing matrix can be impeded by silicon particles, causing the development of internal stress in localized areas and resulting in cracks forming within the film [39,41].

Fratila-Apachitei et al. [42] conducted hard anodization on an AlSi10 alloy under the process conditions (12% H₂SO₄, 30.42 mA·cm⁻², 3000 s). Through SEM and TEM analysis of the anodized samples, they discovered that second-phase particles in the substrate significantly influence the microstructure of the anodic alumina film. To modify the silicon particles in the alloy, they introduced strontium, resulting in well-modified particles measuring less than 5 µm and partially modified particles ranging 5–20 µm. When the oxide encounters a well-modified silicon particle, it envelops the particle completely, leading to defect-free film formation. However, in the case of partially modified silicon particles, the oxide attempts to penetrate them, resulting in a non-uniform thickness of the anodic alumina film [42]. This unevenness is attributed to the lower anodization rate of silicon compared to the aluminum matrix, which causes the anodic alumina film to encroach beneath the silicon particle and partially envelop it within the film [38,40], as seen in Figure 3. Additionally, larger unaltered silicon particles were observed, displaying the presence of oxygen species. This suggests the development of silica (SiO₂) accompanied by oxygen generation, creating oxygen gas-filled voids above the oxidizing particles [38]. The size and shape of these cavities depend on the particle morphology. Throughout the thickness of the anodic alumina film, cavities are present, indicating the distribution of silicon particles in the alloy [38,40]. Moreover, the presence of these particles influences the morphology of the anodic alumina film, with pore termination occurring above the particle and pore branching and deflection around and beneath the particle [38]. While fine silicon particles readily become enclosed in the film, coarse particles require prolonged anodizing times for adequate occlusion [38]. To address this situation, Fratila-Apachitei et al. [42] concluded that achieving a fine cell structure and well-modified silicon particle morphology is crucial for growing anodic alumina films with minimal defects and uniform thickness.

Li et al. [43] investigated the corrosion protection properties of anodic alumina films on an Al-Si alloy with details of the hard anodizing process conditions (17% H₂SO₄, 16 mA·cm⁻², 900 s). The film surface exhibited small cracks, which were believed to be primarily caused by internal stress generated during oxide growth at the substrate/oxide interface [41]. Within the anodic alumina film, needle-like iron-rich areas and silicon-rich areas were detected. The presence of iron-rich areas confirmed the survival of iron-bearing intermetallic particles during the hard anodizing process, locally inhibiting the growth of the anodic film [44]. In the vicinity of silicon-rich areas, the film displayed cracking due to alumina growth around the silicon particles, resulting in localized high stress within the film [41].

Aluminum in the eutectic phase is reported to exhibit primarily the same crystallographic characteristics as the primary $\alpha$-Al dendrites found in unmodified alloys [45]. Heterogeneous nucleation is the primary method for grain refinement, where grains nucleate on foreign nuclei sites and grow slowly within the melt. Effective grain refiners, such as TiAl₃ and TiB₂, must have lattice structures that are perfectly coherent with the aluminum matrix to be effective. Conversely, particles with poor lattice matching have minimal impact on enhancing grain nucleation, leading to an unrefined grain structure [46]. Examples of the microstructures of unmodified, Sr-modified, and Sb-modified alloys are illustrated in Figure 4. Additives in the alloy can improve its ability to form a passive oxide layer, as demonstrated in a recent study by Shan-Liang Xu et al. [47]. The addition of 0.03 wt% of Boron to an Al–Si–Mg alloy encourages the formation of a thicker and more uniform passive film while also boosting the tensile strength of the base alloy [47]. Examining the anodizing behavior of these new alloys can offer valuable insights into the impact of these additives on the anodic layer film of Al-Si alloys.

Zhu et al. [39] investigated how the morphology of unmodified and Sr-modified eutectic silicon particles influences the anodizing behaviour of Al-Si alloys. They analyzed different silicon content levels, ranging from 2.4 to 5.5 wt%. Silicon particles in polygonal flake shapes were observed in the untreated alloys, creating a connected branched network [48]. These entities are depicted in Figure 5. During the anodizing process, the anodic oxide front expanded between the silicon particles. However, due to the limited space between the particles, a significant portion of the aluminum matrix remained unanodizing. Additionally, substantial localized intrinsic stresses were induced by the volumetric expansion of Al₂O₃, leading to the formation of cracks and voids [38,49], as depicted in Figure 5.

Razzouk et al. [40] found a direct link between the formation of oxide and the existence of alloying elements, particularly silicon particles. The dimensions and arrangement of these particles affect the regions of aluminum that are not anodized. The unanodized regions of aluminum surrounded by silicon particles are depicted in Figure 6 along with the cavities associated with silicon particles.

Zhu et al. [50] studied the resistance of corrosion between the aluminum matrix and eutectic silicon particles. They subjected anodized AlSi7Mg0.4 samples to a corrosion test by immersing them in a 3 wt% NaCl solution for 72 h. The interaction between the aluminum and silicon phases resulted in the formation of a micro-galvanic cell, causing the anodic layer to corrode and leading to the creation of corrosion pits on the Al-Si interface. An increase in the defect content within the oxide layer was associated with a more intensified galvanic corrosion underneath the oxide surface [51,52].

2.2. Iron-Rich Intermetallic Particles

Iron is frequently present in Al-Si alloys due to the recycling and casting processes. The presence of Fe is beneficial in preventing die soldering during high-pressure die casting (HPDC). However, Fe is considered an undesirable alloying element because it forms Fe-rich intermetallics like plate-like $\beta$-Al₅FeSi particles, leading to reduced ductility and corrosion resistance [34,53]. According to the chemical composition of the iron-rich intermetallic compounds, they may undergo partial or complete oxidation during the anodizing process [54]. Furthermore, they have the ability to be partially incorporated into the anodic layer, resulting in a wavy boundary with the unanodized aluminum substrate [42]. Specifically, the Al₁₂(FeMn)₃Si and Al₆Fe phases impede the progress of the anodic layer growth due to their higher oxidation energy [44]. However, L.E. Fratila-Apachitei et al. [42] and K. Shimizu et al. [55] reveal that the Al₃Fe particles oxidized at the same rate as the aluminum matrix and could not be entrapped in the oxide layer. During anodizing, the partial or complete dissolution of iron-rich intermetallics creates voids and imperfections within the oxide layer [50,56,57,58]. Iron-rich intermetallic compounds create a galvanic cell with the adjacent Al-Si eutectic, thereby enhancing and exacerbating pitting corrosion beneath the oxide layer [50,56]. Figure 7 shows the corrosion pits on the anodized Al7Si0.4 Mg samples. Some samples were ground to remove 20 µm of material from the surface before the anodizing process, while others were anodized without grinding, leaving the surface as-cast. The corrosion attack seems to have penetrated the oxide layer and was concentrated in the eutectic region, where Fe-rich intermetallic compounds are present within the Al–Si matrix. Removing the casting skin by mechanical grinding reduces the Fe content and Fe-rich intermetallics on the casting surface, leading to an improved oxide layer quality [50].

2.3. Copper-Rich Intermetallic Particles

Copper forms an intermetallic phase with aluminum, which precipitates during solidification as either blocky CuAl₂ or as alternating lamellae of $\alpha$-Al + CuAl₂ [59]. In the presence of iron during solidification, other copper-containing phases can form, such as $\beta$-Al₅FeSi [60]. The CuAl₂ phase may appear as blocky forms or finely scattered $\alpha$-Al and CuAl₂ particles within the interdendritic areas.

The alloy’s high copper content makes it challenging to anodize cast aluminum alloys [61], because the anodizing process generates oxygen when the oxide film grows, leading to film cracking when thick porous films are created [62]. The anodizing behavior of copper-rich compounds is influenced by their chemical composition [27]. Fratila-Apachitei et al. [42] conducted a study on the anodizing response of the Al₂Cu phase in an AlSi10Cu3 alloy. This phase was observed as large globular compounds or irregular particles ranging from 3 to 20 µm. As the anodic front approached a particle rich in copper, the current distribution favored the copper phase because of its lower oxidation energy compared to the surrounding aluminum matrix. Consequently, this resulted in the development of a preferred path for oxide growth inside the particle, which persisted until complete oxidation of the copper-rich compound occurred [42]. Additionally, the Cu-O bond exhibits semiconducting properties, resulting in the generation of gaseous oxygen during the oxidation reaction [35,38]. If the gas pressure becomes sufficient, film cracking may occur [38,63]. Another copper-rich phase that exhibits anodic behavior relative to the Al matrix is the S-phase (Al₂CuMg) [64]. In contrast, intermetallic phases like Al₇Cu₂Fe exhibit cathodic behavior, accelerating the oxidation of the adjacent aluminum matrix. Additionally, these compounds facilitate oxygen reduction reactions, leading to the dissolution of the neighboring aluminum phase, which is a phenomenon referred to as “trenching” [65,66]. The various behaviors of copper-rich intermetallic particles are depicted in Figure 3.

3. Influence of Processing Prior to the Anodizing Process

3.1. Casting Process

The formation of the oxide layer is linked to the casting method used. Labisz et al. [67] compared the anodizing process of Al-Si-based alloys produced via pressure die casting and sand casting, and they found that the anodized layer is thicker for the sand-cast materials compared to those manufactured by pressure die casting. Ridder et al. [49] examined anodized aluminum surfaces from four different manufacturing methods—extruding, permanent mold casting, sand casting, and high-pressure die casting (HPDC). The aluminum foundry alloys used were AlSi7Mg for sand casting and permanent mold casting, AlSi1Mg for extrusion, and AlSi9Cu3 for high-pressure die casting, which were chosen for their typicality in their respective production processes. The research indicated that the uniformity of the oxide layer on the extruded sample could be attributed to the alloy’s low silicon content and the specific fabrication method. In contrast, materials produced through casting processes exhibited oxide layers of non-uniform thickness on their surfaces. The HPDC sample showed the lowest average thickness and the highest variation due to the rapid cooling associated with the HPDC process. This is illustrated in Figure 8, where the samples were embedded in resin to facilitate cross-sectional study.

3.2. Machining Operations

The industry widely uses surface machining operations to cast products to achieve the necessary dimensions. The effect of machining operations on the anodizing process has been investigated in various research studies involving hypoeutectic Al-Si alloys cast using rheocasting and die-casting methods, owing to the alloy’s heterogeneous microstructure and potential defects [68,69,70].

One advantage of the milling process was removing the oxide skin naturally formed during high-pressure die casting (HPDC) at relatively high temperatures. When die castings are extracted from the die at around 300 °C, a thicker surface oxide skin is formed compared to the one formed naturally at room temperature [71]. This oxide skin has a passivation effect that negatively affects the thickness of the anodic oxide layer [71]. Before anodizing, chemical treatments are utilized to eliminate impurities from the surface of the casting, including oil, grime, and the oxide layer. Nevertheless, alkaline cleaners might remove the aluminum phase from the surface, revealing underlying intermetallic compounds [72].

Caliari et al. [68] applied machining operation prior to the anodizing process for several die-cast Al-Si alloys. They found that the milling operation before the anodizing process enabled the removal of surface segregations and consequently exposed surfaces with lower eutectic fraction and coarser Si particles. The increased thickness of the anodic layer of AlSi12Cu1(Fe) alloy primarily resulted from eliminating the existing oxide layer and the surface liquid segregation (SLS) layer [68], as shown in Figure 9.

The anodic layer thickness is closely related to the aluminum phase. When the skin layer is ground and areas with a slower solidification rate are reached, it results in the formation of a substantial, uniform aluminum phase. This was mentioned by Razzouk et al. [40] in their study, where they removed varying amounts of material from the surface of the AlSi12Cu1(Fe) cast alloy before applying an anodizing process under galvanostatic and potentiostatic modes. Figure 10 shows an example of the differences between the regions containing a high amount of silicon compared with the aluminum phase regions, indicating that this alloy has a heterogeneous structure.

A thicker oxide film is not always harder than a thinner oxide film [73]. Yerokhin et al. [74,75] indicated that thin oxide films are better at reducing friction and improving scratch resistance. Furthermore, Fratila-Apachitei et al. [76] discovered that thicker oxide films are more likely to experience significant collapse when subjected to contact pressure. Caliari et al. [68] observed that the preliminary machined substrate reduced the abrasion resistance of the subsequently formed oxide film compared to the as die-cast anodized surface (Figure 9). This change in behavior can be attributed to several factors: Aluminum oxide exists in several grades and allotropic forms, each exhibiting specific physical properties [77]. The hardness of pure alumina ranges from approximately 560 to 2200 HV. However, the aluminum oxide formed through anodizing can exhibit lower hardness values [78]. Makhlouf et al. [79] reported that the hardness of silicon particles measured 1200 ± 120 HV, which is higher than that of the aluminum oxide formed after anodizing. Consequently, it is expected that the eutectic silicon particles embedded in the anodic layer act as reinforcing particles, significantly influencing abrasion behavior. Gillin et al. [75] investigated the abrasion behavior of hard-anodic oxide films obtained from a gravity-cast AlSi7Mg0.3 alloy. They found that the excellent abrasion resistance of the films was partly due to free silicon being entrapped in the oxide film. It is generally recognized that substrates reinforced with coarse particles experience greater wear loss compared to those with fine particles [80]. This behavior accounts for the lower wear resistance observed in the anodic layer grown on milled surfaces. Additionally, coarser eutectic silicon particles are typically detrimental to anodic oxide growth [71], leading to the development of porosity upon oxidation. This characteristic contributes to the overall reduction in abrasion resistance of milled and hard anodized surfaces.

The impact of sandblasting operations on the anodizing process of die-cast surfaces was also examined [68]. Typically, this method produces positive outcomes for anodizing by removing the protective surface layer (skin layer) that is created when the casting is removed from the die at temperatures ranging from 200 to 300 °C [81]. However, the growth of the oxide layer is significantly influenced by the surface microstructure that is revealed after sandblasting. Sandblasting procedures can either promote or impede the growth of the anodic layer depending on the chemical composition of the alloy and the morphology of the compounds. In Al-Si alloys with high copper levels, such as AlSi9Cu3(Fe) alloys, the sandblasted surface’s microstructure looks similar to that of the as die-cast surface, primarily due to the high copper content, which promotes the refinement of silicon particles. As a result, sandblasting processes have a positive effect on the anodizing process by eliminating the skin layer and assisting in forming a thicker anodic layer. Conversely, in Al-Si alloys with lower copper content, such as AlSi11Cu2(Fe) and AlSi12Cu1(Fe) alloys, the microstructure of sandblasted substrates is more heterogeneous, with larger silicon particles compared to the as die-cast surface. As a result, the sandblasted substrate’s anodizing response is poor, which negates the benefits of skin layer removal [68].

4. Influence of Anodizing Parameters

The anodizing process requires specific conditions, which depend on the intended application of the metal part and the desired properties of the anodic film [82]. To ensure successful anodizing, it is important to determine the best conditions, including the electrolyte solution, temperature, and potential or current density.

4.1. Electrolyte

The role of electrolytes is closely related to their composition and significantly influences whether the newly formed oxide layer is porous or a barrier. Electrolytes do not dissolve barrier oxides, or their dissolution rate is considerably slower than their deposition rate. Additionally, barrier layers tend to develop in the presence of non-acidic electrolytes, leading to the formation of an amorphous protective layer over the aluminum substrate surface [83]. A thin and dielectric compact film is produced when using an electrolytes solution like borate [84].

Table 1. Some inert electrolytes used in the production of barrier layers [83].

Non-Acid	Chemical Formula	Conc., (M)	pH
Ammonium Adipate	NH4OCO	150 g/L	6.4
	(CH2)4COONH4
Sodium Borate	Na2B4O7	2.2	7
Sodium Chromate	Na2CrO4	0.1	10
Sodium Hydrogen Phosphate	Na2HPO4	0.1	9.4
Sodium Hydroxide	NaOH	0.01, 0.03 & 0.1	Not specified
Sodium Sulfate	Na2SO4	0.1	5.8

contains a compilation of frequently utilized non-acidic electrolytes [85].

For the production of nanopores ranging in size from approximately 10 to 240 nm, researchers typically use oxalic, phosphoric, and sulfuric acids (see

Table 2. Voltages and durations for three commonly used electrolytes to create a porous oxide layer on an aluminum base [83].

Acid	Conc. (M)	Voltage (Volts)	Pore Size (nm)	Time (Hours)
	0.25	60	75	8.8
	0.3	40	Not specified	Variable
	0.3	40	80	8, Variable
	0.3	40	50	10.5 min
	0.3	60	80	3.8
Oxalic	0.3	40	40–50	40 min, 2
	0.3	40, 50	20, 35	Variable
	0.3	30	40	8, 10
	0.4	40	50	8, 10
	0.5	50	80	8, 10
	0.3	40	22	12, 4, 8, 12 and 16
	Not specified	195	200	variable
	0.4	5 to 40	20 to 75	1 step/variable
	0.4	80	80	1 step
Phosphoric	0.42	87 to 117	64 to 79	1 step/Variable
	0.5	18	70	4, variable
	2.4	15 to 25	13 to 27	2-step/variable
Sulfuric	Not specified	12, 25, 40	25, 50, 100	Not specified
	0.3	25	20	12, 4, 8, 12 and 16

). The anodization voltage is crucial, as it is limited by the specific acid electrolyte and its concentration. Excessively high anodization voltage can cause the oxide barrier layer to burn, leading to non-uniform pore development. These three primary causes can account for this phenomenon: localized heating during production, increased conductivity in the oxide barrier layer at the pore tips, and the ionization of atoms, resulting in the production of additional electrons due to the electric field’s energy. Finally, the breakdown of the oxide barrier layer occurs due to pre-existing fissures [83,84].

The order of conductivity for the most commonly used acid electrolytes in aluminum anodization is H₂SO₄ > (COOH)₂ > H₃PO₄. Consequently, the anodization voltage ranges from 5 to 40 V for sulfuric acid (H₂SO₄), 30 to 140 V for oxalic acid ((COOH)₂), and 80 to 200 V for phosphoric acid (H₃PO₄) [86].

The variance in the features of the oxide layer produced under varying electrolyte conditions has significant ramifications for the surface characteristics and uses of anodized Al-Si alloys. [27]. Chiang et al. [87] conducted a study on the hardness and abrasion resistance of a hypereutectic Al-Si alloy following pulse anodization at 0 °C. Three different electrolyte compositions were utilized: sulfuric acid, a combination of sulfuric and oxalic acids, and a mixed solution of sulfuric and oxalic acids with aluminum sulfate dissolved in it. The findings indicated that incorporating oxalic acid and dissolved aluminum sulfate into the sulfuric acid electrolyte resulted in a gradual increase in oxide thickness and enhanced mechanical properties. Throughout the anodization process, aluminum ions from the aluminum sulfate separate in the electrolyte, improving conduction efficiency. This leads to the development of a thick and refined oxide layer, ultimately resulting in improved hardness and abrasion resistance.

Romdhane et al. [88] performed a study on the process of anodizing AS12 aluminum alloy using different alkaline electrolytes. The AS12 alloy behaves differently during anodization compared to pure aluminum because of its silicon content. To achieve the micro-arc oxidation (MAO) regime for AS12 alloy, high current densities are typically needed. However, the addition of fluorides and silicates allows the MAO regime to be reached at lower current densities, making the process more practical and energy-efficient. When anodizing with a stable current, introducing KF leads to the creation of a thicker oxide layer because the fluorides in the electrolyte accelerate the start of the micro-arc regime. Furthermore, incorporating silicates in the electrolytic solution slightly improves corrosion resistance by acting as sealing/healing agents [88]. High levels of additives could have a negative effect on the anodic properties. A study by Shang et al. [89] showed that an excessively high amount of additives in the electrolyte can negatively affect the quality of the oxide layer in high-silicon aluminum alloy (Al-12.7Si-0.7Mg). The study investigated the effects of three different organic acids added to the electrolytes for anodic oxidation while keeping the sulfuric acid concentration, anodization temperature, anodization time, and current density constant. The findings showed that when organic acids are introduced into the sulfuric acid electrolyte, there is a notable decrease in both the weight and corrosion resistance of the anodic layer. Nevertheless, a well-proportioned mixture of additives enhanced the surface characteristics of the oxide film [89]. The mechanical properties and oxide layer thickness are affected by electrolyte concentration and temperature. Gastón-García et al. [63] performed anodization on an AlSi9Cu3(Fe)(Zn) alloy at 0 °C using sulfuric acid electrolytes with concentrations of 5% and 15% by volume. Regardless of the current density, there were no notable variations in the thickness of the oxide layer when the acid concentration was altered. However, the anodic layer formed at the lower concentration displayed reduced of cracks and defects, resulting in a slightly higher microhardness and enhanced resistance to abrasion. The similar thicknesses of the two oxide layers indicated that the efficiency of the anodizing process was alike for both electrolyte concentrations.

4.2. Anodizing Duration and Electrolyte Temperature

In addition to the electrolyte composition and concentration, temperature and anodizing duration play important roles in achieving the desired properties of the anodic layer produced. Anodizing time impacts the microstructure and immersion stability of anodized aluminum alloys [90]. An increased anodizing time can lead to a decrease in the layer quality. This occurs due to the heat effects that elevate the temperature of the electrolyte and increase its reactivity. As a result, the dissolution rate rises, causing the development of imperfections and porosity on the outer surface of the anodic layer, thereby diminishing its hardness and wear resistance. Multiple research studies have corroborated this process [23,36]. Zhu et al. [23] studied the influence of anodizing time on a Sr-modified AlSi7Mg alloy. The oxide layer thickness grew as the anodizing time elapsed, but the hardness decreased after 30 minutes. The increasing duration of anodizing led to the creation of heating effects at the interface of oxide and electrolyte, resulting in a coarser local porous structure and reduced mechanical properties. The nano-hardness of the anodic layer generally decreases from the oxide–metal interface to the electrolytic–oxide interface. Longer anodizing duration results in a thicker but more stressed oxide layer, and these inherent stresses are released during the growth of the anodic layer, leading to the formation of more cracks and porosities embedded in the anodic layer, consequently decreasing the corrosion resistance.

When the electrolyte temperature decreases, the electrolyte’s reactivity also decreases, which leads to a slower dissolution rate of the anodic layer. Moreover, the reduction in local heating on the oxide surface leads to an oxide layer that is less porous but has better mechanical properties [91]. Caliari et al. [92] studied how the anodizing process of a machined AlSiCu(Fe) alloy substrate is affected by the temperature of the electrolyte below 0 °C. Irrespective of the chemical composition of the alloy, reducing the anodizing temperature from 0 to −4 °C resulted in the formation of a thinner anodic layer. At −4 °C, when subjected to a current density of 2.5 A·dm⁻², increased resistance to current flow prevented the thickening of the anodic layer, leading to a less effective anodizing response.

4.3. Voltage and Current

Galvanostatic anodizing is utilized to investigate the effect of electrical current on the anodizing response of the substrate. Applying an electrical current is part of this method, and the voltage differs depending on the system. Galvanostatic anodizing is especially beneficial for examining the electrical behavior resulting from the development of fractures and defects within the formed oxide. In particular, it is possible to compare the voltage changes over time with the electrical properties of pure anodized aluminum. Voids filled with oxygen gas hinder the ionic transportation process, leading to an increase in the substrate’s resistance [41,63]. The rise in resistivity results in a higher recorded voltage. Consequently, on the voltage–time graph, a higher curve for Al alloy compared to pure Al indicates the development of voids within the anodic layer [40]. Voltage fluctuations in the steady-state region of the voltage–time graph correspond to the formation of cracks in the anodic layer when it cracks and re-anodizes due to high levels of intrinsic stresses [63,93,94]. Fratila-Apachitei et al. [41,76,94] utilized this method to examine how the oxide’s growth evolved under varying anodizing conditions. They examined how the applied current density impacted the development of the anodic layer and observed that higher current density led to the formation of a thicker anodic layer. [41]. The oxide surface experiences heightened local heating effects, leading to increased porosities caused by a greater rate of thermal enhanced dissolution [63,91]. Therefore, the anodic layer is thicker but has low hardness. The effect of the current waveform on the hard anodization of Al-Si cast alloy such as AlSi10 and AlSi10Cu3 was studied. When comparing the direct current to pulse current in various waveforms, no substantial changes in the thickness of the oxide and microhardness were observed [76]. When using potentiostatic anodizing, the applied voltage influenced the barrier layer and the development of the porous layer [4]. As the applied voltage rises, the oxide layer becomes thicker because of the increased flow of electrical current [23]. This leads to a higher current density and, as a result, an accelerated oxide growth rate [95].

Razzouk et al. [40] investigated the effects of anodizing under galvanostatic and potentiostatic modes on the thickness and structure of the anodized AlSi12Cu1(Fe) die-cast alloy. The anodic layers were created in an electrolyte containing 181.1 g·L⁻¹ sulfuric acid, 10.6 g·L⁻¹ aluminum, and 7.1 g·L⁻¹ oxalic acid. Figure 11 depicts the behavior of current density and voltage during the anodizing process. In the potentiostatic mode, once the desired voltage (35 V, 1.6 A·dm⁻² max) was reached, the current density gradually decreased, indicating an increase in resistance and a corresponding change in the anodic layer thickness over the 60-minute period. In contrast, under the galvanostatic mode, Figure 11b shows that after reaching the desired current of 1.6 A·dm⁻², the voltage continuously increased throughout the anodizing process. This increase in voltage is attributed to the insulating nature of the anodic oxide layer, indicating an increase in the anodic layer thickness. The thickest anodic layer was obtained under galvanostatic mode, although the structure of anodized samples exhibited the same characteristics under both modes [40].

5. Influence of Post-Treatment

Among the various post-treatment methods, two fundamental approaches have emerged as particularly promising: sealing using suitable solutions and utilizing the plasma electrolytic oxidation (PEO) process.

The corrosion resistance of wrought aluminum alloys has been extensively studied to understand the beneficial impacts of the sealing process [96,97,98]. However, there is not much research available about how sealing affects anodized cast Al-Si alloys. Zhu [56] investigated the influence of High-Temperature Sealing (HTS) on a rheocast AlSi5.5Mg alloy. The study observed a decrease in the alloy’s corrosion resistance after HTS due to the formation of numerous cracks. These cracks facilitated the start and spread of destructive phenomena throughout the anode layer. During the HTS process, volume expansion is associated with the generation of inherent pressures, particularly around the Si particles. This stress leads to the formation of additional cracks in the sealed oxide layer, especially in regions adjacent to the eutectic structure.

Scampone et al. [99] studied the impact of two sealing processes—hydrothermal sealing (HTS) and cold sealing—in a nickel fluoride solution on the wear and scratch resistance of high-pressure die-cast AlSi9Cu3(Fe) alloy plates anodized in a sulfuric acid electrolyte at 16 °C. The study shows that both sealing processes improve the wear and scratch resistance of the anodized surfaces. The milled substrates, with thicker oxide layers, exhibit greater wear resistance compared to the as die-cast surfaces. Hydrothermal sealing demonstrates better scratch resistance due to the formation of boehmite and bayerite within the porous structure as opposed to the combined precipitation of multiple compounds in cold sealing based on nickel fluoride solutions.

Plasma electrolytic oxidation (PEO) can be used as a post-treatment for anodized aluminum alloys to enhance the properties of the anodic layer. Anodizing serves as a pre-treatment to reduce the energy consumption of the PEO process. PEO is an effective corrosion resistance improvement technique [100,101]. Also, nanoparticles could used in the electrolyte to incorporate inside the ceramic coating and modify its properties [101,102].

Limited studies exist on the application of PEO as a post-treatment for anodized Al-Si alloys. PEO is an effective method to mitigate the adverse effects of silicon and has the potential to address or reduce defects in the anodic layer. The impact of various sealing methods on an AlSi7Mg0.3 alloy was examined by Mohedano et al. [103] with techniques involving cerium salts, nickel, potassium permanganate, and phosphonic acid. The research revealed that all sealing treatments enhanced the alloy’s resistance to corrosion with potassium permanganate and nickel acetate-based sealings showing the most promising outcomes.

In another study, Mohedano et al. [104] investigated the effect of pre-anodizing and frequency on the energy consumption and properties of the resulting PEO coating. Increasing the frequency and current density results in a decrease in the time to the current drop, particularly for specimens with a precursor anodic film, indicating faster attainment of this critical point. In terms of energy consumption, the use of high frequencies and a precursor anodic film has a significant impact, while the applied current appears to have less influence. Using a 20 µm thick precursor anodic porous film with high current (500 mA·cm⁻²) and frequency (400 Hz) during PEO can save up to 76% energy compared to direct PEO treatment. The improved wear and corrosion resistance is due to microstructural refinement from high-frequency processing and an early transition to the “soft-sparking regime”. Notably, the frequency has a larger impact on corrosion resistance than the presence of the anodic precursor.

6. Perspectives and Future Outlook

The anodizing process of aluminum–silicon (Al-Si) cast alloys poses several challenges, which is primarily due to the heterogeneous microstructure and the complex electrochemical reactions involved.

One major issue is the presence of alloying elements such as silicon, iron, and copper, which can form intermetallic compounds and secondary phase particles that significantly affect the anodization process. The size and distribution of Si particles influence the occurrence of defects in the anodic layer, such as cracks, cavities, and unanodized aluminum regions. The shape and size of Si particles can be modified by adding certain elements to the alloy, such as Sr and Sb. Investigating the anodizing behavior of new alloys (e.g., adding boron to an Al–Si–Mg alloy) can provide valuable insights into how these additives impact the anodic layer film of Al-Si alloys. Iron-rich intermetallic particles exhibit different behaviors depending on their chemical composition. During the anodizing process, they have the potential to either undergo partial or complete oxidation or to hinder the formation of the oxide film. Copper-rich intermetallic particles can display both anodic and cathodic behaviors depending on their stoichiometry.

The microstructure of Al-Si cast alloys is significantly influenced by the casting process and the surface condition before anodizing. Enhancing the anodizing response involves the removal of the surface liquid segregation (SLS) that contains intermetallic compounds formed during casting. A thicker anodic layer is achieved after removing the oxide skin from the casting surface. However, thicker anodic layers tend to have more defects than thinner ones, resulting in lower surface hardness and abrasion resistance.

Anodizing parameters control the morphology and thickness of the anodic layer. Generally, optimizing conditions such as electrolyte composition, temperature, and current density can mitigate some adverse effects of the heterogeneous structure.

Post-treatment processes like hydrothermal sealing and plasma electrolytic oxidation (PEO) have shown promise in enhancing the anodic layer’s properties. Hydrothermal sealing can improve scratch resistance by forming boehmite and bayerite within the porous structure of the anodic layer. PEO, when used as a post-treatment, can significantly improve the corrosion resistance and mechanical properties of the anodic layer. Studies have shown that using PEO after anodizing can lead to substantial energy savings and improved wear resistance by refining the microstructure and reducing defects in the oxide layer.

In summary, despite these advancements, anodizing Al-Si cast alloys remains inherently challenging. The intricate relationship between the alloy’s microstructure and the anodizing process requires a comprehensive understanding to achieve better control and improved outcomes. Continued research in alloy modification, anodizing techniques, and post-treatment processes is essential to overcome these challenges and enhance the durability and performance of anodized Al-Si cast components.